MUCH NEEDED RAIN

RIO GRANDE VALLEY

Much of the Rio Grande Valley was fortunate to have received rain today (May 12) from a Mesoscale Convective System (MCS) that formed in the Texas Big Bend area around 3 AM on May 12.  Figure 1, below, shows the Mesoscale Discussion (MD) image issued out by the Strom Prediction Center at the time of the evolving system.

Figure 1. SPC's MCD#752. Written at 2:57 AM CDT 5/12/2011. Actual discussion can be found here: http://www.spc.noaa.gov/products/md/md0752.html

At about 7 AM CDT, the system began to form into a squall line of showers and thunderstorms.  Figures 2 & 3 show infrared satellite imagery and radar, respectively, of the MCS taken at 7:45 AM CDT in its beginning stages as a squall line.

Figure 2. Infrared satellite imagery of MCS at 1245Z (7:45 AM) 5/12/2011.

Figure 3. Radar imagery of MCS taken at 7:40 AM CDT 5/12/2011.

As this system pushed south and east across south, central, and east Texas, it entered a very unstable environment with decent shear and dewpoints.  However, once it started getting closer and closer to the RGV, it started losing its linear form and thus, weakened.  More on this below.  There were a few reports in and around San Antonio area of golf-ball sized hail and wind gusts up to 70 mph.  Luckily, most of the severe weather stayed north of the RGV.  I say luckily because south Texas isn’t used to severe weather, and if severe weather were to strike, I’m not sure how lives would be impacted.

There were a couple of reasons why the bow echo, or squall line, started to lose its form as it was coming through deep south Texas.  For one, parts of the MCS were moving in different directions.  The northern portion of the line was moving east, towards Houston and Galveston.  The center of the line was moving east-southeast towards Corpus Christi.  The tail end of the storm was moving in a southeast motion.  All of this led up to partial breaking of the squall line.  But perhaps the number one reason why the storms started to die as they got closer to the Valley was because of the atmospheric conditions over the Valley.  In my previous blog posts, I talk about a cap inversion, or temperature inversion just above ground level, over the RGV.  It is hard for storms to develop with a sharp temperature inversion. Brownsville’s 12Z (7 AM) sounding doesn’t really show much of a cap inversion because there were actually a few showers nearby at the time of the balloon launch, but the cap was still present for the rest of the Valley.  However, the lower levels began mixing out around the time the MCS pushed through the Valley (around 3 PM CDT), which is why more thunderstorms developed. In addition, wind shear (speed & directional) was not that impressive for storm development or support.  Wind shear is needed for strong, convective storms if you want the storms to last.

Figure 4. Wind Shear diagram that describes the two types of wind shear. Source: WeatherQuestions.com

Every Thursday, NOAA releases out a drought monitor for the entire United States.   It gives you the option to look at the drought conditions by state.  Below, figure 5, is their recent drought outlook for the state of Texas.  Most of the Valley is now under an exceptional drought, the most extreme drought index that NOAA issues.  Even with the recent rains that south Texas got, it won’t help the drought conditions much at all.  The Rio Grande Valley’s total rainfall this year is still well below normal, as much as 5+ inches in some places.  We will most likely stay in a drought through the remainder of the year, unless we get a lot of rain from a tropical system or have a rainy autumn.

Figure 5. U.S. Drought Monitor released May 10, 2011. Most of the RGV is now under an exceptional drought, the rest of the Valley is under an extreme drought.

             *UPDATE* Preliminary Rainfall Totals from NWS Brownsville on 5/12

Figure 6. Preliminary rainfall that fell on 5/12/2011 in the NWS Brownsville County Warning Area. Most rain the Valley has seen since January of this year. Image Courtesy: NWS Brownsville


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About Brian

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