Rain In The Forecast!

RIO GRANDE VALLEY 

After going almost 2 months without a drop of rain, the Valley finally has some relief in sight.  Rain is in the forecast starting Monday afternoon through Tuesday afternoon.  However, I can’t say that your area specifically will get rained on (or the quantity), as the showers will be isolated to scattered.  Best total amounts, I say, would be a little over a quarter of an inch (.25″) if you are lucky. Here, figure 1, is the current (12Z Saturday 4/30) surface analysis, showing the position the cold front [CLICK ON IMAGE TO ZOOM]:

Figure 1. Surface Analysis of North America at 12Z 4/30/2011

Why Is This Front Different Than The Others? 

   Notice the cold front is just south of the panhandle of Texas.  It is forecast to move through the RGV late Monday into early Tuesday morning.  If this cold front were like the previous cold fronts over the past couple of months, it would push through the Valley by Sunday or early Monday morning.  This cold front, though, is a slow mover.  It will take its time traveling south towards the Valley.  Once it reaches the Valley, it will take its time moving out, as well.  As a result, it will have more time to trigger a shower and possibly a thunderstorm or two ahead and even behind the front.  That’s one reason why this front is different.

     Another reason why this front is different is because a digging trough in the upper-atmosphere will position itself over southwestern U.S., bringing upper-level moisture support, shown in figure 2 below.  Many, if not all, of the previous fronts since February did not have upper-level support, which is why the Valley stayed dry.  Another advantage of this trough position in the middle and upper-atmosphere is it will most likely break the capping inversion, a.k.a temperature inversion, that is over the RGV, blamed for preventing any precipitation.  Figure 3 is an example of the cap inversion over the Valley [CLICK ON IMAGES TO ZOOM].

Figure 2. 00Z 4/30 run of NAM model at 500mb out 60 hours(Valid 12Z Mon). This image shows the trough forecast over AZ/NM. Red dash line indicated the trough axis. Drawn in yellow lines represent wind flow at 500 mb.

Figure 3. Example of capping inversion from 00Z 4/30 Brownsville sounding. A capping inversion, also known as temperature inversion, is an increase of temperature with height. It is often very difficult for rain or thunderstorms to form with a strong temperature inversion, like this one.

     Finally, this cold front is different from the previous cold front simply because it will be stronger.  If you have been in south Texas over the past couple of months, you know it hasn’t stayed “cool” very long after a cold front passes through.  Winds and temperatures are usually back to “normal” 24-36 hours after the front goes through the Valley.  I believe this cold front’s effects will be felt a little longer than that.  Some models have high temperatures in the upper 60s and lower 70s on Tuesday afternoon.  I’m not sure how accurate the models are picking up on the front, but I will tell you that it will feel pretty nice on Tuesday and Wednesday if you like temperatures in the upper 70s and lower 80s.  Even Thursday will be somewhat comfortable in the upper 80s/lower 90s; it beats middle to upper 90s!

Forecast

      Until the front approaches Monday evening/Tuesday morning, we are stuck with this windy, hot and humid weather.   The reason why it’s so windy is because the pressure gradient in the Valley is really tight, with low pressure towards the west and high pressure towards the east (see previous post for reasoning).  In fact, the National Weather Service has the Valley (except Starr County) under a Wind Advisory until 6 PM this evening (4/29).  I wouldn’t be surprised if they issued another wind advisory tomorrow (Sun, 5/1).

Edinburg’s 3-Day Forecast

Figure 4. Edinburg's 3-Day Forecast

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About Brian

University of Oklahoma graduate with a degree in Meteorology. Follow me on Twitter: @WeatherInformer
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