Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 10 2023 17:53:43 AWUS01 KWNH 101753 FFGMPD NVZ000-CAZ000-110600- Mesoscale Precipitation Discussion 0094 NWS Weather Prediction Center College Park MD 1251 PM EST Fri Mar 10 2023 Areas affected...Much of Central California, portions of Southern California Concerning...Heavy rainfall...Flash flooding likely Valid 101800Z - 110600Z Summary...Numerous to widespread floods continues across much of Central California, with moderate to heavy rainfall gradually spreading southward into Southern California. Additional localized rainfall totals of 1-3+" are expected, with the Santa Lucia Range and central/southern Sierra Nevada most likely to realize additional 3-5 inches (where locally extreme impacts and flash flood emergencies will be possible). Discussion...A large atmospheric river (AR) continues to impact much of Central CA this morning, with rainfall now gradually spreading southward into Southern CA as well. Rainfall rates have been quite impressive locally, reaching (and even eclipsing) 1"/hr at times. These rates have primarily been confined to upslope portions of the western foothills of the central/southern Sierra Nevada (per MRMS data), but this is also some of the best radar sampled data in Central CA (with the nearby KHNX WSR-88d providing quality observation data). Looking at the relatively dense precipitation observation network, expanding accumulations to the past 6 hours reveals two primary areas of concern: 1) the aforementioned western foothills of the Sierra Nevada (with 6 hour totals ranging from 2-5 inches), and 2) coastal upslope portions of the Santa Lucia Range (with totals ranging from 2-4 inches). Elsewhere, 6 hourly localized totals are as high as 1-2 inches, but are most often below 1" (particularly across the Central Valley where there is a notable gap in moderate/heavy rainfall). As has been widely advertised ahead of this event, concerns are elevated across the areas where heavy rainfall is being realized due to the addition of snowmelt (which is much harder to quantify in real time). Flash Flood Warnings have rapidly expanded over the past several hours across the areas of concern, and at least one Flash Flood Emergency has been issued (specifically for Springville, CA). It is also noteworthy that extensive flooding impacts have also been realized on the eastern slopes of the Sierra Nevada foothills, likely largely due to the aforementioned snowmelt. While the trend over the next 6-12 hours is expected to generally be downward (with respect to low-level moisture transport and associated rainfall rates), it is unclear how much flooding impacts will worsen before recovery can begin. This is primarily due to the snowmelt runoff process, which will likely take some time. Therefore it is recommended that the public remains on high alert for any potential lag effects with regard to flooding, particularly for localities that are farther downstream (at lower elevations) from where the bulk of the rainfall and snow melt are occurring. Meteorologically, 850 mb flow looks to decelerate from the current 30-50 kts to below 30 kts (after 00z). This will dramatically reduce the very efficient moisture transport that has been ongoing, and rainfall rates are expected to fall off in-tandem. Precipitable water values will be slower to decrease, from record to near-record territory of 1.2-1.5 inches to eventually below an inch. This overall waning of the meteorological components (as the associated shortwave trough over the PacNW ejects to the east) is what should limit the overall coverage of flooding impacts across Southern CA (relative to the numerous to widespread flooding being realized across Central CA). Even still, 850 mb flow and highly anomalous tropospheric moisture should allow for a relatively brief period of moderate to heavy rainfall for the LA and San Diego metro areas this afternoon, with the highest risk for flooding across SoCal in the typical vulnerable upslope portions of the Transverse Ranges where localized rainfall rates of 0.5-1.0+"/hr may be realized (mainly between 20z-04z). Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!5u-v_71Cy_hO322M8b9gKJ2-4m7UtfKBwb4TIfgdcMrGnabPdEmK_70chNmqqbbghiee= 4ScjRBs91T0XCGbC7-YhmoE$ for graphic product... ATTN...WFO...HNX...LOX...MTR...REV...SGX...STO...VEF... ATTN...RFC...CNRFC...NWC... LAT...LON 38932021 38931969 38601919 38271863 37891814=20 37461785 36821772 36031764 35451778 34871805=20 34531764 34241677 32811633 32461728 32981813=20 33421903 33812052 34562106 35302126 35862185=20 36542218 37042222 37502129 37732073 38392037=20 =20=20=20=20=20=20=20=20=20=20=20=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) .