Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 24 2023 08:25:59 FOUS30 KWBC 240825 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 325 AM EST Fri Feb 24 2023 Day 1 Valid 12Z Fri Feb 24 2023 - 12Z Sat Feb 25 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREATER LOS ANGELES METRO AREA OF SOUTHERN CALIFORNIA THROUGH SANTA BARBARA... An anomalously strong positively tilted trough in the upper levels will drift south down the coast Friday. Ahead of the trough a large 130 kt southwesterly jet streak starts across the Southwest. The trough cuts off into a strong upper low that is not only highly unusual for this time of year, but also contains quite a bit of energy as shortwave disturbances rotate around the cutoff low. While IVT values will only peak around 500 kg/m/s, the anomalously cold air associated with the cutoff low will be able to take advantage of several hundred miles of fetch to draw that Pacific moisture into the coast. Thus, the strong forcing and abundance of moisture being drawn into the coastal mountains at an orthogonal angle to the ranges will make for a "perfect storm" of favorable ingredients for heavy rain (and high elevation snow) through SoCal. Expect peak rainfall amounts in the Transverse Ranges of over 7 inches by Saturday morning, with some lingering rain into Saturday. Widespread totals of 2-5 inches of rain are expected, including in the Greater Los Angeles area. While snow levels will eventually come down, it will be mainly after most of the heaviest precipitation associated with this system has fallen. Thus, concern not only about the potential for heavy rain, but also fast-moving runoff from the nearby mountains was included with the enhanced risk areas. One other larger change was the Marginal Risk area was expanded to include much of the southern San Joaquin Valley. With deep unidirectional flow throughout the atmosphere, there's highly likely to be significant "slopover" precipitation into the areas north of the Transverse Ranges. 1-2 inches of rain into areas that don't typically see that much rain fall all at once is likely to cause even localized flooding issues, especially in poor drainage areas and urbanized areas, such as Bakersfield. It's important to note that the forecast does include areas where significant snowfall forecast for the higher elevations of the Transverse and Sierra Ranges. The portions of the area the ERO highlights where snow will be the dominant precipitation type of course are under no threat for flooding. Thus, when interpreting the risk areas across Southern California, note that the threat for excessive rainfall is exclusively for lower elevation/warmer locations, and does not actually include the highest peaks.=20 The changes described above were made in coordination with the SGX/San Diego, CA and LOX/Los Angeles, CA forecast offices. Wegman Day 2 The Day 2 outlook will be updated by 0830Z. 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!5oEdkVCh95o2i7-CYJikI7Pvg8qi-gPCZgI9zFk_SH_s= QS3Mrm8vD5SKqLuR-NI9uezOBwsDFxXfFQyhD4sq8xxt4b0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5oEdkVCh95o2i7-CYJikI7Pvg8qi-gPCZgI9zFk_SH_s= QS3Mrm8vD5SKqLuR-NI9uezOBwsDFxXfFQyhD4sqJI2RGIk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5oEdkVCh95o2i7-CYJikI7Pvg8qi-gPCZgI9zFk_SH_s= QS3Mrm8vD5SKqLuR-NI9uezOBwsDFxXfFQyhD4sq3jnVZaI$=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) .