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   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=20   
   jet streak across North TX into the Ozarks, with the ensuing=20   
   right-entrance upper level forcing (northerly low-lvel ageos flow),   
   keeps the surface warm front offshore. Uptick in 850 mb flow=20   
   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=20   
   south-central LA -- a fairly tight gradient between ~500 J/Kg=20   
   inland and ~1500 J/Kg along the coast.=20   
      
   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.=20   
      
   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://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!4L5t_LkqZAE2Tg9RtPh-Z8wFgCeYyBFq1kR8lR36Yt4r=   
   TuHknIm2wlUtu-QV7NZAoT3UKEI6Rxnv0fvtQG6aQUoxnWE$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!4L5t_LkqZAE2Tg9RtPh-Z8wFgCeYyBFq1kR8lR36Yt4r=   
   TuHknIm2wlUtu-QV7NZAoT3UKEI6Rxnv0fvtQG6a9xeLk7E$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!4L5t_LkqZAE2Tg9RtPh-Z8wFgCeYyBFq1kR8lR36Yt4r=   
   TuHknIm2wlUtu-QV7NZAoT3UKEI6Rxnv0fvtQG6aWjuVRtA$=20   
      
   =3D =3D =3D   
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