Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Mar 26 2023 08:29:06 FOUS30 KWBC 260828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 428 AM EDT Sun Mar 26 2023 Day 1 Valid 12Z Sun Mar 26 2023 - 12Z Mon Mar 27 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL ALABAMA AND GEORGIA... There has been quite a notable flip-flop in all of the guidance as pertains to the weather expected to unfold across portions of the Southeast today. A stalled out front remains in place across the Slight and Marginal Risk areas from Louisiana to southern North Carolina. This front is already serving as a focus for convection from central MS through far western GA. The convective activity is only expected to increase for the rest of the predawn hours and into the morning. 30-40 kts of southwesterly 850 mb flow is drawing plentiful Gulf moisture northward into the front. Meanwhile, a pair of subtle disturbances are expected to track eastward along the front, causing local increases in both convective coverage and severity. This is probably the largest change from 24 hours ago, when we had eliminated even a Marginal risk of flash flooding! These disturbances acting to organize the convection along the front now will be quite effective at taking advantage of as much as 2,500 J/kg of MUCAPE and somewhat unusual levels of atmospheric moisture at 2 sigma above normal and PWATs exceeding 1.5 inches. Uncertainty remains above average given the rainfall event is already beginning, as to where the heaviest rain is expected to fall. While most of the CAMs have highlighted the Slight Risk area, the primary area of uncertainty is how a subtle upper level disturbance interacts with the ongoing convection and associated cold pools this evening. The wave will track well north and west of the area, but nonetheless will have enough impact on the ongoing thunderstorms to result in a local increase in storm coverage and intensity. Some of the CAMs suggest this will result in the heaviest convection displaced well to the north of the surface front, highlighting Birmingham and Atlanta, whereas others think the primary forcing will stay with the front further south, resulting simply in an area of strong thunderstorms as the latest salvo in the wave train. The impacts from this difference would be how severe the likely ongoing flooding can get if the stronger storms track over the same areas, or if they're sufficiently displaced to the north to result in fewer impacts (despite the lower FFGs due to yesterday's rain across northern AL). If the former scenario pans out where the storms stay with the main surface front, then there's some potential for extreme rainfall totals exceeding 6 inches (as depicted in the 00Z HRRR primarily), and the potential need for an additional upgrade to a Moderate Risk. As regards training storms, the potential is very high, and in fact is already happening in the Slight risk area of eastern AL/western GA along the front. These storms will continue training at least through the morning hours, at which point the overall convective activity will wane a bit with the typical weakening of the LLJ. Convection then increases again with the approach of the second wave in the late afternoon and evening hours. For all convection today, the motion of the storms will be east-northeastward, which is parallel to the surface front that is not expected to move much during the day. Thus, the flow orienting parallel to the front supports continued training through the day. Already, currently ongoing severe thunderstorm activity is producing local rainfall rates up to 1.5 inches per hour, and that intensity of convection is expected to continue over the next several hours, and again late this afternoon and evening. For the surrounding Marginal Risk, expansions were made in all directions with this morning's update. Some convection has already developed in northwestern Alabama and central and southern Mississippi as of this writing, and continued scattered convective development is likely through the day in both areas. The increased coverage and intensity required an eastward expansion of the Marginal Risk to the Carolina coast, as all of the convection will track along this corridor (albeit likely in a somewhat weaker state than areas further west), and therefore training convection once again will be the dominant factor favoring development of flash flooding, particularly if those stronger storms move over urbanized areas. ....Potential for Future Risk Changes... Depending on storm coverage through the morning, and if guidance remains consistent on continued training storms in much of the Slight Risk area late this afternoon and this evening, a Moderate Risk may be needed. Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6aAiSVwEO0OgkX6vXkHCvN8DvygjLGt9GLrqFHs7KyUS= Mqo469Sc0VmPPZ30pFghS1StCNT2LubFd4KR4-7uWONc7U0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6aAiSVwEO0OgkX6vXkHCvN8DvygjLGt9GLrqFHs7KyUS= Mqo469Sc0VmPPZ30pFghS1StCNT2LubFd4KR4-7uk2Qu1eM$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6aAiSVwEO0OgkX6vXkHCvN8DvygjLGt9GLrqFHs7KyUS= Mqo469Sc0VmPPZ30pFghS1StCNT2LubFd4KR4-7uohJXZlc$=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) .