Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 08 2024 22:05:07 FOUS30 KWBC 082205 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 504 PM EST Fri Mar 08 2024 Day 1 Valid 2158Z Fri Mar 08 2024 - 12Z Sat Mar 09 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ALABAMA, GEORGIA AND MISSISSIPPI... Closer to the Gulf Coast... The axis of a southwesterly 130 to 150 kt upper level jet will provide a persistent source if upper level divergence from eastern Texas into the Tennessee Valley. While this supports cyclogenesis in parts of the central Mississippi Valley...the more important factor will be accelerating low level winds off the Gulf of Mexico that interact with a front slowly moving south across the Gulf Coast states and parts of the Southeast U.S.. In particular...precipitable water values are forecast to rise near 2" from parts of Louisiana into southern Alabama as south winds of 30 to 45 knots at 850 mb result in pronounced moisture transport and low level moisture flux convergence. The convective evolution has been quickening as of late, with the earlier band across central MS completely dissipating and activity across southern LA organizing and coalescing near the southward moving front. This update allows a decent chunk of LA/MS to be eliminated from the SLGT and MDT risk areas, and leaves areas across AL/GA relatively untouched as the previous update appears to be doing a good job with its southern extent. It should be noted that the 18z HREF appears a couple hours slow/too far northwest with the convective evolution thus far. Local amounts in the 6" range remain possible, particularly from portions of the southwest MS/LA border eastward. To the north and east into the Southern Appalachians, the storms that impact Alabama and western Georgia will be weakening as they track northeastward into the western Carolinas. While the storms will be weakening, they will have the benefit of some upslope flow...though the primary flow will be parallel to the mountains. Nonetheless, the moisture-laden nature of the atmosphere should result in 1 to as much as 3 inches of rain later tonight into the western Carolinas. The Slight remains in effect for these areas with little change. Mid-Mississippi, Ohio Valleys to Chicagoland... The previously issued Marginal Risk was modified a bit but significant uncertainty remained. The structure and nature of any showers and thunderstorms that develop in this part of the country will be aided by better forcing but less moisture. Given that there were isolated instances of heavy rainfall in the area a couple of days ago...will keep the Marginal this outlook cycle despite waning confidence.=20 Roth Day 2 Valid 12Z Sat Mar 09 2024 - 12Z Sun Mar 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINAS... ....2030z Update... Generally maintained the same inherited Slight Risk, only tweaking the borders a bit based on the latest data. While the contoured area generally encompasses where the 12z HREF indicates 30-60% neighborhood probabilities for 2" exceedance, it is noted that there is still higher than usual spread amongst the hi-res ensemble members. There is good consensus that an axis of 2-3"+ will occur amongst nearly all the members, but the disagreement concerns whether that area will be across the southern half of the Slight (a bit more likely, indicated by the ARW, HRRR, FV3, and experimental RRFS) or the northern half (less likely, indicated by the ARW2 and NAM-nest). While the confidence farther north into North Carolina is lower, this area coincides with the Piedmont to Coastal Plain transition (fall line), and FFG is notably lower across the clay-like soils of the Piedmont. For this reason, maintained the northern extent of the Slight Risk for now, though it may be adjusted with higher confidence in the subsequent guidance. Elsewhere, the Marginal Risk was maintained (besides a cut-out of the Delmarva Peninsula and surrounding portions of the Chesapeake and Delaware Bays, based on higher confidence of lower totals amongst all the CAMs). Churchill ....Previous Discussion... ....Southeast... The expansion of the Slight Risk area made on Thursday still looks good. The biggest uncertainty being where convection will be on-going at the start of the Day 2 period. The nose of a southwesterly low level jet should be extending from the Gulf of Mexico to the Carolinas...with part of an approaching surface warm front from the Ohio/Tennessee Valleys getting hung up on the terrain. The heaviest rainfall associated with the system should be occurring on Saturday morning before the best focus shifts off the coast during the afternoon and evening. ....Great Lakes into the Northeast... A strong low tracking northeast from the Ohio Valley Saturday morning to New England Sunday morning will drag a powerful cold front behind the front. This will provide ample forcing to lift residual moisture and convection associated with the warm conveyor belt in the form of locally heavy rain across the Marginal Risk area. The lion's share of the available atmospheric moisture will stay over the Southeast, but the ample forcing and some moisture will support some heavier rain. Some adjustments were made to the western periphery of the Marginal Risk area based on the latest QPF. Also made some adjustment to minimize the overlap between the excessive rainfall outlook and areas of snowfall...although some overlap was allowed in regions of rain changing to snow...in regions where there appears to be a risk of rain falling on a snow pack...and where stream flows were already high from recent heavy rains. Bann Day 3 Valid 12Z Sun Mar 10 2024 - 12Z Mon Mar 11 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF COASTAL NEW ENGLAND ON SUNDAY... Thinking remains unchanged with this update, maintaining the Marginal Risk area across portions of coastal New England. Churchill ....Previous Discussion... Some rain and snow should still be falling over the eastern portion of New England as low pressure pulls away from the coast.=20 The risk of excessive rainfall was confined to the coastal regions from New England in part due to the increasing coverage of snow in a strong cold air regime. Areal average rainfall amounts of 1 to 1.5 inches are forecast in a region where there have been multiple rounds of precipitation recently. Once again...minimized the overlap with more inland areas where terrain keeps precipitation type in the form of snow. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!80JqERYx2C3EHTm9u2Nu_Qm2guss7Cwh2xSMoW4kT8po= Gsq8I-QbZUIabeCdypXy4YrBQbp0mcz2-mQMl-X16xfu9Dg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!80JqERYx2C3EHTm9u2Nu_Qm2guss7Cwh2xSMoW4kT8po= Gsq8I-QbZUIabeCdypXy4YrBQbp0mcz2-mQMl-X10SIRmNk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!80JqERYx2C3EHTm9u2Nu_Qm2guss7Cwh2xSMoW4kT8po= Gsq8I-QbZUIabeCdypXy4YrBQbp0mcz2-mQMl-X1WNKayhg$=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) .