Next Best Chance For Rain For South Texas (9/25/2011)

Been busy and away since my last post.

Figure 1. Drought Monitor's drought index for Texas valid on September 20, 2011

Figure 2. Texas rainfall over the past 30 days (8/26-9/25)

Taking a look at the drought conditions across Texas (Figure 1 above) you can see most of Texas is under the “exceptional” (D4) drought level, despite the recent rains in Texas over the past couple of weeks (Figure 2).  It’s going to take a lot more than a few inches of rain to break the drought and dry areas.  Some cities (like Houston for example)across the state are 1-2 feet (12-24″) below their average rainfall for the year.

As for south TX, parts of the Rio Grande Valley have the best drought conditions in the whole state.  This is not to say rain is not needed here.  Harlingen, McAllen, and Brownsville are all well below their average rainfall amounts for the year.  Some cities across the Valley are in jeopardy of breaking records for the driest year, since records have been kept.  September is supposed to be the rainiest month of the year the Valley, but many cities have only seen a limited amount of rain this month.  I’m sure I will have a statistic or graphic of just how dry this month has been, compared to others, at the end of the month or beginning of October (provided there isn’t a drenching of rain across the Valley in the next 5-6 days).

NEXT CHANCE OF RAIN

Figure 3. Current surface fronts (9/25)

There is a stationary front draped across north-central TX , but only bringing in dryer air for the northern part of the state.  This front will stay mostly stationary over the next couple of days and may sag a little south, but will most likely either lift back as a warm front or dissipate before reaching south TX.  Another cold front will be making its way from the north by the middle to end of this week (Wednesday-Friday), however, there may be a sooner chance at some showers and thunderstorms.  For what will cause this, we will have to look at the mid and upper levels of the atmosphere.

Figure 4. 500mb (mid-level) analysis at 12Z 9/25/2011

There is a mid-level low (above) over the IA/IL border that will cause a short wave trough (circled in red above) to round the base of the low.  This will provide instability in the mid-levels and lift at the surface.  There will be enough moisture to spark off a few showers and thunderstorms across south TX starting on Wednesday.  GFS model keeps the forecast quiet and dry, with most of the moisture staying over Mexico.  It does, however, bring showers and storms into south TX by Friday night into Saturday, courtesy of the cold front.  ECMWF (Euro) model is quite the opposite.  It brings a plethora of moisture into south Texas Wednesday through Friday, before the cold front sets in.  Therefore, I will split the difference and say that scattered showers and thunderstorms are possible (30-40%) Wednesday through Friday anywhere south of San Antonio.  Ridge of high pressure sets in for much of the central US and TX for the weekend that will keep conditions mostly dry.

Figure 4. HPC's forecast accumulated rainfall totals over the next 5 days.

The image above (figure 4) gives you an idea of how much rain the Hydrometeorological Prediction Center (HPC) is predicting as a total (accumulated) over the next 5 days.

About Brian

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