textproduct: Brownsville/Rio Grande Valley

KEY MESSAGES

Updated at 1016 PM CST Wed Dec 17 2025

* Expect areas of fog tonight into Thursday morning, with patchy dense fog reducing visibility to a mile or less possible. A Dense Fog Advisory is in effect from midnight to 10 AM for Zapata, Starr, Jim Hogg, Brooks, and inland Kenedy Counties.

* Temperatures will cool slightly on Friday behind a cold front, otherwise we will remain above seasonal norms.

DISCUSSION

Issued at 1016 PM CST Wed Dec 17 2025

Abundant low-level moisture tonight is maintaining soupy, humid conditions. With light winds overnight and radiational cooling, this moisture has begun allowed fog to begin forming in the northern Ranchlands reducing visibilities to a half mile or less. Dewpoint depressions will continue to narrow through sunrise. Areas of fog with visibility less than 4 miles is expected across the RGV, and widespread fog is expected across the northern Ranchlands and Rio Grande Plains with dense fog possible. A Dense Fog Advisory is in effect along the northern Ranchlands and Rio Grande Plains (Zapata, Starr, Jim Hogg, Brooks, and Kenedy Counties) from midnight to 10 AM CST Thursday. The peak coverage of fog will likely occur around sunrise during the morning commute. Fog will begin to dissipate by mid-morning following surface heating and as southeasterly winds increase.

Thursday is expected to be another unseasonably warm day with highs in the 80s ahead of cold frontal passage Thursday night/early Friday. Unfortunately (or not?), impacts felt from the front will be a slight reduction to closer-to-normal (but still above average) high temperatures in the 70s Friday and mostly sunny skies. Fog along the coast and lower RGV is possible ahead of the front overnight Thursday depending on timing. Sooner fropa (ie, prior to midnight) would result in less ability for moist air ahead of the front to cool to its dewpoint, limiting fog potential.

Warm, dry conditions return Saturday through the remainder of the forecast as ridging slowly translates east across CONUS, allowing temperatures to rise back into the 80s for highs and 50/60 deg lows.

AVIATION

(06Z TAFS) Issued at 1016 PM CST Wed Dec 17 2025

MVFR conditions tonight will continue to deteriorate to IFR or LIFR thanks to areas of fog across the Rio Grande Valley, which may become dense at times. Timing-wise, greatest chances exist surrounding sunrise denoted by TEMPO groups, though impacts may be felt as early as 06Z through 16Z. Location-wise, latest short term guidance (SREF, HRRR, NAM) struggle to agree on coverage of dense fog reducing visibilities to LIFR levels in the Valley. HRL is most likely to see impactful fog given its rural location and dewpoint depression already at 0 degF, but IFR/LIFR vsbys are possible at all sites. Fog should dissipate by mid-morning, leaving primarily VFR conditions and southeasterly winds for the remainder of Thursday. Overnight Thursday, a cold front will bring northerly winds.

MARINE

Issued at 1016 PM CST Wed Dec 17 2025

Generally favorable conditions with slight seas and low chances of rain will continue through much of the forecast period. Southerly winds Thursday shift north-northeast and increase to moderate overnight into early Friday with a passing cold front. Winds return to southeasterly by Saturday and continue through the weekend.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 77 63 83 62 / 80 20 0 0 HARLINGEN 77 59 84 57 / 70 20 0 0 MCALLEN 77 61 86 59 / 50 10 0 0 RIO GRANDE CITY 77 56 86 54 / 20 0 0 0 SOUTH PADRE ISLAND 73 66 76 67 / 90 30 0 0 BAYVIEW/PORT ISABEL 76 62 81 61 / 80 20 0 0

BRO WATCHES/WARNINGS/ADVISORIES

TX...Dense Fog Advisory until 10 AM CST Thursday for TXZ248>252.

GM...None.