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...