Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 13 2024 08:15:53 FOUS30 KWBC 130815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 AM EDT Mon May 13 2024 Day 1 Valid 12Z Mon May 13 2024 - 12Z Tue May 14 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ....North Gulf Coast... An incredibly low-confidence forecast panning out for the central Gulf coast today. A low tracking over Kansas and Missouri will=20 drag a cold front across the western Gulf Coast through tonight. Ahead of that cold front, deep convection being supported by mesoscale upper level shortwaves taking advantage of ample Gulf moisture is making this a very difficult forecast. The storms reacting to these upper level impulses are creating outflow boundaries of their own, which in turn is providing the forcing for additional convection. As the storms ongoing across Mississippi and Louisiana now turn and redevelop towards the south and east, their outflow will track along and reinforce a boundary along which additional convection will try to form through the day and into tonight. This boundary will be largely orthogonal to the mean southwesterly flow, which will support training convection. 00Z HREF 40-km neighborhood probabilities are up to 40% for 8 inches of rain along the Mississippi Gulf Coast, and over 70% for 5 inches of rain for the Mobile Metro Area through 12Z. There remain several points of uncertainty in this unfolding forecast scenario. First, how much convection will form along this boundary. Second, will the boundary move. Third, does the boundary move offshore. Fourth, how fast will the storms that form along the boundary move. Let's address these in order: First, how much convection will form along the boundary: CAMs are in poor agreement as to whether the storms that form along the boundary will be swift-moving segments that train towards the ESE, or if in between these segments, additional weaker storms continue forming. This will play a huge role as to whether communities in the MDT risk area see a few rounds of heavy rain with long breaks in between, or if it will be lightly raining with occasional moderate bursts in between the heavy rain producing segments. Of course, the latter would result in much more widespread and significant flash flooding along the boundary. Should the former occur, then it's probable the Moderate Risk is an overforecast. Second, will the boundary move. While the CAMs are in reasonable agreement that the boundary remains in place, the question remains as to where that boundary sets up. The further north the boundary, the more severe the resultant flash flooding.=20 Third, does the boundary move offshore at any point. This of course will play a huge role as to the severity of flash flooding. If the boundary forms or quickly moves offshore, this will dramatically reduce the flash flooding severity and coverage. The CAMs have been trending southward quite quickly, hence the reduction in the coverage of the Moderate, so any further southward shifts will need to result in downgrades since all the heaviest rain, regardless of training, falls offshore. This is a distinct, but still highly uncertain possibility. Fourth, how fast will the storms move. As always, the faster the storm movement, the less severe the flash flooding as any one location sees heavy rain for a shorter amount of time. There is reasonably good agreement than any one cell or segment will be fast enough moving to reduce the flooding threat any one cell or segment may cause. It will take expected training of multiple cells/segments to get the potentially significant flash flooding indicative of a Moderate Risk. All of the above to say, even a small shift in the forecast will lead to large changes in the flooding severity. Stay tuned. Outside of the Moderate Risk, the surrounding Slight risk was expanded to the west to include more areas north and west of Houston with this update. This area has been repeatedly hard hit with multiple days of storms over the past week or so, resulting in very saturated soils that take little additional rain to result in flash flooding. While storm coverage won't be nearly as plentiful along the Upper Texas Coast as further east, even the scattered storms that do form will have significant potential to produce additional flash flooding. For reasons mentioned above, the surrounding Slight was also greatly trimmed on the north side from inherited, as the southward shift in the guidance means the greatest instability fueling the strongest storms also stays south. Much of the rain in central=20 MS/AL is ongoing now. ....Portions of the Midwest... The Slight across portions of Missouri, Iowa, and Illinois was adjusted south by about a row of counties with this update. This follows the warm frontal interface ahead of the developing low over Kansas and eventually southern Missouri. Convection in the warm sector will collide into the warm front, which will remain largely stationary. This in turn may result in localized training along that boundary. The latest CAMs resolve the warm front to be a little bit south of previous runs, and as such the Slight was adjusted accordingly. The cold frontal convection between the 2 Slight areas is highly likely to have a fast forward speed across Arkansas and southern Missouri. Should the convection move over a particularly sensitive area, isolated flash flooding is possible, so the Marginal in this area remains in effect and largely unchanged. Wegman Day 2 Valid 12Z Tue May 14 2024 - 12Z Wed May 15 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE FLORIDA PANHANDLE... ....Florida Panhandle... Storms will be ongoing across the Florida Panhandle to start the period Tuesday morning. They are likely to be progressive, moving along to the east across north Florida through the morning. The uncertainty is how much convection develops behind the first line of storms. The broad unidirectional southwesterly flow would bring any additional convection into the Florida Panhandle through the day Tuesday. When added to any rainfall the area gets Monday night, this could lead to widely scattered instances of flash flooding, particularly between Pensacola and Tallahassee. The area has very high FFGs, even though soils are wetter than normal. Due to the unusually high PWATs associated with the Gulf moisture streaming into the Panhandle ahead of a cold front which will clear out the convection to the east by Tuesday night, any storms that form will be capable of rates of 2 inches per hour. ....Portions of the Southeast, Mid-Atlantic, and Ohio and Tennessee Valleys... North of the Florida Panhandle, the cold front will be racing eastward more quickly than previous model runs had indicated. Thus, now most of the heaviest convection is expected to be largely offshore of the Carolinas and Georgia, and therefore are much less likely to cause flooding. The greatest threat of flooding from these storms would be along the immediate coast, but given the sandy soils, are unlikely to result in flash flooding. Thus, the inherited Slight for Georgia and the Carolinas was downgraded with this update. Further north and west, the parent low and associated upper level cold air will result in widely scattered convection Tuesday and Tuesday night for the Ohio/Tennessee Valleys and over to the Mid- Atlantic. Instability will be modest, but non-zero along with modest atmospheric moisture. Thus, any storms that form, especially those that undergo cell mergers or training, have the potential to produce isolated instances of flash flooding. The inherited Marginal risk was kept largely the same, but was expanded a bit to include the DC and Baltimore metros due to low FFGs from recent heavy rainfall.=20 Wegman Day 3 Valid 12Z Wed May 15 2024 - 12Z Thu May 16 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE SOUTHERN PLAINS AS WELL AS FOR PORTIONS OF NORTHEAST FLORIDA... ....Southern Plains... Convection that develops on the leading edge of return flow associated with an LLJ that will move north along the west Texas dryline is expected to be isolated to widely scattered in coverage Wednesday afternoon, becoming more numerous by Wednesday night. There remains considerable uncertainty as to how this convection will organize...and given the highly variable nature of the soils across north Texas and western Oklahoma, confidence is not yet there for more than a Marginal Risk at this time. However, with better confidence a Slight may be needed, particularly in north Texas. For western Oklahoma, while similar coverage and severity of convection is expected...very dry soils there will be better able to absorb any rainfall, and so upgrades here are much less likely.=20 ....Northeast Florida... The trailing end of a cold front will stall across Northeast Florida on Wednesday. South of the front, abundant Gulf moisture will stream eastward along the boundary, providing a focus for convection to develop and move. Thus, a small Marginal remains for northeast Florida, as this area is difficult to flood due to flat and largely swampy terrain, but any training convection in this area could result in flash flooding, particularly in any urban areas impacted. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4RCGPn5UZ2DbFb-7HGUUC6OraLpct0gJjmX5G6orQ0Yq= rLCs0dDratkfoZoBZcImQW_R-G0EC8ea2YdrTqfZMoldFMg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4RCGPn5UZ2DbFb-7HGUUC6OraLpct0gJjmX5G6orQ0Yq= rLCs0dDratkfoZoBZcImQW_R-G0EC8ea2YdrTqfZdTnRKrk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4RCGPn5UZ2DbFb-7HGUUC6OraLpct0gJjmX5G6orQ0Yq= rLCs0dDratkfoZoBZcImQW_R-G0EC8ea2YdrTqfZYRsFCZk$=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) .