textproduct: Brownsville/Rio Grande Valley

SHORT TERM

(Today through Friday) Issued at 320 AM CST Thu Dec 26 2024

Weather conditions overall will be quiet, with above average temperatures. Fog will be an issue this morning and Friday morning. Dew point depressions are low this morning, and weaker winds will be the key to fog coverage and impact on visibility both nights. This will be especially true tonight when the NBM depicts a solid fog signal for much of the CWA east of the extreme upper Valley. Visibility could drop to between 3 and 5 miles, with isolated dense fog dropping visibility to less than a half mile in spots. WASUP shows a low percent (10%) of less than a mile visibility Friday morning, this being across eastern sections, and short range model guidance shows fog and low ceilings as well. This makes sense since WPC has analyzed a dryline parsing the CWA into a western third and an eastern two thirds, with better low level moisture availability across the eastern 2/3s.

A weak surface frontal boundary from yesterday is still lingering in the vicinity, slightly upstream, while a mid level trough with underlying surface low pressure moves across north Texas. This feature will support moderate southeast to south winds today. Southwest 850 mb winds for the mid and lower Valley will support very warm temps, pushing highs into the lower to mid 80s this afternoon. This will be roughly 15 degrees above average at most locations. Lows tonight will be in the 50s and 60s while highs on Friday will again be toasty for this time of year in the mid 80s.

After a first short wave moves across today, another will follow quickly behind on Friday. It, too, will be a shallow feature moving across north Texas. However, another surge of high pressure (still rather weak) will show up as a cold front stalling out just to our north Friday afternoon. Finally, rip risk has been elevated to moderate today and tonight due to slightly higher wave heights and a longer swell period.

LONG TERM

(Friday night through Wednesday) Issued at 320 AM CST Thu Dec 26 2024

Key Messages:

* Unseasonably warm, near record-breaking temperatures expected to persist through the remainder of 2024.

* Large-scale pattern change and cooler changes expected to commence to start the New Year.

The last few days of December 2024 will feel more like late October. Under the influence of mid-upper ridging on a west-northwest zonal to semi-zonal flow pattern, unseasonably warm, near record-breaking temperatures, will persist across the local forecast area through the remainder of the year. During this time, overnight low temperatures at Brownsville/South Padre Island International Airport (KBRO) are progged to range in the mid 60s to near 70F degrees, while daytime highs are progged to be in the low to mid 80s. To further put into context on how extreme these temperatures will be, normal lows for this time of year at KBRO is in the lower 50s and highs in the lower 70s. So, we're generally talking 10-15+ degrees above average with 850 mb to 925 mb (near sfc temps) running +1 to +2 STDEVs above normal. However, a cool down is expected to take place to start the New Year (2025).

We start off the extended Friday night, where overnight lows will remain well above climatology with values in the mid-upper 50s northwest to the mid-upper 60s along the mid-lower RGV. During the day on Saturday, daytime highs are progged to be in the low to mid 80s, which happens to be 10-15+ degrees above normal or +1 to +2 STDEVs above normal. The all-time record high for December 28 is 86F degrees set back in 1921. Ahead of a weakening cool front, overnight lows Saturday night will remain warm and again warmer than climatological levels with values in the 50s across the Northern Ranchlands and the lower to mid 60s along the mid-lower RGV.

Saturday night into Sunday, a weak cold front is expected to pass through the region. Given how weak the front will be, an airmass change is not expected to take place, and in fact, will be hardly noticeable as sensible/ambient weather conditions of well above normal temperatures continue. Daytime highs on Sunday following this weak cool fropa will top out in the upper 70s to lower 80s, which is still 5-10+ degrees warmer than normal for late December standards. That said, the all-time record high of 87F degrees set back in 2021 & 1970 will be harder to achieve here.

Temperatures warm slightly on Monday with highs projected in the mid- upper 80s. The all-time high of 90F degrees set back in 1945 will also be harder to achieve, despite the warm up from Sunday. Overnight lows Sunday and Monday night will remain well above average with values in the upper 50s across the Northern Ranchlands to the mid 60s along the Rio Grande Valley.

Monday into Tuesday (New Year's Eve), in conjunction with a declining/negatively trending Arctic Oscillation (-AO), forecast models/ensembles are advertising a southeastward advancing cold fronts emerging out of the Northern Rockies and Plains. This looks to be the first of a series of cold fronts that will be tracking from Canada and the Arctic regions southward on the leeward side of a developing western U.S. ridge into the Lower 48 primarily east of the Rockies. This is all part of a large scale pattern change that will bring cooler changes to Deep South Texas and the RGV and areas east of the Rockies as deep troughing develops, while a warming trend amid ridging develops over the western U.S. to start the New Year.

The worst of the cold during the first part of January will be located well to our northeast across portions of the central and eastern U.S. No extreme cold is expected across Deep South Texas and the RGV with this cool snap during the first part of January. The airmass coming into the Lower 48 will be more of a modified Canadian airmass as opposed to a true Arctic airmass. The cooler changes to come will be noticeable across Deep South Texas and the RGV given how extremely warm the temperatures have been in late December. However, anomalies will be near normal levels and the temperatures will feel very comfortable to start the New Year with highs in the upper 60s to mid 70s and lows mainly in the 50s. These temperatures will be driven by cool air advection (CAA) on breezy northerly winds 10-20 mph.

Finally, the long-term period, by-and-large, will remain dry/rain- free due to mid-upper ridging, sfc high pressure influences, and dry cool/cold frontal passages. There may be some low-grade (isolated) chances for showers at around 20% on Wednesday (New Years Day) and Thursday with the best chances being near or over the Gulf Waters.

AVIATION

(12Z TAFS) Issued at 451 AM CST Thu Dec 26 2024

MVFR ceilings/visibility at KBRO and KHRL (not KMFE) this morning. Winds are slightly higher this morning (compared to yesterday), keeping the fog from manifesting lower visibility. Still, can't rule out a drop in the next two to three hours. Conditions will improve after sunrise, with winds increasing to moderate and breezy. Another round of fog will be possible tonight.

MARINE

Issued at 320 AM CST Thu Dec 26 2024

Today through Friday...I expect mainly light to moderate southeast to south winds and moderate seas in the short term. Slightly stronger south winds will occur offshore today, meeting small craft should exercise caution conditions.

Friday night through Thursday....Light to moderate winds and low to moderate seas will persist from Friday night to Tuesday. However, by the start of the New Year (Wednesday), marine conditions could become a more unfavorable as Small Craft Exercise Caution (SCEC) to Small Craft Advisory conditions could develop in association with the cold fropa.

PRELIMINARY POINT TEMPS/POPS

BROWNSVILLE 83 69 83 69 / 0 0 0 0 HARLINGEN 85 63 84 65 / 0 0 0 0 MCALLEN 86 62 87 67 / 0 0 0 0 RIO GRANDE CITY 82 54 83 59 / 0 0 0 0 SOUTH PADRE ISLAND 78 69 77 70 / 0 10 0 0 BAYVIEW/PORT ISABEL 83 66 81 66 / 0 0 0 0

BRO WATCHES/WARNINGS/ADVISORIES

TX...None. GM...None.