textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 1028 PM CDT Sun May 17 2026

Key Messages:

* Daily or near daily chances for showers and thunderstorms is expected through the upcoming workweek and possibly through the end of May as a major shift in the weather pattern takes shape.

* Strong to severe thunderstorms along with heavy rainfall and localized flooding is possible through the upcoming workweek.

* Hot and humid conditions will take place through Tuesday; maximum heat indices between 105-111F degrees will result in widespread Moderate (Level 2 of 4) to Major (Level 3 of 4) Heat Risk through Tuesday.

* A Small Craft Advisory has been extended till 9 AM CDT Tuesday; a High Risk for Rip Currents may need to be expected till Tuesday as well.

DISCUSSION

Issued at 1028 PM CDT Sun May 17 2026

Building heat with 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 are the two main weather headlines through the forecast period. We'll first begin talking about the unsettled weather pattern followed by the building heat through midweek.

Global computer and AI/ML models/ensembles continues 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 sings that mid-upper troughing along with multiple impulses/perturbations will undercut high pressure/ridging aloft. This large-scale setup will help to yield incessant day-to-day or near day-to-day chances for showers and thunderstorms through at least next week and maybe through the remainder of May.

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. More than normal favorable jet dynamics will be in place through next 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.

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. Strong to severe thunderstorms are possible given the atmospheric and forcing ingredients highlighted above. The Storm Prediction Center (SPC) has highlighted 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 SWODY3.

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) has placed 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 (Wednesday) and Day 5 (Thursday) ERO.

Most of the convection through next week is anticipated to initiate off the Sierra Madre before possibly rolling off the front range and translating east-northeastward into our CWA. So far, for Monday night, low (20%) chance PoPs are forecasted generally along the Rio Grande Valley. Tuesday night, rain chances increase with low-medium (30-40%) PoPs for showers and thunderstorms. Wednesday-Wednesday night, there is a low-medium (20-40%) chances for showers and thunderstorms. Thursday will feature medium (30-60%) chances. By Thursday night, rain chances increase with widespread categorical or medium-high (50-80%) chances. Friday through Saturday, there remain medium (30-60%) chances across Deep South Texas. Finally, Saturday night through next Monday, there exists medium (30-50%) chances. Overall, there generally is a 30-60% chance for showers and thunderstorms through the forecast period with Thursday night yielding the greatest chances between 50-80%.

The other news will be the heat! Global forecast models/ensembles continue to depict a strengthening 588-591 dam sub-tropical heat ridge developing over the region early next week. This feature will result in hotter than normal temperatures through Tuesday, and maybe even extending into Wednesday. Forecast high temperatures for Monday and 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 both Monday and Tuesday. These values will result in Moderate (Level 2 of 4) to Major (Level 3 of 4) Heat Risk both days. 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 Moderate (Level 2 of 4) over the region on Wednesday.

Thursday through next Sunday, high temperatures will range between the upper 80s to lower 90s. This largely will be due 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

(00Z TAFS) Issued at 606 PM CDT Sun May 17 2026

Through 00z Tuesday....VFR-MVFR conditions will prevail through the 00z TAF cycle. VFR conditions will be more prevalent during the afternoon-evening hours with MVFR conditions more prevalent during the night-morning hours.

Breezy southeast winds will continue through the forecast period with speeds between 12-17 knots and gusts between 25-30 knots.

MARINE

Issued at 1028 PM CDT Sun May 17 2026

A Small Craft Advisory has been extended till Tuesday morning due to continued breezy southeasterly winds and elevated wave heights. Outside of the threat for showers and thunderstorms, Tuesday through Wednesday, adverse (Small Craft Exercise Caution) conditions are possible. Again, outside of the threat for showers and thunderstorms Wednesday night through next Sunday, marine conditions are expected to improve with moderate seas and low to moderate winds.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 81 95 81 94 / 10 10 20 30 HARLINGEN 79 96 79 95 / 10 20 10 10 MCALLEN 81 97 81 97 / 10 20 20 10 RIO GRANDE CITY 79 99 79 98 / 20 10 20 10 SOUTH PADRE ISLAND 81 86 81 86 / 10 10 10 20 BAYVIEW/PORT ISABEL 81 93 80 92 / 10 10 10 20

BRO WATCHES/WARNINGS/ADVISORIES

TX...High Rip Current Risk from 7 AM CDT Monday through Monday evening for TXZ451-454-455.

GM...Small Craft Advisory until 9 AM CDT Tuesday for GMZ130-132-135- 150-155-170-175.