Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Nov 21 2023 00:19:46 FOUS30 KWBC 210019 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 718 PM EST Mon Nov 20 2023 Day 1 Valid 01Z Tue Nov 21 2023 - 12Z Tue Nov 21 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL GULF COAST STATES... Increasing upper-level difluence ahead of the positively-tilted longwave trough has favored large-scale ascent for widespread moderate-heavy rainfall, particularly across parts of the ArkLaMiss where the convections has been showing some training character as it aligns with the 850-400 hPa mean flow. The favorable dynamical and thermodynamical setup within the Marginal Risk area (ML CAPEs 500-2000 J/Kg with ~70 kts of effective bulk shear which will eventually evolve the convective organization into a squall line) has fostered hourly rain totals ~2". The CAM guidance as a whole has been too slow/too far northwest, which extends to the 23z HRRR. A stripe of 2"+ has fallen across central LA and the heavy rain axis is headed towards south-central MS generally south of Jackson, which appears to be headed towards focusing near the 31-32N latitude band across MS. Where it has yet to rain, 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 support maintaining the Marginal Risk area with the expectation of a more localized flash flood risk. Localized pockets of Slight Risk impacts cannot be ruled out, but should be constrained to urban areas. By 06z, the thunderstorm pattern should become more progressive and local 2-4" amounts should become less likely. Roth/Snell/Hurley 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 very minor changes were made to the Marginal Risk area on the day 2 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. If any area has slightly elevated chances for 4"+ amounts and increased isolated flash flood/rapid runoff concerns, it's along the Blue Ridge Mountains of central/northern VA. Here, 12z HREF depicts high probs (60-70%) for 24-hr QPF to exceed 10 year ARI values. 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 or urban influence, 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/Snell 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. Snell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7HVyi3n7Zer2q_pG1AItRzuAs0bia-7yDN__mLr6wW7v= omF1okBT2lYyDXOtnvm379pqJH5R1p4Abw224LvPNmn6nfY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7HVyi3n7Zer2q_pG1AItRzuAs0bia-7yDN__mLr6wW7v= omF1okBT2lYyDXOtnvm379pqJH5R1p4Abw224LvPRP_n2mU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7HVyi3n7Zer2q_pG1AItRzuAs0bia-7yDN__mLr6wW7v= omF1okBT2lYyDXOtnvm379pqJH5R1p4Abw224LvPEe3jfCg$=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) .