Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 20 2023 20:07:24 FOUS30 KWBC 202007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 306 PM EST Wed Dec 20 2023 Day 1 Valid 16Z Wed Dec 20 2023 - 12Z Thu Dec 21 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....16Z Update... Only minor tweaks to the overall composition of the risk areas, but maintained overall continuity from previous forecast. The overall synoptic and expected mesoscale evolution of the event remains on track as broad cyclonic flow around the closed-low off the CA coast will slowly propagate southward with main moisture trajectory aligning orthogonal to the terrain to the northwest of Los Angeles by later this afternoon and evening. Some minor adjustments to the probabilities were made within the original forecast to reflect the latest 12Z suite of guidance. ....Previous Forecast... Latest upper air analysis continues to show a deep, well-defined closed upper low offshore the California coast as it slowly migrates 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 2-4" 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 (60-70 percent chance of at least 5" per HREF neighborhood probability), and even a non-zero (very slight) chance of 8" totals. The latest hi-res guidance and 12Z HREF neighborhood probabilistic also indicate a high probability of hourly totals at least 0.5" (80-90 percent) and even a slight to moderate chance (30-45 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/Kleebauer Day 2 Valid 12Z Thu Dec 21 2023 - 12Z Fri Dec 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN CALIFORNIA... ....20Z Update... Minimal changes to the D2 risk areas, including the Moderate risk in place over the Los Angeles metro and the terrain to the northwest. Models continue to signal a prolonged heavy rain event for the south-facing slopes of the Santa Ynez to the north of Santa Barbara proper where two-day totals are approaching 8-10" on hi-res deterministic guidance. 12Z HREF blended mean was exceptionally bullish for the western edge of the Santa Ynez with totals of 3-6" increasingly likely within the time frame of 12Z Thursday through 00z Friday before heavy rain focus shifts southeast into LA proper. Los Angeles will see its heaviest rainfall likely between 00-12Z Friday with some guidance indicating upwards of 2-3" within the time frame as 700mb VV panels depict a strong low to mid-level ascent field traversing over the metro area. This correlates with an extended period of heavy rainfall within the LA metro, as well as the hills just off the north as southeast flow over the terrain will enhance the upslope component and provide another local maximum just east of the Santa Ynez. Heavy rain will be possible down towards San Diego proper, but the forecast is for the heaviest rain to remain focused over upstream where flow will be more orthogonal to the coast leading to training. Overall, there is little change in the current forecasted evolution of the upper level disturbance and expected rainfall footprint within portions of Southern California, thus maintained the Moderate risk and will assess the need for any upgrades pending the result of what occurs during D1. Kleebauer ....Previous Discussion... A strong, anomalously deep closed upper low will slowly meander off the southern California coast Thursday into 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 ascent, while embedded vort maxes rotating around the low will work to provide extra/enhanced lift at times, with the latest model guidance suggesting one wave coming through Thursday morning followed surge later in the evening/overnight Friday morning. 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 of 500-750 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 expected today/tonight) 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.=20 Taylor Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9KXc0Ev44iz9vFouCcrbFaKs64Od10kW2APOmGSxzPQB= 2f4D8M1910N7JkHVDkMCfNgwOL0RDHNGqcPJsO5sTnCs6-k$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9KXc0Ev44iz9vFouCcrbFaKs64Od10kW2APOmGSxzPQB= 2f4D8M1910N7JkHVDkMCfNgwOL0RDHNGqcPJsO5sZPFuOqY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9KXc0Ev44iz9vFouCcrbFaKs64Od10kW2APOmGSxzPQB= 2f4D8M1910N7JkHVDkMCfNgwOL0RDHNGqcPJsO5suJhNNB4$=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) .