Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Dec 31 2022 20:15:11 FOUS30 KWBC 312015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 314 PM EST Sat Dec 31 2022 Day 1 Valid 16Z Sat Dec 31 2022 - 12Z Sun Jan 01 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CALIFORNIA... Strong atmospheric river (AR) continues to impact northern California this morning with a frontal boundary draped WSW to ENE out of the Pacific. Moisture anomalies (PW and IVT) per 12Z RAOBs/obs are above the 99th percentile from near SFO eastward across the Sierra and will maintain its axis through the early afternoon hours. Weak surface wave along the front will slow the front's southeastward progression over the next several hours, but as that passes through the Sacramento Valley the front will pick up speed as sharper height falls move into the area. Observed hourly rainfall rates have eclipsed 1"/hr along the coast and over the coastal ranges as well as into the Sierra foothills, with lower rates in the Sacramento Valley but still near and just over 0.50"/hr. Strong moisture flux eastward will result in more spillover east of the Sierra than normal, and maintained a small eastward extension of the Marginal Risk into northwestern NV as the weak wave translates ESE-ward early this afternoon. Thereafter (after 00Z), precipitation will be mostly out of NorCal as the AR shifts southward. For central/southern areas of CA, AR will be more progressive this afternoon and evening especially after 00Z (central areas) and 04-05Z (southern areas). 110kt jet will still provide broad lift over the region with plenty of moisture (PWs at least about the 90th percentile) but the speed of the front will limit the QPF. Nevertheless, strong moisture flux and dynamics along with some instability (MUCAPE ~200 J/kg and some negative LIs) will support some rainfall rates > 0.50-0.75"/hr, especially over the Transverse Ranges as the orthogonal flow aides in lift on 30-50kt 850mb wind. 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 negated 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 was maintained for SoCal, but confined to the terrain east of San Diego. Fracasso Day 2 Valid 12Z Sun Jan 01 2023 - 12Z Mon Jan 02 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTHERN ARIZONA... Progressive AR will continue through the Southwest on Day 2 (Sunday into very early Monday) beneath a 110kt jet streak and a sharpening mid-level trough that will be closing off into an upper low. Moisture plume off the East Pacific will quickly move through the region, supporting a relatively brief window for some convectively-enhanced rainfall rates over 0.50"/hr per some CAM guidance (max near 0.75"/hr) outside the terrain. This should be below FFG for the area, but cannot rule out an isolated FF threat with a surge of PW values 0.50-0.75" and some CAPE of a couple hundred J/kg in advance of the frontal passage. Fracasso 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!7VzESONl9YVq68FX8mQZCFPiLAH1QYI7Sey967UAeaDF= DVztrmnsrtIP8tQ7n1abWEohTmltQiYjXrs6voXpfpc0KYI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7VzESONl9YVq68FX8mQZCFPiLAH1QYI7Sey967UAeaDF= DVztrmnsrtIP8tQ7n1abWEohTmltQiYjXrs6voXpxghopfs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7VzESONl9YVq68FX8mQZCFPiLAH1QYI7Sey967UAeaDF= DVztrmnsrtIP8tQ7n1abWEohTmltQiYjXrs6voXpu5QphTs$=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) .