Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Dec 21 2023 08:18:06 FOUS30 KWBC 210818 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 317 AM EST Thu Dec 21 2023 Day 1 Valid 12Z Thu Dec 21 2023 - 12Z Fri Dec 22 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 Valid 12Z Thu Dec 21 2023 - 12Z Fri Dec 22 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN PORTIONS OF CALIFORNIA AND ARIZONA... The strong/deep closed upper level low is expected to be centered just offshore southern California early Friday morning, then begins to pick up speed as it moves eastward toward central Arizona by early Saturday morning, opening up as it does so. This will put much of southern California, far southern Nevada, and Arizona in a favored region of deep-layer forcing/lift while anomalous S-SSW low-level flow positions a plume of higher moisture initially into southern California then across Arizona during the afternoon/evening hours. Moisture anomalies with this system are expected to remain well above normal, near 3 standard deviations above normal for this time of year, and precipitable water values are forecast to reach or exceed 1" at times, especially across southwest to south-central Arizona. The GFS and ECMWF IVT forecasts from CW3E show an impressive surge approaching 500+ kg/m/s into central Arizona. During the afternoon, some limited instability may develop across the Desert regions of CA and eastward into southern Arizona, which may allow for some localized higher rain rates approaching 0.50-1.0"/hr at times per the 00Z CAM guidance. Overall, a broad area of 0.75-1.5" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 2.5" possible in any thunderstorms. Per collaboration with WFOs SGX, PSR, and TWC, we have bumped a large portion of the Marginal Risk to a Slight Risk across southern CA into much of southern AZ. This given the wetter QPF trends, along with a corresponding increase in HREF exceedance probabilities through 00Z 12/23 (particularly the probabilities of at least 0.50"/hr and 1.00"/3hr). Meanwhile, the latest HREF probs of 6hr QPF exceeding the 2 year ARI peaks between 60-80% across either side of the CA-AZ border between 12Z Fri-00Z Sat. Also supporting the upgrade to a Slight Risk is the recent CSU First-Guess Fields, including the UFVS-verified version. Hurley 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!6l5SaNR9pkOKHHtD6syF-RXcSWBOJTAJBgmqZsJjRedc= HeC4KayBlSaD-4XRYDpNjZ0U_5oe2JDG02QpoRGkUs7E7kY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6l5SaNR9pkOKHHtD6syF-RXcSWBOJTAJBgmqZsJjRedc= HeC4KayBlSaD-4XRYDpNjZ0U_5oe2JDG02QpoRGkqAmrlYc$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6l5SaNR9pkOKHHtD6syF-RXcSWBOJTAJBgmqZsJjRedc= HeC4KayBlSaD-4XRYDpNjZ0U_5oe2JDG02QpoRGk3RtS1-E$=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) .