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 Southeast TX. MU CAPE in and near the Middle TX Coast is 1000+ J/kg, and has been declining. Effective bulk shear of 30-45 kts has led to occasional cell organization, with organized activity moving east while less organized convection moves northeast (an environment favorable for cell mergers). Mesocyclone formation is possible in this environment. The combination of convection along a warm front in and near Houston at the present time and incoming convection associated with a cold front moving in from the 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 cell training. The atmosphere is saturated with precipitable water values around 1.5". The mesoscale guidance shows a bit of dispersion, with some indicating a maximum west of Houston, the HRRR insisting on the Houston Metro area itself, while other pieces of guidance are near or a bit north of Beaumont. This allowed the Marginal Risk to be maintained and expanded. Widely scattered instances of flash flooding are possible as hourly rain amounts maximize near 2.5" and local totals reach 5". Should this occur over an urban area, 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: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt