Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 08 2024 20:12:06 FOUS30 KWBC 082012 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 310 PM EST Fri Mar 08 2024 Day 1 Valid 16Z Fri Mar 08 2024 - 12Z Sat Mar 09 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF ALABAMA, GEORGIA, AND MISSISSIPPI... ....16z Update... Relatively disorganized convection has been ongoing for much of the morning across portions of the Southeast, mainly across eastern TX into central LA/MS thus far. Rainfall rates locally have already been reaching 1-2"/hr (per MRMS), and convection is expected to continue to organize more linearly along a developing warm front. This should result in hourly accumulations reaching 1-2"/hr along the training axis, most favored from east-central LA northeastward into southwest MS, south-central AL, and west-central GA. Where these values repeat through the evening and overnight hours, localized totals are expected to reach 4-6" (most favored across the Moderate Risk area, which was expanded a bit southwest to include a bit more of southwest AL and adjacent portions of MS). 12z HREF 40-km neighborhood probabilities for 5" exceedance are indicated to be between 30-60% for southern MS, south-central AL, and west-central GA (which includes the Moderate Risk area and surroundings), and they remain maximized across the AL/GA border region just to the south of the Birmingham and Atlanta metro areas (relatively unchanged since the prior update, despite some southwest trend in the training axis. Churchill ....Previous Discussion... No changes in the overall synoptic reasoning for heavy to excessive rainfall developing in parts of the Gulf Coast and Southeast U.S. today and spreading eastward by overnight hours.=20 However...a southward shift and some westward expansion was needed to the Moderate Risk area given a consistent signal from the 00Z convective allowing models.=20 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 quasi-stationary front draped across the Gulf Coast states and parts of the Southeast U.S.. In particular...precipitable water values are forecast to exceed 2 inches from parts of Louisiana into southern Alabama as south winds of 30 to 45 knts at 850 mb result in pronounced moisture transport and low level moisture flux convergence. The 08/00Z HREF showed greater than 10 percent neighborhood probabilities that peak at over 20 percent probability of rainfall exceeding 2 inches in an hour within the Moderate Risk area and where the probability of 3 hour rainfall exceeding 3 hour flash flood guidance exceeds 30 percent by 09/00Z. The 08/00Z HREF still depicted exceedance of the 5 inch threshold at 30-50%, respectively. Given several rounds over the past few days of heavy rainfall across parts of the outlook area and with the potential for training cell/repeat convection that can result in spots getting more than 8 inches of storm total rainfall amounts...the potential exists for instances of flash flooding (with localized instances of significant flash flooding). A broader Slight Risk area encompasses much of the Southeast, though the chances for these high-end amounts should generally be confined to a training west-east axis from MS/AL/GA along the advancing warm front. 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 Bann 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 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5Ht_-qmxQVRBvSg3MNrYl-F6Ew8tlEDaWDbFlyCBIBB-= 7L-oiHA9CnMiJBVNKwxmkf5Zk8vqy9cWiFOMfnE0OeABluc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5Ht_-qmxQVRBvSg3MNrYl-F6Ew8tlEDaWDbFlyCBIBB-= 7L-oiHA9CnMiJBVNKwxmkf5Zk8vqy9cWiFOMfnE0RYEVOK4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5Ht_-qmxQVRBvSg3MNrYl-F6Ew8tlEDaWDbFlyCBIBB-= 7L-oiHA9CnMiJBVNKwxmkf5Zk8vqy9cWiFOMfnE0SBvvMQY$=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) .