Saturday, October 8, 2011

Heavy rain where it's needed the most

If you remember, during the late summer there was a lot of talk about the wildfires in central and west Texas.  That area has been stuck in extreme or exceptional drought conditions for much of this year. Here's the official drought monitor image from NOAA as of October 4th:

You can see that pretty much all of Texas is in some pretty dire straits when it comes to needing rainfall.  But it appears that there is some relief in sight.  Take a look at the radar composite tonight over central Texas:

A large area of slowly moving, rather heavy rain has planted itself over much of central Texas.  There have been some strong wind gusts reported with some of the more intense cores, but no real severe weather.  Just lots of heavy rain.  I'll update this on Sunday morning with the 24-hour rainfall totals from across central Texas.

So what's helping to cause this rain?  Another one of these cut-off lows--like the one that was parked over the midwest and mid-Atlantic states a week ago--has set up over the Rockies, as seen on the 300mb analysis from this evening:
What's interesting about this particular setup is that there doesn't seem to be a strong or organized surface low associated with this upper-level low.  A check of the surface map shows that there's an "inverted trough" of low pressure, but really only seems like its there because you have to have a relatively "low" pressure point between the sprawling high over the eastern US (which has brought delightful temperatures to much of the upper midwest) and the high over the northern Rockies today.
I find it somewhat unusual to see the weather pattern more dominated by sprawling high pressure centers as opposed to powerful lows.  But, here it is.  Notice how as we approach winter the pressure pattern up north is really starting to get intense.  A powerful surface low is analyzed over the southeastern tip of Alaska and there's a very tight pressure gradient across northern Ontario and Quebec.  Tight pressure gradients imply strong winds, and you can see the cold temperatures up there as well.  Soon that sort of stuff will be headed our way...

However, with respect to today's setup, steady southerly and southeasterly winds on the western periphery of the east-coast high are helping to bring lots of relatively deep moisture out of the Gulf of Mexico and into the southern Plains.  This 850mb analysis of relative humidity shows a nice plume of moisture from the Gulf up into central Texas.
This moisture is obviously contributing to their heavy rainfall there.

Of course, while this rain is a welcome relief to drought-weary Texans, heavy prolonged rainfall after periods of drought can create flash floods.  If the ground is very, very dry it often can't absorb more water at the same rate that the rainfall is dropping water to the ground.  The result can be a lot of standing water and runoff into streams.  So--even though it's nice to get the rain, Texans should be vigilant for unpredictable flash floods.

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