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   FOUS30 KWBC 031957   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   257 PM EST Wed Dec 3 2025   
      
   Day 1   
   Valid 16Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...   
      
   ...16Z update...   
      
   A notable mid-level dry layer observed on this morning's OSPO ALPW   
   imagery will moisten up over the next 24 hours as winds in the   
   lower/middle levels back in advance of an approaching mid-level   
   trough currently over the Southwest. Since the surface front is   
   forecast to barely edge inland along the middle/upper Texas coast,   
   most parcels within the outlook area will be elevated. Recent RAP   
   guidance supports a plume of 500-1500 J/kg MUCAPE moving northward   
   across the middle Texas coast into eastern Texas, western and   
   central Louisiana. WSW steering flow is expected to allow training   
   of showers/thunderstorms along elevated axes of convergence   
   aligned with the mean flow supporting training and repeating of   
   heavy rain.   
      
   The 12Z HREF and 06Z REFS support 12 hour maxima of 3-5 inches   
   (ending 12Z Thursday) from the upper Texas coast into southwestern   
   Louisiana with probabilities for 3 inches from the HREF at 50-60   
   percent and 70-80+ percent from the REFS. Peak hourly rainfall of 2   
   to 3 inches appears likely, although 3+ inch hourly totals cannot   
   be completely ruled out near the coast. Minor edits were made to   
   the existing Marginal Risk area for this update.   
      
   Otto   
      
   ...previous discussion follows...   
      
      
   CIRA LPW shows the slug of enhanced moisture along the central   
   Mexican Gulf coast starting to lift northward through the western   
   Gulf jet under response from the digging mid-level shortwave across   
   the Four Corners of the Southwest U.S. Twenty to 30kts of southerly   
   low level flow will advect the moisture north, bring the surface   
   front toward the central Texas coast through late afternoon into   
   the evening; CAPE values will rise into the 2000-3000 J/kg range.   
   As night-falls, stronger isentropic ascent will increase through   
   depth as the upper-level southwesterly jet stream flow expands and   
   strengthens providing the solid ascent plane for sufficient   
   moisture convergence with total PWats over 2" for expanding region   
   of elevated thunderstorm activity across the central and upper   
   Texas Coast.   
      
   Northward expansion remains the greatest uncertainty but overall   
   00z CAMs continue denoting solid rainfall rate potential mainly   
   along and south of the I-10 corridor with cell motions being slow   
   but also slightly veering to more easterly with solid back-building   
   potential to allow for some repeating training. HREF probability   
   driven by the 00z CAMs, still suggest best signals will remain   
   off-shore but there was also a slight northeastward trend in some   
   of the convective elements into southwest to south-central LA with   
   streaks of 2-3" totals. This will retain the solid Marginal Risk   
   (5-15% coverage) of FFG exceedance with isolated rapid inundation   
   flooding possible.   
      
   Gallina   
      
      
   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=20   
   probabilities, the 12Z RRFS Mean, and recent deterministic RRFS=20   
   runs. With the HREF/RRFS guidance showing the heaviest amounts=20   
   remaining more confined to the coast, pulled the Marginal Risk a=20   
   little bit further south and west with this update. While the=20   
   footprint for heavy amounts may not be as broad, the previously=20   
   noted HREF and RRFS guidance continue to indicate heavy amounts (3+   
   inches) are likely within the highlighted area, with the greatest=20   
   threat centered along the southwest Louisiana coast.   
      
   Pereira   
      
   Previous Discussion...   
   Upglide/warm-advective elevated convective activity should be=20   
   ongoing start of the forecast period (04.12z) across the central=20   
   Gulf coast. Steering flow will continue to flatten within fairly=20   
   unidirectional west-southwesterly deep layer flow. Right entrance=20   
   ascent to 150-170kt jet centered over the Ohio Valley will keep=20   
   isentropic moisture flux across the surface front that will=20   
   oriented along and south of the coast. A weak mid-level shortwave=20   
   will slide from west to east along the southern periphery of the=20   
   jet streak maintaining 20-25kt of west-southwesterly 850mb flow=20   
   which given 1.75-2" total PWats and modestly unstable ribbon (up to   
   1000 J/kg) and should maintain the elevated convective=20   
   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=20   
   perhaps as far as western FL Panhandle by the end of the forecast=20   
   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=20   
   remain in place for the day 3 period as well. The subtle, flat mid-   
   level shortwave feature will be translating quickly across the=20   
   South, through the Carolinas and offshore. The connection to the=20   
   western Gulf moisture pool/theta-E source will continue to further=20   
   narrow and stretch through an elongated isentropic ascent plane in=20   
   the wake of the wave draped across SC, central GA toward the=20   
   central Gulf coast. By this time period, the ascent across the=20   
   front will be fairly oblique and lapse rates will have moderated=20   
   toward limited elevated MUCAPE below 500 J/kg...becoming more of an   
   Atmospheric River with prolonged moderate rates with 24hr totals=20   
   of 1-2" across the length of the stream with scattered localized=20   
   maxima of 2-3". To be expected, much like the day 2 period, there=20   
   remains a latitudinal spread to the placement of the AR stream and=20   
   the width of the inherited Marginal Risk reflects this. However, at   
   this point, FFG exceedance will be more likely in the longer=20   
   duration of 6, 12 and 24hr exceedance, with any flash flooding=20   
   potential most likely upstream across LA/S AL, where multiple days=20   
   of rainfall should saturate the upper soil profiles and FFG values=20   
   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!8c_KQFoDBPEoIDDmiNh2rMlW6mXBlutM_vHYAwBN47QC=   
   iPKVt1ho6kn74odDeVTvlbCf64TxClXxdsjb0yoJ88ANgWI$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!8c_KQFoDBPEoIDDmiNh2rMlW6mXBlutM_vHYAwBN47QC=   
   iPKVt1ho6kn74odDeVTvlbCf64TxClXxdsjb0yoJKPwyZJU$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!8c_KQFoDBPEoIDDmiNh2rMlW6mXBlutM_vHYAwBN47QC=   
   iPKVt1ho6kn74odDeVTvlbCf64TxClXxdsjb0yoJJFjP2oo$=20   
      
   =3D =3D =3D   
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