Quantitative Precipitation Forecast
Issued by NWS
Issued by NWS
133 FOUS30 KWBC 040105 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 805 PM EST Wed Dec 3 2025 Day 1 Valid 01Z Thu Dec 04 2025 - 12Z Thu Dec 04 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA... ...0100 UTC Update... Trimmed the Marginal Outlook on both sides (esp. west) based on the latest guidance trends, including recent HRRR runs and recent (18Z) HREF rainfall exceedance probabilities. The outlook now extends from the Upper TX coast into southwest and south-central LA. Surface-based instability will be non-existent as the upper level jet streak across North TX into the Ozarks, with the ensuing right-entrance upper level forcing (northerly low-lvel ageos flow), keeps the surface warm front offshore. Uptick in 850 mb flow however along with PWs climbing to around 2" will bring a bubble of elevated CAPE to the coastal areas between the Upper TX Coast and south-central LA -- a fairly tight gradient between ~500 J/Kg inland and ~1500 J/Kg along the coast. Much of the outlook area is in at least a D1 drought, so to no surprise the FFGs are quite high (2-3"/hr and 3-4" in 3hrs). Based on the latest HREF 1/3/6 exceedance probabilities, the outlook area was pared a bit. Either way, it appears to be a low-end Marginal, with neighborhood probabilities of flash inundation closer to 5%, and mostly over urban and flood-prone areas. Hurley Day 2 Valid 12Z Thu Dec 04 2025 - 12Z Fri Dec 05 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL GULF COAST... 20Z Update... Made some small adjustments based on the 12Z HREF/RRFS probabilities, the 12Z RRFS Mean, and recent deterministic RRFS runs. With the HREF/RRFS guidance showing the heaviest amounts remaining more confined to the coast, pulled the Marginal Risk a little bit further south and west with this update. While the footprint for heavy amounts may not be as broad, the previously noted HREF and RRFS guidance continue to indicate heavy amounts (3+ inches) are likely within the highlighted area, with the greatest threat centered along the southwest Louisiana coast. Pereira Previous Discussion... Upglide/warm-advective elevated convective activity should be ongoing start of the forecast period (04.12z) across the central Gulf coast. Steering flow will continue to flatten within fairly unidirectional west-southwesterly deep layer flow. Right entrance ascent to 150-170kt jet centered over the Ohio Valley will keep isentropic moisture flux across the surface front that will oriented along and south of the coast. A weak mid-level shortwave will slide from west to east along the southern periphery of the jet streak maintaining 20-25kt of west-southwesterly 850mb flow which given 1.75-2" total PWats and modestly unstable ribbon (up to 1000 J/kg) and should maintain the elevated convective development. Internal training of back-building convective elements should be the greatest potential for higher rainfall totals across the Bayous of south-central LA. A slow eastward propagation of the SSW- NNE oriented line will occur with the translation of the flat shortwave energy and potentially expand into coastal MS/AL and perhaps as far as western FL Panhandle by the end of the forecast period. There remains some north/south spread in placement of heaviest rainfall cores/training corridor. The GFS continues a traditional northward bias along with the NAM relative to the ECMWF and other global guidance; with some suggestion of overall totals in excess of 5" in spots. Hi-Res CAMs generally trend toward the southern solutions with heaviest rainfall along and south of I-10, which appears more plausible given the overall flow orientation (unstable air upstream over the waters). Still, there is some solid suggestion of rainfall rates/totals and environmental conditions that may warrant an upgrade to a Slight Risk, however, the split in placement and wider overall total rainfall total spread continues to be too great to delineate one at this time, but will continue to evaluate with future updates. As such, the broad Marginal from Galveston Bay to the Pensacola area of FL remains, though have broadened it slightly northward to account for the latitudinal variance. Gallina Day 3 Valid 12Z Fri Dec 05 2025 - 12Z Sat Dec 06 2025 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL GULF COAST THROUGH CENTRAL GEORGIA... 20Z Update... Made only minor adjustments based on the 12Z guidance. Pereira Previous discussion... The deep layer unidirectional west-southwesterly flow regime will remain in place for the day 3 period as well. The subtle, flat mid- level shortwave feature will be translating quickly across the South, through the Carolinas and offshore. The connection to the western Gulf moisture pool/theta-E source will continue to further narrow and stretch through an elongated isentropic ascent plane in the wake of the wave draped across SC, central GA toward the central Gulf coast. By this time period, the ascent across the front will be fairly oblique and lapse rates will have moderated toward limited elevated MUCAPE below 500 J/kg...becoming more of an Atmospheric River with prolonged moderate rates with 24hr totals of 1-2" across the length of the stream with scattered localized maxima of 2-3". To be expected, much like the day 2 period, there remains a latitudinal spread to the placement of the AR stream and the width of the inherited Marginal Risk reflects this. However, at this point, FFG exceedance will be more likely in the longer duration of 6, 12 and 24hr exceedance, with any flash flooding potential most likely upstream across LA/S AL, where multiple days of rainfall should saturate the upper soil profiles and FFG values may be much lower than currently analyzed. Gallina Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt