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   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   734 PM EST Sat Nov 29 2025   
      
   Day 1   
   Valid 01Z Sun Nov 30 2025 - 12Z Sun Nov 30 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHEAST TEXAS...   
      
   Inflow at 850 hPa is confluent in the vicinity of I-10 across=20   
   Southeast TX. MU CAPE in and near the Middle TX Coast is 1000+=20   
   J/kg, and has been declining. Effective bulk shear of 30-45 kts has   
   led to occasional cell organization, with organized activity=20   
   moving east while less organized convection moves northeast (an=20   
   environment favorable for cell mergers). Mesocyclone formation is   
   possible in this environment. The combination of convection along=20   
   a warm front in and near Houston at the present time and incoming=20   
   convection associated with a cold front moving in from the=20   
   northwest could also lead to cell mergers at 04z or so across   
   Southeast TX. MU CAPE is forecast to retreat southwest and weaken   
   further, which could lead to backbuilding and short periods of=20   
   cell training. The atmosphere is saturated with precipitable water   
   values around 1.5".=20   
      
   The mesoscale guidance shows a bit of dispersion, with some=20   
   indicating a maximum west of Houston, the HRRR insisting on the=20   
   Houston Metro area itself, while other pieces of guidance are near=20   
   or a bit north of Beaumont. This allowed the Marginal Risk to be=20   
   maintained and expanded. Widely scattered instances of flash=20   
   flooding are possible as hourly rain amounts maximize near 2.5" and   
   local totals reach 5". Should this occur over an urban area,=20   
   Slight Risk impacts would be possible.   
      
      
   Day 2   
   Valid 12Z Sun Nov 30 2025 - 12Z Mon Dec 01 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Orrison   
      
      
   Day 3   
   Valid 12Z Mon Dec 01 2025 - 12Z Tue Dec 02 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE   
   NORTHWEST AND CENTRAL GULF COAST INTO THE SOUTHEAST...   
      
   2030Z Update...   
      
   The 12Z suite of guidance is generally a bit faster with the   
   ejection of shortwave energy and surface wave activity across the   
   Gulf Coast states and into the Southeast for this period. However,   
   there will still be an expansion area of convection focusing across   
   areas of southeast TX/LA and east-northeastward across the Deep   
   South and interior parts of the Southeast. There remains   
   uncertainty with just how much instability can pool northward away   
   from the Gulf Coast as most of the guidance focuses the stronger   
   instability along and offshore of the Gulf Coast, but there will be   
   plenty of warm air advection, moisture transport and at least some   
   elevated instability to focus an elongated corridor of heavy   
   showers and thunderstorms. Some localized swaths of cell-training   
   will be possible which may result in some 2 to 4 inch rainfall   
   totals. However, given the dry antecedent conditions, this is   
   expected to generally be an isolated flash flood threat and mainly   
   around the more urbanized locations. Therefore, aside from some   
   modest tweaks to account for the latest guidance, the Marginal Risk   
   area is maintained.   
      
   Orrison   
      
   Previous discussion...   
      
   A fast-moving progressive shortwave trough that extends from an   
   expansive upper level low over Canada will provide the upper level   
   forcing for cyclogenesis along a potent cold front that will   
   roughly parallel the Gulf Coast and Eastern Seaboard. The low will   
   begin to form along the Texas Gulf coast during the day Monday,   
   then track ENE across the Deep South, ending the period near the   
   South Carolina/Georgia border. As the low tracks along the Gulf   
   Coast, it will continually add Gulf moisture to its circulation,   
   while also thriving on the baroclinicity brought on by a   
   reinforcing cold air mass moving southeast from the Midwest. PWATs   
   in the warm, moist Gulf air mass will exceed 1.75 inches in some   
   areas. However, any instability will likely remain along the   
   immediate coast, with very little instability inland. This supports   
   an overrunning rainfall scenario, rather than an overly convective   
   one. A prolonged period of light to moderate rain is expected   
   across much of the South as a result. The heaviest rain is forecast   
   from near New Orleans northeast to the southern Appalachians. It's   
   in this corridor that the highest probability of flash flooding   
   exists. The qualifier to this statement however is that FFGs across   
   the Deep South are quite high as a result of recent dry weather   
   and therefore very dry soils. This will favor most of the rainfall   
   associated with this low being beneficial to affected areas.   
   Isolated flash flooding is most likely in urban or flood-prone   
   areas, as well as in the flashier streams that drain the Southern   
   Appalachians.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!99qYPmeuKt8Kx9fHtZORYeMwe37hPeki-zNNsgZRJ6UE=   
   Z92PxxX5_VPxsqE5Xas8AWornqKKuuefZy-nBlh7lnJbgwU$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!99qYPmeuKt8Kx9fHtZORYeMwe37hPeki-zNNsgZRJ6UE=   
   Z92PxxX5_VPxsqE5Xas8AWornqKKuuefZy-nBlh7R8egRS8$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!99qYPmeuKt8Kx9fHtZORYeMwe37hPeki-zNNsgZRJ6UE=   
   Z92PxxX5_VPxsqE5Xas8AWornqKKuuefZy-nBlh7KD2IE-s$=20   
      
   =3D =3D =3D   
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