Subj : HVYRAIN: Excess Rain D1 To : All From : Mike Powell Date : Thu Aug 14 2025 08:52:52 FOUS30 KWBC 140754 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 354 AM EDT Thu Aug 14 2025 Day 1 Valid 12Z Thu Aug 14 2025 - 12Z Fri Aug 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC STATES... ....Southern New England through the Lower Mississippi Valley... Surface cold front draped from New England through the Ozarks will drift gradually southeast but generally stall across the region Thursday as the parent trough begins to fill, leaving gradually more zonal flow over the eastern CONUS. Although heights will gradually rise, weak shortwave impulses embedded within the flow will maintain periodic ascent atop the front to create waves of low pressure rippling northeast to produce locally enhanced lift, and scattered to numerous thunderstorms are again expected across the east on Thursday, primarily from southern New England through the Gulf Coast. Thunderstorms that develop will all have the potential to produce pockets of heavy rainfall as thermodynamics remain impressive. PWs of more than 2 inches will remain in a plume arcing within return flow around offshore high pressure from the Florida Panhandle through the northern Mid-Atlantic states. This PW will overlap with elevated MUCAPE of 1000-2000 J/kg to create the favorable thermodynamics for heavy rainfall, and the HREF indicates scattered neighborhood probabilities for 2"/hr rain rates across much of the area supporting the broad inherited MRGL risk. However, there appears to be at least a signal for some more organized convection with a greater threat for excessive rainfall across the eastern Carolinas and into Southeast Virginia. This region will benefit from additionally focused ascent through the presence of the cold front and a surface trough, upon which a more pronounced shortwave is progged by the consensus of the models to lift northeast during peak heating. This will promote at least modest bulk shear to support multi-cell clusters across this area, while most of the rest of the east should generally feature pulse type thunderstorms. Additionally, mean 0-6km winds and propagation vectors are aligned and parallel to the boundaries, suggesting an enhanced training risk as well, which additionally supports the elevated HREF and REFS probabilities for 3"/24hrs (and lower but still notable 5"/24 hrs). For this reason the inherited SLGT risk was adjusted to spread farther south into the eastern Carolinas. ....Southwest... Mid-level ridge centered over NM to start Thursday morning will weaken in response to height falls approaching from CA as a trough digs down the Pacific coast. This will have the two- pronged affect of lowering heights across the Desert Southwest/Four Corners region, while also leading to increased cyclonic flow emerging from near Baja to push a potent shortwave northward across AZ and into UT. Additionally, this increasingly south flow, especially between 850-700mb, will transport elevated PWs northward, and a plume above 1" is progged to surge almost to UT by the end of the forecast period. Where this combines with MUCAPE above 500 J/kg, slow moving storms (0-6km mean winds of just around 5 kts to the north) could become more organized leading to a longer duration of heavy rainfall rates of 0.5-1"/hr. The total coverage is still somewhat uncertain so the inherited MRGL risk still appears appropriate, but this was expanded north to match recent guidance, and convection is likely to be more widespread Thursday than it was on Wednesday. ...Upper Mississippi Valley... A warm front will lift northeast towards Minnesota today, accompanied by a surge of 850mb winds to 20-30 kts advecting anomalous PWs of 1.5 to 1.75 inches northeastward (+1 to +1.5 standard deviations above the climo mean). This front will be driven eastward by a potent but flat and fast- moving shortwave embedded within the nearly zonal flow aloft, leading to a period of enhanced ascent aided by the warm front and accompanying isentropic lift of the 850mb wind. This will result in scattered thunderstorms across the area, with bulk shear of 25-35 kts potentially supporting some organization into clusters. The guidance is highly variable in both the placement and intensity of convection today, and the QPF footprint and ensemble probabilities suffer as a result. Still, with the impressive thermodynamics in place, thunderstorms that develop could support rainfall rates of 1-2"/hr with at least short-term training possible, and the inherited MRGL risk was curtailed for latest guidance but continued. ....Pacific Northwest... A rare August atmospheric river (AR) is progged to make landfall this evening north of 45N into WA state with IVT above 500 kg/m/s as reflected by both GEFS and ECENS probabilities above 90%. A weak shortwave embedded within otherwise zonal flow aloft will help produce ascent into this plume of IVT, but instability will be negligible and jet level ascent appears to be displaced north of the core of the AR. While deterministic model QPF output and accompanying ensembles support a threat for heavy rainfall, especially in the Olympics, this will be occurring atop extremely dry soils and should generally be beneficial rainfall. Additionally, in the AR/ERO verification data set since 2019, there has never been an impactful (MRGL risk or higher) AR in the month of August. For this reason, despite some heavy rain spreading onshore this evening, no excessive rainfall impacts are expected. Weiss $$ --- SBBSecho 3.28-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .