Area Forecast Discussion
Issued by NWS Brownsville, TX
Issued by NWS Brownsville, TX
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008 FXUS64 KBRO 191816 AFDBRO Area Forecast Discussion National Weather Service Brownsville TX 116 PM CDT Thu Sep 19 2024 ...New AVIATION... .SHORT TERM... (Today through Friday) Issued at 404 AM CDT Thu Sep 19 2024 Key messages: * Oppressive heat continues today, with heat index values ranging from between 105 to 112. * A Special Weather Statement for elevated heat indices is in effect this afternoon. * There is a low chance (15 to 25%) of showers and isolated storms this afternoon. Better chance (20 to 35%) of showers/storms Friday afternoon. Midlevel high pressure over Texas will continue to minimize rain chances today. Any showers and thunderstorms that do develop will be isolated in nature and most likely develop along the sea breeze boundary. With that said, PWATs are still around 2 inches, which is equivalent to the 75-90th percentile of SPC sounding climatology for the Brownsville, TX upper air site. Any showers or thunderstorms that do develop will be capable of producing brief heavy rainfall. However, the overall upper level ridging over our area will favor mostly dry weather and efficient daytime heating so have largely kept PoPs at or below 20 percent. Given the lack of large scale synoptics, showers or isolated storms that develop would be triggered by the sea breeze boundary. Forecasting heat index values for this afternoon is quite tricky, mainly because of the potential for boundary layer mixing. Hi-res guidance are quite consistent in depicting an uptick in southeast winds this afternoon, gusting to 15-20 mph. Forecast soundings indicate an inverted v profile, or an increase in dew point depression with height. The National Blend of Models (NBM) is likely too moist with the boundary layer, so blended in the 25th percentile for dew points especially west and along the US-77/I-69E corridor. Our CWA`s Special Weather Statement (SPS) criteria is 108-111 while Heat Advisory criteria is 111-115 for two or more hours over 50 percent of the forecast zone. The thinking is that dew points should get into the 73-76 range away from our coastal zones. With ambient air temperatures around 94-98, that air temperature/dew point combination would yield heat indices in the 107-111 range, maybe locally 112-113 out towards southern Hidalgo. The wild card would be the delayed onset of mixing, but this would be a low probability scenario. Regardless of SPS or Heat Advisory, today will once again be uncomfortably if not oppressively warm with temperatures only cooling into the 77-81 range this morning across the RGV and dew points staying elevated in the 76-79 range. Whether the heat index value is 110 or 112, please exercise precaution if spending extended amount of time outdoors today. With sunset, there are some indications of convection developing across our western zones overnight tonight but given the overall upper level ridging in place, confidence in any widespread convection is low. With the same air mass in place, it would be another muggy night with lows generally in the 76-82 range. Heading into Friday, model guidance suggests a slight erosion of the upper level ridge. This would favor a slight uptick in shower and thunderstorm chances for Friday afternoon. Therefore, PoPs are marginally higher than the past couple of days, rising into the 20 to 35 percent range. Current maximum heat index values for Friday afternoon exhibit a wider spread, ranging from 102 to 110. A SPS may once again be needed, but the confidence is lower given the more widespread rainfall chances. && .LONG TERM... (Friday night through Wednesday) Issued at 404 AM CDT Thu Sep 19 2024 To begin the long term Friday night, a mid-to-upper level ridge will continue to hold its grip over Deep South, situated to to the east- southeast of an upper level low and associated trough, centered over southern California. Throughout the course of the weekend, the upper level low will advance northeastward and over the Rocky Mountains, riding along the ridge`s western and northern side, gradually flattening the ridge`s curvature and overall strength. As the ridge still holds over Deep South Texas, chances of showers and thunderstorms are limited through Sunday across much of the CWA, aside from a 20% to 30% chance of isolated afternoon showers and thunderstorms along the seabreeze boundary for the coastal counties (Cameron/Willacy/Kenedy) and the middle RGV. By Monday morning, the ridge will be significantly flattened and stretching across the Gulf coast states. Meanwhile, a strong upper level ridge will push into the Pacific Northwest states and dig the trough, mentioned above, southeastward, causing the ridge over Texas to weaken into the middle of next week. Meanwhile, at the surface, an inverted trough over the Gulf of Mexico may fuel more moisture into Deep South Texas, bringing an uptick in afternoon showers and thunderstorms through Wednesday, rising POP`s to at least 20% across all of the CWA for Monday and as high as near 40% across the mid to lower RGV and coastal counties. By Wednesday, further breakdown of the ridge aloft could result in at least a 30% chance of rain across all of Deep South Texas as well as an increased risk of convection, up to 45%, with heavy downpours across the coastal counties and the mid RGV. Temperatures are expected to be near normal with inland Deep South Texas daily maximum temperatures reaching into the low to mid 90s and minimum temperatures falling into the mid to upper 70s, at least through the weekend. Special Weather Statements (SPS) could still be needed for the mid and upper RGV, especially on Saturday, as heat indices may briefly hit 111 F in these areas for an hour or two. As cloud coverage and chances of showers and thunderstorms build through Wednesday, daily minimum temperatures may range from the lower to mid 70s and highs could peak into the lower 90s with mid 90s still possible across the mid to upper RGV Monday through Wednesday. As for the barrier islands, daily high temperatures in the mid to upper 80s and lows in the upper 70s to lower 80s are expected. It should also be noted that the National Hurricane Center (NHC) has given an area of the northwestern Caribbean Sea and southeastern Gulf of Mexico has a medium (40%) chance of tropical development in the next 7 days. They indicate that a broad area of low pressure may form in the western or northwestern Caribbean Sea over the weekend, or early next week, and could undergo gradual development into a tropical depression as it tracks to the north or northwest over the northwestern Caribbean and into the southeastern Gulf of mexico into the middle of next week. We will continue to monitor and relay NHC updates as changes in intensity and, or track could alter the upcoming forecasts. && .AVIATION... (18Z TAFS) Issued at 1232 PM CDT Thu Sep 19 2024 Main aviation impact this afternoon will be isolated showers and thunderstorms associated with the sea breeze with best chances from 18-23Z accompanying a primarily southeast flow. Flight categories are forecast to remain primarily VFR this afternoon and overnight outside of the convection. There is medium (30-40%) probability of patches of MVFR to IFR ceilings at KMFE starting as early as 8Z and lasting through 15Z, cloud bases at KMFE may drop briefly below 1000 feet as was the case Thursday morning. Daytime convection is forecast again Friday with PROB30s in place for 16z at KHRL and KBRO to highlight uncertainty in spatial and temporal coverage of isolated and scattered showers. && .MARINE... Issued at 404 AM CDT Thu Sep 19 2024 Now through Friday...Generally favorable marine conditions will prevail with seas 1-2 feet and southeast or east southeast winds at 6-12 mph, with 15-20 mph gusts at times this afternoon into early evening. There is a 15-25 percent chance of showers and thunderstorms this afternoon, rising to 20-35 percent chance Friday afternoon. Waves and winds may be briefly higher during thunderstorms. Friday night through Wednesday...Light to moderate winds and light seas of 1 to 3 feet are expected through Tuesday and possibly building to 4 feet on Wednesday. Chances of daily showers and thunderstorms remain slight through the weekend but could still result in brief periods of heavy rainfall, gusty winds and elevated seas. Chances of showers and thunderstorms increase slightly on Monday through Wednesday. && .PRELIMINARY POINT TEMPS/POPS... BROWNSVILLE 92 80 92 79 / 20 10 40 10 HARLINGEN 94 76 92 75 / 30 0 40 0 MCALLEN 98 80 97 79 / 20 10 30 0 RIO GRANDE CITY 97 78 95 77 / 20 20 20 10 SOUTH PADRE ISLAND 88 82 88 81 / 20 20 30 20 BAYVIEW/PORT ISABEL 90 78 90 77 / 20 10 30 10 && .BRO WATCHES/WARNINGS/ADVISORIES... TX...None. GM...None. && $$ SHORT TERM...99 LONG TERM....65-Irish AVIATION...69-HK/59-GB