Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 03 2022 00:39:55 FOUS30 KWBC 030039 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 738 PM EST Fri Dec 02 2022 Day 1 Valid 01Z Sat Dec 03 2022 - 12Z Sat Dec 03 2022 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Coastal Ranges of Southern California... Moisture will be gradually returning overnight along a quasi-stationary front draped over southern California and extending offshore down to the southwest. The moisture transport as driven by the low-level flow will be rather modest with only about 20 to 25 kts of southwest flow expected along the front and into the windward slopes of the coastal ranges. However, sufficient convergence/forcing along the front and the arrival of weak vort energy embedded within the deeper layer southwest flow aloft should favor an increase in shower activity overnight, and especially in the 03Z to 09Z time frame. The available moisture and forcing should favor some rainfall rates getting into the 0.25" to 0.50"/hour range at least occasionally, and the 18Z HREF shows some low-end probabilities over Santa Barbara and San Luis Obispo counties for some brief 0.50"+/hour rates which is favored also by the latest HRRR guidance. The HRRR suggests the overall heaviest rainfall threat overnight tending to be over the Santa Ynez mountains in Santa Barbara County. As much as 1 to 1.5+ inches of rain is forecast over the terrain here, and there is a burn scar of note (Alisal, 2021) that will need to be monitored should heavier rainfall rates materialize. So, while there is currently not a risk area highlighted for the coastal ranges of southern California, there is a non-zero threat of excessive rainfall, with a highly isolated and localized concern for runoff problems later tonight. Orrison 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... ....2030z Update... The Marginal Risk areas were largely unchanged for this update, but additional refinements were made based mainly on the new 12z HREF suite. Most significantly, this resulted in a more confined MRGL area across southeast AZ and southern NM (and to a lesser extent for the CA risk areas). These refined MRGLs still encompass all areas of potential convective activity (based on the HREF members), and the overall meteorological reasoning is on track with little to no change in expected precipitation amounts (with longer term rainfall totals primarily driving the Marginal flash flood risk, as hourly rainfall rates are generally expected to remain below 0.5"/hr). Churchill ....Previous Discussion... ....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 rainfall exceeding flash flood guidance is less than 5 percent. Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6BjcZNVY3Jvha5uFqPw8eKqkoXtwaJVqJtk6yhpJmMD7= 2ba_oL7JJdG4YZDOf0W088Fyk0Qmu3CyEnPqRaCBprOijk0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6BjcZNVY3Jvha5uFqPw8eKqkoXtwaJVqJtk6yhpJmMD7= 2ba_oL7JJdG4YZDOf0W088Fyk0Qmu3CyEnPqRaCBK1NE9RU$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6BjcZNVY3Jvha5uFqPw8eKqkoXtwaJVqJtk6yhpJmMD7= 2ba_oL7JJdG4YZDOf0W088Fyk0Qmu3CyEnPqRaCBfF7PeNo$=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) .