Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Nov 20 2023 15:55:35 FOUS30 KWBC 201555 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1054 AM EST Mon Nov 20 2023 Day 1 Valid 16Z Mon Nov 20 2023 - 12Z Tue Nov 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MID-SOUTH AND CENTRAL GULF COAST STATES... Only very minor changes were made to the Marginal Risk area for the day 1 outlook. Increasing upper-level difluence ahead of the positively-tilted longwave trough will favor large-scale ascent for widespread moderate-heavy rainfall, particularly across parts of the Mid-South between the Bootheel of MO and the ArkLaMiss where the ageostrophic circulation within the left-exit region of the 130-150kt upper level jet will enhance the divergence aloft/deep-layer ascent and southerly low-level moisture transport. 850 mb moisture flux anomalies climb to around 3 standard deviations above normal late Monday-Monday night ahead of the cold front, with the low-level response also allowing for an uptick in deep-layer instability, even elevated instability north of the triple point. The favorable dynamical and thermodynamical setup within the Marginal Risk area (some convective contribution with MUCAPEs ~500 J/Kg north and ~1000+ J/Kg south) should foster peak hourly rainfall rates between 1-2+ inches, especially Monday night where the uptick in the southerly LLJ would favor increased upwind propagation and thus enhance the risk for cell training. The CAM guidance is especially bullish with the QPF along/near the track of the triple point (elongated area of 2-3+ inches), given the overlap of favorable synoptic and thermodynamic forcing. This would be from portions of AR into northwest MS, the Bootheel of MO, and western TN. Another potential stripe of localized 2-3+ inch totals exists across the warm sector from northern LA to central MS, where numerous supercells are possible and could track over similar areas. This area is also where 12z HREF neighborhood probs for at least 3"/3-hr are highest (10-20%). Relatively low soil moisture currently within the top 40 cm (<25%, which is below the 20th percentile for the entire outlook area), along with the current FFG and relatively short duration of peak low-level moisture transport and deep-layer instability, support maintaining the Marginal Risk area with the expectation of a more localized flash flood risk (especially in urban areas). Hurley/Snell Day 2 Valid 12Z Tue Nov 21 2023 - 12Z Wed Nov 22 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PORTIONS OF THE SOUTHEAST TO MID-ATLANTIC... Similar to the Day 1 ERO, only minor changes were made to the Marginal Risk area in the Day 2 ERO from yesterday's Day 3 outlook. Approaching longwave upper trough and attendant 130-150 kt jet streak on the lee side will favor a robust, elongated warm conveyor belt (WCB) with upper level moisture origins from the eastern Pacific south of Baja. Favorable along-stream upper level forcing (deep-layer Qs convergence and ascent) should linger a little while longer on day 2 as the upper trough becomes more neutrally-tilted (thus retarding the eastward progression somewhat). As noted in the previous discussions, instability will be very limited, especially across the northern half of the outlook area. However, PWs 1.0-1.5" or ~2 standard deviations above normal for late November, along with 850-700 mb moisture flux anomalies between +3 and +4, will foster widespread rainfall totals between 1-3+", highest along the eastern slopes of the southern-central Appalachians give the terrain enhancement. Much of the region is experiencing very dry ground conditions and drought, with relative soil moisture within the top 40cm layer currently below the 20th percentile, and in many areas below the 10th percentile. Therefore much of this rainfall is expected to be beneficial, while the lack of even elevated instability should limit the short-term rainfall rates. However, isolated areas of flash flooding will be possible where drainage is impaired, especially with the fallen foliage, and/or where the soils have become more hydrophobic given the recent dry spell (30 day rainfall <20% of normal across the outlook area per AHPS). Hurley Day 3 Valid 12Z Wed Nov 22 2023 - 12Z Thu Nov 23 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_3h-txV7BTelbf8rwmNeob3X1C_hfFvn0pks4wiTPnSy= l019zbMzK9_WdtEPVHcAiOccR5Srz9mPJZHoSKI3O5rYXKQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_3h-txV7BTelbf8rwmNeob3X1C_hfFvn0pks4wiTPnSy= l019zbMzK9_WdtEPVHcAiOccR5Srz9mPJZHoSKI3jThjmXw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_3h-txV7BTelbf8rwmNeob3X1C_hfFvn0pks4wiTPnSy= l019zbMzK9_WdtEPVHcAiOccR5Srz9mPJZHoSKI3PI4SwO0$=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) .