Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 05 2022 08:31:04 FOUS30 KWBC 050831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 AM EST Mon Dec 05 2022 Day 1 Valid 12Z 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... 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 Mon Dec 05 2022 - 12Z Tue Dec 06 2022 The probability of excessive rainfall exceeding flash flood guidance is less than 5 percent. In coordination with all of the forecast offices in KY & TN, there was unanimous agreement that the forecast rainfall of less than 1.5 inches over the 24 hour period of this forecast period was not enough to result in even isolated flash flooding. Soils in this area, especially across KY, are very dry and the light nature of any rainfall should easily be able to absorb into the soils. CAMs guidance for this period shows that only widely scattered to scattered light to moderate intensity showers that will be pretty fast moving is what can be expected across KY and TN on Tuesday. It should be noted that much of the guidance has trended north into northern TN & KY with the axis of heaviest rainfall. This further displaces the expected rainfall Tuesday from that expected today/Monday. Thus, the soils will not have much benefit from previous days' rainfall. Across the Northeast, here too rainfall will be on either side of 1 inch for the day, which is below all thresholds for flooding, per coordination with the impacted forecast offices. Thus, no Marginals were considered. Wegman 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!5kN9qLIHClwIDSv_9UJ2dalJMTIWWg80jtxepx6rDQRp= _pmX_toNLpyuviESRFKqquztLkuTZBEuKKCk80iaHfXIrEs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5kN9qLIHClwIDSv_9UJ2dalJMTIWWg80jtxepx6rDQRp= _pmX_toNLpyuviESRFKqquztLkuTZBEuKKCk80iaJJKBGi8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5kN9qLIHClwIDSv_9UJ2dalJMTIWWg80jtxepx6rDQRp= _pmX_toNLpyuviESRFKqquztLkuTZBEuKKCk80iaNxn37gc$=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) .