Subj : MESO: Hilary - Heavy Rain To : All From : Mike Powell Date : Sun Aug 20 2023 14:56:00 AWUS01 KWNH 201801 FFGMPD AZZ000-NVZ000-CAZ000-210000- Mesoscale Precipitation Discussion 0952 NWS Weather Prediction Center College Park MD 201 PM EDT Sun Aug 20 2023 Areas affected...much of southern California and adjacent areas of southern Nevada and far western Arizona Concerning...Heavy rainfall...Flash flooding likely Valid 201800Z - 210000Z Summary...Tropical Storm Hilary continues to make steady north-northwestward progress toward the Southwestern U.S.. Areas of heavy rainfall should cause flash flooding at least through 00Z. Discussion...Widespread areas of light to moderate rainfall continue to stream northward ahead of the approach of Hilary, which was recently located near the northern Baja California Peninsula. Rain rates have generally remained in the 0.2-0.5 inch/hr range across southern California so far, with the heaviest rain recently noted in/near higher terrain north of Los Angeles and just south of El Centro in far northwestern Mexico over the past hour or so. These rain rates have resulted in a few impacts so far. Models/CAMs suggest that rain rates within this general axis (from Los Angeles northward toward Death Valley and southern Nevada near Pahrump) could increase as the center of Hilary gets closer, with spots of 0.5+ inch/hr rainfall expected as low-level wind fields increase over time. A somewhat more concerning scenario should unfold from along and just east of the Peninsular and eastern Transverse ranges into the southern Sierra through 00Z. As Hilary moves closer to these areas, low-level wind fields will increase dramatically, with a distinct southeasterly component of flow interacting favorably with local orography to provide lift. Additionally, strong low-level warm/moist advection and surface heating should allow for added destabilization to both 1) bolster updraft strength and coverage while 2) eventually increasing rain rates. Eventually, heavier precipitation (such as the near 1 inch/hr rain rates observed just south fo El Centro) should migrate northward into southern California and further increase the flash flood risk. These rates could persist for a few hours in tandem with Hilary's storm motion. Areas of 2-4 inch rainfall totals are likely, and this thinking is generally in line with the 12Z runs of the high-resolution Nam and HREF. Additional thunderstorm activity should also materialize along the Lower Colorado River Valley, although the flash flood potential is somewhat in question there as fast storm motions limit the amount of rainfall in any given spot. It appears that more organized training of convection will be needed for a more appreciable flash flood threat to materialize across eastern portions of the discussion area (closer to the Lower Colorado River) through the evening. Cook ATTN...WFO...HNX...LKN...LOX...PSR...REV...SGX...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 38231643 37511562 36391459 35181471 34111442 33031429 32421479 32301716 32801790 33951911 34971939 36641939 37651876 38221764 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .