textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 1106 PM CDT Mon May 18 2026

Key Messages:

* Day-to-day or near day-to-day chances for showers and thunderstorms is expected through next weekend and possibly beyond amid a major shift in the weather pattern.

* Strong to severe thunderstorms along with heavy rainfall and localized flooding is possible through next weekend.

* Hot and humid conditions will take place once again on Tuesday; maximum heat indices between 105-111F degrees will result in widespread Major (Level 3 of 4) Heat Risk.

* A Small Craft Advisory remains in effect till 9 AM CDT Tuesday; a High Risk for Rip Currents is also in effect through at least Tuesday evening.

DISCUSSION

Issued at 1106 PM CDT Mon May 18 2026

Hot temperatures featuring Moderate (Level 2 of 4) to Major (Level 3 of 4) Heat Risk through Tuesday (possibly into Wednesday), and an increasingly unsettled weather pattern punctuating a significant pattern change from dry to wet remains to be the two main weather headlines through the forecast period.

...ACTIVE/WET WEATHER DEVELOPS THIS WEEK; MULTIPLE ROUNDS OF STRONG TO SEVERE THUNDERSTORMS AND HEAVY RAINFALL IS POSSIBLE...

Global computer and AI/ML models/ensembles continue to advertise a significant pattern change from dry to wet beginning this week. A split-flow 500 mb pattern featuring what will become an increasingly active and persistent southwest flow aloft sub-tropical jet or southern branch jet stream is in place. This will serve as a conduit for multiple impulses/perturbations tracking over the region over the next several days. Later in the week through next weekend, there are signs that mid-upper troughing along with multiple impulses/perturbations will undercut high pressure/ridging aloft.

Closer to the sfc, a frontal boundary will sag southward across the U.S. before stalling and meandering to our north across the southern Plains. A better than normal jet structure will be in place through this week. Additionally, southerly flow near the sfc will yield a pool of deep tropical moisture helping to increase the atmospheric water content over the region through the week ahead. This is justified with sample maximum precipitable water (PWAT) values between 1.80-2.50 inches, some 1-3 STDEVs above normal and above the 90th percentile of sounding climatology for mid to late May. Furthermore, high instability values will be present through this week with steep mid level lapse rates between 6.5-8.0 C/km, SBCAPE values between 2,000-4,000 J/kg, MLCAPE values between 1,000- 2,000 J/kg, a moderately to very unstable atmosphere with Showalter values ranging between -2 and -6.

All in all, the combination of a favorable mid-upper level 500 mb steering pattern, favorable jet stream dynamics, high atmospheric moisture content, and high instability will support a noticeable change to a wetter/active weather pattern this week and possibly beyond. This includes incessant day-to-day or near day-to-day chances for showers and thunderstorms through this week, and maybe through the remainder of May. The best chances for rain this week will be later in the week into next weekend. Strong to severe thunderstorms are possible given the atmospheric and forcing ingredients highlighted above. The Storm Prediction Center (SPC) continues to have the northwestern half of our area under a Marginal Risk (Level 1 of 5) for severe thunderstorms on Tuesday. This includes all of Zapata and Jim Hogg Counties, and most of Brooks and Starr Counties in it's latest SWODY2.

Given the high atmospheric moisture content available, heavy rainfall and instances of flooding is possible through the week, especially later in the week. This is especially true for slow moving thunderstorms that repeatedly track over the same areas. The Weather Prediction Center (WPC) continues to have the entire forecast area or all of Deep South Texas under a Marginal Risk (Level 1 of 4) for Excessive Rainfall in it's Day 4 (Thursday) and Day 5 (Friday) ERO.

Through tonight, a thundershower or two is possible. Latest radar scans reveal a blossoming thunderstorm moving northeast off the slopes of the Sierra Madre with batches of showers further to the north and west. Will have to monitor trends through tonight. For now, we have low (20%) chance PoPs across much of the area. Should the thunderstorm to our southwest off the Sierra madre hold together, we may need to bump PoPs later tonight. For Tuesday night into Wednesday morning, several hi-res CAM guidances continue to advertise a squall line of strong to severe thunderstorms or MCS originating from central Texas and advancing southward through Deep South Texas late Tuesday into Wednesday between midnight and 9 AM Wednesday morning. Currently, we have low-medium (20-50%) chances for showers and thunderstorms across the region late Tuesday night into Wednesday morning. This may need bumping up depending on things evolve. Again the Storm Prediction Center (SPC) has a Marginal Risk for severe thunderstorms Tuesday night into Wednesday, currently favoring Zapata, Jim Hogg, Starr, and Brooks Counties.

Wednesday night through the early parts of next week, additional precipitation chances exists. During this timeframe, we could see another one or two squall lines or MCSs with a heavy rainfall threat move through Deep South Texas and the Rio Grande Valley. Wednesday night-Thursday night, we have 30-70% chances for showers and thunderstorms over the region. Friday night through Saturday night, we have widespread categorical chances ranging between 50-75%. Sunday through Monday, there's a 40-60% chance for showers and thunderstorms across the region. Finally, Monday night through Tuesday night, there will be low chances ranging between 20-30%.

Through Saturday, quantitative precipitation forecasts (QPF) or rainfall amounts are expected to range between 1-2 inches with locally higher amounts across Deep South Texas and the Rio Grande Valley. The rain to come will certainly prove to be beneficial, helping to further improve the D1 (Moderate) to D2 (Severe) Drought status across the region.

...ANOTHER HOT DAY EXPECTED ON TUESDAY WITH WIDESPREAD MAJOR HEAT RISK...

The other weather headline through Tuesday will be the heat! Global forecast models/ensembles continue to depict a sharp 588-591 dam sub- tropical heat ridge over the region. This feature will result in another day of hotter than normal temperatures on Tuesday, and maybe even extending into Wednesday. Similar to today, forecast high temperatures for Tuesday are expected to be in the mid 90s to lower 100s with the hottest temps west of IH-69C/US-281. Dewpoints in the mid 70s will result in max heat indices or apparent temperature values climbing to between 105-111F degrees on Tuesday. These values will once again yield Moderate (Level 2 of 4) to Major (Level 3 of 4) Heat Risk. Cloud over will be a factor in these temperatures being realized.

By Wednesday, the aforementioned heat ridge relaxes which should result in a slightly cooler day. Shower and thunderstorm chances Tuesday night into Wednesday as well as cloud cover could also play a role in the slightly lower temperatures from Monday and Tuesday. Nonetheless, it will be another warmer than normal day with high temperatures in the low to mid 90s. With heat indices ranging between 100-108F degrees, Heat Risk scores will be mainly fall in the Moderate (Level 2 of 4) category over the region on Wednesday.

Thursday through next Tuesday, high temperatures will range between the upper 80s to lower 90s. This largely will be attributed to the shower and thunderstorm chances during this timeframe. Tonight through Tuesday night, overnight lows are progged to be in the upper 70s to lower 80s. Wednesday night through next Sunday night, overnight lows are expected to be mainly in the 70s.

AVIATION

(12Z TAFS) Issued at 606 AM CDT Tue May 19 2026

A cloud deck moving north will lift through the morning, returning primarily VFR through the afternoon. SCT to OVC cloud cover should remain at VFR levels, but brief drops to MVFR are possible. Southeasterly winds will become gustier to around 25 kts this morning and persist into the evening.

MVFR returns around sunset. There is a chance of showers and thunderstorms overnight. Latest hi res guidance suggests a line of thunderstorms from the north will approach the RGV tonight, but there is uncertainty on whether the line can be sustained this far south. Most likely timing would be in the final hours of the current 12Z TAF cycle.

MARINE

Issued at 1106 PM CDT Mon May 18 2026

A Small Craft Advisory remains in effect till 9 AM CDT Tuesday due to residual elevated wave heights. Outside of the threat for showers and thunderstorms, Tuesday through Wednesday, adverse (Small Craft Exercise Caution) conditions are likely. Again, outside of the threat for showers and thunderstorms Wednesday night through next Monday, marine conditions are expected to improve with moderate seas and low to moderate winds.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 94 81 93 80 / 10 10 20 0 HARLINGEN 96 79 94 77 / 10 20 20 0 MCALLEN 97 80 95 79 / 10 20 20 20 RIO GRANDE CITY 99 78 95 76 / 10 30 30 50 SOUTH PADRE ISLAND 86 81 85 81 / 10 10 20 0 BAYVIEW/PORT ISABEL 92 81 91 79 / 10 10 20 0

BRO WATCHES/WARNINGS/ADVISORIES

TX...High Rip Current Risk through this evening for TXZ451-454-455.

GM...Small Craft Advisory until 7 PM CDT this evening for GMZ130-132- 135-150-155.

Small Craft Advisory until 9 AM CDT this morning for GMZ170-175.