Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 02 2023 22:59:15 AWUS01 KWNH 022259 FFGMPD AZZ000-UTZ000-NVZ000-CAZ000-030400- Mesoscale Precipitation Discussion 1027 NWS Weather Prediction Center College Park MD 658 PM EDT Sat Sep 02 2023 Areas affected...Portions of the Great Basin into the Deserts of CA and AZ Concerning...Heavy rainfall...Flash flooding likely Valid 022257Z - 030400Z Summary...Widespread showers and thunderstorms will persist into this evening across the Desert Southwest and Great Basin. Rainfall rates in the more intense convection will approach 2"/hr, which through training could produce 2-3" of rain with locally higher amounts. Flash flooding is likely. Discussion...Another active day of convection continues across CA/AZ/NV as reflected by widespread showers and thunderstorms on the regional radar mosaic. Rainfall rates as estimated by local radars continue to reach above 1.5"/hr, which has resulted in mesonet observed rainfall above 1.5" in some areas, and 6-hr MRMS rainfall has high as 2.5". FLASH response to these higher rainfall rates has been impressive as well, reaching more than 200% in the QPE-FFG ratio, and as high as 800 cfs/smi for CREST unit streamflow. This has resulted in a myriad of flash flood warnings already, with more flash flooding expected through the evening. The environment remains extremely supportive to heavy rain thanks to a pool of +2.5 to +3 sigma PWs being drawn northward coincident with SBCAPE according to the SPC RAP of 1500-2500 J/kg. Within these robust thermodynamics, forcing for ascent is impressive in the vicinity of a potent shortwave noted in WV imagery lifting northward, interacting with modest height falls from the west and subtle upper level diffluence from the distant RRQ of a poleward arcing jet streak over OR/CA. Together, this will likely result in a continuation of thunderstorm development for several more hours until instability gets exhausted, with the greatest coverage likely invof the mid-level shortwave, or in the immediate vicinity of the mid-level trough to the west. In any of these locations, rainfall rates could reach 2"/hr as reflected by HREF neighborhood probabilities, and HRRR 15-min rainfall as high as 0.5". 850-300mb winds, being used a proxy for storm motions, indicate cells will generally move quickly northward at 15-30 kts. However, generally aligned Corfidi vectors indicate a strong likelihood for training, with some redevelopment into the greater instability south leading to some areas getting multiple rounds of heavy rainfall. Where this occurs, the HREF exceedance probabilities for 3"/6hrs reach 15-25%. This region continues to be vulnerable from 14-day rainfall that has been more than 600% of normal according to AHPS, resulting in USGS streamflow anomalies that are well above the 90th percentile. Additionally, this region has many sensitive burn scars, canyons, and usually dry washes that are even more vulnerable to flash flooding. It is likely that there is enough ascent into the favorable thermodynamics to persist convection well into the evening, with a northward push of more widespread activity also expected. This will likely lead to numerous instances of flash flooding during the next several hours. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-Le4CKTsxVf14TLhq9TxZw63fNU0xfcYvldswJ0NlDAZqAX2pkunre3-4lpyPzjhEaPL= 0trBpI3OBhYfGMnv-1-gwx8$ for graphic product... ATTN...WFO...FGZ...LKN...PSR...REV...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 40021858 39881729 39481582 38591368 37761228=20 36791188 36091198 34691243 33501310 33331387=20 33481473 33681544 34031597 34611617 35351694=20 36301743 37721821 39151861=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) .