Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 15 2025 15:57:51 FOUS30 KWBC 151556 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1156 AM EDT Sat Mar 15 2025 Day 1 Valid 16Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-SOUTH INTO THE TENNESSEE VALLEY... ....16Z Update... The shortwave trough over Texas and Oklahoma, which is south of=20 the primary low circulation now over Wisconsin continues to drive much of the convection, severe, and flash flooding across the Lower Mississippi and Tennessee Valleys this morning. This shortwave is stronger than much of the previous guidance had suggested, which in turn has kept the primary area of storms that stretches from Louisiana to western Tennessee as of this writing further west. Thus, the heaviest rainfall totals this morning have been along the Mississippi River, with a secondary area of storms over northern Alabama into portions of eastern Tennessee. As the shortwave trough continues to get its act together, it will begin to shift eastward, resulting in the line of storms also moving east, with increasing forward speed. This in turn will gradually diminish the flash flooding threat as the line of storms will not have time to drop prodigious amounts of rain in these more eastward areas of Alabama, Georgia, and the Southern Appalachians. Given this, ERO upgrades were proposed along the Mississippi River where rainfall amounts over 3 inches in far southeastern Arkansas have already occurred, and the storms will continue to impact the area for another few hours before the significant eastward shift of the line begins. Conversely, because the storms have stayed=20 further west than expected, there has been less rain into the=20 southern Appalachians as the stronger storms with the eastward extent of the rain shield arm have stayed further west towards=20 Chattanooga, and impacted the mountains far less. While all of these areas will see the main line of storms go through later today into tonight, the lack of prior rainfall should reduce the flooding impacts from the line significantly. For western North Carolina, there should still be an upslope component to the southerly flow along the line of storms as it moves through late tonight, which may locally enhance the rainfall as well as begin the rain sooner due to the added lift. The Slight Risk for this area has been maintained, while the Moderate for extreme southwest North Carolina and northern Georgia was downgraded with this update. The 12Z HREF guidance suite has also followed suit with the=20 westward shift in the rainfall. A higher end Moderate has been extended southwestward to include northwestern Mississippi along the Mississippi River, Memphis, and extends northeastward into the Nashville area. Neighborhood probabilities in this area have increased to as high as 80% for 5 inches or more of rain. However, with the chances for 8 inches or more of rain only at 10-20% at most, we remain shy of criteria necessitating a High Risk upgrade. ....Pacific Northwest... No changes were made to the previous forecast. The heaviest rain will begin late this afternoon and persist through much of the night. With most of the expected rainfall capable of isolated flash flooding not having begun yet, there was no need determined for any changes. Wegman Day 2 Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EAST COAST, FROM THE SOUTHEAST TO NEW ENGLAND, AND FOR PORTIONS OF THE PACIFIC NORTHWEST... ....East Coast... As the aforementioned low lifts northward into Canada, what's left is a weakening cold front as the upper-level pattern becomes a bit less amplified (shortwave over the Southeast lifting a bit and cutting off from northern/polar stream). The result still looks to be scattered localized totals of 1-2" over a large region from the Southeast (central FL, southeast GA, and Carolinas) into the Mid- Atlantic and portions of the Northeast/New England (though some 2-3" totals are possible in the coastal Carolinas region). The Marginal risk was maintained as these relatively limited totals should only present a localized flash flood risk as the front begins a faster progression towards the Atlantic Coast. ....Pacific Northwest... The aforementioned AR looks to bring in stronger moisture transport into Day 2 period with the potential for IVT in the 600-800 kg/ms range near the OR/CA border region, peaking for a period early in the day on Sunday. Model consensus suggests an additional 2-4" of QPF through Day 2, and a targeted upgrade to Slight risk remains possible (depending on the evolution of the forecast QPF in the Day 1 period). Churchill/Putnam Day 3 Valid 12Z Mon Mar 17 2025 - 12Z Tue Mar 18 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ta7HKOvXEMqbLkh5fq7czs_zshr6XaOFxcki0Qqbole= A7_HMf0WRIbCJFycvY19-OoqH1xb5N-2HNawNkeFvQyyIKk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ta7HKOvXEMqbLkh5fq7czs_zshr6XaOFxcki0Qqbole= A7_HMf0WRIbCJFycvY19-OoqH1xb5N-2HNawNkeFxMiaYrg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ta7HKOvXEMqbLkh5fq7czs_zshr6XaOFxcki0Qqbole= A7_HMf0WRIbCJFycvY19-OoqH1xb5N-2HNawNkeFymRn3iM$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .