Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Feb 24 2023 08:30:57 FOUS30 KWBC 240830 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 329 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 Valid 12Z Sat Feb 25 2023 - 12Z Sun Feb 26 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOS ANGELES AND SAN DIEGO BASINS... The upper level low bringing heavy rainfall across much of Southern California on Day 1/Friday will begin to move eastward a bit more quickly on Saturday. This will force the surface cold front to finally move eastward into the coast as well. As a result, the primary period when the heaviest rainfall is expected is Saturday morning across the Los Angeles and San Diego areas. By late Saturday afternoon, the upper level low will move overhead, resulting in the rainfall becoming more cellular/convective and more scattered. While this won't completely end the threat for excessive rainfall late Saturday afternoon and Saturday night, it will greatly diminish that threat. Any excessive rainfall from cellular convection will be isolated to the localized area. Despite the main threat for heavy rain relegated to the first 6-9 hours of the day, expect that rain will be heavy enough for 1-3 inches of additional rainfall on top of any rain that fell Friday night. The highest rainfall totals will be in the Peninsular Ranges, whose higher elevations can also expect some of the precipitation to be in the form of snow. This is likely to cause flooding issues. Thus, in coordination with the SGX/San Diego, CA and LOX/Los Angeles, CA forecast offices, upgraded much of coastal Southern California to a Slight Risk. Wegman Day 3 Valid 12Z Sun Feb 26 2023 - 12Z Mon Feb 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL PLAINS THROUGH THE UPPER MISSISSIPPI RIVER VALLEY... The upper level low that caused the heavy rain Friday through Saturday morning in southern California will eject eastward into the Plains and rapidly increase its forward speed. The cutoff low will begin re-merging back into the jet stream as it does so, becoming a negatively tilted trough Sunday night. The low-turned-trough will support lee cyclogenesis, and the strengthening surface low will take advantage of abundant Gulf moisture being drawn northward ahead of the low on a very strong 70 kt LLJ. This will push anomalous moisture with PWATs exceeding 1.25 inches across the Plains Sunday night. This moisture will feed precipitation ahead of the low. That said, instability will be lacking somewhat. While it won't be non-existent, values will generally stay under 750 J/kg, and will only reach those levels briefly Sunday evening. Thus, expect some convective elements embedded within a mostly stationary boundary that the low will travel along as it moves from the Central Plains into the Midwest. Isolated rainfall totals exceeding 2 inches may cause localized flash flooding, especially in any urbanized or poor drainage areas. Changes from the inherited forecast were minimal. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8K-yHDB_KwNr5onjpgrIJAfy6nTSfYPspclHjYSf3ZKJ= h941yESuyrUlTNhel4s23CbpwM9sjhwn93QRe0oR-zfTK8s$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8K-yHDB_KwNr5onjpgrIJAfy6nTSfYPspclHjYSf3ZKJ= h941yESuyrUlTNhel4s23CbpwM9sjhwn93QRe0oRa_MM3Vo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8K-yHDB_KwNr5onjpgrIJAfy6nTSfYPspclHjYSf3ZKJ= h941yESuyrUlTNhel4s23CbpwM9sjhwn93QRe0oRnAYPiPw$=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) .