Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 25 2025 22:23:18 AWUS01 KWNH 252223 FFGMPD NVZ000-CAZ000-ORZ000-260415- Mesoscale Precipitation Discussion 0800 NWS Weather Prediction Center College Park MD 622 PM EDT Fri Jul 25 2025 Areas affected...Northern California and far Northwest Nevada Concerning...Heavy rainfall...Flash flooding possible Valid 252221Z - 260415Z Summary...Showers and thunderstorms will continue to pivot slowly across portions of northern CA through this evening. Rainfall rates within convection could reach 1-1.5"/hr, leading to 1-2" of rain or slightly more in some areas. This may cause isolated instances of flash flooding. Discussion...The GOES-E day-cloud phase microphysics RGB this afternoon shows rapid growth of updrafts into Cb along the northern Sierra Nevada and into the other high terrain of northern California including the Shasta/Siskiyou and Klamath regions. These deepening updrafts are leading to numerous thunderstorms which are depicted via the regional radar mosaic. This activity is building in response to steepening lapse rates ahead of an upper low clearly indicated on WV imagery pivoting northward from near San Francisco, leading to a region of impressive upper level directional diffluence. At the same time, light onshore flow is pushing PWs to above 1 inch along the immediate coast, with a gradient falling to around 0.6 inches across the Sierra, both of which are still well above normal and approaching the 90th percentile in some areas. This elevated moisture combined with modest, but sufficient, MLCAPE of 250-500 J/kg are providing favorable thermodynamics to support heavy rainfall which has been measured via MRMS to be locally above 1"/1hr and as much as 0.4"/15 mins. As the aftn progresses, the northward advance of the upper low should provide continued synoptic ascent, while additionally supporting increased instability potentially surging as high as 750 J/kg. In this environment storms should continue to develop, expanding in both coverage and intensity as reflected by simulated reflectivity in the high res CAMs. Additionally, the HREF neighborhood probabilities for 1"/hr rates peak above 40% from around 23Z to 02Z, further indicating the continued enhancement in the next few hours. These storms will move slowly in the vicinity of this upper low as well, noted by 0-6km mean winds of just around 5 kts, and similarly weak and chaotic propagation vectors which could indicate short-term training at times. While storms should remain of the pulse variety due to a lack of meaningful shear, the coverage of cells will likely lead to mergers and collisions, locally enhancing the intensity and duration of heavy rainfall to produce 1-2" of rain, or locally higher (HREF PMM nearly 3 inches) in a few areas. In general, the 0-10cm soil percentiles from NASA SPoRT are quite dry, indicating a high filtration capacity which should be able to absorb much of this rain. However, these intense rates, especially during any mergers, or where duration lengthens due to collisions, could cause runoff concerns. This will be most likely atop any more vulnerable terrain or sensitive burn scars where isolated flash flooding instances could occur. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_6UhQKQLilOYvWpQSDVezXB51xCMr58eVbq2fA7o3iWKE0WVujjRwc6ximNsGqFuEh8z= knvLvM6A6Uq46V-l-PuTeqc$ for graphic product... ATTN...WFO...EKA...LKN...MFR...REV...STO... ATTN...RFC...PTR...RSA...NWC... LAT...LON 42112125 42082006 41811920 41391895 40911896=20 40561914 40181945 39551957 39141966 38681959=20 38641979 38832020 39142059 39522097 39922129=20 40322155 40622185 40592224 40252234 39932236=20 39522272 39532326 39942338 40472353 41332347=20 41712299 41992219=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .