Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 05 2022 20:30:33 FOUS30 KWBC 052030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 PM EST Mon Dec 05 2022 Day 1 Valid 16Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR NORTHERN ALABAMA, FAR SOUTHERN TENNESSEE, AND NORTHERN GEORGIA... ....16Z Update... Only very minor modifications were made to the Marginal Risk area to account for the latest Hi-res guidance with no changes to the Slight Risk area for this update. The axis of highest rainfall amounts remained well aligned with the current Slight Risk area with generally 2-4 inches expected. A band of light to moderate rainfall was already occurring this morning across the outlook area with the max rainfall rates as estimated by the latest pass of the MRMS Q3 Multi-Sensor output around 0.25"/hr. Expect rainfall rates to increase a bit into this evening and overnight but as noted in the prior discussion, the lack of any notable CAPE values will keep the chances of rates any greater than around 0.5"/hr isolated. Thus, even where the higher amounts of rainfall occur, expect the chances of flash flooding to remain isolated to scattered as the rates will keep the overall totals well-distributed throughout the outlook period. Putnam ....Previous Discussion... Southwesterly flow of Gulf moisture will collide with a cold front and associated jet streak across the Slight Risk region today. Meanwhile, surface ridging banked up against the east side of the Appalachians will further add to the southerly flow to the west. Lingering 850 mb moisture from the tropical East Pacific will be enhanced by the Gulf moisture. Therefore, there is potential for a widespread 2-4 inches of rain today and especially into tonight in the Slight Risk area. As in previous days, the area with the best chance of seeing the higher rainfall totals will be over north to northeastern Alabama. The highly anomalous moisture noted in previous discussions remains with PWATs to 1.5 inches equating to moisture amounts that are 2.5 standard deviations above normal, or in the 90-95 percentile range of climatology. The lack of instability remains the primary limiting factor to excessive rainfall amounts being realized. This is due in no small part to the time of year, where lack of solar influence is actively reducing the instability. MUCAPE values in the latest guidance remains under 500 J/kg west of the Slight Risk area, and 200 J/kg in the Slight Risk area and points east. This will greatly limit the potential for the highly anomalous moisture to achieve some verticality in the cloud to grow into larger raindrops. Thus, even in the Slight Risk area the confidence is somewhat reduced, which ties with the latest model trends of gradually reducing the peak forecasted rainfall amounts in this area. Weak lobes of vorticity in the upper levels may locally help with lift, but that alone can only do so much. Soil moisture is right around average for this time of year, which implies any recent rainfall has soaked in as much as it is going to. CAMs guidance shows training segments of occasionally moderate to heavy rainfall from west to east across the Slight Risk area. Changes from inherited include trimming the western end of the Slight entirely out of MS, as all guidance suggests rainfall amounts that could lead to flash flooding...2 inches or more, if they occur, will be further east. Nonetheless, western areas of TN & MS aren't completely out of the woods, and the Marginal remains in these areas. In coordination with the MRX/Morristown, TN and GSP/Greenville-Spartanburg, SC forecast offices, kept the Slight out of the mountains of NC and far northeastern GA, as any upslope enhancement should still only result in isolated flash flooding. Wegman Day 2 Valid 12Z Tue Dec 06 2022 - 12Z Wed Dec 07 2022 The probability of excessive rainfall exceeding flash flood guidance is less than 5 percent. Putnam 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!8kdLYKKr5lMy7Eam-VVcJUenPIFP0V-ZeOA7ZV2G4JO5= 9vuzXy1rNpDwGHxPo7tPoJVDrnmb743g6yFEdTal4qUCaMI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8kdLYKKr5lMy7Eam-VVcJUenPIFP0V-ZeOA7ZV2G4JO5= 9vuzXy1rNpDwGHxPo7tPoJVDrnmb743g6yFEdTal_RM-zhU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8kdLYKKr5lMy7Eam-VVcJUenPIFP0V-ZeOA7ZV2G4JO5= 9vuzXy1rNpDwGHxPo7tPoJVDrnmb743g6yFEdTalGcZunH8$=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) .