Outbreak

Nothing technical here tonight--just awe at how terrible the fury of nature can be.

Currently, the majority of the eastern US is under a tornado watch:

Fig 1 -- SPC tornado watches (red) and severe thunderstorm watches (blue) as of 0041Z, April 27, 2011.

With dozens and dozens of tornado reports so far.  Mississippi and Alabama have been particularly hit hard this afternoon--and storms are still continuing to move east.

Fig 2 -- SPC tornado, wind, and hail reports for April 27, 2011 as of 0041Z, April 28, 2011.

So to sum up the setup, here's the 00Z sounding from Birmingham, Alabama this evening:

Fig 3 -- Birmingham, AL, sounding from 00Z, April 28, 2011.

That's a practically uncapped environment with a surface-based CAPE calculate at 3458 J/kg--an enormous amount of convective potential energy from the surface layer.  The entire lower half of the sounding is potentially unstable--any significant lift is going to fire off storms.  And when those updrafts go up, what kind of wind shear are they running into?  Winds go from southerly at 15 knots at the surface immediately to 65 knots out of the southwest at 850mb.  This results in a 0-1km storm-relative helicity of 470 m^2/s^2.  That's an incredible amount of low-level wind shear, meaning a high potential for rotation in storms.  This is an environment with strong tornadic potential.

Everything just seems to be coming together for this event.  A deep, neutrally-titled trough in the 500-250mb layer aloft over the Mississippi Valley and a strong jet stream around its base are providing divergence aloft over much of the eastern US.  With surface dewpoints in the mid 60s or greater all the way from Tennessee up through Maryland, juicy air with lots of energy has nowhere to go but up.  Violently.

If you live anywhere where there's a tornado watch, please be extra vigilant.  We've already seen strong tornadoes move through places like Tuscaloosa, Birmingham, and even near Washington, DC.  To quote the SPC, this is a particularly dangerous situation.  Stay safe.

Squall Lines and Rotation on Radar

I've been away for a week--and what a week it has been.  Repeated rounds of tornadoes, including one that destroyed dozens of homes and part of Lambert International Airport in Saint Louis.

Unfortunately, it's not over yet.  The SPC convective outlooks currently have moderate risks of severe weather posted for the next three days.  It's round after round of severe weather.

I want to quickly look at the model forecasts to show why we aren't getting out of the woods yet.  Here is this morning's European model 500mb analysis.  I've drawn dashed lines through two troughs--one in the central US which is causing tonight's severe weather in the lower Mississippi Valley and the other just now impacting the Pacific northwest with a strong jet streak right behind it.

Fig 1 -- ECMWF 500mb analysis of winds (colors) and heights (contours) at 12Z, April 25, 2011.  From the HOOT website.

The forecast for 24 hours later, tomorrow morning, shows the central US trough moving eastward and becoming more negatively tilted over the lower Ohio River valley.  In the meantime, the Pacific northwest trough is beginning to cross the Rocky Mountains.

Fig 2 -- ECMWF 24 hour forecast of 500mb winds (colors) and heights (contours) at 12Z, April 26, 2011. From the HOOT website.

The strong jet streak behind the trough over the Rocky Mountains hints that that trough will continue to deepen as it exits the mountains and enters the plains.  The eastern trough seems to become absorbed back into the flow while the western trough really digs in by Wednesday.

Fig 3 -- ECMWF 48 hour forecast of 500mb winds (colors) and heights (contours) at 12Z, April 27, 2011. From the HOOT website.

There's even yet another shortwave trough approaching off the Pacific northwest again by Wednesday.  As we keeping having troughs move through every other day, the chances for severe weather will keep coming and coming.

But today I wanted to focus more on observing severe weather on radar, mostly in describing squall lines and bow echoes.  Tornadoes are one major element of severe weather.  However, the strong winds in squall lines cause tremendous amounts of damage too--and over a much larger area.  What causes the very strong winds at the surface in these lines of storms?

Let's re-visit the basic model of a thunderstorm. As a storm (or several storms) move from west to east, they are fed by warm moist air rising in the updraft region.  As that air rises, it cools and the water vapor condenses into rain.  The now colder, rain laden air sinks rapidly back down to the surface in the downdraft region.  Often the lift needed to get that warm, moist air initially rising is provided by convergence along a boundary like a cold front which helps move the storms along.  Think of the cold front like a big snow plow, moving along and forcing warm moist air up and into the storm as it plows forward.  Notice that the cold air sinking in the downdraft region sinks behind the cold front--this influx of cold air at the surface actually helps reinforce the cold front boundary as the cold downdraft air spreads out.  In this way, the storms can actually become self-sustaining as their own downdrafts spread out and lift up surrounding warmer, moist air, further feeding the updraft.

Fig 4 -- Schematic of a cross-section through an idealized thunderstorm.  The storm is moving to the right.

So we can see one reason why there are strong winds at the surface as the storm moves through--all that cold, rain-laden air crashing back down to the surface would make for gusty conditions indeed.  But what makes squall lines in particular so destructive?

Remember in my previous discussions about tornado formation how the low-level winds just above the surface tend to really increase in strength right around sunset.  This phenomenon is called the "nocturnal low-level wind maximum" or the "low-level jet" (these actually are different things, but the nocturnal low-level wind maximum is a kind of low-level jet...).  So let's say that we have a strong jet of winds aloft just off the surface, like what we often see around sunset.  What would that do to the storms?

Let's imagine a low-level jet of strong winds approaching the storm from the rear.  What happens as the storm interacts with the jet?  The rapidly sinking air in the downdraft region of the storm forces that jet of winds downward until it runs into the surface.  This is what tends to give squall lines their particularly powerful straight-line winds--a low-level jet of air that is pushed down to the surface.  In this case, the jet is often called the "rear inflow jet".  Not only does this interaction bring those very strong winds down to the surface, but all that air rushing in behind the storm can force the cold front or outflow boundary (that snow plow that is helping to lift up the warm-moist air in the updraft) to surge forward rather quickly.  Thus, the storms in the area where the low-level jet is intersecting the boundary will tend to rush forward faster than storms further from the center of the jet.  This is what creates the "bow echo" structure so often seen on radar--the storms "bow out" or rush forward where the strong winds of the center of the low-level jet are being forced to the surface, causing those particular storms to surge ahead.  No wonder that the strongest winds are often found on the leading edge of a bow-echo.

Fig 5 -- Schematic of a cross-section through an idealized thunderstorm interacting with a low-level jet flowing in from the left.  The downdraft forces the jet down to the surface, causing very strong winds at the surface.  Furthermore, this strong air at the surface causes the storms in the area right in the middle of the interaction with the jet to surge forward, forming a bow echo.

So that's the basic reasoning behind bow echoes and why they have such strong winds.  It's all because of the downdraft bringing down a jet of stronger winds from just above the surface.  There are lots of nuances and details to how this structure works, but that's the general idea.  For more detailed descriptions of these structures (in more technical terms) I would suggest papers like Smull and Houze, 1987  , Houze et. al 1989  or Biggerstaff and Houze 1991.So how does this look on radar?

Here's a reflectivity image of a squall line/bow echo from last week as it approached Champaign/Urbana, IL:

Fig 6 -- KILX 0.5 degree base reflectivity image from 0002Z, 4/20/2011.

Can you imagine the structures I described above?  Rain falls in the downdraft region, so we know where all of the downdraft area is--all the high reflectivity areas of red and yellow behind the leading edge of the line.  Notice the bowing structure of the leading edge of the reflectivity.  You can imagine that there are strong winds being forced down in that rain-filled downdraft area, causing the leading edge to surge forward.  We can see this on the base velocity image, too.

Fig 7 -- KILX 0.5 degree base velocity image from 0002Z, 4/20/2011.

Remember red colors indicate air moving away from the radar while green colors indicate air moving toward the radar.  In this image, the radar is toward the top of the image to the left of the center.  I've kept the schematic arrows reminding us of how the low-level jet air is being forced down to the surface.  Notice that there is a maximum of air moving away from the radar (bright red colors) right behind the leading edge of the line. This is where those upper-level winds are really being dragged down to the surface.  Immediately in front of the line is air moving toward the radar--in the updraft region.  The strong convergence between the air being dragged down to the surface and the warm air out in front is helping to force that warm moist air up ahead of the storms and keeping them going.

I wanted to zoom into the velocity image for a moment to talk more about properly interpreting radar velocity images.  Remember--the radar can only measure air moving toward or away from it--so if we try to determine wind direction, those are our only two options.  I talked in a previous post about how strong inbound air (green colors) right next to strong outbound air (red colors) can be evidence of rotation.  But here we have strong reds and greens right next to each other along the leading edge of the line.  This is NOT rotation--this is convergence.  Here's a closer view:

Fig 8 -- Zoomed in version of figure 7.  The circled areas indicate potential rotation.

You always have to keep in mind where the radar is located to determine what direction the winds are going with respect to the radar.  In the image above, the radar is off the upper-left corner.  So, when I start drawing arrows to show wind direction, all of my arrows in the green section must point directly toward the radar.  Likewise, all my arrows in the red section must point directly away from the radar.  When we do this, we see that the winds in the southern part of the line approach each other head-on--this is convergence, not rotation. For there to be rotation, we would need the arrows to be moving past each other showing spin--not right at each other showing convergence.  However, there are a few small areas where we can indeed see some rotation.  Notice the areas further north on the line where I put blue circles.  There are small "chinks" in the otherwise straight line--small areas where if you were to draw those arrows, they would point past each other instead of directly head on.  Can you see how this would happen?  It's kind of difficult to see at first.  But, with some practice, these little "chinks" in the line where the velocity arrows would be pointing past each other instead of right at each other stand out and can provide evidence of embedded rotation.

Here's another example.  This is from this afternoon's squall line moving through Arkansas.  Here's the reflectivity image.

Fig 9 -- 0.5 degree base reflectivity from KLZK, 0101Z April 26, 2011

Once again we see a relatively nice bowing line on reflectivity.  Can you imagine a jet of air descending down right in the middle of that bow and pushing it forward?  I hope so by now.  Notice that I circled a little area where high reflectivity returns seem to stick out a little bit ahead of the line. What would cause a very small area of the downdraft to shoot out even further ahead of the front of the line?  Could there be rotation there, where rotating air is wrapping the downdraft around and pushing it out ahead even faster than the descending low-level jet air?  If so, then this little reflectivity bump could be a small hook echo--we should investigate.

Here's the base velocity image corresponding to what we see above.

Fig 10 -- 0.5 degree base velocity from KLZK, 0101Z April 26, 2011

Remember--all the arrows I draw have to be pointing directly toward or directly away from the radar.  You can see the location of the radar as the small blue dot labeled "KLZK" just above and to the left of the center of the image.  Because we're so close to the radar, you can see the the inferred wind directions change depending on where they are in relation to the radar.  Nevertheless, we see that almost everywhere there is just implied convergence--evidence of strong winds and strong storms.  However, I left the circle on our area of interest.  Notice that there is indeed a little "chink" in the line separating the inbound and outbound (the red and green) velocities there!  Let's zoom in even closer:

Fig 11  -- Zoomed in figure 10,  Slight rotation is implied in the circled area.

You can see the location of the radar in the very upper left corner of the image.  If we draw our wind velocity arrows with that in mind, we see that right where that "chink" is in the circled area, the arrows can go past each other instead of hitting each other head-on.  This does imply some weak rotation there instead of just straight convergence.  So...probably based on that reflectivity bump and the fact that the velocity image does show some potential for rotation, the NWS put that area under a tornado warning.

You can see that not all bright reds next to bright greens on radar velocity images mean rotation--often they mean convergence or divergence.  You must keep in mind where the radar is located and what that means for the implied wind directions to accurately assess the radar velocity image.  The simple rule is this--draw a line directly out from the radar location to your area of interest.  If the line separating the reds and greens is perpendicular to the line drawn straight out from the radar, then it is implied convergence or divergence.  If the line separating the reds and greens is parallel to the line drawn straight out from the radar, it could imply rotation (particularly if it's a very small, localized region).  Here's the above image with that guideline applied to it:

Fig 12 -- Figure 11 annotated to describe areas of convergence versus rotation.

See the difference between the orientation of the red-green boundaries?

Here's another, more advanced but still cool thing that we can see on radar.  With certain radar viewing software, we can examine vertical cross-sections of the reflectivity and velocity fields shown by the radar and compare them to the idealized schematics I drew above.

Here's a reflectivity cross-section through the heart of the squall line in figures 9-12 as it approached Memphis this evening:

Fig 13 -- Base reflectivity cross-section from KNQA at 0313Z, April 26, 2011.

Remember, this is a cross-section--I basically just took a vertical slice right through the bow echo to show its vertical structure.  So, up on this image is actually up--height increases in the vertical with the ground at the bottom.  The radar is located just off the right side of this image.  I drew in the arrows showing the rough locations and orientations of the updraft and downdraft.  It's difficult to really see these structures on reflectivity, but we know that the heaviest rain is in the downdraft region.  So, the big column of dark reds and even pinks near the surface is associated with the downdraft.

Things get more interesting when we look at the velocity cross-section:

Fig 14 -- Storm-relative velocity cross-section from KNQA at 0313Z, April 26, 2011.

Now we can really see a lot of the structures I was describing.  Remember, the radar is located just off the right side of this image.  So--red colors show air moving away from the radar and green colors show air moving toward the radar.  You can see that well-defined column of red colors showing air moving up and away from the radar on the right side of the image.  This shows the updraft region very well--air moving up and back into the storm.  Furthermore, we can trace the low-level jet, which is marked by air heading toward the radar.  We can see a ribbon of green colors, indicating air moving toward the radar, that runs from the left the side of the image before coming down to the surface right behind the updraft.  This very nicely shows the low-level jet being pushed down to the surface by the downdraft.  You can see the strong convergence along the leading edge of that jet forced to the surface as it runs into the updraft (I marked this with the little cold front symbol at the bottom of the image).  That convergence is helping to lift the warm air up into the updraft and keeping the storm going.  Pretty amazing that you can see all these structures on radar.

Hopefully you can now look at squall lines and bow echoes on radar and understand a little bit more of what's going on.  The powerful, straight-line winds associated with bow echoes have very distinct radar signatures that can provide advance warning of their approach.  Furthermore, sometimes rotation can be embedded in that leading line, but it takes a keen eye to spot the velocity signature corresponding to that.

Quick glance at severe weather this afternoon

As many people know by now, the SPC has issued a moderate risk of severe storms this afternoon for parts of the midwest from Arkansas up through the lower Ohio River valley.

Fig 1 -- SPC Day 1 convective outlook for April 19, 2011.

SPC surface mesoanalysis at 1PM CDT (18Z) shows a low pressure centered over central Missouri with a large warm, moist sector extending to the east.  This warm sector is bounded by a warm front to the north across southern Illinois and southern Indiana and also by a cold front to the west from central Missouri down through eastern Oklahoma.

Fig 2 -- SPC Mesoanalysis of surface mean sea-level pressure (black contours), temperature (solid red and brown contours), dewpoint (colored shadings) and winds barbs) for 18Z (1PM CDT), April 19, 2011.

If we take a look at the current visible satellite image, we can already see lots of small cumulus clouds popping up throughout the warm sector.  There's overcast conditions with thick low cloud banks north of warm front and even west of the cold front, to an extent.

Fig 3 -- Visible Satellite image from GOES-E at 1815Z (115 PM CDT), April 19, 2011.  From the College of DuPage.

Why are we seeing all of these little cumulus clouds pop up but no towering cumulonimbus clouds?  The answer is the capping inversion.  Here's this morning's 12Z sounding from Springfield, MO:

Fig 4 -- 12Z Sounding from KSGF on April 19, 2011.  From the SPC.

Notice how the temperature warms abruptly with height right at around 850mb in the upper left panel of the diagram.  Below that, the air is saturated--the dewpoint and the temperature are practically the same.  With the sun out, it has warmed up a bit since early this morning--the current temperature in Springfield is 80 degrees.  So, that lowest level beneath the capping inversion has now become more unstable--if the surface temperature has risen up to 80 degrees, the temperature probably decreases rather strongly with height up to the base of the capping inversion now.  With a lot of moisture near the surface as well, small clouds are able to start convecting because of the steep low-level temperature drop off (making it unstable) and all the moisture.  However, because of the capping inversion, the clouds can't grow above that strong warming with height.  This is why we see a whole lot of small, low-level cumulus clouds, but no big thunderstorms yet--the clouds are being "capped" by the capping inversion.  Of course, as the cold front moves through, things will start getting lifted.  The capping inversion layer here is actually potentially unstable--if lifted, it will immediately start weakening.  But potential instability is the subject for another blog...

In contrast to Springfield, MO's sounding, Lincoln, Illinois's 12Z sounding shows a slightly different picture:

Fig 5 -- 12Z sounding from KILX on April 19, 2011.  From the SPC.

Lincoln, Illinois was north of the warm front this morning, underneath that area of thick low-level clouds we saw on the visible satellite image.  You can see this thick cloudiness on the sounding above--the air is saturated all the way up to around 700mb--the dewpoint and temperature are the same through that entire layer.  There is also a rather strong inversion present from around 850-950mb.  Because that cloud deck has been around all day, there hasn't been much sunlight getting through and, consequently, very little warming.  In fact, at 2PM CDT, Lincoln's reported surface temperature was only 50 degrees--not the best for strong convection.  Furthermore, that inversion layer is NOT potential unstable--lifting will not quickly erode through that layer.  Granted the warm front will move north--and that will bring much warmer air behind it.  However, because Lincoln has already spent much of the day under clouds and is still very chilly, the odds of maintaining strong thunderstorms here and further north will be somewhat lower.  This is part of the reason why the risk drops off north of the warm front.

That was just a quick look at some of the setup for today.  The SPC is calling for large hail and strong winds with a few isolated strong torandoes in today's event.  Much of Missouri and southern Illinois is under a tornado watch as those cumulus clouds keep building under the cap, waiting for the cap to erode or for the cold front to come marching on through.  If you live in a threatened area, please stay alert this afternoon.  The past few weeks have shown us just how fast and destructive these storms can be.

How to recognize rotation on Doppler radar

One of the most frequent questions I am asked by people is how we can see tornadoes on our Doppler radar.  Given that the shortwave that has trekked across the southern US the last few days has produced (and is still producing...) dozens of tornadoes, I collected a few images from radars to try and show the different signatures associated with tornadoes.  I believe that the more people know about looking for threatening weather, even in this sophisticated way, the safer we can be.  Nevertheless, personal diagnosis of radar images is no substitute for warnings from the experts at the National Weather Service.  Always pay attention to the warnings and advisories issued by the Weather Service--they're there to protect you.

Let's start with some basics.  The basic structure of a thunderstorm consists of two parts--warm, moist air rising in an updraft, and cool, rain-laden air sinking in a downdraft.

The kinds of thunderstorms that most often produce long-lived or strong tornadoes are called "supercells".  At the heart of a supercell storm is a rotating column of air called the mesocyclone.  Just because this rotating column of air is present doesn't mean that the storm is producing or will produce a tornado--all supercells have a rotating mesocyclone, but only some produce a tornado. The presence of rotation in the middle of a storm will somewhat twist and turn how the updraft and downdraft actually look.  Let's switch to an idealized block diagram looking down from the top of a storm.  The warmer, moister air is usually to the south or southeast of the storm, so that's where the storm starts drawing in its moisture.  Because there is a rotating core in supercells, it can draw in warm moist air somewhat laterally instead of just vertically.  This air that is drawn in is called the "inflow".

The inflow rises up as it enters the storm, forming the updraft.  As the warm, moist air rises, it cools off, and as the air cools off, water vapor condenses back out into liquid water and rain is formed.  Eventually, at the top of the cloud, there is rather cold, rain-filled (or hail and ice filled too) air that is no longer buoyant because it's so cold.  So, it starts sinking.  Now, for the storm to keep growing and maintain itself, the downdraft can't sink into the updraft--this would choke off the updraft and kill the storm.  But in an organized storm like a supercell, this doesn't happen.  The downdraft air takes one of two paths (usually).  A lot of the air gets pushed out in front of the updraft (to the east, typically) by the strong upper-level jets that we find at the top of the storm.  The same jet streaks that are providing the divergence aloft to support the updraft can also carry the cold, rain-filled air off ahead of the storm and away from the updraft.  Thus, we'd expect to see a lot of radar returns out ahead of the mesocyclone area, since radar beams reflect off of raindrops.  This large area of precipitation and sinking air out in front of the mesocyclone is called the "front flank downdraft".

However, because the center of the storm is rotating, some of the downdraft air can get wrapped around the back side of the storm as well.  Thus, in most supercells, we see an intense, though narrow band of rain wrapped around on the back side of the mesocyclone.  This part of the downdraft is usually called the "rear flank downdraft".  Here's a block diagram of the two paths I just metioned:

So what would our resulting radar reflectivity image look like for this kind of situation.  We expect to see lots of returns out ahead of the storm because that's where most of the rain-filled downdraft air is going.  There would also probably be a weaker area of returns (or no returns) where the inflow region happened to be, since that warm, buoyant and rising and rain is probably not falling through that area.  Then we also expect a narrow band of radar reflectivity returns behind the mesocyclone corresponding to that portion of the downdraft that gets wrapped around by the rotating air.  The light green outline here shows generally what we'd expect to see on radar:

Basically we expect to see radar reflectivity returns everywhere that the downdraft is dominant.  Does this structure start to look familiar?  This pattern of reflectivity return is the iconic signature of a supercell thunderstorm.  Notice how since the rear flank downdraft--the downdraft air getting wrapped around the mesocyclone--is forced out into the inflow air, and there is a protruding area of reflectivity return on the southern side of the storm.  This defines what is commonly called the "hook echo" that you so often hear about on TV. Hook echoes are a sign of a strong rear flank downdraft coupled with a strong inflow region.  This is only really feasible if you have very strong rotation in the storm.  Notice how there is no real reflectivity return in the inflow region.  Because air is rising in this region, no rain is falling there and we get no radar return.  However, this seems to "notch out" the otherwise round reflectivity signature of the storm.  This area of no return cutting into the storm is referred to as the "inflow notch".  It helps define the hook echo and shows you the area where the storm is bringing in its warm, moist air.  A deep inflow notch often means strong inflow and updrafts and correspondingly a strong storm.

So what does this look like on an actual radar reflectivity image?  Here's a picture of a radar reflectivity image from a supercell thunderstorm from this afternoon in North Carolina that was producing a tornado at the time:

Radar 0.5 degree Base Reflectivity image from KRAX, 2040Z, April 16, 2011. 

I've overlaid labels of several of the features we talked about.  You can see all that enhanced return to the north and east of the center of the storm--that's all rain and hail falling in the front-flank downdraft region.  That sharp cut into the reflectivity on the eastern side is the inflow notch--this is the area where the storm is drawing in its warm, moist air.  Rising motion dominates here, and as such no rain is falling (and we see no reflectivity returns).  However, there is a clear, hook-shaped appendage hanging down from the storm--a classic hook echo.  This is that other part of the downdraft air (the rear flank downdraft) that is wrapped around the mesocyclone as it descends, bringing rain (and, consequently, radar returns) with it.  Some of that enhanced return in the big ball at the end of the hook might actually be the radar reflecting off of debris being kicked up by a tornado on the ground.  The hook echo and the inflow notch wrap around the area of strongest rotation--the mesocyclone.  If the mesocyclone becomes very tightly wrapped at low-levels, it can form a tornado.

So that's all and good for reflectivity images--we can recognize these kinds of structures that show us what rotation is doing to the environment.  But what about seeing rotation itself--looking at the wind velocities?  Our Doppler radars can do this, and that's a big reason the Doppler capability was added to the radars.  They do have some limitations, though.  But first...let's get back to concepts.

We have an area of strong rotation in the mesocyclone. Winds there are probably much faster than they are elsewhere, and they also change speed and direction over a very small spatial area.  Our Doppler radars can measure wind speeds, but only in two directions--air moving toward the radar and air moving away from the radar.  The radar measures wind speed by looking at phase differences caused by moving targets along the radar beam.  However, things moving across the radar beam perpendicular to it do not cause phase shifts along the beam.  So, we can actually only measure wind in two directions--towards and away.  Turns out that's all we really need.

Let's go back to the mesocyclone diagram and insert a theoretical radar location.  Since the radar measures winds moving toward and away from it, the radar's location is going to dictate what the return looks like.  In the image below I've randomly added a radar located to the southeast of the storm.

As the radar looks toward the storm, its beam follows a path similar to the dotted line.  You can see that to the right of the dotted line, winds in the mesocyclone (which is rotating counter-clockwise here as most mesocyclones do...but that's a topic for another blog...) are generally moving away from the radar.  But, to the left of the dotted line, winds are generally moving toward the radar.  We can assume that if there's a compact space where winds suddenly shift from moving away from the radar to moving toward the radar, there's probably something rotating there.  Unfortunately, we just can't tell what's going on when winds are blowing perpendicular to the radar beam (unless you use another radar located at another location, for instance).  But, in general, anything rotating should have one side strongly blowing away from the radar and the other side strongly blowing toward the radar.

What will this look like on a radar velocity image?  Usually, warm colors like reds and oranges are used for winds blowing away from the radar.  Cool colors like greens and blues are used for winds blowing toward the radar.  If we assume that the rotation is very concentrated in the mesocyclone, we'd then expect the radar velocity return to look something like this:

We'd expect a small area of bright reds (or oranges, or other warm colors) to be located right next to a small area of bright greens (or some other cool color) with a line parallel to the radar beam running in between them.  This is important--the red and green enhancements must be on either side of a line drawn radially outward from the center of the radar.  If they are not, it implies other things (like convergence or divergence--which are also important, but not rotation...).  The reds once again indicate air moving away from the radar and the greens indicate air moving toward the area.  If they are right next to each other, that implies that the winds are changing very rapidly across a small-distance--and that there's probably strong rotation.

What does this look like in real life?  Here's the base velocity image corresponding to the radar reflectivity image of the storm in North Carolina that I showed above.

Radar 0.5 degree Base Velocity image from KRAX, 2040Z, April 16, 2011.

Note that in the radar image above, the radar location is actually off to the upper left side of the image.  I've drawn a black dotted line along what is approximately the radar beam path.  Notice how there's a very localized area of bright green colors right next to an area of bright red colors exactly where we guessed the mesocyclone to be from the reflectivity image.  If you follow the arrows I drew showing the approximate rotation of the mesocyclone, you'll see that the bright green side corresponds to air moving toward the radar and the bright red side corresponds to air moving away from the radar.  So...here we have a rotation signature right where we expected it to be--in the mesocyclone region.  This close pairing of bright red and green colors on the radar is known as a "velocity couplet". It's a hallmark of rotation.

So those are the basics.  Now let's look at a couple more examples, just for practice.  Here's a reflectivity image of the powerful supercell and tornado that did lots of damage right as it moved over downtown Raleigh, North Carolina.

Radar 0.5 degree Base Reflectivity image from KRAX, 1940Z, April 16, 2011.

Once again, off the north and the east of the storm is the front flank downdraft--where most of the downdraft air, hail and rain is falling.  We see an inflow notch cutting in and a strong hook echo wrapping around the bottom side of the storm.  Right in between we would expect to see the mesocyclone and tornado.  You can really visualize in these reflectivity images how the mesocyclone is wrapping things up around it, forming that hook echo. How does this look on the velocity image?

Radar 0.5 degree Base Velocity image from KRAX, 1940Z, April 16, 2011.

The couplet is not as strong, but there's lots of air inbound to the radar on the southern side of the mesocyclone.  You can actually see where the radar is located in this image--in the middle of that black circle on the lower right side.  Once again, if you match up the arrows I've drawn showing the rotation to the colors, you can see that there's green on the side where the air is blowing toward the radar and red where the air is blowing away from the radar.  It matches up very well.

Of course, you'll notice on these velocity images that there are a lot of places where green and red velocity colors are right next to each other--and obviously they don't all mean rotation.  This is why I also discussed the reflectivity images--it's best to use the reflectivity images to know where to look for rotation before looking at the velocity images to verify your suspicions. If you just use the velocity images, you can sometimes get rather lost.

But, then again, sometimes the reflectivity images don't tell you everything and don't show the clear structures we saw in today's North Carolina storms.  For example, here's another tornadic supercell in Oklahoma two days ago in reflectivity:

Radar 0.5 degree Base Reflectivity image from KTLX, 202Z, April 15, 2011.

Not the clearest-cut structure here.  You might guess that that sharp cut of lower reflectivity values into the southern side of the storm could be an inflow notch.  But, then the hook to the west of that really doesn't look like a hook.  This is what's called an "HP supercell" for 'high precipitation".  When there's a whole lot of rain or hail, sometimes there is so much reflectivity return in different places that it can mask the signatures of rotation that would otherwise have been seen in the reflectivity.  But it's a strong enough storm that a look at velocity is warranted.  Here it is:

Radar 0.5 degree Base Velocity image from KTLX, 202Z, April 15, 2011..

Wow--that's some intense rotation right there.  I drew in the arrows again to show the area we're looking at.  The radar is located off to the northwest of this storm.  That's a very strong velocity couplet indeed.  But, the evidence of this rotation was somewhat hidden in the reflectivity image.  Also, the further you get from the radar, not only does your resolution decrease, but your radar beam is getting higher and higher above the ground.  This storm is rather far from the radar, and we're probably not looking at the "low-level" features of this storm.  Nevertheless, this storm is clearly rotating and with a strong velocity couplet like that, it's no wonder it was producing a tornado.

As I said before, often it can be somewhat difficult to really tell what is rotation and what's not by just looking at the velocity image alone  Often, as we saw above, the reflectivity image may not be very much help either.  In these cases, you just have to investigate and be persistent--check higher tilts of the radar to see if there's clearer signs of rotation aloft, look at a loop of images to see any suspicious signs in the recent past, and so on. However, the velocity couplets for strong storms will usually stand out.  Just as an exercise, here's a wider view of the reflectivity images of the storms in eastern Oklahoma on Thursday evening:

Radar 0.5 degree Base Reflectivity image from KSRX, 231Z, April 15, 2011.

A mess of several nasty storms.  Could that be a very fat hook echo on the southwest side of the furthest northeast cell?  Or what about the smaller appendage sticking out of that same cell just to the east of the fat hook?  And what about these other cells further to the southwest--they're definitely intense, but do they have rotation in them?

Here's the raw velocity image for that same time:

Radar 0.5 degree Base Velocity image from KSRX, 231Z, April 15, 2011.

There's a lot going on in that velocity image.  Can you spot areas that may be rotation?  On the image below I've circled some of the areas that I would want to look at more closely.  Some are pretty obvious...others are just suspicious enough to warrant a look:

Well, that's all I have for now.  I hope this has given you enough background to start spotting potentially rotating storms on radar.  Once again--just because a storm is rotating doesn't mean it's producing a tornado. And, even more importantly, just because you don't think you see any rotation on the radar but the Weather Service has a warning out does not mean you should ignore the warning. There are professional meteorologists whose job it is to analyze radar images and lots of other data to determine what is going on and who to warm.  Trust their judgment and take cover when there's a warning.

More moderate risks...and differences

We're definitely entering the heart of the spring severe weather season now.  In the wake of last weekend's moderate risks in northern Iowa and Wisconsin where several large and damaging tornadoes were reported, yet another potent shortwave is digging into the central part of the country.

The SPC has issued a moderate risk for eastern Oklahoma and western Arkansas today:

Fig 1 -- SPC day 1 convective outlook for April 14, 2011.  From the SPC.

And then as this trough and surface low move eastward, another moderate risk has been issued for an area in central Mississippi and Alabama with a slight risk area throughout much of the southeast.

Fig 2 -- SPC day 2 convective outlook for April 15, 2011. From the SPC.

Based on this morning's 12Z analyses, there is a sharp shortwave across the intermountain west at 300mb:

Fig 3 -- 300mb analysis of winds (colors) and heights (contours) valid 12Z, April 14, 2011.  From the HOOT website.

Now, compare this to the 12Z RUC model (the Rapid Update Cycle model) initialization:

Fig 4 -- 12Z RUC initialization for April 14, 2011.  From the HOOT website.

Overall the two are very similar--and they should be, as the RUC model should be initialized off of the 12Z observations.  But I do notice some small, but important differences.  The most critical is in the placement of the axis of that trough in the west.  In the objective (computer-drawn) analysis of the observations in figure 3, the axis of the trough is placed through far western Idaho and into central Utah.  However, in the RUC initialization, the trough axis seems to be located in eastern Idaho and down into central Colorado--further east than in the objective analysis.  Which do we believe?

One thing we can check is the water vapor imagery from that time.  Water vapor images retrieve radiation that is typically emitted from the upper-levels of the troposphere--around the 300mb level we're looking at in the maps above.  The strong jet streaks up there perturb and contort the water vapor field so that we can rather easly trace the location of most major jet axes aloft on the water vapor images.  Here's the water vapor image from 12Z this morning:

Fig 5 -- 12Z GOES-W water vapor image for April 14, 2011.  From the HOOT website.

See the sharp contrast between the drier air (the blacker shades) and the moister air (the white and blue shades) that stretches from eastern Nevada through southwestern Utah and into northern Arizona and New Mexico?  That roughly marks the center of the jet streak aloft at that time.  I tried to highlight it using my mouse that has a very poor response time on the image below:

Fig 6 -- Same as figure 5, but annotated with approximate location of the center of the jet streak.

If we assume that the winds are more or less parallel to the height contours (which isn't a bad assumption--we should be near geostrophic balance up there...) then we see that the actual trough axis agrees more with the RUC model initialization--it probably stretches into eastern Colorado at 12Z instead of being further west in Utah.  So...the model seems more believable after all.  This once again highlights the need to be cautious when looking at objective analyses--analyses that were done by a computer trying to fit contours to sparse data.  Using higher-resolution observations like the satellite imagery can help to correct any errors.

As the exit region of this jet streak aloft emerges from over the Rockies this afternoon, we've seen a surface low develop which is now in northwestern Oklahoma as of early this afternoon:

Fig 7 -- RUC surface analysis of temperature (colors), mean sea-level pressure (contours) and winds (barbs) for 18Z, April 14, 2011.  From the HOOT website.

 Note the wind field and temperature fields are a little different from what we usually see.  The warm sector to the east of the low is very expansive--the warm front boundary between colder air to the north and warmer air to the south extends northward out of the analyzed low pressure center before turning sharply eastward in northern Kansas and stretching across the midwest into western Pennsylvania.  This seems like an odd structure for a warm front, as usually we don't see them heading northward out of a low-pressure center.  Also the winds to the northeast of the low are more easterly than southerly--this doesn't do much for advecting in moister air needed to fuel thunderstorms.  Therefore, we might guess that the greater threat for severe storms would be further south to the southeast of the low where winds are southerly all the way down to the Gulf of Mexico.  This area probably has more moisture advecting in.  A check of the dewpoint analysis from the RUC shows that this is indeed where the higher dewpoints are located:

Fig 8 -- RUC 15Z dewpoint temperature analysis with wind barbs for April 14, 2011. From the HOOT website.

And we begin to see why the SPC has placed their moderate risk in that particular region.  Why not further south where the moisture is greater? The capping inversion seems to be stronger there and our jet streak aloft seems to be staying further north.  Without that upper air support that comes from the divergence induced by the jet streak exit region, it becomes much more difficult for storms to form.

Upper-level support is fine, but what about forcing near the surface?  We need something to provide lift.  Usually this is accomplished by a frontal boundary, but we saw that the warm front was way too far north and the cold front really didn't seem to have fully developed yet.  However, the dewpoint map shows the primary mechanism being looked to for lift--convergence of the winds along the dryline.  There is a notable dryline present in the dewpoint map from early this afternoon--the sharp boundary between moister air to the east and drier air to the west in western Oklahoma and north Texas is accompanied by winds shifting from southerly to the east and westerly to the west.  This means winds are converging near the dryline, and convergence at the surface typically forces air to rise.  As the dryline mixes eastward across the state of Oklahoma this afternoon, that convergence zone will move with it and storms will form as surface air is lifted.

Why haven't storms fired already?  Our old friend the capping inversion is still in place--but just barely.  The Norman forecast office launched a special sounding at 18Z this afternoon (around 1 PM CDT).  Here it is:

Fig 9 -- 18Z sounding from Norman, OK (KOUN) on April 14, 2011. From the SPC.

A small capping inversion remains right at around 800mb.  However, the lapse rate (the rate temperature is changing with height) below that inversion is very steep--nearly dry adiabatic.  This is a highly unstable layer--and that probably means that a lot of mixing of air is going on in that near-surface layer.  With such a small cap remaining, this mixing will almost assuredly erode the cap away in the near future.  The dewpoint is registered at 56 degrees Fahrenheit at the surface here.  Any small increases in moisture due to advection from the south will be magnified as increases in the convective potential energy.  Already there is a surface CAPE value of some 2064 J/kg.  Very unstable.

And I won't say much about wind shear, but it's rather strong--winds shift from 10 knots out of the south at the surface to 40 knots out of the southwest at 700mb in the sounding above.  Not the largest shear I've seen, but definitely there.  Furthermore, as evening approaches and the low-level jet kicks up, that shear will undoubtedly increase.  However, by then the threat will have shifted to eastern Oklahoma as the dryline will have already mixed through Norman.

One final interesting note about the movement of this low-pressure center.  The RUC model has the low pressure center moving northward along that north-south oriented warm front boundary before settling in at the point where the warm front suddenly turned east.  Here's the forecast surface map from the RUC model for 10 PM CDT tonight:

Fig 10 -- RUC 12 hour forecast of surface temperature (colors), winds (barbs) and mean sea-level pressure (contours) for 03Z, April 15, 2011.  From the HOOT website.

So the low is forecast to move almost due north throughout the day, according to the RUC model.  Also we can see that as the low moves north the cold front is finally beginning to show up, and it is beginning to overtake the dryline from central Kansas down into northern Oklahoma.  At least according to this model.

However, if we look at a map of surface pressure changes in the last three hours, we see something slightly different:

Fig 11 -- 3-hour pressure changes and wind vectors as of 19Z, April 14, 2011.  From the College of DuPage.

The area of strongest pressure falls over the last few hours has been in far eastern Kansas near the Kansas City area.  To me, this implies that the low pressure center is actually moving more easterly than the RUC model would suggest.  This would speed up the timeline of events a little bit as then the dryline would push through Oklahoma somewhat faster. If these strong pressure falls are a sign of the low pressure center's movement, then I wouldn't be surprised to see the risk area extended slightly eastward as the day wears on.  We can also watch the water vapor image to try and trace the location of the jet streak's exit region over time this afternoon.  If it seems that the jet streak is also moving further east, the surface low will probably try to stay with it as that divergence aloft is what's supporting the low.  We'll just have to see.

Regardless, I expect some impressive-looking storms in eastern Oklahoma today.  The tornadic potential will be high, so be alert and listen for warnings if you live in an at-risk area.

So what makes a PDS watch?

Last night was a lot more active than I expected.  Several tornadic supercells did severe damage in northern Iowa.  These storms fed off of convergence associated with the warm front that was lifting north through the state at the time.  I'll admit that in my last blog post I wasn't really looking at the warm front at all.  But, it is a front and there is convergence along it--enough to force some powerful storms.

Today the SPC has moved the moderate risk further north into Wisconsin and eastern Minnesota, mostly.  At 3:30 PM CDT this afternoon, the Storm Prediction Center has issued their first tornado watch of the day, and it's a special type of watch that many people aren't as familiar with--a PDS tornado watch, where PDS stands for Particularly Dangerous Situation.  What makes this kind of watch special?

Fig 1 -- SPC graphic showing extent and risks associated with PDS tornado watch 120.  as of 320 PM, CDT, April 10, 2011.

PDS watches are pretty rare--according to a study by Dean and Schaefer (2005), by 2005 only 216 out of 3058 tornado watches (around 7%) were "particularly dangerous situations".  I personally see these kinds of watches issued more frequently on the high plains and south central US as opposed to the upper midwest.

So what are the qualifications to be a PDS tornado watch as opposed to a normal tornado watch?  Well, as far as I can tell from reading the background literature, technically there aren't any hard numbers to describe criteria that differentiate between the two.  To list a watch as a PDS watch is a choice by the SPC forecaster to emphasize their belief that this could be an extremely destructive event.  Typically when the ingredients seem to have come together to produce violent tornadoes (EF 2 or greater) or they are expecting extremely strong winds over a large area or hail of unusually large size the forecaster will issue the watch as a PDS watch.  According to that same study by Dean and Schaefer (2005),  in PDS tornado watches on average, the number of reports of EF 2 or greater tornadoes is three times as high as in normal tornado watches.  I've also found that it's not only how potentially bad the threat could be, but also how widespread this extreme weather is expect to be that often dictates a PDS watch.  Both tornado watches and severe thunderstorm watches can be issued as PDS watches.

The explanation for this particular watch is right in the watch text:

...THIS IS A PARTICULARLY DANGEROUS SITUATION...
   
   DESTRUCTIVE TORNADOES...LARGE HAIL TO 4 INCHES IN DIAMETER...
   THUNDERSTORM WIND GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE
   POSSIBLE IN THESE AREAS.

--From the SPC PDS Tornado watch 120 text.

So what is the setup this afternoon so far?

The cold front has asserted itself and overtaken the dryline down into central Iowa.  The warm front has also moved up into northern Wisconsin by the middle of this afternoon.  Here's the SPC mesoanalysis graphic for 1900Z ( 2 PM CST) of surface temperature, winds, and moisture.  I've added my analysis of where the low pressure center is and where the frontal boundaries seem to be.

Fig 2 -- SPC 1900Z Mesoanalysis of surface pressure (Black contours) temperature (red solid and blue dashed contours) and dewpoint (colored shadings) for April 10, 2011.

We can see that northwestern Wisconsin is right in that "sweet spot" just to the southeast of the low pressure center where the most severe storms tend to form.  Also, there are parts of this mesoanalysis graphic I don't trust.  For example, notice how in north central Wisconsin there seems to be this dry bulge in the dewpoint shadings behind the warm front.  I don't trust this analysis--looking at actual reports  in that area, they show
dewpoints in the low 60s across all of northern Wisconsin--enough that they should be included in that shading.  Also notice how the mesoanalysis seems to ignore Lake Michigan--all of the contours go around it.  I don't really believe that either.  This is just more proof that you can't trust computer analyses blindly...

So we've got good moisture in that area.  But there's moisture all through that warm sector down into northern Illinois and eastern Iowa.  Why is the strong "particularly dangerous" tornadic situation being highlighted further north?

We have strong low-level wind shear throughout the area.  Here are the wind observations (from vertical wind profilers) at 1500m and 6000m at 2PM CDT:

Fig 3 -- VAD wind profiler winds at 1500m (red) and 6000m (blue) at 1900Z, April 10, 2011.

Notice that throughout Wisconsin, the winds veer with height--their direction changes from southerly at 1500m (the red barbs) to more southwesterly at 6000m (the blue barbs).  Furthermore, wind speeds also increase--from 20 or so knots at 1500m to 70-80 knots at 6000m.  So, we have a combination of both directional shear and speed shear--a good combination for rotating storms.

However, where will there be the most wind shear?  The blue wind barbs in the above image show the winds at 1500m--what if we went all the way down to the surface?  Go back to figure 2 above (the surface map) and click on it to enlarge it.  Look at the difference in the direction of the surface winds in northern Wisconsin as opposed to in northern Illinois.  The winds at the surface in northern Wisconsin have a more southeasterly direction as opposed to the winds in northern Illinois that have a more southerly direction.  This means that the winds in northern Wisconsin turn more with height in the near-surface layer than they do further south.  It doesn't seem like that much of a difference, but it can be enough to increase the potential for tornadic storms greatly.

What about instability?  The 1800Z sounding out of Minneapolis shows this:

Fig 4 -- 1800Z sounding from KMPX on April 10, 2011.

There still was a capping inversion present in the 800-850mb layer at 18Z (around 1PM CDT).  This is what has been holding back convective development so far.  Above that, though, steep lapse rates (the temperature falling off rather rapidly with height) are contributing to a CAPE value of 1824 J/kg--definitely unstable enough for storms at Minneapolis, and probably more unstable to the east.  With such a capping inversion in place, however, I'd expect the first storms to be firing off in the areas with the greatest potential for lift--the frontal boundaries.  Later on this afternoon as we get into the evening, perhaps there will have been enough heating during the day to erode the capping inversion enough for storms to fire out ahead of the fronts.  This may coincide with that dangerous time around sunset where the low-level winds really start picking up in speed.  This will increase that low-level wind shear even further and could make for some very tornadic storms.

We can see in the visible satellite image that storms are already beginning to fire--and right along the cold front, too, like we'd expect to see with a capping inversion in place:

Fig 5 -- GOES-E visible satellite imagery from 2015Z, April 10, 2011.

I've learned my lesson from yesterday--I'll also be watching that warm front draped across far northern Wisconsin and the upper peninsula of Michigan to look for more development there.  I'm also concerned with the "triple point" further south where the dryline intersects the cold front.  In the surface map above, I've analyzed that in central Iowa.  As this system moves east-northeastward, wind shear may maximize around that triple point as surface winds adjust to the complex pressure fields there.  As such, as storms fire further south along the cold front/dryline, there could be enough shear to get some stronger-rotating storms in that area too.

It's going to be an active afternoon and evening--a particularly dangerous one, according to the SPC.  Here's a summary of what I'll be looking at in the coming hours:

1. Storms will continue to increase in coverage along the cold front and probably start firing further south along the dryline in central Iowa as well.  If these storms, though with lift provided by the cold front, can stay surface-based by drawing in warm, moist surface air from just ahead of the cold front, they could become tornadic.  Large hail will also be a concern with these storms. 
2. Keeping an eye on the warm frontal boundary in northern Wisconsin as another focal point for convective development.
3. As the afternoon wears on into evening, we may see some storms fire out ahead of the advancing cold front.  These storms have a higher potential for being surface-based and, if they can tap into the strong wind shear, particularly in northern Wisconsin, could produce some long-lived tornadoes.
4. Also somewhat concerned about enhanced low-level shear near the "triple point" intersection of the dryline and cold front moving eastward through eastern Iowa.  
5. After night falls, low-level winds will pick up even more.  This increased shear could increase the tornado threat much more broadly across the entire region.  However, if those high winds in the low levels mix down behind the cold front, it may accelerate the front and form storms into a squall line along the advancing cold front.  These storms will pack some very, very strong winds.  I'm guessing this will be the main threat late this evening for eastern Wisconsin and across Lake Michigan into Michigan.

So...if you live in those areas, be aware this evening.  Keep in touch with a good weather information source and be aware of the changing weather around you.