Subj : HVYRAIN: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 06 2022 00:29:33 FOUS30 KWBC 060029 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 728 PM EST Mon Dec 05 2022 Day 1 Valid 16Z Mon Dec 05 2022 - 12Z Tue Dec 06 2022 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR TENNESSEE, PORTIONS OF NORTHERN MISSISSIPPI, ALABAMA, AND GEORGIA AS WELL AS ADJACENT COUNTIES OF THE CAROLINAS... ....Northern GA...Eastern TN...into Western Carolinas... Favorable upper-level forcing along/ahead of weak shortwave directly below the apex of the anticyclonically curved subtropical jet streak (as noted in GOES-WV) continues to progress eastward through Northern GA into the Carolinas over the next few hours.=20 This combined with weak isentropic ascent with the anomalously higher total PWAT values (still over 1.5") continues to support increased rates particularly along the leading edge of the shortwave, but this is expected to wane quickly over the next 2-3 hours moving further away from the source region, as well as falling into the drier low level environment east of the Appalachian ridge. As such, have pulled the Slight Risk in favor of a continued Marginal Risk.=20 ....Delta Region of MS Valley into Central TN... Further upstream, a pocket of enhanced mid-level instability due to higher theta-E surge through LA and the Lower MS valley with numerous Hi-Res CAMs and global guidance supporting 500-750 J/kg of CAPE. As the air lifts north and impinges with the wavy warm front across Southern AR across NW MS into N AL, deep layer flow flattens and becomes more parallel to the boundary while concurrently boundary layer is expected to ascend isentropically across the boundary. This may allow for shallow/narrow convective features across the Delta Region after 06z, slowly strengthening toward 12z. While rainfall rates will continue to be generally low given the updrafts probably will also be narrow in appearance/coverage, there is some lower probabilities for streaks of enhanced rainfall totals with possibilities of 1-1.5" totals mainly along/north of the front across W TN. Given enhanced rainfall totals today, there remains a non-zero risk for excessive rainfall and isolated flooding concerns through the end of the outlook period. As such, have contoured the western edges of the Marginal Risk to account for Hi-Res CAMs and best HREF probabilities, up to the central KY/TN boarder. Gallina 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!5VshrEyXi7ZXbpqTGqT6cfLgCpVBuDs5doNCFULQqlyM= sYTevfxNOGdm-ac_vV6XHtdQQyggm885A_do6uuaATTWbdI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5VshrEyXi7ZXbpqTGqT6cfLgCpVBuDs5doNCFULQqlyM= sYTevfxNOGdm-ac_vV6XHtdQQyggm885A_do6uuaLQFpRTY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5VshrEyXi7ZXbpqTGqT6cfLgCpVBuDs5doNCFULQqlyM= sYTevfxNOGdm-ac_vV6XHtdQQyggm885A_do6uua3Y8jxQs$=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) .