Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Mar 20 2023 20:26:49 FOUS30 KWBC 202026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Mon Mar 20 2023 Day 1 Valid 16Z Mon Mar 20 2023 - 12Z Tue Mar 21 2023 ....16Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 2 Valid 12Z Tue Mar 21 2023 - 12Z Wed Mar 22 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL ACROSS COASTAL AREAS OF SOUTHERN CALIFORNIA... ....2030Z Update... ....California... In coordination with the SGX/San Diego, CA and LOX/Los Angeles, CA forecast offices, the Slight Risk area for coastal SoCal and the adjacent Transverse and Peninsular Ranges has been upgraded to a Moderate Risk with this afternoon's update. The primary change with this update includes significantly higher QPF forecast for the Moderate Risk area. With the inclusion of the CAMs guidance, and the atmospheric river set to begin late tonight, much of that higher resolution guidance has a good handle on the evolution of the upcoming A.R. Thus, the forecast has increased rainfall, particularly in the Transverse and Peninsular Ranges. The primary factors contributing to the likelihood of flash flooding continue to be favorable antecedent conditions in the form of nearly saturated soils, heavy rain that may come down as quickly as 0.75 inches/hour at times, highly anomalous atmospheric moisture that is +4 sigma above normal with PWATs approaching 1 inch, and associated strong wind gusts that will increase the likelihood for downed trees and debris that may clog storm drains in the many urbanized portions of the Moderate Risk area. Rainfall amounts in the densely populated valleys and coastal areas will generally range between 1.5 and 2.5 inches. When adding runoff from the steep nearby mountains where rainfall totals may exceed 4 inches, numerous flash floods are likely. The timing of the heaviest rain also favors added impacts, as they will likely be ongoing during the Tuesday morning commute. There are a few factors working against flash flooding worth noting. The primary one is the low snow levels, specifically the short duration of higher snow levels. The snow levels will come up to between 6.500 and 8,000 ft overnight tonight with higher values further south. This will correspond with the heaviest precipitation rates. MUCAPE up to 500 J/kg will accompany the heaviest rain, primarily forced by the very cold air following behind the A.R. moving above the warm and moist air at the surface. Once the cold front swings through, the snow levels will crash down to 4,000 ft by Tuesday night. Secondly, a lack of higher elevation rain. Due to the generally low snow levels, the heaviest precipitation will be over portions of the Transverse Ranges where it will fall as all or mostly snow, totaling a few feet in some places by the time the A.R. wraps up. Due to snow falling at the higher elevations, that is less water that will run off into the rivers. Third, the system remains quite progressive. As mentioned above the snow levels will only remain elevated for about 6-12 hours (highest further south). After that the cold front will swing through, causing the snow levels to fall. While the precipitation won't end behind the cold front due to continued onshore and upslope flow, it will lighten up considerably, and more of it will fall as snow at the higher elevations. The heaviest rain will fall with the warmest temperatures, however will be relatively short in duration. Thus, with all of the above considered, this is a lower-end Moderate Risk (closer to 40% chance of exceeding flash flood guidance, rather than 70%). Further north into central California the inherited Slight and Marginal Risks are largely unchanged. This is largely due to the heaviest rain impacting southern areas, and snow levels may never rise above 5,000 feet for the entire event. Thus, despite obviously favorable antecedent conditions, the lower forecasted rain totals and much of the precipitation falling as snow farther down the mountains will greatly limit the flash flooding threat with latitude.=20 ....Central Arizona... Relatively little has changed here despite the inclusion of higher resolution short-term guidance. If anything, forecast storm total rainfall has actually dropped a few tenths of an inch in this area as compared with 24 hours ago. Significantly less total rainfall is expected here as compared with the mountains of SoCal, with peak precipitation expected to be around 2 inches along the Mogollon Rim. Snow levels will be a supporting factor for flooding rains in AZ as compared with California, as the higher snow levels in the 8,000 to 9,000 foot range will persist for a bit longer, generally 12-18 hours. The cold front will follow behind the heaviest rain and will move through quickly, so once again the progressive nature of the heavy rain will limit total rainfall, and therefore reduce resultant impacts. Very few changes were made from the overnight update. Wegman ....Previous Discussion... ....Southern California... A strong closed low diving southeast across the Northeast Pacific and inland into central California and the Great Basin will direct another atmospheric river (AR) into California and Arizona through Tuesday and the overnight hours. Strong 850 to 700 mb moisture flux expected on the south side of this closed low, impacting areas from the central California coast range , southeast through the Transverse and Peninsular Ranges and into the southern Sierra during the day on Tuesday, progressively spreading inland into the Southwest Tuesday night into early Wednesday . The National Water Model has been showing several streams with high flows across the central California coast range, southeast into coastal southern California, including the Transverse and Peninsular Ranges, which coincides with anomalous soil moisture values as per NASA SPoRT maps. The latest guidance continue to depict a broad area of 1 to 3+ inches over the southern to central Valley/foothills of the Sierra with the heaviest precip here expected to fall as snow in the Sierra, with no significant contribution to runoff from snow melt expected. The snowpack in the Transverse Ranges is very limited to small areas of the highest elevations which will greatly limit or negate any snowmelt contributing to any flooding. The Marginal and Slight Risk areas remain in good order but required minimal adjustments to reflect the latest WPC QPF and trends. ....Central Arizona... Higher snow levels are expected for the terrain of central Arizona as the AR reaches the area, which will limited snowmelt along the higher elevations of the Mogollon Rim to contribute more to any flash flooding in the area. With PW anomalies creeping up to +_4 and +5 standard deviations, most of the guidance are signaling up to 3 inches of rainfall for the higher elevations, with lesser amounts forecast for locations to the south and west. The conditions for excessive rainfall and rapid runoff will be more favor for points north and east of the Phoenix metropolitan area. The inherited Slight and Marginal Risk areas remain reflective of the level of threat for flooding concerns. Therefore, no adjustments were made with this issuance. Campbell Day 3 Valid 12Z Wed Mar 22 2023 - 12Z Thu Mar 23 2023 ...2030Z Update... The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-CvgYWmnkiAhtS_zV-Qi7jPIcrgw3iCnANIaLkt_b31k= kmnMKnexb3QmAGiwlsHJ1aa87M6wPAPBP6PZJ8fGqJLLVXg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-CvgYWmnkiAhtS_zV-Qi7jPIcrgw3iCnANIaLkt_b31k= kmnMKnexb3QmAGiwlsHJ1aa87M6wPAPBP6PZJ8fG8NJlGsE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-CvgYWmnkiAhtS_zV-Qi7jPIcrgw3iCnANIaLkt_b31k= kmnMKnexb3QmAGiwlsHJ1aa87M6wPAPBP6PZJ8fGmnM6IlI$=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) .