Today's classic case of isentropic lift

An incredible band of snow and rain currently stretches across the eastern  half of the country, as seen here on the radar composite from around 0100 UTC tonight:

Widespread precipitation from Texas all the way up through New York.  You'll notice in the middle of that band there is an area of relatively strong reflectivity.  This doesn't necessarily mean it is precipitating heavier in that area; rather it's a symptom of what we call the "bright band" effect that occurs when the radar is sampling mixed-phase precipitation.  It turns out that melting snow is more reflective to our WSR-88D radar beams than either pure snow or pure rain.  Usually this effect is somewhat localized and doesn't always show up well on radar composites, but here we have a very clear "bright band" that shows the separation between all snow to the north and all rain to the south.

You'll also notice that the character of the reflectivity changes across the band.  To the north the reflectivity looks a lot smoother and the northern edge of the precipitation looks almost "wispy".  That's a good indication that the radar is seeing snow.  This is opposed to the precipitation to the south which is much more "blobby" and irregular---characteristic of the radar seeing rain.  A striking example of how valuable our network of radars can be for determining precipitation types (even without looking at dual-pol products!)

But there's a bit more I wanted to talk about with respect to this band.  And it's going to get a little technical, so stop now if you just want to enjoy the fun radar images.  Here we overlay the Storm Prediction Center's mesoanalysis for 0100 UTC:

There's a lot going on on this map, but I want to focus on the surface temperature contours (the red solid and blue dashed lines).  You can see where the strong temperature gradient associated with a frontal zone is located--the temperature contours are really "squashed" together in a band from central Mississippi through northern Alabama and eastern Tennessee.  But notice that the precipitation itself is actually well behind the surface front---it doesn't begin in earnest until central Tennessee into eastern Kentucky.  Why the separation?  Why is the "lift" so far behind the surface front?

This is a classic case of what we call isentropic lift.   The surface front is only the leading edge of a dome of colder air.  This edge slopes back to the north and rises in height the further north you go.  Let's draw a cross-section through an analysis of what's going on now.  Below is a cross section (from the College of DuPage site) from New Orleans, Louisiana (on the left) to Green Bay, WI (on the right):

The red lines you see there are called "isentropes", lines of equal potential temperature.  You can see that there is a "wedge" of colder potential temperatures, and this wedge slopes up to the right (to the north).  That's describing the structure of the dome-like cold air mass sitting to the north.  If you look at the wind barbs in this dome of cold air (particularly between the surface and 850 hPa), you'll notice that they all have a strong northerly component (note: even though in this cross section "north" is technically to the right, with wind barbs "north" is still oriented "up").  This makes sense---cold air advecting out of the north.  If we look on the left end of the diagram outside of the cold air (down near New Orleans) the winds have a strong southerly component---warm air advecting out of the south.

It turns out that, as long as air remains unsaturated, as it is advected along it will maintain the same potential temperature.  In other words, if air starts at a potential temperature, it will follow that same potential temperature line wherever it goes.  So let's take air just above the ground at New Orleans.  It has a potential temperature on that map of just under 300 K. As that air moves northward, it is going to stay on the 300 K isentrope.  As we said before, all the isentropes are tilted upwards as we head north.  So,  warm air moving in from the south will be forced to rise following its isentropes as it moves northward.

The air that rises will keep following the isentrope until it's saturated.  We can see from the radar images that the air must be lifted for quite a ways before it hits saturation, as the precipitation band is so far behind the surface front.  But it's all laid out for us in that cross-section!  You can even see in the cross section above that the moist air (to the south; green contours are moisture) has a lobe lifted up and over the cold air, just like we'd expect if this were happening!

We can also look at a single isentrope and see what is happening along that particular isentrope.  Let's look at the 296 K isentrope.  We see in the cross section above that the height of that isentrope above the ground changes quite a bit as you move around horizontally.  We can make a map showing the height of that isentrope above the ground, and the moisture and winds along that isentrope.  Here is such a map, again from College of DuPage:

The heights are the black contours on this map and they are given in pressure levels.  Remember that pressure decreases with actual height.  So, we see lower pressures to the north (some of the black contours get below 400 hPa up in Canada) and higher pressures to the south (around 850 hPa in Louisiana).  This agrees with that cross section---as we go north, the 296K isentrope gets higher above the ground.  Notice the winds on this map; they are mostly southerly over the southern US. Combine this with the fact that this surface is higher off the ground as you move north and we can again conclude that the air moving northward will be rising along with this isentropic surface.  We also see how much moisture is being brought northward with this air---relative humidities over 85% for much of that region! So we have moist air being forced to rise by this flow, a classic isentropic lift setup.

A warm winter rain with isentropic lift

Today an upper-level shortwave moving across the Great Plains (as seen in this 500 mb map)...

 ...and associated lift with this feature is helping to bring lots of rainy weather to much of the central part of the country.  A surface low is trying to develop along the baroclinic zone (a zone of horizontal temperature gradients) that is interacting with the jet streak aloft.  Here's the radar and surface analysis from late this morning:

Areas of rain with embedded thunderstorms.  This is somewhat unique for this time of year (though not completely unheard of), particularly in the upper midwest.  Usually we'd be expecting snow by mid-December--not heavy rain.  But, temperatures are far from snow-producing with this weather event.  Here's the GFS forecast surface temperature map for 18Z this morning:

Notice that there is a large, expansive warm sector associated with this developing cyclone--temperatures in the 40s stretch as far north as northern Illinois and Indiana, and east all the way through Virginia and Maryland.  Even up into Minnesota we're still well into the 40s at the surface--much too warm for any snow.

The horizontal temperature gradients are not to strong with this cyclone as of yet, and that may be helping to slow its development.  It looks like there's a cold front trying to form back across Nebraska and western Kansas, but the air behind it really isn't that cold.  Furthermore, the winds behind the front have a strong westerly component, meaning they are coming off the high elevations of the Rockies and the high plains and down to lower elevations further east.  This sinking motion of the air will cause it to warm, further weakening any developing temperature gradients.  So, in short, this storm doesn't look like it's going much of anywhere on the cold air side.

The temperatures aloft also don't look very conducive for snow formation.  One parameter we often look at to get a quick first guess if it's going to snow or not is something called thickness charts, specifically with reference to a "critical thickness".  I wrote a blog post about this a while back, and you can read that here.  Basically, as the atmosphere gets warmer, air expands and the "thickness" between two pressure levels increases.  The opposite happens when the atmosphere gets colder--the thickness decreases.  Here's a map of the GFS 6-hour forecast of 1000mb-500mb thickness for 18Z today:

Thicknesses are shown in the blue and red dashed lines.  The blue solid line is the 5400 meter thickness line--often used as a "critical" thickness value.  North of this line where the thicknesses are lower, the lower atmosphere tends to be cold enough to support snow.  South of this line where thicknesses are higher, the lower atmosphere tends to be too warm to produce snow.  This doesn't always hold true, but it's often a good guess.  You can see here that the "critical" thickness line is well to the north of the areas of precipitation--back across the northern plains and up through Lake Superior.  This is just another way of seeing how expansive that warm sector of this wave has become.

With all this warm advection to the east of the shortwave aloft, it would make sense that we're getting lots of clouds and rain, if only we could prove that there was rising motion going on.  Lots of warmth and moisture aren't enough by themselves to cause clouds and precipitation--we need some mechanism to lift that air so it cools to its dewpoint and starts condensing into those clouds and precipitation.

One peculiar thing about air motions in the atmosphere is that often the air tends to follow "adiabatic" or "isentropic" surfaces.  As long as a parcel of air moves along an adiabatic surface, it does not gain or lose any energy.  This is one reason air tends to follow these surfaces--it takes very little effort to do so.  So, if the isentropic surface happens to be tilted upward and the winds are blowing air along this upward-tilted isentropic surface, the air will naturally want to rise along the surface.  This can be a source of widespread lifting motion and lead to lots of clouds and precipitation.

Here's a map of one particular isentropic surface this morning--the 295 Kelvin isentropic surface.  The blue contours are the height of the surface above the ground in terms of pressure level.  This means that the lower the numbers get, the higher the isentropic surface is above the ground.  Also shown are the winds along this surface and the theta-e (a way of looking at moisture and temperature) of the air at this level.

We see, following the blue contours, that this particular isentropic surface tilts upward to the north.  In south Texas, this isentropic surface is at the 950mb pressure level.  It's up at the 900mb pressure level by central Oklahoma, then rapidly rises through the central plains and upper midwest to 750 mb by central Wisconsin back towards central Colorado.  Furthermore, look at the winds on this level from Texas up through the central plains and into the upper midwest.  They're blowing generally from south to north.  Also, high theta-e values (the green shading) indicate very moist air.

So what does this mean?  We have southerly winds pushing moist air up along a surface that tilts upward as it goes further north.  So, as air parcels are pushed northward by these winds, they rise along with the isentropic surface (until they become saturated).  As this air rises, it will cool until the dewpoint is reached and clouds and precipitation start to form.  So, on this map, we're seeing a strong lifting mechanism from Texas up through the central plains and into the Mississippi River valley and the midwest.  We call this "isentropic lift".   And this explains a lot of why in the infrared satellite image...

...we see a big swath of clouds in that exact same region.  Air is being pushed northward along a surface that tilts upward in that direction.  So the air rises right along with it.  The result?  A widespread area of clouds and precipitation anywhere we're seeing strong isentropic lift.

Heavy Rain in the East

Today finds some very wet weather on the eastern coast of the US.  Here's the national radar mosaic from early this afternoon:

Fig 1 -- Base reflectivity radar mosaic for 1946Z, Nov 30, 2010. From the College of DuPage website.

 

There's a large area of precipitation extending all the way from the middle Atlantic states down to the Gulf Coast.  There are even tornado watches for parts of the southeast.  Pretty potent system.  It seems even more so when you look at the surface map from 12Z this morning:

Fig 2 -- Sea-level pressure (contoured) and temperature (shaded) from the RUC analysis at 12Z, Nov 30, 2010.  From the HOOT website.

There's a very deep surface low that's pretty obvious over western Lake Superior at this point.  By this analysis, the minimum closed contour of pressure is 998 mb--fairly strong.  We can also see the sharp cold front associated with this low in three ways on the map above:

1. Since one of the "adages" I mentioned in an earlier post was that pressure tends to fall as a cold front approaches and then rise again in its wake, we can conclude that a cold front tends to lie in a local pressure trough.  We can see an elongation on the contours around the low pressure center along a line stretching from near Chicago, though Indiana and down into northern Alabama.  We would suspect that some sort of boundary would like in this pressure trough.
2. There is a shift in the winds along that same line.  To the west, winds are out of the west and become more northwesterly to northerly the further south you go.  To the east, winds are generally southerly.  This implies an area of convergence in the winds along that line. Convergence like that is typically associated with a front.  But, in connection with our first observation above, if there is a low pressure trough along that same boundary, since air tends to flow from areas of high pressure to low pressure, we would expect winds to head toward our trough.  Thus, this convergence makes even more sense.
3. There is a strong temperature gradient across the front, particularly to the south.  Since the technical definition of a front is a strong gradient in potential temperature, this is the surest sign we have a front there.

However, one thing I would point out is how weak the temperature gradient gets once we get close to the low.  In fact, you've got similar temperatures surrounding the low itself--the temperature gradient doesn't really sharpen up until you get into Illinois and points south.  This make me think that the low is beginning to get occluded (which often occurs as a cyclone becomes stacked in the vertical).  To see our vertical support, let's look at the winds aloft:

Fig 3 -- 300 mb wind and height analysis from 12Z, Nov 30, 2010.  From the HOOT website.

Once again, a nice jet streak overhead, though this is just behind the front.  We are all the way up at 300 mb, though, so the thermal gradients may tilt with height causing the jet to be back over Illinois instead of directly over the front.  Our surface low is still located over the "exit" region of this rather cyclonically curved jet (it's even over the left exit region, which is better for divergence).  So we can still infer that the surface low is still being supported by the winds aloft.  However, note how the real core of the jet streak--where the strongest winds are--is down over Missouri, Arkansas, Oklahoma and Texas.  It's not coincidental that our strongest temperature gradients are further south.  But we also saw in our surface analysis that temperature gradients seemed to be weakening around the surface low.  This might explain why the jet seems so much weaker and less organized further north.

So what can we conclude about this cyclone, then?  Considering the weakening temperature gradients around the low over western Lake Superior, stronger temperature gradients further south, and consequently the better upper air support further south, I might suspect that the northern low is going to slowly weaken in favor of a stronger low somewhere further south.

Now this was at 12Z--what has happened since then?  Here's a look at the pressure falls as of 20Z this afternoon:

Fig 4 -- 3-hour pressure falls and wind vectors as of 20Z, Nov 30, 2010.  From the College of DuPage website.

The problem with using the pressure falls map this far north is that there are very sparse observations over northern Ontario and northwestern Quebec.  Therefore it's difficult to tell what the low is doing as it moves into that part of Canada.  However, we can see a general area of pressure rises over Minnesota and Wisconsin associated with the low passing by them earlier.  What's interesting is the elongated area of pressure falls along the eastern slopes of the Appalachians--somewhat further south than we'd expect given the typical storm track.  This is probably pressure falls in association with the cold front approaching (remember the front lies in a pressure trough).  Yet, in the absence of a strong signal for where the  surface low is going to move--could this also indicate some cyclogenesis further south?  Perhaps.  We'd have to wait and see.

Of course, one thing that this cyclone is definitely doing is bringing a lot of rain. However, it's not just along the cold front as we can see in the radar image back in figure 1.  So, what is providing the lift in that broad region?  The answer is isentropic lift.  When air moves, it tends to want to do so without gaining or losing any energy, or rather it moves isentropically--keeping the same entropy.  This means that the air may rise or sink depending on whatever path satisfies this condition.  We measure the entropy by using potential temperature which accounts for the energy in both the actual temperature and the pressure.  Two parcels of air at the same potential temperature have the same entropy.  Therefore, a map of a constant potential temperature surface is an isentropic map.  Since parcels want to conserve their entropy, if a parcel starts out at a certain potential temperaure, it wants to stay at that potential temperature as it moves around. Therefore, we can start inferring how air is going to move based on the structure of isentropic surfaces.

Below is the 300 Kelvin Isentropic surface from 12Z this morning.

Fig 5 -- 300K Isentropic Surface with heights and winds from 12Z, Nov 30, 2010.  From the HOOT website.

This shows the contours of height (in terms of presure level) of the 300K isentropic surface.  Remember, if air starts on this surface, it wants to stay on this surface.  Green shading indicates moisture at this level, and the wind barbs are wind along this level.

Take a look at what's happening in the southeast and on the east coast.  Note now the pressure contours decrease as you go further north.  Since pressure decreases with height in the atmosphere, this implies that the isentropic surface is higher off the ground to the north and closer to the ground to the south.  Also note the winds in this area.  They are all blowing from south to north in a region with lots of moisture.  We can conclude that there is very moist air on this surface being advected from south to north.  But as it moves north, this air must follow the surface.  Since the surface is getting higher as we move north, the air must be rising too.  This is a phenomenon known as isentropic lift--and there's a lot of it going on in the eastern US.  That's what's causing the precipitation to form over such a large area and to be so heavy--lots of isentropic lift.

Of course, once air becomes saturated it doesn't follow potential temperature surfaces anymore, but once the air is saturated--we're getting condensation and rain.  So it still gives a good solid reasoning behind all the rain in the eastern US...

Tornado and Snow

Well--quite the day.  And at least from the Seattle end of things, it's just beginning...

But first--unfortunately, this morning's tornado possibility in northern Illinois verified.  A strong tornado formed just east of Rockford, Illinois, and caused significant damage in the town of Caledonia.  This same supercell produced additional tornadoes along its path as it moved across northern Illinois and southern Wisconsin.  I grew up riding my bike through Caledonia all the time--I can only hope that there are no fatalities and that the damage isn't too bad. Tornado watches continue along this frontal boundary as it pushes east, so we're not out of it yet...

I'm still trying to get some radar images from the Milwaukee radar as the tornado strengthened.  I was watching as this went on but was unable to grab any screen captures.  There was a loosely-defined hook structure in the reflectivity field.  However, the true strength really came through in the velocity field.  It should be noted that these storms were moving northeast at almost 50 knots--fast movement can often mask internal rotation signatures.  This is why it's always important to use storm-relative velocity fields when trying to assess rotation..

I also wanted to update the snow situation.  We're looking at winter storm warnings throughout the Puget Sound region, with strong northerly winds and snow expected to continue this evening.  Here are the surface observations for western Washington from

Fig 1 -- Surface Observations from 100Z, Nov 22 2010.

The lowest pressure currently observed is just under 1000 mb, which is stronger than we were expecting from our models.  The low pressure center is actually on the western side of the Olympic Mountains and slowly tracking southeast.  However, note this isn't very well reflected in the wind field--we can see that the winds along the coast are indeed curving counter-clockwise around the low like we'd expect.  But--the winds throughout Puget Sound are very strong and out of the north.  This is opposite of what we'd expect for being on that side of the low.  So what's going on?

Take a look at the wind profiler images for the lowest 3.5 km for the last day or so.  Particularly focus on the most recent four profiles (the four left-most profiles).

Fig 2 -- Wind profiler history for the lowest 3.5 km at Sand Point.  Current as of 200Z, Nov 23, 2010

The labels at the bottom are of the form date/Zulu-time.  Notice how we had relatively light winds until 2300Z.  Then the winds became northerly near the surface while the winds aloft stayed relatively out of the west-southwest.  As the hours have gone on since then, we see that the layer of northerly winds has gotten deeper and deeper with southwesterly winds aloft.  This represents a strong wedge of cold air that's moving south across the Puget Sound region near the surface.  Aloft, we still see westerly to southwesterly flow--which is more the direction we'd expect from having a low-pressure to the west-southwest of the area!

Why is this important?  Because a setup like this is going to enhance snowfall amounts across the Puget Sound region.  We have a wedge of cold air moving in at the surface, but above the low-pressure center is still advecting in moist air that's coming in off the Pacific Ocean.  As this moister air is lifted over the cold wedge, we'll see the moisture condense out and more precipitation.  Furthermore, since the air new the surface is getting so much colder (cold air advection associated with the northerly winds), that precipitation is going to fall as snow--and any liquid near the surface will soon be frozen.  I believe that we're seeing an area of enhanced isentropic lift in association with this cold wedge coming south--a topic I'll get into in a later blog.

In the meantime, time to stay indoors and stay warm in Seattle tonight!