Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Sep 01 2023 23:26:46 AWUS01 KWNH 012326 FFGMPD AZZ000-NVZ000-CAZ000-020300- Mesoscale Precipitation Discussion 1021 NWS Weather Prediction Center College Park MD 726 PM EDT Fri Sep 01 2023 Areas affected...Low and High Deserts of CA, western AZ Concerning...Heavy rainfall...Flash flooding possible Valid 012324Z - 020300Z Summary...Showers and thunderstorms will continue to develop and train to the north this evening. Rainfall rates will likely eclipse 1"/hr, which through this training could produce 1-2" of additional rainfall with locally higher amounts. Flash flooding is likely. Discussion...The GOES-E IR imagery this afternoon depicts areas of rapidly cooling cloud tops centered over the low and high deserts of southern CA. Beneath these cooling cloud tops, thunderstorms are widespread as noted in the local radars, with estimated rainfall rates reaching above 2"/hr in some areas. This rainfall has been persistent this afternoon, resulting in mesonet measured rain as high as 2-3" in some areas, with 6-hr rainfall measured by MRMS approaching 4". Flash flood warnings and reports are widespread across the area, and this is likely to continue for several more hours. The primary feature responsible for this activity is a potent closed low dropping across northern CA which is squeezing southerly flow into the Great Basin upstream of a pronounced mid-level ridge over the Great Plains. Within this flow, subtle shortwave impulses, some of which are being convectively enhanced by thunderstorms, are shedding northward, which when combined with a favorably placed 90kt jet streak to the north to provide RRQ diffluent ascent, is providing impressive deep layer lift over the area. This ascent into robust thermodynamics characterized by PWs of 1.8-2.0 inches and SBCAPE around 2000 J/kg is fueling the widespread heavy rain production. Additionally, this squeezed southerly flow is driving 850-700mb moisture flux to as much as +5 standard deviations above the climo mean according to the SREF, furthering the support for these impressive rain rates. While the guidance is in good agreement that the most intense convection and associated rainfall rates will gradually lift north into tonight, there is still expected to be a several hour period of training heavy rain rates from the southern CA deserts and across much of western AZ. The UA WRF and HREF neighborhood probabilities both indicate a high likelihood of 1-2"/hr rates, potentially briefly reaching 3"/hr at times (0.75"/15 min in the HRRR sub-hourly fields), resulting in axes of 1-2" of rainfall. The HREF also indicates a 30-40% chance for more than 3" in isolated locations. These rates will almost certainly overwhelm the FFG, which is only 0.5-1"/1hr in many areas, noted by the HREF exceedance probabilities peaking above 70%. This suggests that any of these heavy rain rates will rapidly lead to runoff and flash flooding, with the greatest impacts, which could be significant, expected in urban areas or where the most pronounced training occurs. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_kEldHMNFO3UfIMudPXKJUZUo2DQJi9ZXQFmvSQXU1YC30usCS3IQnwR_KQcXIuIAevR= j3jET1mnsxMV0VkdVdyXdw8$ for graphic product... ATTN...WFO...FGZ...PSR...SGX...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 35581573 35451473 35221378 34741332 33851337=20 33371358 32801397 32391499 32321582 32571655=20 33521696 34331726 34991716 35431698 35531673=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) .