Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 20 2023 08:17:09 FOUS30 KWBC 200817 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 316 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 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 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 Valid 12Z Fri Dec 22 2023 - 12Z Sat Dec 23 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHERN 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 forcing/lift while southerly 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 500-700 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 1"/hr at times. Overall, a broad area of 0.5-1.0" totals are expected for the 24-hr period ending 12Z Saturday, with some localized/embedded higher amounts up to 1.5" possible in any thunderstorms. Some localized flash flooding may develop, especially across southern Arizona if higher rates do materialize, but for now until confidence in ingredients and rain rate potential increases, a Marginal Risk reasonably covers the flood threat at this time though if trends continue, an upgrade to a higher risk category may be needed for parts of Arizona. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-4OTPaxcusymqI3FMAdwF0yNjLGI9BJdeE4wk6yIzlpb= KKd1fUB80Sduw7xDHnzIh-LHGiNVK8Kq7fSbC4-m3pj-44Y$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-4OTPaxcusymqI3FMAdwF0yNjLGI9BJdeE4wk6yIzlpb= KKd1fUB80Sduw7xDHnzIh-LHGiNVK8Kq7fSbC4-mhvdAvU8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-4OTPaxcusymqI3FMAdwF0yNjLGI9BJdeE4wk6yIzlpb= KKd1fUB80Sduw7xDHnzIh-LHGiNVK8Kq7fSbC4-mpwNbDRY$=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) .