Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 20 2023 20:26:17 FOUS30 KWBC 202026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 PM EST Fri Jan 20 2023 Day 1 Valid 16Z Fri Jan 20 2023 - 12Z Sat Jan 21 2023 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Wegman Day 2 Valid 12Z Sat Jan 21 2023 - 12Z Sun Jan 22 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE GULF COAST AND SOUTHEAST... ....2030Z Update... The biggest change from inherited was to remove essentially the entire Florida Peninsula from the Marginal Risk area for the Southeast. Otherwise no changes were needed. The front that will be producing the potential for isolated flash flooding from excessive rainfall will produce multiple rounds of rain across the Marginal Risk area. Associated rainfall is unlikely to extend across the Florida Peninsula, and any rain that impacts northern areas is unlikely to amount to enough to cause an isolated flash flooding risk. Soil moisture depictions from NASA SPoRT indicate that much of the Marginal risk area is drier than normal, so any storms that have the potential to cause flash flooding will have to be training storms, move over a particularly sensitive area, such as urbanized areas, or merge with other storms over the same area. Thus, the threat is definitely on the lower end of the Marginal Risk scale. As with the previous discussion, the addition of a Marginal Risk for RI and far eastern MA was considered with this update. The combination of a slight eastward shift in the axis of heaviest rainfall into higher FFG areas on Cape Cod and similar rainfall totals forecast for this area as inherited once again precluded this. This area will continue to be monitored. Wegman ....Previous Discussion... A southern stream trough will amplify as it moves through the Great Plains on Saturday. A surface low pressure system will glide across the Gulf Coast and direct moisture across the region and the eastern seaboard through Sunday. Modest southerly mid-level flow will support PWATs in the 1-2" range and hourly rates of around 0.25-0.5in/hr over portions of the central Gulf Coast and Southeast. Increased frontogenesis is expected in the late afternoon hours of Saturday as diurnal heating potentially spawns convection. Fortunately, the soils in the Southeast are relatively dry so the 1-2" of forecasted rainfall won't cause widespread flash flooding concerns. There's still a chance for isolated flash flooding, therefore a broad marginal risk is in place. Kebede Day 3 Valid 12Z Sun Jan 22 2023 - 12Z Mon Jan 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST... ....2030Z Update... A slow moving squall line/line of storms will be ongoing Sunday morning at the start of the Day 3 period. Most of the models and the few CAMs that go out that far indicate that the line should be east of the inherited western end of the Marginal Risk area, so a few rows of counties in Alabama and northwestern Georgia, including metro Atlanta, were removed with this update. Conversely, the storms will likely be at their strongest and will be repeatedly training over much of the Florida Panhandle Sunday morning, before gradually shifting east into the afternoon. Thus, the Marginal Risk was expanded southwest to the Gulf Coast. Many of the western areas of the Marginal Risk in FL/AL/GA/SC will have the soils moistened from rain on Saturday night, likely lowering FFGs in these areas from present levels. Elsewhere, much of the Carolinas and the Hampton Roads area of southeast Virginia remains in the Marginal Risk. The storms that develop further southwest over Florida and Georgia will congeal into a larger area of less heavy, but much steadier rain into the Carolinas and Virginia. There will certainly still be convective elements embedded in the broader rain shield, so isolated flash flooding remains possible. Rainfall in these areas will begin later than areas further south/west. Wegman ....Previous Discussion... A southern stream trough will phase with its northern stream reflection and amplify over Ohio/Tennessee Valley on Sunday. Mid-level flow out of the South will continue moisture advection into the Southeast and eventually up the East Coast later into Monday morning. There's a decent signal for heavy rainfall to develop over the Southeast Coast along a strengthening cold front and out ahead of a deepening low pressure system. PWATs between 1.5-2" and rainfall rates between 0.25-0.5in/hr may form along an axis of enhanced frontogenesis Sunday evening. The latest ECMWF trends have suggested a QPF footprint that is closer to the coast and inland areas while the GFS has been more consistent in this regard as well as the potential for enhanced rainfall tracking up the East coast and into New England. A marginal extending up into the Northeast was considered, but 24hr amounts between 0.5-1" aren't high enough to extend it northward at this time. Kebede Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_636ZnNGYRZlaM2qum8juWRdDgrVPYz19aH6OV83jKeU= 2Zqb5spH7GmlZWrlKkUrRwNIDHemYJzcsDfu_eG0t-82FLs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_636ZnNGYRZlaM2qum8juWRdDgrVPYz19aH6OV83jKeU= 2Zqb5spH7GmlZWrlKkUrRwNIDHemYJzcsDfu_eG0MzTAG4Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_636ZnNGYRZlaM2qum8juWRdDgrVPYz19aH6OV83jKeU= 2Zqb5spH7GmlZWrlKkUrRwNIDHemYJzcsDfu_eG0o__w5zA$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .