Rio Grande Valley 

*Please Click All Images to Enhance Picture*

I know that I have made previous blogs and/or videos about rain chances over the past 2 or 3 months, with little to no rain resulting.  The current dynamics over the Rio Grande Valley have proved that it is very difficult to get showers and thunderstorms going.  I wrote a blog on the La Niña effects on the Rio Grande Valley this year and can be found if you click here.  What I basically wrote was that the RGV is forecast to have above normal temperatures and below normal precipitation as long as the La Niña conditions persist.  It may not neutralize (normalize) until June, or even possibly July.  This means we will stick to this weather pattern until the effects of La Niña neutralize.  This is not to say that we can’t get ANY rain or break from the heat until then, though.

…Discussion: Tuesday Afternoon through Friday…

As of Tuesday afternoon, there is a deep 500mb low in the western CONUS, amplifying a ridge for much of the Southeast and Midwest regions (figure 1).  This low will continue progressing east-northeast over the next 48 hours, enhancing severe weather for the Southern Plains at the surface, associated with a cold front and high dew points.

Figure 1. 12Z 5/10/2011 500mb Map from SPC. Solid black line are isoheights, dashed red lines are lines of constant temperatures, blue flags are wind barbs from stations.

Most of the severe weather will stay well north of the RGV over the next few days.  However, the trough associated with this upper level low will dig down enough to help bring a change for mid-levels.  This trough may be strong enough to break the temperature inversion in the lower levels of the atmosphere that has been over the Valley over the past few months.  This all means, along with the help of a surface cold front north of the Valley, a chance of showers and thunderstorms is in the forecast for Thursday afternoon into Friday.  The surface high pressure, that is currently bringing in the southeasterly winds, in the eastern Gulf of Mexico  will weaken enough to help trigger storms along and ahead of the cold front.  Whether the cold front has enough support to push through the Valley is another question.  As of right now, models are struggling to come up with one solution.  If it does have enough support, it will push through the Valley sometime on Saturday.   Below, figure 2, is current (15Z) surface map showing fronts and pressures.

Figure 2. 15Z 5/10/11 surface map. Drawn in blue arrow indicates direction of cold front by Friday. Drawn in yellow arrow and circle indicates the low pressure movement by Friday.

Figure 3 is a composite map from 18Z model data.  There are many plotting options, but I just plotted the simple ones (500mb heights, 850mb winds/WAA, surface obs) because it can get quite messy.  The purpose for using composite maps is to show the big picture using only 1 map, instead of a series of maps looking at every layer in the atmosphere.  It is also clear and easier to see where there is potential for precipitation and severe weather.

Figure 3. 18Z data composite map (different levels of the atomosphere and indices stacked on one map) of the U.S. This is to show potential areas of severe weather. I circled the potential areas of showers and thunderstorms in yellow.

Figure 4 shows what the Storm Prediction Center (SPC) is forecasting for day 3 in their convective outlook.  In this case, day 3 is Thursday, May 12.  You can find the description and explanation of every risk here.  They include south TX, east TX, and northeast TX in their slight risk for Thursday.

Figure 4. SPC's 3-Day (for Thursday) outlook of severe weather. They have the RGV under a slight risk, meaning there is a slight risk of severe storms according to them. This will be monitored each day until then.

Edinburg’s 3-Day Forecast:

Although I didn’t included it in my forecast, there is still a slight chance of isolated showers and a thunderstorm Friday.  I left it out of my forecast because it will be ever so slight.  It is hard to say how much rain will fall over the next 3 or 4 days, or if we even get any rain at all.  You may be lucky if you get a downpour where you’re at.  The best case scenario will be locally up to three-quarters of an inch (.75″) of rain between now and Saturday.

Figure 5. Edinburg's 3-day forecast. Issued 5/10/11.


About Brian

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