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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1100 AM 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=20   
   lower/middle levels back in advance of an approaching mid-level=20   
   trough currently over the Southwest. Since the surface front is=20   
   forecast to barely edge inland along the middle/upper Texas coast,=20   
   most parcels within the outlook area will be elevated. Recent RAP=20   
   guidance supports a plume of 500-1500 J/kg MUCAPE moving northward=20   
   across the middle Texas coast into eastern Texas, western and=20   
   central Louisiana. WSW steering flow is expected to allow training=20   
   of showers/thunderstorms along elevated axes of convergence=20   
   aligned with the mean flow supporting training and repeating of=20   
   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=20   
   percent and 70-80+ percent from the REFS. Peak hourly rainfall of 2   
   to 3 inches appears likely, although 3+ inch hourly totals cannot=20   
   be completely ruled out near the coast. Minor edits were made to=20   
   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...   
      
   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...   
      
   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!4uEpngNdmkAsVs3XbjJF1IXUN604SCNaGB-vkhF2aOet=   
   WRO1HgiTS3MAkDB7n8aYm2jGpT3iQJQFX2CZdJ3YogcwOFA$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!4uEpngNdmkAsVs3XbjJF1IXUN604SCNaGB-vkhF2aOet=   
   WRO1HgiTS3MAkDB7n8aYm2jGpT3iQJQFX2CZdJ3YQOWQbmk$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!4uEpngNdmkAsVs3XbjJF1IXUN604SCNaGB-vkhF2aOet=   
   WRO1HgiTS3MAkDB7n8aYm2jGpT3iQJQFX2CZdJ3YxmV2GJk$=20   
      
   =3D =3D =3D   
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