Why divergence aloft leads to rising motion

Last night was quite the night for severe weather--several quasi-linear convective systems moved across the southern US.  There's another moderate risk out for today in the central Appalachians associated with this same powerful cyclone.  I didn't even mention the snowfall potential on the northern side of the cyclone...  Lots going on with this one.

But today I wanted to cover a more fundamental topic that I've been asked a lot of questions about.  Three readers of this blog have sent me messages asking this fundamental question--

Why does horizontal divergence aloft lead to rising motion?  Why can't air also come down from above?


This is a very good question--admittedly one that also bothered me when I was first learning about these patterns during my undergraduate days.  So today I thought I'd try to explain what's going on aloft when we see divergence and convergence.

One area everyone seems to be able to agree on is what happens at the surface when we have horizontal divergence or convergence.  Take divergence at the surface, for example.  If air is spreading apart, it creates a semi-vaccuum, or area of lower pressure in the middle of the area where the air is spreading out.  As the saying goes, nature abhors a vacuum and tries to fill it.  It's simply a matter of air moving from higher to lower pressure.  Since the ground is below and air doesn't come out of the ground, the only way to fill that semi-vacuum left by the diverging air is to bring more air down from above.  So divergence at the surface leads to downward motion.

Fig 1 -- Divergence at the surface leads to downward motion.  Simple conservation of mass.

Likewise we can have a similar argument for horizontal convergence at the surface.  If all this air is coming together, it's going to increase the pressure in the area where the air is converging.  This makes that pressure higher than the surrounding ambient pressure and the atmosphere will want to try to evacuate air out of the area of convergence to try and equalize things again.  Since the solid ground is below and air can't be forced into the ground, then the air has to go up.  Therefore, we see that horizontal convergence at the surface leads to rising motion.

Fig 2 -- Horizontal convergence at the surface leads to upward motion.

So all is well and good at the surface where we have the solid ground serving as a boundary to prevent fluid motion in one direction.  Therefore, by default, the air will have to move in the other direction.  But what about aloft at the jet stream level?  There's no solid ground up there... 

Most of our primary jet streams (and, consequently, most of our convergence and divergence aloft) occur at or just below the tropopause level (for reasons that would make another good blog post sometime later).  The tropopause is the boundary between the lowest level of the atmosphere, the troposphere, and the next highest layer, the stratosphere.  We see this level all the time on our vertical soundings as a kink in the temperature profile.

Fig 3 -- Sample sounding from KILX at 00Z, Sunday, Feb. 27, 2011.

Our temperature trace on these vertical soundings is given by the red line.  With some minor exceptions (they're actually very important exceptions, but not for the scope of this particular post) we see that the temperature tends to decrease with height--this makes sense.  It gets colder the further up we go in the atmosphere.  At least, it does until we hit the level of the tropopause.  Right at about 200mb on the sounding above, the lapse rate (that is, the rate at which temperature is changing with height) suddenly makes nearly a right angle.  Instead of cooling with height, we see that the temperature almost remains the same (remember on these kinds of skew-T vertical charts that the temperature lines are slanted 45 degrees to the right...) or even warms slightly.  The rate of temperature change is much less than it was below.  That abrupt transition of the temperature curve marks the level of the tropopause.

Why does the temperature stop cooling off at that level?  Above the tropopause lies the stratosphere and within the stratosphere is the ozone layer.  Most of us learn in elementary or high school that the ozone layer helps block the harmful UV radiation from the sun (hence why the ozone hole is/was such a big issue).  Ozone blocks that UV radiation by absorbing it.  As the layer absorbs radiation, it gets warmer.  So all that ozone up in the stratosphere is why temperatures don't cool off with height up there.  Kind of amazing that we can see that so clearly on our temperature profiles.

Also, notice on the above sounding that I've circled the maximum wind speed in the wind barbs on the right.  Note that the maximum winds occur right around the tropopause level or just below it.  So, as I said, when we talk about the jet stream aloft, we're usually talking about right up under that tropopause level.

Now this abrupt change in the lapse rate at the tropopause signifies another property of the tropopause and the stratosphere above it.  Because temperatures tend to stay the same or even warm a bit with height, this layer is what we call statically stable.  Static stability is directly linked to vertical motion.  Vertical motion tends to be dampened or inhibited in areas of high static stability.  In areas of low static stability (or, what we usually would call relatively unstable areas), it's a lot easier for there to be vertical motions.  Because the air tends to warm a bit with height above the tropopause, it tends to be far more stable than air throughout the troposphere below where the air tends to cool off with height.  I could spend another entire blog post explaining why this is, so I'm just going to leave it at that for now.  The key link is--stratosphere=>stable=>limited vertical motion.  And also troposphere=>less stable=>more possibility of vertical motion.

One proxy that meteorologists use to measure stability is a quantity called potential temperature (another good topic for another blog post).  It turns out as air moves around in the atmosphere, it likes to keep the same potential temperature.  Therefore, if the potential temperature gradient with height is very strong (if the contours are very close together), as air moves around it is forced to stay very close to the same potential temperature value it started with.  If the gradient of potential temperature is weaker (if the contours are further apart), the air has a bit more freedom of how it can move around.

Fig 4 -- Comparison of very stable and less stable environments.  Air parcels want to stay at the same potential temperature (the same value of theta in the above images).  If the air is stable, the theta gradient is very strong (the top panel) and the parcel does not have a lot of potential for vertical motion.  If the air is less stable, the theta gradient is weaker (the bottom panel) and the air parcel has a larger potential for vertical motion.

Below is an averaged vertical cross section across the northern hemisphere of potential temperature.  Notice how below around 200mb the potential temperature gradients are pretty weak.  But suddenly, right at around the 150-200mb levels, we start seeing lots and lots of potential temperature contours--potential temperature is increasing rapidly with height.  This is another way we see the tropopause--it's the level where we start seeing a very strong potential temperature gradient.  A strong potential temperature gradient means strong static stability.

Fig 5 -- Vertical cross section of potential temperature across the northern hemisphere from July 18, 2010.  Latitude increases as you go to the right.  The troposphere is marked by very weak potential temperature gradients whereas the more stable stratosphere above has very strong potential temperature gradients (tightly packed contours).

So hopefully by now I've managed to convince you that at and above the tropopause the air is very stable.  Because the air is so stable, vertical motions are inhibited.  But you've probably actually seen this particular phenomenon yourself.  Ever look at a big thunderstorm with its anvil cloud shape?  Why is it shaped like an anvil?  Because as all that air rushes upward in the thunderstorm, eventually it hits the tropopause.  When it hits the tropopause, the strong stability reduces the vertical motion.  There's still all that air lifting in the updraft below, though.  So what does the air do?  It spreads out.

Fig 6 -- Schematic of a thunderstorm anvil cloud overlayed on a photo taken by the author in Anchorage, Alaska, during the summer of 2010.

So you can literally see the abrupt transition between the less stable air below and the very stable air above the tropopause as the level where the thunderstorm anvil spreads out!  All because the strong static stability above the tropopause works against vertical motion, forcing the air to go somewhere else instead of up.  Therefore, the air spreads out instead.

But this gets us back to our fundamental question of what happens with divergence and convergence in the jets aloft.  Take divergence for example.  The original question posed was why divergence aloft didn't mean downward motion from above to fill the void being left by the diverging air.

Fig 7 -- Why can't we see downwward motion and upward motion if we have divergence in the jet stream winds aloft?

The above schematic would make perfect sense--except for the fact that our jet stream winds and their associated divergence (or convergence) almost always occur just below the tropopause.  We now know that the tropopause and the air above it is statically stable (it tends to warm with height) and therefore it inhibits vertical motions.  Therefore, any vertical motion above the tropopause will be zero or very, very small in comparison to the vertical motion possible in the less stable troposphere below.  Thus we assume that the dominant vertical motion by far will be upward motion from below.

Fig 8 -- Vertical motions of any kind are dampened or inhibited by the stable air above the tropopause.  The vertical motion of air in the troposphere below is much greater.

Therefore we get our final model of what we expect to see with divergence aloft--strong upward motion from below to compensate for the diverging air above.

Fig 9 -- Final model showing why we expect upward motion from below as the main compensator for divergence aloft.

If we look at convergence aloft, we see a similar story.  Any vertical motion above the tropopause, no matter the direction, will be inhibited--the primary compensation left has to be motion below in the less stable troposphere.  Thus, we see downward motion below as a response to convergence aloft.

Fig 10 -- Final model for what happens in response to convergence aloft.  Downward motion below is the favored response.

Whew.  That was a long post.  But I hope it has helped to explain that burning question about how the atmosphere responds to convergence and divergence in the upper level jets.  This discussion also brought up a whole lot of other topics for other, future blog posts...

Thanks to my blog readers who asked me questions about this topic!  I'm very grateful for your interest and I have a lot of fun trying to compose these responses.  So, keep your questions coming!

Moderate risk again...and model uncertainty

Another day, another moderate risk of severe weather in the southern US forecast for tomorrow.  Severe weather season seems to be here...

Fig 1 -- SPC day 2 convective outlook from 1730Z, Saturday, Feb. 26, 2011.

The evolution of this system seems to be straightforward--at first.  We start with the current upper-level conditions.  Like we saw with the last severe weather system, we start out with a deep trough off the coast of California this morning:

Fig 2 -- 300mb height (contoured) and winds (shaded) at 12Z, Feb. 26, 2011. 

This trough is forecast to move eastward and become more neutrally tilted (the axis will be pointed more north-south) by Sunday morning.  Both the GFS (shifting to 500mb here so that I can do a comparison...):

Fig 3 -- GFS 36 hour forecast of 500 mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011. 

And the ECMWF:

Fig 4 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 00Z, Monday, Feb. 28, 2011.

Show similar upper-air patterns on Sunday evening.  The exit region of the jet around the base of the trough (particularly the left exit region since this jet seems to be straight and not very curved) would be the favored location for divergence aloft.  Divergence aloft means low pressure and lift at the surface.  So we'd generally expect widespread lift in the eastern Oklahoma-Arkansas-southern Missouri region on Sunday night.  This corresponds well with the western part of the SPC's convective risk above.

However, from here on out things start diverging in the models (no pun intended...).  Compare the GFS forecast for 500mb on Monday morning:

Fig 5 -- GFS 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 12Z, Monday, Fe.b 28, 2011.

To the ECMWF forecast at the same time:

Fig 6 -- ECMWF 48 hour forecast of 500mb heights (contoured) and winds (shaded) at 12Z, Monday, Feb. 28, 2011.

There are subtle, but significant differences in the wind field between the two models.  Notice the location of the maximum wind in the jet.  The ECMWF has the maximum over southeastern Missouri whereas the GFS has the maximum back over southeastern Oklahoma.  The ECMWF also places the critical coupling area between the anti-cyclonically curved jet over eastern Canada and the left exit region of the jet over the central US somewhat further west and north than in the GFS model.  These differences make it difficult to predict how the surface low is expected to move from Sunday night into Monday morning, since the areas of divergence aloft associated with the jet patterns are different in the two models.

But I'm not the only one having difficulty placing the surface low.  We'll look at what the GFS forecasts to happen at the surface overnight.  On Sunday night at 00Z, the center of the low is decently well-defined over western Oklahoma.

Fig 7 -- GFS 36 hour forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) at 00Z, Monday, Feb. 28, 2011.

Note the strong baroclinic zone stretching east from the low center.  A baroclinic zone is just a fancy name for an area with a strong temperature gradient (that technically should be moving around as well, but...that's difficult to tell).  We would normally associate the temperature gradient stretching east from the low with the warm front.  But, compare the winds to the south and the north of that boundary--those are pretty strong winds to the north of it, blowing out of the north.  If the cold air to the north is advancing, this looks to be a cold front instead of a warm front.  But that wouldn't be consistent with our normal depiction of a surface low-pressure system--a cold front typically doesn't stretch to the northeast of the low center.  We'd expect to see a cold front in that location if the low center was actually further northeast --say, over Missouri or Illinois.  But here is what the GFS shows for the surface pattern just six hours later:

Fig 8 -- GFS 42 hour forecast of surface temperature (shaded), sea-level pressure (contoured) and winds (barbs) at 06Z, Monday, Feb. 28, 2011.  

The center of low pressure has been shifted to the northwest--now over Illinois and Indiana!  This can't be the same low pressure center--even driving on the interstate at 70 mph it takes 14 hours or so to reach Indiana from Oklahoma.  That would imply that the surface low would have had to have moved along at somewhere around 140 mph to move that far in that amount of time.  Highly unlikely.  So we have a reformation of the low pressure center further to the northeast along the baroclinic zone.  But this GFS model only gives us one perspective on where and when this transition will happen.

Here is the SPC's SREF forecast for low pressure centers from all of its different ensemble member models.  At 00Z Sunday night, we see that there's generally good agreement in the placement of the low center in northwestern Oklahoma/southwestern Kansas.

Fig 9 -- SREF 33 hour forecast of member surface low pressure centers for 00Z, Monday, Feb. 28, 2011.

Moving just six hours later, we can see that there is considerable spread among all the members, where they have the position of the surface low (or lows--some model members have two low centers at the same time) at various places along the baroclinic zone:

Fig 10 -- SREF 39 hour forecast of member surface low pressure centers for 06Z, Monday, Feb. 28, 2011.

So there is some ambiguity where the actual center of the low pressure is going to be.  As many storm chasers know, the further one gets from the low pressure center, supercell probabilities tend to weaken.  The low pressure center serves as a focal point for low-level wind shear and the further you get from that, usually the wind shear will tend to weaken somewhat.  Not always the case, though.

So why this ambiguity in tracking where the low center is going to be?  It all goes back into the feedback between the the temperature gradients below, which drive the winds aloft, which generate divergence aloft, which forms low pressure at the surface, which advects the temperature gradients around...and the feedback continues.  Those subtle differences in the placement of the jet maxima and the dynamics of the winds aloft that we saw in the GFS and ECMWF models earlier have drastic implications for just where the surface low will be at any given time.

So what caused the divergence of the GFS and ECMWF upper air pattern forecasts?  After all--they seemed to agree very well through early Sunday evening.  Then they started differing.  But by Sunday evening, we know that convection is going to start forming.  Everyone agrees on that.  The place where convection is likely to form would be along the fronts (the baroclinic zones) where the low-level convergence and lift are maximized.  So we're likely going to have lots of storms firing along the fronts--which is also right underneath the jets.  What do storms do?  They tend to lift lots of warm, moist air into the upper atmosphere and can draw down cool air from aloft.  This radically alters the thermal structure of the atmosphere in areas of convection.  Since the position and strength of the jets themselves is tied to the thermal structure of the atmosphere, this in turn alters the jet pattern.

So, differences in how models handle convection can feed back into altering how the models forecast large-scale, upper-level patterns.  If some models fire more convection or lift more warm air aloft in the convection, they can change the wind pattern aloft to keep things in balance.  Models that aren't as vigorous in their convective prediction may not alter their wind patterns aloft as much.  I'm guessing that differences in how the GFS and ECMWF forecast convection are leading to the differences in their upper air patterns.  I would also suggest that the same thing is happening with the different SREF model members--and, since it all feeds back, that's why 's so hard to place the low pressure center...

Regardless, there's good agreement that moisture at low levels will be there:

Fig 11 - - 33 hour forecast of 2m dewpoint temperature from the mean of the SREF model members at 00Z, Monday, Feb. 28, 2011.

The mean of the SREF models suggests 60+ degree Fahrenheit dewpoints as far north as southern Missouri and southern Illinois on Sunday night (note the strong gradient of moisture over central Oklahoma and Texas showing a classic dryline or cold front pattern).  There is some concern about an elevated mixed layer capping convection over Oklahoma--temperatures are somewhat warm at 850mb aloft.  But if that can be overcome, there is more than enough moisture to get some good storms.  Winds aloft are conducive for lift and shear also looks respectable.  So the potential for severe weather is definitely there.

Snow in Seattle (and a moderate risk!)

Quick post tonight as I have some other things I need to take care of.  But, I promised to talk about snow in Seattle today...

First, as many people have noted, the SPC has upgraded to a moderate risk for the severe weather potential tomorrow in the southern US:

Fig 1 -- SPC day 2 convective outlook as of 1730Z, Feb. 23, 2011.  From the SPC website.

So things are continuing to shape up there...

However today as promised I wanted to focus on the snow event in Seattle--and how locationally variant it has been.  Models have shuffled back and forth amazingly over just where the snow is going to fall.  Here is what the local 4km UW-WRF model was forecasting would be the 24 hour snowfall accumulations as of tomorrow morning--basically, how much snow (and where) the model predicted would fall with this event.  This first image is what the model was saying on Tuesday morning:

Fig 2 -- 48 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Back then, note the heaviest snow was forecast to be along the northern slope of the Olympics, a lot along the Cascades, and basically heavy snow to the north of Seattle.  Compare that to what the Tuesday evening run of the model said for this same time of snow accumulation.

Fig 3 -- 36 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Dramatically different. The snow accumulations moved much, much further south.  In this model run the heaviest snows seem to fall under a band that goes right over downtown Seattle.  The snowfall amounts are also forecast to be in the 12 inch+ range across the Seattle area.  That strong, thick band over the central Puget Sound region looks a lot like an enhanced Puget Sound convergence zone.  The formerly snowed-in north also seems oddly dry as compared to the previous model run.  Now for this morning's model run:

Fig 3 -- 24 hour forecast of the previous 24 hours of snowfall accumulations at 12Z, Thursday, Feb. 24, 2011.

Now the heaviest snows have moved back north again, with only two or three inches over Seattle.  This is one reason there has been a lot of confusion surrounding this event--the models just haven't agreed.

So far today, the northern snow solution has been favored--five to six inches of snow had fallen in areas north of Everett as of the middle of this afternoon.  But the snow is now beginning to move south--and is definitely linked to a convergence zone.  Here's a radar image from early this evening:

Fig 4 -- KATX 0.5 degree base reflectivity from 0115Z, Feb. 23, 2011.  

Surface observations are overlaid on the radar image above.  Note the line of enhanced reflectivities marching south through Seattle at that time.  This line was accompanied by a huge graupel downpour and has now given way to snow behind it.  That definitely looks like it's being enhanced by the convergence zone--note that the wind barbs to the north of the line are showing winds out of the north and to the south of the line we see winds out of the south.  Classic convergence zone.

But is that really a convergence zone?  That band of enhanced snow seemed to be moving pretty far south and east an hour and half later:

Fig 5 -  KATX 0.5 degree base reflectivity from 0245Z, Feb. 23, 2011.

It looks more like a front now.  In fact, most analyses on the local news call this an "arctic front" pushing south through the area.  The location of that front is going to have a lot to do with the location of the surface low off the coast.  So this front's movements will also factor into what locations are going to receive enhanced snow amounts.

So what are we expecting to happen?  Currently the surface low is sitting just off of Cape Flattery--the northwest tip of the Olympic peninsula.  As that low shifts south, the "arctic front" (and the cold air to the north of it) will shift south as well.  Note, however, in the radar image that the snow is rather diffuse--it's not one big huge sheet of reflectivities like we usually see.  This hints that there isn't as much lift as we'd be expecting for a widespread snow event.  This also leaves only three real sources of lift:

1. Orographic lift as air is forced to rise up over the mountains (the Olympics and Cascades).
2. Convergence due to the Puget Sound convergence zone or convergence along the Strait of Juan de Fuca. 
3. Convergence along the cold front moving south through the sound.

This means if you're not in the mountains, the snow is probably going to be rather localized.  As the local convergence zone(s) fidgets around tonight, lift over the convergence zone will cause areas of snow to move around throughout the Puget Sound region, probably trending further south as the evening wears on.  I really can't see us getting that much accumulations--certainly not the foot that was hinted last night.  I'd guess only an inch or two at most in the Seattle area.  However, if the convergence zone decides to park over a certain area, the snow could really accumulate.

Of course, the mountains will get a ton of snow.  They have built in lift around them...

So, in short, this isn't really like the big snow event of last November.  Roads may get a bit slick, but things shouldn't be too bad...

Severe weather for the south-central US

A lot is happening in the weather across the country over the next few days.  Today I'm going to focus on the severe weather threat Thursday into Thursday night for the south-central part of the country.  Tomorrow I'm going to talk about the snow event that is shaping up for Wednesday night into Thursday here in Seattle.

The shortwave trough that moved through a week or so ago that initially spawned some severe weather hype passed through without much of a fuss in terms of severe weather--big snow event, though.  This shortwave moving through this week, however, has a bit more certainty to it with regards to it being a severe weather producer.  So what's different about this week?

Let's look at the current 500mb map:

Fig 1 -- RUC 18Z analysis of 500mb winds (shaded and heights (contoured) on Tue., Feb. 22, 2011.  From the HOOT website.

 Right now we have pretty zonal (west-east) flow across the eastern half of the country with a bit of a ridge building over the Great Lakes.  However, two troughs are approaching from the west.  One is somewhat off the map but just entering the Pacific northwest (more on that one tomorrow).  The second is the cut-off low height center off the southern California coast.  This cut-off low is forecast to be picked up into the main west-to-east flow and move into the southwestern US tonight and tomorrow.  By Thursday morning, the GFS shows this at 500mb:

Fig 2 -- GFS 42 hour forecast of 500mb winds (shaded) and heights (contoured)  for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

Notice that there seem to be two main jet streams in the forecast map above--one involves jet streaks oriented west to east across the northern tier of the country.  However, another is oriented southwest to northeast stretching from northern Mexico into the midwest.  The presence of two jet streams like this is important--it makes this a potential severe thunderstorm event instead of another snowstorm.  Why is this?

During much of the winter, we typically see only one major jet stream across the US--the polar jet.  Remember that strong winds aloft form as a response to temperature gradients below.  As such, this one polar jet lies over the "polar front"--the boundary separating frigid arctic air from warmer, moister subtropical air.

Fig 3 -- Schematic of the single polar jet.

As warm, moist air is advected northward to the east of a low pressure center, it moves into areas that were previously underneath frigid arctic air.  This not only limits instability (we'd want it to be really warm at the surface and cold aloft to get good instability) but also causes the warm air to cool off considerably as it heads north.  As long as you are north of the polar jet, you're usually having very cold weather.

So what happens when the polar jet moves further north, particularly as we start warming up during the spring and into summer?  It leaves behind a somewhat cooler (but not frigid) air mass over the country.  However we also still get warm, moist, subtropical air trying to intrude north.  This time, however, it's moving into a less-cold air mass.  Furthermore, there's still a boundary between the cool air over the continent and the frigid air up north underneath the polar jet.  So we have two areas of temperature gradients--one between the frigid air up north and the cool air over the central continent and another between that same cool air over the central continent and warm, moist air coming from the Gulf of Mexico.  With two temperature gradients, we see two jets:

Fig 4 -- Typical spring time pattern with the polar jet further north and the "subtropical" jet to the south.

The southern jet is typically called the "subtropical jet", though usually not until it has become more organized later in the year.  Why is this setup better for severe weather?  Instead of the warm, moist semi-tropical air moving into frigid arctic air, it's moving into a moderated air mass that's typically cool and very dry.  Since the middle part of the country has warmed up some (as compared to when it was under the arctic air), it's much easier to warm the surface enough to generate strong instability.  It also means that frontally-generated precipitation can usually fall as rain instead of us worrying about temperatures near the surface being cold enough for snow.

So we have a jet pattern similar to this forecast for this Thursday morning.  Does the surface temperature profile match this model?  It certainly does.

Fig 5 -- GFS 42 hour forecast of surface pressure (contoured), temperature (shaded) and winds (barbs) for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

The cold, arctic air is confined to the northern part of the country--north of the polar jet.  There are cool temperatures in a region extending from the central plains through the upper midwest, and the warm, moist tropical air is located in the southern plains and lower Mississippi valley.  We can see a surface low also forming in west Texas underneath the divergent exit region of a jet streak in the subtropical jet stream (compare it to figure 2 above).  We still see strong thermal contrasts around the developing surface low, indicating a cold front stretching south and a warm front stretching east.  However, the frigid arctic air we would be expecting for a winter storm is left well to the north.

Is there moisture in that expanding warm sector across the southern plains?  The GFS seems to think there will be:

Fig 6 -- GFS 42 hour forecast of surface dewpoint temperature for 12Z, Thursday, Feb. 24, 2011.  From the HOOT website.

There are 60+ degree Fahrenheit dewpoints forecast for much of east Texas , Louisiana and Mississippi.  As the low moves eastward, the moisture moves too:

Fig 6 -- GFS 60 hour forecast of surface dewpoint temperature for 06Z, Friday, Feb. 25, 2011.  From the HOOT website.

So we have moisture, and the frontal boundaries will definitely provide some lift.  What about instability and wind shear?  Here's the GFS forecast sounding for Thursday evening in Jackson, Mississippi:

Fig 7 -- GFS 60 hour forecast sounding for Jackson, Mississippi (KHKS) at 00Z, Friday, Feb. 25, 2011.  From Earl's Skew-T Page.

This forecast sounding shows strong directional wind shear from southerly winds at the surface to southwesterly winds aloft--a good directional shear for severe storms.  There's also speed shear--from 10 knots at the surface to 40 knots at 10,000 feet.  With this combination, the hodograph has a nice, clockwise curvature that we like to see for severe weather.  The lapse rate isn't extremely steep--in fact it looks close to moist adiabatic for much of the way up.  Correspondingly, only 120 J/kg of surface-based CAPE is being forecast for that sounding.  This is something to watch--we'd want stronger instability for very severe storms.  However, the lift provided by the front should be enough to fire off some storms.  This is also just a sample sounding at one time at one point--instability can be greater elsewhere.  In fact, the GFS SB-CAPE forecast map looks like this:

Fig 8 -- GFS 54 hour forecast Surface-Based CAPE forecast for 00Z, Friday, Feb. 25, 2011. From the HOOT website.

The best CAPE values are forecast to pool along the frontal boundary from southeastern Oklahoma down through Texas--and even then they aren't terribly impressive. There's also a small area out in front of the front in southern Arkansas and northern Louisiana where the instability seems to be enhanced.  This area would probably see the best chance for tornadic storms--with wind shear like what we saw in that forecast sounding, there's definitely the potential for rotating storms.  However, if the front becomes the main lifting mechanism, this will probably reduce to a hail-wind event, at least in the storms along the front.

The OWL-WRF model does fire some storms out ahead of the main front, but the structure seems very unorganized:

Fig 9 -- OWL-WRF 60 hour forecast of simulated composite reflectivity at 00Z, Friday, Feb. 25, 2011.  From the HOOT website.

So this will be something to watch as we get closer.  Quite understandably, though, the SPC has issued a slight risk of severe weather for Thursday in that general area:

Fig 10 -- SPC Day 3 Convective outlook, issued 0824Z, Feb. 22, 2011.  From the SPC website.

And so we're right on schedule for our first severe weather event of the spring season.

Tomorrow I'll turn my attention to the snow event up in Seattle that may even be ongoing by tomorrow evening.  Lots of weather going on...

What makes a Red Flag Warning?

We're getting into one time of year (another is in the mid to late summer) when we start to see a fair number of Red Flag Warnings crop up across the country.

Fig 1 -- US Warnings, watches and advisories as of 1918Z, Feb 19, 2011.

In the warnings and advisories map above, the dark pink areas in the trans-Pecos region of western Texas and southern New Mexico and also in southern Virginia and northern North Carolina are areas with red flag warnings.  The light pink and blue colors across the northern plains and upper midwest are associated with a potential winter storm moving through in the next few days.

During my time in Oklahoma, I saw a lot of red flag warnings come and go for the state, particularly on the high plains to the west.  But I never figured out just what exactly that unique warning meant.  I knew it had something to do with fire danger, but what conditions were needed to merit a red flag warning?

It turns out there are three criteria, all of which make sense in the context of fire danger:

1. Relative humidity at the surface that is less than or equal to 15%.
2. "20 foot winds" of 20 mph or more and/or gusts to 35 mph.
3. National Fire Danger Rating System danger level of "high" or greater.

These conditions must occur or be forecast to occur for three hours or more for there to be a red flag warning issued.  So let's look at these three criteria in the context of the weather right now.

Surprisingly, very few people seem to generate relative humidity maps at the surface.  As meteorologists we tend to be far more concerned with dewpoints at the surface as opposed to relative humidity.  Usually this is because relative humidity is "relative" to the temperature (hence its name).  Thus it makes a somewhat poor variable for actually looking at how much moisture is in the air.

With that said, I had to go to Intellicast to find a relative humidity map at the surface for today:

Fig 2 -- Surface relative humidity as of 2:00 PM EST.  From Intellicast.

Conveniently, we see areas of very low relative humidity right where the red flag warnings have been issued.  With a 10% RH area in Virginia and North Carolina, it's clear to see why the 15% or less criterion was met there.  However, it's more difficult to see in west Texas.  This Intellicast map shows an isolated pocket of 70% RH right near that area.  So why are we looking at a red flag warning if this criteria doesn't seem to be met?

Let's check this morning's sounding from El Paso:

Fig 3 -- This morning's 12Z sounding from El Paso, TX (EPZ) on Feb. 19, 2011.  From the HOOT website.

Note how in this morning's sounding from El Paso, the morning cold air near the surface is very visible.  Radiative cooling overnight means the surface cooled down a lot---look how cold the temperature at the very bottom of the profile is compared to the air just a little bit above.  Remember what we said about relative humidity--it's "relative" to the temperature.  In some ways (though not explicitly), the relative humidity measures how close that temperature profile above is to the moisture profile.  If the temperature at the surface warms up (which it does throughout the day) but the amount of moisture stays the same, the temperature and moisture profiles will separate more and the relative humidity will go down.  Plus, the air above the surface is also dry.  Strong winds of 20+ knots shown in the sounding (along with somewhat steep lapse rates above the surface cold layer) mean that there is probably a fair amount of mixing going on.  Through this mixing process, the dry air aloft will "mix down" to the surface and dry things out even more. Though the surface layer is already pretty dry--the dewpoint in the above sounding is below freezing at the surface.

So we have reason to believe that as things are warming up today the relative humidity will drop and the surface layer relative humidity will fall.  So we can extrapolate that the first criterion will be satisfied in west Texas if it hasn't been already.

What about the next one--the winds at "20 feet" need to be at least 20 mph or gusting to 35 mph. How do we check that?

There's an odd disconnect between this particular criterion and what we actually measure.  In typical weather stations, winds are measured at 2 meters and 10 meters above the ground--or, at about 6.5 feet and 33 feet.  The meteorological community likes to keep things in the international standard units, using meters and hectopascals and degrees Celsius.  But there are those occasional areas where we have to defer to the public's use of Imperial Units here in the US.  Hence at the surface, we use miles per hour and degrees Fahrenheit and whatnot.  I'm guessing this "feet" specification comes from the fact that these red flag warnings are designed for widespread public use, so 20 feet is easier to understand. (But why not 30 feet?  Why not the 6.5 feet?  These are things I need to look up...)

So here are the wind observations as of early this afternoon:

Fig 4 -- Surface observations for the CONUS at 19Z, Feb. 19, 2011.  From the HOOT website.

Of course, in meteorology at the surface we often try to cater to the marine crowd since they are heavy users of wind forecasts.  As such, our wind barbs are all in knots.  The 20 mph threshold we are looking for corresponds to about 17.4 knots.  So we'd be looking for winds in at least the 15-20 knot range.

It's somewhat of a hard sell in both regions based on these observations--also note these are 10 m or 33 foot winds, which tend to be slightly higher than what we'd see at 20 feet.  In west Texas there is a big hole in this observation map--to the east in west central Texas there are somewhat weaker winds around 10 knots.  However, we do see winds in the 20 knot range throughout central New Mexico.  On the El Paso sounding above we also saw winds at 20 knots out of the southwest a little bit above the surface.  With vertical mixing going on, gusts to or above 20 knots are definitely possible.  So we can assume that the winds will meet the criterion in west Texas.

In Virginia and North Carolina it's a bit harder to tell.  There are some 20 knot wind observations in the foothills of the Applachians.  Also note that the general flow is out of the west--downslope out of the mountains.  This could contribute to some adiabatic warming which would drop the RH values even further--just a side note there.  But anyhow, through much of the central part of these states, winds are only showing sustained values at 10 knots.  Let's check this morning's sounding from Greensboro, NC.

Fig 5 -- Sounding from 12Z this morning at Greensboro, NC (GSO) from Feb. 19, 2011.  From the HOOT website.

We can see the very dry lower part of the atmosphere.  But note the wind structure.  While the winds at the surface are weak, immediately off the ground they jump to 30 knots out of the northwest--then 50 knots at the next level up.  These are some very strong winds very close to the surface.  Even with the not-so-steep lapse rates shown here, as the surface warms during the day we should see some vertical mixing develop.  This mixing won't have to extend very far to start tapping into those high winds just above the surface and transporting some strong wind gusts down to the surface.  As such, with strong winds so close to the surface, I'd say that at least a forecast of high enough winds to meet our red flag criterion at the surface seems justified.

So what about the final condition?  The National Fire Danger Rating System describes itself as a bunch of models that try to forecast fire danger not only due to the weather, but also due to topography and the current quality of the fuel material on the surface.  For instance, lots of dry vegetation in the area would increase the fire danger and so on.  They have a somewhat rudimentary, but still very educational website here that details what they do and how they do it.  They also publish these fire danger maps based on current observations and on model forecasts.  Here is their forecast map for today:

Fig 6 -- Forecast fire danger map from the NFDRS for Feb. 19, 2011.

It's interesting how they develop these maps.  It appears that each reporting station has a "circle of influence" where they kind of assume that the entire area around the reporting station has the same fire risk.  Where these circles overlap, there is some kind of algorithm that blends the forecasts from the stations that overlap.  Anyhow, there is a 5-level scale as shown on the legend of the map--low, moderate, high, very high and extreme.  For a red flag warning, all there needs to be is a "high" risk--so anything yellow, orange or red on the map.  We do indeed see that west Texas and southern New Mexico are at the "high" level of risk.  Central Virginia and North Carolina actually get up into the "very high" risk.  So, based on these maps, that final criterion is indeed met in both those areas.

There's a look at what goes into a red flag warning.  Red flag warnings usually result in burning bans or cautionary statements that any sort of wildfire that starts will have a strong chance of escalating quickly.  Be cautious about any sort of open flames when under a red flag warning...

Looking at a the potential for a strong storm later next week in the Pacific Northwest and with warmer temperatures in the central US, all that snow from the blizzards is going to start to melt.  Flooding concerns are about to get huge--so start paying attention if you live in a flood prone area...

Pattern shift -- when will the convective season start?

The first half of February has been one major snowfall after another.  This includes several snowfalls in places that aren't used to seeing a lot of snow--like central Oklahoma.  The bitter cold weather and several snow events in the central US have been due to a persistent pattern aloft since the beginning of February.

Fig 1 -- Hemispheric plot of 500mb heights (shaded) and mean sea level pressure (contoured) valid 00Z, Feb 8, 2011.

The above pattern is similar to what we've had in place for the past two weeks.  Note the very high amplitude ridge off the west coast of the US. This is contrasted by the general troughing (the cooler colors extending further south) across much of the central US.  Short waves (which are the source of many of our storms) will tend to follow the jet stream, and the jet stream (from our thermal wind arguments) will tend to follow the area with the strongest height gradient (assuming height is a proxy for temperature, which it isn't the best, but still...).  This means that all of our short waves will tend to follow the sharp gradient in colors on the map above, which is right around where the blues transition through greens and into yellows.  This implies a storm track where the shortwaves dive south along the eastern slopes of the Rockies, dig across the southern Plains and then move north up the east coast.  The result? Snow-producing storms a lot further south than they normally are and no northward advection of warm air into the central US--so it stays cold.

But a pattern change is currently happening.  Here in Seattle we're getting our first trough to come on shore in the last two weeks--this means that that ridge that has been just parked over the west coast is finally moving inland...

Fig 2 -- Hemispheric plot of 500mb heights (shaded) and mean sea level pressure (contoured) for 12Z, Feb 12, 2011.

Note how there's no longer a ridge off the west coast--instead the ridge has been shunted further south and inland, with its axis (we're looking at the color shadings here, by the way) now over Nevada.  That large-scale troughing over the eastern US no longer extends as far south as it used to.  Warm air looks to be returning to the central US...hooray!

But note another difference in these pictures.  There is a persistent low-pressure center in the north Atlantic that's usually referred to as the Icelandic low (even though it can meander away from Iceland).  We can see it in both the images above as that big, heavily-contoured surface low in the north Atlantic.  Notice how in the first image back on February 8th, the Icelandic low is not as deep as it is in the current image.  In fact, it seems that as the trough over the eastern US is lifting out, some of the "energy" associated with that trough might be deepening the Icelandic low.  The fluctuations in the strength of the Icelandic low are a phenomenon that is called the "North Atlantic Oscillation", and its dynamics are still being researched.  However, there is an index that measures the NAO which basically looks at the strength of the Icelandic low.  Here's a graph of how the NAO has varied over the past few months (we're only really concerned with the top frame).

Fig 3 -- NAO index for the previous four months (top panel). From the CPC NAO page.

We note that ever since the end of January, we've been in a "positive" phase of the NAO--the NAO index measured positive values which correspond to higher than usual pressures in the Icelandic low--just like we saw, the low has not been as deep as it is getting now.  However, we can also see that the recent trend in the NAO over the past week or so has been downward--the low pressure center in the north Atlantic is getting deeper again.  The red lines at the end of the graph indicate various model projections of how the NAO may evolve. It's interesting to note that several of them bring the NAO back up again, which could be evidence of more troughing over the eastern US as a result.  But, these are just models...

Speaking of models, I've started seeing a lot of buzz in various weather discussion areas online about the potential showing up in our models for the first severe weather event of the plains this season.  What's the buzz about?  Well, most of what I've seen has been about the model forecasts for next Friday, which at this point have a surface low moving across the northern plains with rather warm air (compared to the frigid air we've been seeing in the central US) brought up in the warm sector.

Fig 4 -- GFS 144 hour forecast of sea-level pressure (contours), temperature (shaded) and  winds (barbs) for 12Z, Friday, Feb. 18, 2011.

Those are 50 degree temperature projected to get all the way up to northern Illinois by the end of the week--pretty impressive considering how bitterly cold it has been recently.  Of course, in the image above, there's also a relatively strong cold front associated with this low.  At the forecast time shown here, the front stretches from near Sioux Falls, SD through Nebraska, western Kansas and down into western Oklahoma.  Considering the GFS forecast for dewpoint temperatures at the same time:

Fig 5 -- GFS 144 hour forecast of dewpoint temp (shaded) and winds (barbs) for 12Z, Friday, Feb. 18, 2011.

We see marginal dewpoints, maybe in the low 50s at best, across eastern Oklahoma and down into Texas.  So there is some northward moisture advection with this low and with convergence along the cold front, maybe we'll see some storms.  But this is still at 144 hours out--no one should trust any model on details this far out.  We'll see how this evolves in the forecasts for this week...

So when does the convective season usually start in the plains?  I did a quick survey of the past ten years on the SPC's severe weather event archive and documented the date of the first events in the year that had significant severe weather reports of any type in the plains.  I did not include events that were primarily in the southeastern US and also used some discretion as to when the first event was supposed to be--for example, the random early January severe weather outbreaks we often see were not included.  Averaging across the ten years, the average date I came up with was...

March 2nd.

Though we did see first events occurring as early as February 5th and as late as March 23rd.  There also have been month-long gaps between the first and second severe weather events on the plains.  So it's a bit rough.  But, on average--March 2nd seems to be the time when our severe weather kicks up.  But we're already within the envelope of possible dates based on past records--so it's time to start thinking ahead for this year's convective season.

The Puget Sound Convergence Zone and Convection

Ok--I was going to do a post about the large scale pattern in upper-air features over the past two weeks and how that will hopefully start breaking down by the end of this week.  But then on the way back from the University today, I started getting text messages from people...

"Was that hail that just fell?"  "Hail?  Graupel?"  "Is it supposed to snow today?"

Hmm...we were in the upper 40s today in terms of temperature at the surface...I was pretty sure we weren't expecting any kind of frozen precipitation...

But sure enough, when I turned on this evening's news, their lead story was about the downpour of hail that hit Seattle this afternoon.  Interesting...  So I fired up the radar and cycled back to the time of the report.  The radar showed this:

Fig 1 -- KATX 0.5 degree base reflectivity at 0048Z, Feb. 8, 2011.

The little cell I point out with the red arrow is the closest I could come to something looking remotely like convection going on.  A cross-section through the radar returns in that cell is also not very impressive:

Fig 2 -- KATX base reflectivity cross section through the cell identified in figure 1.

There is a small core in the middle of the cell with some vertical coherency.  The reports said the "hail" was only pea-sized or smaller (hardly worth calling "hail" in my opinion...) and as such we wouldn't expect to see THAT large of a reflectivity return from the hail compared with the surrounding rain.  The top of the core only makes it up to an estimated 5000-6000 feet too--not very tall.  But apparently this was vigorous enough convection to produce some small hail.  It's too bad our local sounding site is way out at Quileute out on the Pacific coast so our upper-air profile over Seattle is a bit of a mystery...

So how do we get convection in Seattle?  This place usually does not get the strong fronts and temperature gradients nor the instability aloft needed to produce deep convection.  So we live with shallow convection.  Still...there has to be a mechanism to provide the lift necessary to get convection going.  This is where the Puget Sound convergence zone comes in.

To describe this phenomenon, we first need to focus on the geography of the Puget Sound area.  To the east of the city of Seattle and Puget Sound lie the Cascade Mountains, running north to south.  However, to the west of Puget Sound lies the Olympic Peninsula and the relatively high, but isolated Olympic Mountains.

Fig 3 -- Topographic map of western Washington.  Higher elevations are shown in the tans and browns.  The orange colors indicate urban areas.  To the west of Seattle lie the Olympic Mountains.

Puget Sound convergence zones set up when the winds in the low levels of the atmosphere have a strong westerly component.  Let's consider what happens to low level winds out of the west once they reach the western shore of Washington.  Immediately they are confronted by the isolated high terrain of the Olympic Mountains.  All that air rushing in has two choices--it can either rise up over the mountains or go around them.  Now, we do see a fair amount of lift on the windward side of the Olympics--the Hoh rainforest lies on the western side of the Olympics for that very reason.  But generally the atmosphere suppresses large-scale rapid vertical motions like that (there are many reasons for this, but we'll just accept that for now).  Instead, much of the air is forced to go around the Olympic Mountains.

Fig 4 -- Schematic of westerly flow separating to go around the Olympic Mountains.  A lee-side low is formed.

However, when all that air splits to go around the mountains, what happens on the eastern side of the mountains?  All the air flowing around the mountains creates a sort of void in the atmosphere immediately behind the mountain range.  This is signified by a lowering of the pressure there.

But what does the atmosphere do in response to a lowering of pressure?  Air will rush in to try and fill this "void" that has been left on the eastern side of the mountains.  As a result, the wind that split to go around the mountains will be sucked back toward the low pressure that has formed on the lee side of the mountains.

Fig 5 -- The lee-side low draws the winds inward on the lee side of the mountains.  As the winds from the north and the south meet, a convergence zone is formed.

Of course, air is being sucked in toward the low pressure from both the north and the south.  When these two wind streams meet, there's a region of rather strong convergence (indicated by the dotted line in the figure above).  This convergence can provide enough lift to get shallow convection over Seattle and form small hailstorms like we saw today.  So that's the Puget Sound convergence zone in a nutshell.

So did we have those kinds of conditions today?  You bet we did.  But with a slight twist.  Here's the forecast 925 mb (low-level) chart for 00Z this evening (near the time when the hail over Seattle occurred):

Fig 6 -- UW 4km WRF 12 hour forecast of 925 mb  temperature (shaded) and winds (barbs) at 00Z, Feb. 8, 2011.

We can see here that off the Washington coast, the winds were out of the northwest.  This isn't straight out of the west, but you can imagine that a similar effect occurs.  In this case, the convergence zone would form further south than indicated in the diagram above and would also be oriented in a northwest-to-southeast direction.

Fig 7 -- Same as in figure 5 but adjusted for more northwesterly winds.  The convergence zone is located further south and oriented from northwest to southeast.

So we'd expect to see a convergence in our low-level winds south of Seattle somewhere, though the location tends to meander with time.  What I showed above is a model forecast for 925mb winds.  Do we see this convergence reflected in the observations?  Here's the surface map of observations from 0100Z this evening (about the time of the hail in Seattle):

Fig 8 -- Surface METAR observations from western Washington at 0100Z, Feb. 8, 2011. 

I've added several blue arrow roughly paralleling the wind barbs in those areas.  We see a clear flow of wind around the Olympic Mountains and decent convergence at the surface right through the southern part of Puget Sound (the dashed red line).  Also, the lowest pressure in the area (1022.8 mb) is the observation at Shelton, which I circled in orange.  This is pretty close to where we would expect see that lee-side low pressure form.  So--this is a classic convergence zone case.

Furthermore, we can look at the radar radial velocities from this time to see convergence there.

Fig 9 -- KATX 0.5 degree radial base velocities at 0101Z, Feb. 8, 2011.  Arrows showing the rough direction indicated by these colors demonstrate convergence.

The KATX radar is in the northern part of this image.  If we remember that green colors indicate air moving toward the radar and red colors indicate air moving away from the radar, we can see convergence right along that line we were expecting to find it in figure 8.  So the radar velocities also show some convergence there.

It gets more complex though--a second convergence zone seems to be forming across the northern Puget Sound area and the Strait of Juan de Fuca.  Can we have dual convergence zones?  Interesting possibility.  The surface winds don't show as clear of convergence there.  Could this be convective instability released in a direction parallel to a jet over open water (similar to a lake-effect snow band coming off of the Strait of Juan de Fuca)?  Perhaps.  More investigation would be needed.  But that's about all I can cover in one blog without going too long...