textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 1235 AM CDT Thu May 21 2026

Key Messages:

* The Storm Prediction Center (SPC) maintains a Marginal Risk (level 1 of 5) of isolated severe thunderstorms for Hidalgo and Brooks counties and points westward overnight tonight with the primary threat of damaging winds of 58+ mph and a secondary threat of large (1+ inch) hail. Isolated strong thunderstorms, one or two of which may briefly become severe, are possible further east during the pre-dawn and early Thursday morning hours.

* The Weather Prediction Center (WPC) expands a Marginal Risk (level 1 of 4) of isolated flash flooding from excessive and heavy rainfall portions for portions of the Upper RGV and Northern Ranchlands tonight to all of deep south Texas on Thursday, except northeastern Kenedy County, which is included within a Slight Risk (level 2 of 4).

* Daily rounds of strong to isolated severe thunderstorms continue into next week, though location, timing and severe probabilities are yet to be defined and will depend on previous rounds of convection. However, WPC holds daily Marginal to Slight Risks of excessive rainfall for portions or all of the region through Sunday.

* Warm and muggy conditions as well as afternoon heat indices ranging from 95-105 F continue to result in a mainly Minor Heat Risk (level 1 of 4), while the Lower RGV and portions of the coastal counties can expect a Moderate Heat Risk (Level 2 of 4).

* A moderate risk of rip currents continues through Thursday afternoon.

DISCUSSION

Issued at 1037 PM CDT Wed May 20 2026

A very active and interesting weather pattern continues across deep south Texas through this weekend and into next week. A mid/upper level trough near the Baja Peninsula slowly advances eastward to central and southern Texas over the weekend, followed by another trough moving over the Desert Southwest early next week. Forcing aloft, very high lower level moisture content, enhanced lift and convection firing off the Sierra Madre lead to daily elevated chances of rain from potential rounds of showers and thunderstorms, some of which are capable of producing severe weather and heavy rain. In the meantime, a separate trough digs from the Pacific Northwest to the central Rockies into Friday morning, aiding in deepening the trough to the southwest as well as amplifying a series of downstream shortwaves passing northeastward and over southern Texas, bringing the initial potential severe rounds to our region tonight and Thursday night. Already this evening, slow moving strong thunderstorms developed over Zapata County as well as Starr County, where a SPS and Flood Advisory were issued for winds up to 50 mph, pea-sized and heavy rain. This cluster of cells continued north and led to an SPS for western Jim Hogg County for winds gusting up to 40 mph.

For the remainder of tonight, chances of rain increase to as much as a medium to likely (40-80%) chance along/west US-281 and both the HRRR and RRFS feature strong to isolated severe thunderstorms rolling into the Rio Grande Plains, the Northern Ranchlands and the Upper/Middle RGV late tonight, perhaps between around 1-3 AM. Modeled SBCAPE, DCAPE, 0-3 km lapse rates, and PWAT of 2,000+ J/kg, 1,200+ J/kg, 7.0+ C/km and 1.6+ in., respectively, as well as inverted V sounding signatures indicate a continued primary threat of damaging winds of 58+ mph. Simultaneously, MUCAPE and mid-level lapse rates of 1,500-2,000 J/kg and 6.7-7.2+ C/km could produce large (1+ inch) hail, though weaker 850-300 mb mean winds (<20 kts) make hail a second threat. Therefore, SPC has extended the Marginal Risk (level 1/5) of isolated severe thunderstorms for western Deep South Texas to as far east as Hidalgo and Brooks counties. Additionally, WPC highlights a Marginal Risk for excessive rainfall leading to isolated flash flooding or hazardous roadway ponding tonight across Zapata, northwestern Starr, northwestern Jim Hogg and the northwestern tip of Brooks counties. Showers and thunderstorms are anticipated to gradually weaken as they move into the Lower RGV and coastal counties during the pre-dawn and early Thursday morning hours, though a severe thunderstorm or two cannot be completely ruled out. Showers and thunderstorms generally diminish across the region throughout Thursday, thus decreasing chances of rain to a low (15-30%) chance in the afternoon. Thursday evening, another shortwave could bring another round of strong to isolated severe thunderstorms, which could prompt SPC to introduce at least a Marginal Risk of isolated thunderstorms in upcoming updates. Yet, this will depend on how long clouds linger into daytime as well as the degree of instability recovery into the afternoon and evening. Due to the ongoing possibility of heavy rain, WPC highlights all of the CWA within a Marginal Risk of excessive rain for Thursday as well a Slight Risk (level 2 of 5) for the northeastern tip of Kenedy County.

The next focus of the forecast shifts to this weekend as the trough moves through northern Mexico into central/southern Texas and enhances forcing aloft and a procession of mid-level impulses bring as much as a medium to likely (40-70%) chance of rain Saturday night. The GFS is hinting at this period as tenably the most active of the next 7 days, particularly Saturday afternoon/night, during which there could be a more widespread coverage of severe thunderstorms. Just as is the case over the next 24-36 hours, confidence is too low for any probabilities of severe to be discerned, but WPC expands a Marginal Risk of excessive rainfall across the northern tips of Zapata, Jim Hogg and Brooks counties on Friday to all of deep south Texas on Saturday as well as a Sight Risk for northeastern Brooks and the northeastern half of Kenedy counties, followed by a Marginal Risk along/east of US-281 on Sunday. Chances of rain continue into the beginning of next week.

Between now and Saturday evening, the Lower and Middle RGV could have already received 1.0-1.5 inches of rain while the Upper RGV and the Northern Ranchlands could see 1.5-2.0 inches. The 90th percentile, which indicates only a 10% probability of rainfall totals meeting or exceeding, indicates a very low potential for 2-3 inches along/east US-281 and 3-4 inches further west. However, 3-4+ inches are possible where locally heavy rain develops and/or persists within each round. By next Monday morning, we currently anticipate 1.0-2.0 inches across the RGV and the Rio Grande Plains while the Northern Ranchlands could receive closer to 2.0-2.5 inches.

Temperatures are generally expected to maximize in the upper 80s/low 90s during the afternoon and minimize in the 70s overnight. A Minor Heat Risk is expected each afternoon for most of the region as heat indices peak around 95-105 F. Meanwhile, a Moderate Heat Risk is expected for the Lower RGV and portions of the coastal counties, where light to gentle onshore easterly to southeasterly winds advect higher relative humidity.

Finally, a moderate risk of rip currents continues through Thursday afternoon.

AVIATION

(06Z TAFS) Issued at 1235 AM CDT Thu May 21 2026

Convection is building to the west of the aerodromes, and this will have the potential to affect them during the overnight hours. For the daylight hours of Thursday, model guidance suggests that another round of convection will affect the aerodromes during the afternoon hours, and flying conditions may be more adverse than currently indicated in the TAFs.

MARINE

Issued at 1037 PM CDT Wed May 20 2026

Gentle to moderate easterly to southeasterly winds and slight to moderate (2-5 ft) seas continue through the forecast. An ongoing low to medium (20-50%) chance of rain continues along with potential daily rounds of showers and thunderstorms persist, a few of which may be strong to severe, capable of producing large (1+ inch) hail and strong winds (36+ knots). Activity could peak in coverage and strength on Saturday, when there is as much as a medium to likely (50-70%) chance of rain.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 88 78 92 79 / 50 20 10 30 HARLINGEN 88 74 92 76 / 50 30 20 30 MCALLEN 88 76 94 78 / 60 30 10 40 RIO GRANDE CITY 88 73 94 76 / 50 40 10 40 SOUTH PADRE ISLAND 84 79 85 81 / 60 20 10 20 BAYVIEW/PORT ISABEL 87 77 90 79 / 60 30 10 30

BRO WATCHES/WARNINGS/ADVISORIES

None.