Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jan 05 2024 07:18:57 FOUS30 KWBC 050718 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 218 AM EST Fri Jan 05 2024 Day 1 Valid 12Z Fri Jan 05 2024 - 12Z Sat Jan 06 2024 ....Gulf Coast and Southeast...=20=20=20=20 A strong and fast-moving system will track northeastward from Texas to the Southeast by Saturday morning. The result will be an increasingly favorable divergence pattern aloft associated with the shortwave itself, which will support deepening of a surface low that will track across the Southeast near and ahead of the shortwave. The low will take advantage of increasing Gulf moisture along a coastal front/steering line to result in showers and potentially organized thunderstorms. The storms will be capable of heavy rainfall, as precipitable water values increase to as high as 1.75". Inflow at 850 hPa and effective bulk shear build to 50+ kts and some areas could see MU CAPE rise to 500+ J/kg. Decent low- to mid-level frontogenesis should also exist. IVT into the region mimics a small scale, but progressive, atmospheric river which should lend itself to rainfall efficiency. The ingredients should allow for hourly rain totals to 2" and local amounts to 4". The fast forward speed of the storms potentially limits how much rain any one area sees, so if organized convection forms, mesocyclones will need to form or cell mergers would need to occur to create any heavy rain related issues. Soils all across the Marginal Risk area are either drier or much drier than normal, so the rivers and streams could take on some rainfall. The result will be any isolated flash flooding most likely occurring in flood-sensitive urban and poor-drainage areas. Changes were noticeable this cycle and the Marginal Risk was constrained to where 0.5"+ an hour totals and where decent probabilities of 2"+ existed for the 24 hour period on the 00z HREF guidance.=20 Roth Day 2 Valid 12Z Sat Jan 06 2024 - 12Z Sun Jan 07 2024 In and near the Mid-Atlantic...=20 A pair of mid-level shortwaves should allow for diffluence aloft near the East Coast. Inflow of ~50 kts at 850 hPa is expected off the Atlantic. Low- to mid-level frontogenesis is likely to do the heavy lifting as instability will be quite low -- at or below 250 J/kg -- which should be able to yield hourly rain totals of 0.5"+ at times. The signal for heavy rainfall exists, but the guidance is not overly strong with the QPF signal as the system overall should remain progressive. Portions of this region experienced heavy rainfall within the past week, with pockets receiving 300%+ of their average weekly rainfall for this time of the year, which is the main reason the Marginal Risk area has been kept from continuity. Overall, thought there was just enough there for the Marginal Risk to carry forward, but some adjustments were made based on newly available guidance QPF-wise and to better account for the precipitation type expected with a little bit of overlap into the expected snow area (in case the guidance continues to edge northwest on the expected rain/snow line as the system moves through general southwest flow aloft downstream of a more significant deep cyclone). The Carolinas were removed from the risk area as the 850 hPa inflow looks increasingly parallel to their coastline/less onshore as the best moisture is nearby, which is more likely to keep convection more tied to the Gulf Stream offshore. =20 Roth Day 3 Valid 12Z Sun Jan 07 2024 - 12Z Mon Jan 08 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5s8kwbIIae6J5VEaohQyyNVW-u4Dhcg1AC-iz-temVR_= E4vuTGwyoX8brC-_hYZ3ShwoiPw6G8X9dOqfThGgzYCuLOs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5s8kwbIIae6J5VEaohQyyNVW-u4Dhcg1AC-iz-temVR_= E4vuTGwyoX8brC-_hYZ3ShwoiPw6G8X9dOqfThGgQmKTjCE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5s8kwbIIae6J5VEaohQyyNVW-u4Dhcg1AC-iz-temVR_= E4vuTGwyoX8brC-_hYZ3ShwoiPw6G8X9dOqfThGgWvJGSsM$=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) .