Monday, November 7, 2011

Late-season severe weather in the southern plains

It's been a busy two weeks for me, so I apologize for not doing more blog posts.  Hopefully this week I'll be able to get back into the regular swing of things.

Today we have an interesting weather event setting up for the southern plains.  In an upper-air scenario that looks more like what we see in springtime, we have an upper-level trough across the desert southwest this morning.

The jet streak and wind fields associated with this upper-level trough are providing enough upper-level divergence to support a growing area of low-pressure, currently analyzed in west Texas.

See the trough of lower surface pressures extending northeastward through western Oklahoma out from the center of the low?  That trough marks the location of a relatively stationary front.  You can also see a bit of convergence in the surface winds along that boundary.  Notice that throughout much of eastern Texas and Oklahoma the winds are out of the south or southeast--straight out of the Gulf of Mexico.  This is bringing in some unusually humid air to the region.  Here's the Oklahoma Mesonet's dewpoint map for this morning:

You can see dewpoints in the mid-60s across much of the eastern part of the state.  However, once you cross that stationary frontal boundary across northwestern Oklahoma dewpoints drop into the mid-40s.  A strong moisture gradient is another sign of a front.

One other thing we can check is the temperature structure aloft to see if the atmosphere is unstable enough to support convection. Here's this morning's 12Z sounding from Norman, OK:

You can see that we are fairly saturated at low levels (we should be with dewpoints in the 60s at this time of year...) so we look to see if the lapse rates (the rate temperature is changing with height) are greater than the saturated adiabatic lapse rate to find instability.  On average, the saturated (or moist) adiabatic lapse rate is around 6.4 degrees Celsius per kilometer.  In the lower-left corner of the skew-T diagram above, they list the lapse rates over various layers of the atmosphere.  You can see that (except for the layer right at the surface) all the lapse rates are greater than that 6.4 degrees Celsius per kilometer benchmark.  This tells us that the atmosphere is indeed unstable with respect to saturated air.  This is good for convection.

So it appears we have all the ingredients--upper-level support, low-level convergence near a surface boundary, a conditionally unstable atmosphere, and lots of moisture near the surface.  November just seems a little late in the year to be expecting severe weather.  However, take a look at this climatology of tornado events in the Oklahoma City area from the National Severe Storms Laboratory:
The x-axis is given as the day of the year (out of 365).  Today (Nov. 7) is day 311, so that's where we are on the graph.  You can see the clear maximum during the spring time, but there is a secondary, smaller maximum in the autumn.  The autumn is once again a period when the orientation of the upper-level jets and the cold polar air are fluctuating rather wildly, so it makes sense that we see another period of slightly more active weather now.  As such, seeing tornadoes in early November is by no means historically unprecedented.  It's just not what we usually think about during this time of year.

With respect to the development of severe weather today, the Storm Prediction Center has already put out a tornado watch for north Texas and southwestern Oklahoma, citing strong low-level wind shear for the tornadic potential.  Already storms are forming around that stationary frontal boundary, as we can see in this visible satellite image:

What's interesting is that there's a lot of cloud cover filling in the entire "warm sector" of this storm--the entire region where there is warm moist air advecting in from the Gulf of Mexico.  The extensive cloud cover over eastern Oklahoma and eastern Texas is going to prevent those areas from seeing a lot of sunlight today and, consequently, they won't warm up as much at the surface. This works against promoting strong instability in those regions.

But notice that in western Oklahoma, the cloud deck isn't completely overcast.  There are lots of small cumulus clouds but no big mass of clouds blocking out the sun.  This is allowing western Oklahoma to heat up far more than eastern Oklahoma, making it more unstable there.  We can see these temperature differences between overcast eastern Oklahoma and partly sunny western Oklahoma in the Mesonet temperature observations:

Western Oklahoma has warmed into the mid-70s while eastern Oklahoma is still mostly in the mid-60s.  Those clouds make a big difference.  You can also see that it's much colder behind that front in northwestern Oklahoma.  Anyhow, this is one reason why the Storm Prediction Center is focusing their severe weather threat on western Oklahoma--they're going to warm up more, making them more unstable, and they're closer to the convergence associated with the frontal boundary.  All in all, it will be an interesting day to watch things develop.  Particularly if you have access to the dual-pol radar data coming out of the KVNX radar in northern Oklahoma...

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