Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 20 2023 08:16:40 FOUS30 KWBC 200815 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 314 AM EST Wed Dec 20 2023 Day 1 Valid 12Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... An early morning analysis showed a deep, well defined closed upper low offshore the California coast that is slowly moving south/southeast. Impressive height falls associated with this feature and strong forcing for ascent will overtake the region throughout the period, with an increase expected in locally heavy rainfall brushing the coastal areas of central California today then spreading southward into portions of southern California later on as the associated surface front approaches from the west. In addition to the broad forcing, the orographic forcing will help drive an increase in shower and perhaps thunderstorms, particularly for portions from the Bay Area southward. IVT values per the CW3E page indicate that modest values upwards of 500 kg/m/s are expected along the immediate coast and moisture anomalies are expected to be near/above the 90th percentile and roughly 3 standard deviations above normal. The latest model guidance shows a broad area of 2-3" is expected along the coastal ranges across central California southward into the Transverse Ranges northwest of Los Angeles. Across the Transverse Range, the magnitude of the IVT plume along with the more perpendicular flow/direction will enhance rain totals where localized 5-6"+ totals will be possible through 12Z Thursday (60 percent chance of at least 5" per HREF and even a non-zero (very slight) chance of 8" totals. The latest hi-res guidance and 00Z HREF neighborhood probabilistic also indicate a moderate to high probability of hourly totals at least 0.5" (60-80 percent) and even a slight chance (20-30 percent) of hourly totals greater than 1" for several hours, especially across the Transverse Ranges. This magnitude and rate of rainfall is likely to cause scattered/numerous instances of flash flooding, some of which could be significant and higher end with the possibility of flash, urban, and mud/debris flows over recent burn scars. Taylor Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5qmQrrEhH_U9heToafBv25KRLXCJPkckD-mYD6wkOFMR= OERKNXCUFxpGugB458JlSyRmcgSJ55b0dncTTKmT9-9ozlM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5qmQrrEhH_U9heToafBv25KRLXCJPkckD-mYD6wkOFMR= OERKNXCUFxpGugB458JlSyRmcgSJ55b0dncTTKmTWrs_6Gw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5qmQrrEhH_U9heToafBv25KRLXCJPkckD-mYD6wkOFMR= OERKNXCUFxpGugB458JlSyRmcgSJ55b0dncTTKmThxUrAD8$=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) .