FOUS30 KWBC 200800 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 AM EDT Tue May 20 2025 Day 1 Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE OHIO AND TENNESSEE VALLEYS... A deeply negatively tilted longwave trough stretching from Montana to the mid-Mississippi Valley will continue flattening by becoming even more negatively tilted through the period as the trough itself lifts back into the broader more zonal jet stream. While this trough will continue to support a surface low as it moves east from Iowa to roughly the IN/OH/MI tripoint by 12Z Wed, the low will be weakening with time. The low will be filling in part because the zonal pattern will gradually decouple the low from the plume of Gulf moisture as the LLJ that has been supporting the storms over the middle of the country the past couple days gets shunted south back towards the Gulf Coast. The result of this will be decreasing amounts of atmospheric moisture available for the storms with time, resulting in a subsequently decreasing flash flooding threat. However, for this period, the southwesterly influx of Gulf moisture will feed training storms across the Ohio and Tennessee Valleys, including portions of eastern Kentucky and southern West Virginia which were hard hit with heavy rain and severe storms over the past week or so. The inherited Slight Risk area was trimmed on the northwest side due to a lack of instability over much of Illinois, but expanded a few rows of counties across Tennessee towards the south and west. No significant changes were made to the surrounding Marginal Risk. Wegman Day 2 Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF THE CENTRAL APPALACHIANS... A Slight Risk upgrade was introduced with this update for a portion of southwestern Pennsylvania, northern West Virginia, eastern Ohio, and far western Maryland. New shortwave energy will begin to tug an upper level low north of Lake Superior southward into the Midwest on Wednesday. Meanwhile, an active subtropical jet and associated 110 kt jet streak will also round the base of that upper level low. The cold air aloft associated with the upper low is expected to locally increase instability across the Slight Risk area Wednesday afternoon, while the multiple shortwaves moving through result in a rather compact area with multiple rounds of showers and thunderstorms moving through in rapid succession. This portion of the central Appalachians is particularly sensitive to flash flooding, as well as the urban concerns in and around the Pittsburgh metro. While there is still limited guidance on the nature of the storms, the RRFS solution suggests the above convective evolution Wednesday and into Wednesday night. Two big questions still remain...how far north will significant instability get and how will this instability align with the greatest forcing, likely to set up in and just north of the Slight Risk area. A total elimination of instability shouldn't prevent training convection, but it's likely to be mostly showers and unlikely to result in more than isolated flash flooding. Further, any northward shift in the track of the repeating shortwave impulses could also reduce the flash flooding threat. Meanwhile a more northward expansion of the instability or a slightly further south track to the upper level energy could both enhance the flash flooding threat. Thus, the consensus was a low end Slight with a southward bias towards the greater instability. Elsewhere, the Marginal Risk across the Northeast was trimmed largely due to a certain lack of instability. While areas such as NYC are likely to see periods of light rain for much of the period, the stratiform nature should limit rainfall rates to a half inch per hour at most, precluding any more than some ponding on roads. The Marginal Risk in deep south Texas was trimmed on its eastern side but expanded north. Convection is expected to develop along the mountains of Mexico, but is unlikely to drift too far east off those mountains, narrowing the flash flooding threat to the immediate Rio Grande Valley. Wegman Day 3 Valid 12Z Thu May 22 2025 - 12Z Fri May 23 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The inherited Marginal across eastern New England was downgraded with this update in coordination with BOX/Taunton, MA and OKX/Upton, NY forecast offices. The broad 1-2 inches of rainfall expected through the period will be from a wound up low that will develop off the coast. The low should shove any instability to its east out to sea, resulting in only stratiform rain across New England. While there may be a few urban concerns in Boston and Providence, the low rainfall rates should favor more ponding and very little, if any, flash flooding. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt