Tuesday, November 15, 2011

Flow with no low

A quick look at the infrared satellite image over the eastern US today shows an elongated area of high clouds and deep convection stretching from Texas up through the eastern Great Lakes.
Looking at the radar composite also confirms that there's a lot of convection and rain going on associated with these clouds.  The Storm Prediction Center even has a slight risk for severe weather out for the southern Mississippi valley.
However, this pattern doesn't look much like what we typically see in mid-latitude cyclones.  Usually we have a pretty well-defined low pressure center with fronts.  Above this low pressure center there's often a jet streak well-positioned to give divergence aloft and the lift needed to generate lots of clouds and precipitation.  But where is the surface low on the above images?  There's no characteristic swirl of the cloud pattern or precipitation (except for the little low over eastern North Dakota).  Looking at a surface map of temperature and pressure also doesn't reveal any strong low-pressure centers.

What we do see, however, is what meteorologists call a baroclinic zone--a zone where there are relatively strong horizontal temperature gradients.  Notice that along the same corridor where we're seeing clouds and rain, there's a pretty well-defined temperature gradient with 60s to the south (he green colors) and 40s to the north (the light blue colors).  This looks kind of like a front, and you might even analyze it as a stationary front.  Baroclinic zones like this are often associated with convergence in the wind field, and you can see that there are signs of some convergence in the surface winds, particularly over the southern plains.  It turns out that weak convergence in the lower atmosphere along this baroclinic zone is providing just enough lift to support all of this rain and cloudiness.

One other feature of baroclinic zones, however, is that they provide the "fuel" for surface low pressure centers to feed on and deepen.  There's a lot of potential energy that can be tapped in a region where the temperature is changing rapidly.  We also know that baroclinic zones can strengthen the winds aloft (through the thermal wind relation).  In fact, a check of the 300mb winds aloft shows that we do indeed have a strong jet streak roughly paralleling the baroclinic zone.

Since this baroclinic zone probably slopes northward as you go up in the atmosphere (for reasons I'll explain in another blog) it makes sense that we see the jet streak to the north of the strongest temperature gradients at the surface.

But let's look at this upper-air setup for a moment.  Notice that there does look to be a shortwave trough of sorts across the central plains--troughs often bring in vorticity that can aid in surface low development.  We also might try applying that four-quadrant jet streak model I've talked about in previous blog posts to look for regions of convergence and divergence associated with this jet streak.  From that model, we know that divergence aloft often occurs in the right entrance region and the left exit region of a straight jet streak.  If we imagine air entering the jet streak over the Great Lakes in the map above, it's moving from southwest to northeast.  This would place the right entrance region somewhere over northern Texas, Oklahoma, Arkansas or Missouri.  It would place the left exit region somewhere over Ontario and Quebec.

Now, divergence aloft leads to upward motion and a lowering of the pressure at the surface.  Here's a diagram from another blog post I did about this particular fact:

So, if we see an area of potential divergence aloft (particularly over a low-level baroclinic zone), we might look for a surface low to form there.  The surface plot I showed above was at 16Z (about 10 AM CST).  Here's a plot from 20Z--just four hours later:
It now looks like we have a surface low center developing in north Texas and another area of low pressure developing in Ontario.  Pretty amazing.  We could have probably forecasted something like this to happen because of the three ingredients I have been talking about:
  1. The presence of a rather well-defined baroclinic zone in the lower atmosphere.
  2. A shortwave trough axis moving in aloft.
  3. The divergent region of a jet streak aloft moving overhead, causing the pressure at the surface to fall.
So now we are finally getting our better-defined surface lows.  The question is which low will actually become dominant and start directing the flow.  Low pressure centers need to feed on the energy in baroclinic zones to grow and deepen.   Notice on the western side of  the Ontario low there is a very strong pressure gradient, implying very strong winds out of the northwest.  Also, the temperatures accompanying those winds are bitterly cold--those pinks and purple represent temperatures in the teens.   In contrast, there don't look to be very strong temperature gradients around the Texas low--in fact it seems kind of orphaned out there away from the baroclinic zone.  You can also see that the primary baroclinic zone (the region where temperature is changing rapidly) looks to have moved further north--it's up over northern Illinois through northern Kansas now. 

These factors point to the northern low becoming the dominant low pressure center here.  With very cold air racing down the back-side of the low, this surge of air will quickly overtake the existing, somewhat stationary baroclinic zone and turn it into a full-fledged cold front.  And, indeed, that's what the models are forecasting.  Here's the NAM model forecast for tomorrow at 15Z:

The low center in eastern Ontario still isn't very well defined, but flow around it has brought in much colder air, sharpening the baroclinic zone again and turning it into a cold front.  However, this baroclinic zone/cold front is still in the same geographical area as it was today, so I wouldn't expect any break from the rain any time soon...

1 comment:

  1. Excellent post, Luke. Thanks for taking the time to explain a phenomenon easy to overlook.