textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 1108 PM CST Sat Dec 13 2025

* Fog, mist, low stratus possible again tonight. * Warmer than normal temperatures and mainly rain-free weather will prevail through next week. * Our next best chance for rain will be on Sunday in response to a cold front; medium (30-60%) chances on Sunday.

* Another chance for showers and storms will be in the Tuesday through Wednesday timeframe.

* Hazardous marine and coastal conditions are expected to develop on Sunday and persist through Monday in response to the cold front.

DISCUSSION

Issued at 1108 PM CST Sat Dec 13 2025

The forecast period for Deep South Texas and the Lower Rio Grande Valley will continue to consist of warmer than normal temps with a couple chances for rain.

Environmentally, tonight will be similar to the past couple of nights (e.g light winds, clear skies, and plenty of low level moisture) and therefore suitable for the potential of fog, mist, and/or low stratus development. Due to the persistence in the pattern and expectations for sufficient radiational cooling through tonight, have opted to issue a Dense Fog Advisory from midnight tonight to 8 AM CST Sunday morning for all of Deep South Texas including the Rio Grande Valley.

Big changes to the weather pattern takes place on Sunday as a strong southward advancing cold front approaches the region. This cold front is associated with an Arctic 1040-1045 mb sfc high pressure system that will be established over the Midwest U.S. Increased sfc convergence and a nearby weak shortwave trough will increase the prospects for showers (maybe an isolated thunder or two as MLCAPE values will be as high as 500 J/kg) to develop over the region on Sunday. We've maintained medium (30-60%) chances for showers across Deep South Texas on Sunday. Rain chances diminish from north to south as the cold front sweeps through the region and shifts into northeastern Mexico Sunday evening/night.

The other aspect of this cold front will be the winds. Windy conditions are expected to develop on Sunday. Courtesy of how strong the Arctic high is over the Midwest, along and behind the cold fropa, enhanced pressure gradients or tightening isobars will result in blustery northerly winds developing. Late morning through the afternoon and into the evening hours, strong north winds with speeds between 20-30 mph could gusts as high as 35-40 mph, particularly along and east of IH-69C. The strongest of winds will be closer to the coast. In fact, gale force winds are possible over the Gulf Waters (see MARINE SECTION for more details). This winds will decrease in time as the evening progresses.

Tuesday through Wednesday is another period that we have to continue to monitor for the potential of showers and thunderstorms. During this time period, a return flow out of the south on the backside of a broad sfc high pressure system centered over the Southeastern U.S. and on the leeward side of a rather potent 575 mb mid-upper low pressure system over northern Mexico will result in increased moisture and warm air advection (WAA) Tuesday through Wednesday. Increased atmospheric moisture content and instability interacting with a nearby shortwave trough or weak frontal boundary plus impulses riding along the southwesterly jet aloft will result in increased prospects for showers and thunderstorms in the Tuesday through Wednesday timeframe. Low-medium (20-40%) PoPs have been established over Deep South Texas during this time period. The best chances looks to be Tuesday evening through Wednesday and the upper level low and associated perturbations get into closer proximity to the region. Areas along and east of IH-69C has the highest chances/risk. Again, we will continue to monitor these trends in the days ahead.

A downtrend in temperatures can be expected Sunday through Monday before temperatures begin moderating/warming on Tuesday. Late next week into next weekend, the cold and wintry weather across the northern U.S. begins to retreat northward into Canada as flat ridging (low amplitude 500 mb pattern) envelopes a vast majority of the Lower 48, marking a major large scale weather pattern change towards the milder, less wintry side. For us here in Deep South Texas, this means a stabilized, warmer than normal pattern will solidly be in place with less risk of cool frontal passages and less fluctuations in temperatures.

Overall, temps will continue to average out warmer than normal for the balance of the forecast period with daytime highs in the 80s most locations each day except for Sunday through Tuesday. Ahead of the cold front on Sunday, high temperatures will top out in the 70s most places early in the day before falling. Monday is progged to be the coolest day of the forecast period with daytime highs struggling to make it out of the 60s. Finally, on Tuesday, temperatures will begin to moderate as daytime highs are expected to climb back into the 70s most places. Wednesday through at least next Sunday, above normal temperatures will be driven by a 585-588 mb ridge. Overnight low temperatures will mainly be in the 60s during the forecast period. Sunday night into Monday will be the coolest with overnight lows in the 40s across the Northern Ranchlands and 50s along the RGV. Monday night into Tuesday, overnight lows will be in the 50s most places.

AVIATION

(06Z TAFS) Issued at 1108 PM CST Sat Dec 13 2025

Key Aviation IDSS Messages:

* IFR-VLIFR conditions are possible again tonight due to fog (possibly dense), mist, and/or low stratus.

* Strong cold front arrives Sunday morning from north to south; winds will shift out of the north and become blustery late morning through the afternoon hours. Northerly winds could gusts as high as 25-30 kts.

* Rain showers will be associated with the arrival and passage of the cold front on Sunday.

Over the next few hours, the main weather concern like the previous couple of nights will be the potential for another round of fog (possibly dense), mist, and low stratus developing. Ahead of an approaching cold front, the environmental conditions this evening/tonight will be very similar to previous nights. That is light winds, mainly clear skies, and copious amounts of low level moisture. Given the persistence in the pattern and expectation of sufficient radiational cooling taking place through tonight, TEMPO groups have been added through tonight at all of the TAF sites to account for the potential for fog/mist/low stratus developing yet again. Any fog or low stratus that develops will have the capabilities of reducing flight categories down to MVFR and eventually IFR-VLIFR levels.

Big changes to the weather pattern is expected to take place over the next 6-12 hours. A strong cold front is expected to push through the region from north to south during the morning hours, resulting in a wind shift out of the north along with the chances for showers beginning mid-late morning and persisting into the afternoon hours. Fog (possibly dense) and low stratus will be ongoing ahead and just behind the cold front. It may take time for the drier air to work its way into the region. That said, it's possible that MVFR-IFR conditions could continue into the afternoon hours.

Overall, expect for flying conditions to deteriorate over the next couple of hours from VFR conditions to MVFR-LIFR conditions (possibly instances of VLIFR conditions) due to fog, mist, and/or low stratus. MVFR-VLIFR conditions are then expected to persist through the remainder of the 06z TAF period with showers possibly accompanying these conditions mid-late morning into the afternoon hours.

Winds will be light and variable to calm tonight. In response to a cold fropa, winds will shift out of the north Sunday morning. An enhanced pressure gradient associated with the cold front will result in northerly winds becoming blustery late morning through the afternoon on Sunday with speeds between 15-20 kts and peak gusts between 25-30 kts. This could impact east-west oriented runways on Sunday.

MARINE

Issued at 1108 PM CST Sat Dec 13 2025

Deteriorating marine conditions is expected to take place over the next 6-12 hours as a cold front approaches the region. Along and behind the cold fropa, late Sunday morning, winds will shift out of the north and become gusty over the Laguna Madre and the Gulf Waters. A Small Craft Advisory remains in effect for the Laguna Madre and a Gale Warning is in effect for the Gulf Waters through at least Sunday night. Adverse to hazardous marine conditions will linger into Monday. However, conditions will improve Monday night with low to moderate winds and seas returning and persisting through the remainder of the forecast period. Showers and thunderstorms are possible over the Gulf Waters on Sunday and again in the Tuesday through Wednesday time period.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 83 67 75 54 / 10 10 50 20 HARLINGEN 85 62 72 51 / 0 10 40 10 MCALLEN 87 66 74 52 / 0 10 50 10 RIO GRANDE CITY 84 64 72 49 / 0 0 60 10 SOUTH PADRE ISLAND 76 67 71 60 / 20 10 50 20 BAYVIEW/PORT ISABEL 81 64 72 55 / 10 10 50 20

BRO WATCHES/WARNINGS/ADVISORIES

TX...Dense Fog Advisory until 8 AM CST Sunday for TXZ248>255-351- 353>355-451-454-455.

GM...Small Craft Advisory from 9 AM Sunday to 6 AM CST Monday for GMZ130-132-135.

Gale Warning from 9 AM Sunday to 6 AM CST Monday for GMZ150-155- 170-175.