textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 516 AM CST Sun Dec 21 2025 * A Dense Fog Advisory is in effect for Zapata, Jim Hogg, Brooks and Starr counties from 2 AM tonight until 10 AM Sunday morning as fog reduces visibilities to a quarter mile or less.

* Unseasonably warm temperatures continue ranging from around 15 degrees F above average on Sunday to 10-15 degrees above average into next weekend.

* Moisture pooling south of a stalled frontal boundary over south Texas is expected to result in a low (15-20%) chance of rain near the coast and the eastern half of inland Deep South Texas on Monday.

* Additional Special Weather Statements and/or Dense Fog Advisories are possible as calm to light winds and enhanced moisture support the development of fog Sunday night.

DISCUSSION

Issued at 1041 PM CST Sat Dec 20 2025

Over the past couple of hours, clear skies and efficient radiational cooling have allowed for temperatures to drop progressively closer to the dewpoint, with relative humidity values generally ranging from 72-94% at this time across Deep South Texas. Patchy fog has already started to develop with visibilities as low as 4-5 miles in Brownsville and Falfurrias. As dewpoint depressions continue to narrow along with clear skies and light to calm winds, areas of patchy to widespread fog are expected to continue to develop. The best chance for dense fog is across Zapata, Jim Hogg, Brooks and Starr counties, where a Dense Fog Advisory goes into effect from 2 AM to 10 AM as visibilities are likely to reduce to 0.25 miles or less. The remainder of Deep South Texas and the Rio Grande Valley is under a Special Weather Statement as areas of fog generally reduce visibility to 1-3 miles, with pockets of 1 mile or less through 10 AM.

Otherwise, ridging from mid/upper level high pressure and southeasterly to southerly surface winds from surface highs over the southern US and the Gulf are expected to maintain mostly dry conditions across the region, with above average temperatures continuing over the next 7 days. On Sunday, a cold front is progged to slowly move into southern Texas, likely stalling north of the northern ranchlands, resulting in a wind shift from southerly on Sunday morning to southeasterly throughout the afternoon and evening hours. Moisture pooling and weak lift south of the frontal boundary are expected to increase chances of rain mainly over the Gulf waters during the day on Monday, though a low (15-20%) chance of rain is possible along the coast and inland as far west as eastern portions of Jim Hogg and Starr counties. The boundary is expected to lift northward as a warm front overnight Sunday, which will reduce chances of rain back to less than 10%. However, enhanced moisture from onshore southeasterly flow and light to calm winds are likely to support the development of areas of patchy fog Sunday night, with additional Special Weather Statements and/or Dense Fog Advisories are possible.

Enhanced southerly flow south of the front on Sunday is expected to result in record-breaking, or near record-breaking, maximum temperatures, with mid to upper 80s likely across the Rio Grande Valley, which is close to 15 degrees above average. Warm air advection from southeasterly to southerly winds will keep temperatures 10-15 degrees above normal as daily highs reach into the 80s inland and mid 70s along the coast, before falling into the 60s overnight. Additional high temperature records could be broken.

A low risk of rip currents continues through the day on Sunday.

AVIATION

(12Z TAFS) Issued at 516 AM CST Sun Dec 21 2025

While some shallow ground fog has been seen for brief periods of time, only HRL has seen persistent fog that has resulted in IFR conditions. Conditions are expected to improve for HRL after sunrise. VFR conditions are expected for the rest of the day and through the evening hours. The models do indicate that a round of fog is possible again late into the overnight hours and near sunrise tomorrow.

MARINE

Issued at 1041 PM CST Sat Dec 20 2025

Patchy to widespread areas of fog or dense fog have developed this evening and a Dense Fog Advisory remains in effect over the nearshore (0-20 nm) Gulf waters and the Laguna Madre until 11 AM as visibilities less than 1 nm persist. Fog is possibly again Sunday night. Otherwise, light to gentle, moderate at times, southeasterly to southerly winds and slight to moderate (2-4 feet) seas continue into next weekend. Moisture pooling and lift ahead of a frontal boundary stalling over southern Texas is expected to result in a low to medium (20-30%) chance of rain during the daytime on Monday. Thunderstorms are possible as well. Otherwise, dry conditions continue with chances of rain less than 10%.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 85 68 83 68 / 0 10 20 10 HARLINGEN 86 65 84 65 / 0 0 20 10 MCALLEN 87 69 85 69 / 0 0 20 10 RIO GRANDE CITY 86 65 85 66 / 0 10 20 0 SOUTH PADRE ISLAND 77 70 76 69 / 0 10 20 10 BAYVIEW/PORT ISABEL 83 67 81 66 / 0 10 20 10

BRO WATCHES/WARNINGS/ADVISORIES

TX...Dense Fog Advisory until 10 AM CST this morning for TXZ248>250- 252.

GM...Dense Fog Advisory until 11 AM CST this morning for GMZ130-132- 135-150-155.