Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Mar 22 2025 22:47:06 FOUS30 KWBC 222246 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 646 PM EDT Sat Mar 22 2025 Day 1 Valid 01Z Sun Mar 23 2025 - 12Z Sun Mar 23 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth/Gallina Day 2 Valid 12Z Sun Mar 23 2025 - 12Z Mon Mar 24 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF WESTERN WASHINGTON STATE AS WELL AS FOR PORTIONS OF THE LOWER MISSISSIPPI AND TENNESSEE VALLEYS... ....Western Washington State and Northwest Oregon... 21z update: Little changes were required with the Marginal for the weak AR signal pushing through on day 2. Given entire day 2 period is within the Hi-Res CAM window, there is a bit higher confidence to sculpt the risk area to the terrain and slowly lifting rain/snow line across the Cascades in timing/placement. Other changes were to including northwest Oregon coastal range as well as carving out some of the downstream rain-shadow areas of W WA. ---Prior Discussion--- The trailing cold front of a low moving into British Columbia will move into the Pacific coast of Washington State on Sunday. The front will usher in a period of strong westerly fetch off the Pacific. This will keep a steady supply of moist upslope flow into the Olympics and Washington Cascades. 3 to 5 inches of rain are expected into those ranges Sunday and Sunday night. The inherited Marginal Risk was largely unchanged, with the greatest chances of isolated flash flooding on the west facing slopes of those ranges. The Skokomish River remains the river with the highest potential for flooding given its low thresholds for flooding. Further, the soils are already near saturation in this area given the multiple rounds of rain the area has seen over the last several weeks, so most of the rainfall expected should convert to runoff. ....Portions of the Lower Mississippi and Tennessee Valleys... 21z update: As noted below, arrival of instability from warmer Gulf air mass continues to be a bit slower filtering into the Cumberland Plateau and points north and east. Additionally, the overall convective pattern seems to be favoring more of scattered but intense convective cells along the frontal zone with some flanking development trailing to allow for W to E streaks of enhanced rainfall totals across western into Middle Tennessee and points southwest into the Lower Mississippi Valley mainly after 21z, peaking in the 03-06z period across portions of N MS, before becoming much more isolated through the remainder of the period downstream. HREF probabilities are also a bit more diminished with only a few hints of 2"/hr rates or 2"/3hrs suggesting the overall risk category of Marginal is more than necessary. The overall broader areal coverage can be more attributed to the higherly likelihood of those intense totals likely to be in shorter duration/sub-hourly manner resulting in isolated FFG exceedance with greatest flash flooding risk within urban/traditional poor drainage areas. If trends continue, the risk area may need to be further reduced in coverage or even the category itself. Gallina ---Prior Discussion--- A slow-moving cold front will move southeastward across portions of the Mid-South Sunday afternoon through Sunday night. The front will be supported by an LLJ of Gulf moisture, which will be aligned southwest to northeast, or parallel to the frontal interface. This will support some training convection, despite only modest Gulf moisture. The potential for some training over areas that got heavy rain a few days ago could lead to isolated instances of flash flooding, especially in low-lying flood-prone areas and urban areas. FFGs are quite high across the Mississippi Valley but decrease markedly across the Tennessee Valley due to recent heavy rain. However, the further north you go, the less instability there will be, which will effectively cap how strong the storms will be. The front will have enough forward speed to also limit the potential for training as well. Given all of these above factors, there have been few changes to the ERO for this area. Further northeast into West Virginia, the latest forecast rainfall has come down significantly from previous forecasts. Due to a lack of instability. the rainfall is not expected to remain heavy for long as individual cells track northeastward along the front. Despite very low FFGs, expect much of the rainfall in this area to be stratiform, greatly limiting the flash flood potential, despite some topography the rain will have to overcome to cause flooding. With the broader forecast now under a half inch in much of northern West Virginia, the inherited Marginal Risk in this area has been downgraded. Wegman Day 3 Valid 12Z Mon Mar 24 2025 - 12Z Tue Mar 25 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL GULF COAST... 21z update: Frontal zone timing placement appears to be a tad faster and continues to diminish the probability for broad swaths of heavy rainfall. However, orientation of the front in the morning is generally flat to diurnal onshore flow and with weakening deep layer flow, there is suggestions that any thunderstorms that do develop or maintain throughout the morning may linger over susceptible urban centers along I-10/12 from Baton Rouge east into Southeast AL/Western FL Panhandle. While the overall trends continue to decrease, have retained the low-end Marginal Risk to allow for a cycle or two of Hi-Res CAMs to help resolve these smaller scale interaction/urban intersection. Gallina ---Prior Discussion--- The front that moved across the Lower Mississippi Valley on Sunday will continue to the Gulf Coast on Monday before stalling. There remains some potential for instability and modest moisture (by Gulf Coast standards) to support heavy rainfall from training storms, but the primary flash flooding threat will be in the urban areas from New Orleans east through Pensacola and the Florida Panhandle. The inherited Marginal Risk area was trimmed on the northern end where high FFGs and a meager expected rainfall should preclude any flash flooding concerns into interior southern Alabama and Georgia. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-8zIpORREKjL7n5nR_BDf8isK3oNZBN_UhCaZiYpRwzK= 3BwUPHVycunQleZoZwk0YSLW3ENDXMs6gEND-WAK30Oxbjc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-8zIpORREKjL7n5nR_BDf8isK3oNZBN_UhCaZiYpRwzK= 3BwUPHVycunQleZoZwk0YSLW3ENDXMs6gEND-WAKxnEBZKc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-8zIpORREKjL7n5nR_BDf8isK3oNZBN_UhCaZiYpRwzK= 3BwUPHVycunQleZoZwk0YSLW3ENDXMs6gEND-WAKBTIVu9Q$=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) .