Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Dec 05 2022 20:31:04 FOUS30 KWBC 052030 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 330 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 Valid 12Z Wed Dec 07 2022 - 12Z Thu Dec 08 2022 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE OZARKS TO THE LOWER OHIO AND TENNESSEE VALLEYS... ....16Z Update... Increasing upper-level energy and a meandering surface boundary will help to promote widespread continuous thunderstorms Wednesday with anomalously high moisture increasing the chance for heavy rainfall. Uncertainty on where impactful heavy rainfall will occur, and the subsequent coverage of the current Slight Risk area, is tied to the eventual location of the surface front. The model guidance is in agreement on the potential for 2-4" of rain over eastern Oklahoma, northern Arkansas, and southern Missouri where storms are most likely to last through the period. Uncertainty increases with eastward extent into the lower Ohio and Tennessee Valleys due to both the eventual location of the surface front and associated axis of heaviest rainfall, as well as the duration of rainfall, which may end sooner on Wednesday evening. The eastward extent of the Slight Risk captures where there is enough overlap in the latest model guidance to narrow down a region most likely to see higher rain totals, and where rain from the prior days will lead to wetter antecedent conditions. A small adjustment was made to include metro Nashville given the locally higher risk for flooding in urban areas. There is a chance for higher rainfall totals in the 2-3" range further east into northeastern Tennessee and southeastern Kentucky, as well as similar rain rates to areas within the Slight Risk, and an eastward extension of the Slight Risk may be necessary in future outlooks as confidence increases. A portion of the southwestern extent of the Slight Risk was removed in coordination with the OUN/Norman, OK office while the Marginal Risk was extended southwest into northwest Texas. There was a trend in the latest guidance for the axis of higher rainfall to extend further to the southwest. However, there is still a good bit of uncertainty in the eventual location of the surface front to further narrow down an area of increased risk, and dry antecedent conditions should also keep any instances of flash flooding isolated. Putnam ....Previous Discussion... The Slight Risk inherited from Day 4 was expanded for this forecast update. A stalled out front will get a large invigoration of upper level energy moving over it with a hyperactive 140 kt jet streak. The southwesterly jet streak will start out the day stretched over the Plains to the Midwest. By Thursday morning it will extend into the Northeast. This will put the Slight Risk area in the favorable right entrance region of the jet streak. Meanwhile, a strengthening shortwave will race eastward in that jet streak from the Southwest to the central Plains, intensifying all the while. This shortwave will dip the jet stream and support cyclogenesis across Kansas Wednesday night. This will draw polar air over much of the northern tier of the country southward down the lee of the Rockies. This cold air will clash with a continued supply of Gulf moisture streaming northward across Texas into the Slight Risk area. The moisture remains very highly anomalous for this time of year. PWATs to 1.5 inches is 4 standard deviations above the climatological normal...representing 95-99% climatological percentile range. As with previous days, while moisture will not be an issue precluding heavy rainfall, instability sure will be. At most instability in the Slight Risk area gets to around 500 J/kg, particularly in OK, with lesser values further east. With that said, unlike Tuesday, the aforementioned factors should work to overcome this, resulting in 2-4 inches of rain in OK & AR. Further east, rainfall totals will be lower...in the 1-3" range, but previous days rainfall may make up for that with more saturated soils. Confidence is still somewhat low on where the axis of heaviest rainfall will set up. The cyclogenesis over KS supports the area over OK & AR, a southwestward expansion in the Slight Risk from previous forecasts...but it's much lower further east, partly due to uncertainties in antecedent conditions across KY & TN. It's likely further adjustments will be needed in this region. The expansion across OK was coordinated with the TSA/Tulsa, OK forecast office. The uncertainty with the eastward extent of the rainfall was coordinated with the JKL/Jackson, KY, LMK/Louisville, KY, and OKX/Nashville, TN forecast offices. The inherited Marginal Risk across Maine was dropped in coordination with the CAR/Caribou, ME forecast office. Rainfall amounts of 1-1.5" are not expected to be enough to cause flash flooding despite the saturated soils. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!90Lxw2Xlos8J1wGKEiRbOB55fbD55SaBJDBmrTq9CCXf= 9w042lXSpM1RlFf38GPXf603U0qoaJl0Qzb-RYjpB6akP48$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!90Lxw2Xlos8J1wGKEiRbOB55fbD55SaBJDBmrTq9CCXf= 9w042lXSpM1RlFf38GPXf603U0qoaJl0Qzb-RYjpaTKe-3Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!90Lxw2Xlos8J1wGKEiRbOB55fbD55SaBJDBmrTq9CCXf= 9w042lXSpM1RlFf38GPXf603U0qoaJl0Qzb-RYjpWxlnMX0$=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) .