Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 31 2022 08:29:30 FOUS30 KWBC 310829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 328 AM EST Sat Dec 31 2022 Day 1 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... The Slight Risk is largely unchanged with this morning's update. The 2 inherited Slight Risk areas were merged into one combined area to match the Flood Watches already out for the Central Valley. However, the overall area of the newly combined Slight Risk is unchanged. The atmospheric river (AR) aimed at northern CA at the time of this writing will reintensify later this morning as a strong upper level disturbance approaches. This will begin to move the AR southward later this morning. However, until that happens, expect periods of heavy rain to continue to impact northern CA down to the Bay Area. With the support of the left exit region of a 120 knot jet streak, the rain will have no trouble intruding further inland into northwestern NV. In this area...temperatures at the time of this writing are in the 40s and 50s. With the cold air behind the AR going to be held up until it starts moving south, expect 1-2 inches of rain into northwestern NV, which could lead to isolated flash flooding when combined with snowmelt. Thus, the Marginal Risk area was expanded eastward to include more of that area. For Southern California later today, the AR will be increasingly buckled ahead of the strong disturbance moving southeast down the CA coast. This will align the predominant wind-flow orthogonal to the Transverse Ranges, and at an angle to the Peninsular Ranges. This will result in local maxima of rainfall totals on the windward sides of those mountains. However, the fast speed of movement of the upper level disturbance means by the time the AR reaches SoCal, it will be moving fast enough that only a few hours of heavy rainfall are expected in any one area. Thus, the heavy intensity of the rain will be canceled out by the fast movement of the AR itself. Nonetheless, urban areas of the L.A. basin, burn scars, and largely saturated soils will all work to lower FFG values such that the inherited Slight Risk for these areas remains, but confidence is lower here. Snow levels in SoCal will largely hover between 7,000 and 8,000 feet, so the very highest peaks may see some snow, but the vast majority of the total precipitation is expected to fall as rain. Wegman Day 2 Valid 12Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... No changes to the inherited Marginal Risk area were made with this morning's forecast update. The digging trough from New Years night will press eastward into Arizona on New Year's Day Sunday. Expect rainfall to begin early New Year's morning and continue through the day across the Marginal Risk area. Generally expect rainfall totals of 1-2 inches, but with locally higher amounts in the mountains east of Phoenix. Meanwhile further north along the Mogollon Rim and points north, expect snow levels to be low enough that any rain that occurs initially will quickly change over to snow. PWATs of 0.5 to 0.75 inches, or +3 standard deviations above normal, will start out the day in the Marginal Risk area. Thus, moisture amounts will not be a problem on Sunday. Snow levels are expected to begin the day around 8,000 feet...but drop to around 5,000 feet by sunset on Sunday. This should allow many lower elevation areas to see all rain. Wegman 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!5mBTjuv9TLQ_jSYpByqNK_61M_ZhIbIq1pVzJYIkDrtE= Rh-p7vpVLAkE2xvMP8D8SzvFo9RinnC3okM0ly7k7XBDu44$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5mBTjuv9TLQ_jSYpByqNK_61M_ZhIbIq1pVzJYIkDrtE= Rh-p7vpVLAkE2xvMP8D8SzvFo9RinnC3okM0ly7k5WZDVgA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5mBTjuv9TLQ_jSYpByqNK_61M_ZhIbIq1pVzJYIkDrtE= Rh-p7vpVLAkE2xvMP8D8SzvFo9RinnC3okM0ly7kkr2mke4$=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) .