Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 20 2023 00:57:30 AWUS01 KWNH 200057 FFGMPD AZZ000-UTZ000-NVZ000-CAZ000-200655- Mesoscale Precipitation Discussion 0950 NWS Weather Prediction Center College Park MD 855 PM EDT Sat Aug 19 2023 Areas affected...Lower Colorado River Valley Concerning...Heavy rainfall...Flash flooding possible Valid 200055Z - 200655Z SUMMARY...Showers and thunderstorms may tend to increase in coverage a bit more going through the late evening hours. Locally heavy rainfall rates are expected to result in some additional flash flooding concerns ahead of Hurricane Hilary. DISCUSSION...Radar imagery continues to show a fairly broad area of stratiform rain across much of the Lower CO River Valley as deep moisture and warm advection ahead of Hurricane Hilary continues to surge north-northwest from the Baja Peninsula, the Gulf of California, and adjacent areas of northwest mainland Mexico. The clouds and rainfall over the last several hours has continued to keep the boundary layer instability to a minimum across southeast CA, southwest AZ and up into far southern NV. However, there has been sufficient levels of solar insolation and heating across areas of northwest AZ and into southwest UT for greater instability to pool across these areas, and this coupled with strengthening low to mid-level moisture flux ahead of Hilary and orographics has led to an uptick in shower and thunderstorm activity over the last couple of hours. Convection is expected at least in the short-term to generally expand in coverage across northwest AZ, far southwest UT and parts of southern NV given better thermodynamics and also aid from improving right-entrance region upper jet dynamics across the region in between Hilary and the upper low/trough offshore of central CA. Rainfall rates across these areas may reach 1 inch in as little as 30 minutes with the stronger storms. Additional flash flooding will be possible as a result from this in itself given some of the area slot canyon and dry wash sensitivities. The greatest uncertainty just through this evening is over southeast CA and adjacent areas of far western AZ. Despite the more compromised instability profiles, there should be at least some potential for convection to begin to regenerate in more earnest by late this evening, and especially over southeast CA where strengthening upper jet support and increasing low-level moisture transport/convergence up from the Gulf of California will begin to combine with some of the terrain/orography to favor redeveloping clusters of heavier showers. The last couple runs of the experimental WoFS support this. Going through 06Z, the WoFS does suggest the potential for some spotty 1 to 3 inch amounts locally near and to the east of the Peninsular Range and including the San Jacinto mountains. Areas of flash flooding will be possible as a result for the first part of the night, with much more significant and direct impacts from Hilary's rains expected to arrive later tonight through early Sunday. Expect more MPDs to be issued accordingly later tonight. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!9qNuMtwEGCiBH5n3H9Jtjt_ygcUkjDAjbZlL1P-_6C6BOqX1SpNbU4kWSifY7qrsPY4T= BR624SNnGC1osBH7XMSeCL8$ for graphic product... ATTN...WFO...FGZ...PSR...SGX...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 37961387 37521304 36531271 35611285 34831346=20 33911432 32851481 32601534 32611633 33031682=20 33711692 34781694 36361704 37051674 37601601=20 37941499=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) .