Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 21 2023 08:16:06 FOUS30 KWBC 210816 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 314 AM EST Thu Dec 21 2023 Day 1 Valid 01Z Thu Dec 21 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... A strong, anomalously deep closed upper low will slowly meander off the southern California coast through Thursday night, setting up a prolonged and potentially significant rainfall event for portions of Southern California including the Transverse Ranges and portions of the Los Angeles metro. The position of the upper low will put the area in the favored area of forcing for deep-layer ascent (maximizing upper difluence/divergence), while embedded vort maxes rotating around the low will work to provide extra/enhanced lift at times. Most recent model guidance continues to depict two main waves -- one coming through this morning and afternoon, followed by another surge overnight and into Friday morning. Most of the guidance has the second wave coming in a bit farther downstream, i.e. east of the LA-SD metros and across the deserts. In any case, the ingredients will feature fairly anomalous moisture, with PW values between 3-4 standard deviations above normal and IVT values 400-500 kg/m/s. Some instability (MUCAPEs 250-500 J/kg) is forecast to develop as well along the immediate coast, which will help to realize the higher rain rate potential, where hourly totals may exceed 1" at times, particularly for the favored south facing slopes of the Transverse Ranges. The other factor that increases the QPF and flood threat is the relatively slow motion of the storm, which may allow for some prolonged rain bands to set up. The favorable southerly flow nearly perpendicular to the terrain will enhance the orographic ascent and should enhance rainfall totals, which may reach 3-5" for the 24-hr period ending 12Z Friday. 48-hr totals (including rain from Wednesday-Wed night) may exceed 6-8" (localized 10") in the areas northwest of Los Angeles in the Transverse Ranges, and this is likely to lead to significant flooding, including flash and urban flooding as well as potential for rock slides, mud slides, and debris flows over sensitive burn scars like the South Fire burn scar in Ventura County. Very few changes were made to the existing Moderate Risk area, which covers both the terrain flood risk as well as the potential for flash flooding for western LA metro areas. Hurley/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!9MeVR2fYisw-H669F7vRxTOXLhN1v8-A97nbg1LgZMtS= da7e9c60mdjx2IDpHCNoylSepaK-amnUIvDQf2ZWkSMft2M$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9MeVR2fYisw-H669F7vRxTOXLhN1v8-A97nbg1LgZMtS= da7e9c60mdjx2IDpHCNoylSepaK-amnUIvDQf2ZWDXJuKgU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9MeVR2fYisw-H669F7vRxTOXLhN1v8-A97nbg1LgZMtS= da7e9c60mdjx2IDpHCNoylSepaK-amnUIvDQf2ZWs3kxIcM$=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) .