Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Dec 02 2022 15:45:18 FOUS30 KWBC 021545 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1044 AM EST Fri Dec 02 2022 Day 1 Valid 16Z Fri Dec 02 2022 - 12Z Sat Dec 03 2022 The probability of excessive rainfall is less than 5 percent. In coordination with the LOX/Los Angeles, CA forecast office, the previous Marginal Risk for the Transverse Ranges northwest of L.A. was removed with the prior forecast update. The rainfall that occurred overnight and this morning was not heavy enough to raise area river gauge levels much, and there were no reports of flash flooding. After a break in rainfall for much of today, showers will resume with a reinforcing disturbance after sunset tonight. However, the rainfall is not expected to become heavy enough (with rates peaking at 0.25"/hr) such that any flash flooding will occur until after the 12Z transition into Day 2. Churchill/Wegman Day 2 Valid 12Z Sat Dec 03 2022 - 12Z Sun Dec 04 2022 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL CALIFORNIA COAST, THE WEST-FACING SLOPES OF THE SIERRA NEVADA RANGE IN CENTRAL CALIFORNIA, AND PORTIONS OF THE SOUTHWEST... ....California... An upper level low centered off the OR/WA coast to start the day Saturday will drift southeastward towards the northern CA coast by Sunday morning. A disturbance circulating around the increasingly cut off low will phase with a weaker southern jet stream disturbance as both move into the central CA coast. This process will begin Friday night in the Day 1 period, but the effects will be largely felt on Saturday as the phased disturbance moves into central CA. The heaviest rain will begin the day Saturday near the Point Conception portion of the Central CA coast, but move northward through the morning, where it will stall south of Big Sur. The coastal mountains here will all be below the freezing level from top to bottom, so the 2-4 inches of rain forecast for these areas will all fall as liquid. Highly anomalous moisture will be present with PWATs over an inch, which is as much as 3 standard deviations above normal for this time of year. A 30% chance of rainfall exceeding FFGs is forecast for Saturday afternoon. Further inland, a new Marginal Risk area was introduced to include the west facing slopes of the Sierra Nevada range. Snow levels of 7k-8k feet will allow much of the precipitation expected in the Sierras to fall as rain. Thus, the runoff into the Central Valley will be significant. Forecasted precipitation into the Sierras will be in the 3 to 5 inches range for Saturday and Saturday night. Thus, expect isolated flash flooding with any runoff, especially when combined with snowmelt over the numerous burn scars in this portion of the Sierras. ....Southeast Arizona into Southern New Mexico... A weak atmospheric river of Pacific moisture is expected to develop and become enhanced by the right entrance region of a 130 kt jet over the 4 Corners Region. This will cause a narrow area of rainfall to develop that will initially lift north from Mexico Saturday morning, but subsequently stall out a bit further north from Tucson east into the Gila National Forecast of southwest New Mexico. While large rainfall values will be relatively isolated, training rainfall with convective elements is likely through the day across this corridor. This could lead to isolated flash flooding, as wetter areas pick up between 1-1.5 inches of rain. There is some uncertainty as to where this area of rain will stall out, so subsequent updates may have the confidence to shrink the Marginal Risk area to highlight the heavier rain areas. Light to moderate rain will continue into Saturday night. Wegman Day 3 Valid 12Z Sun Dec 04 2022 - 12Z Mon Dec 05 2022 The probability of excessive rainfall is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4uJd5Ysg5v2A8xmD9Vhpx94aucBCK0Rj_30EQFUAx4Fl= 5bnw2l5KcRbJFvqYLKU2Gg2pJyaLRZCelXq45oPoRDmSEtw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4uJd5Ysg5v2A8xmD9Vhpx94aucBCK0Rj_30EQFUAx4Fl= 5bnw2l5KcRbJFvqYLKU2Gg2pJyaLRZCelXq45oPoQpARIAQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4uJd5Ysg5v2A8xmD9Vhpx94aucBCK0Rj_30EQFUAx4Fl= 5bnw2l5KcRbJFvqYLKU2Gg2pJyaLRZCelXq45oPoR8yBqKs$=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) .