Flash Flood Guidance
Issued by NWS
Issued by NWS
994 AWUS01 KWNH 201944 FFGMPD AZZ000-NVZ000-CAZ000-210130- Mesoscale Precipitation Discussion 1036 NWS Weather Prediction Center College Park MD 343 PM EDT Fri Sep 20 2024 Areas affected...Southern CA...Far Southern NV....Far Northwest AZ... Concerning...Heavy rainfall...Flash flooding possible Valid 201945Z - 210130Z SUMMARY...Slow moving, back-building cells with rates of 1"/hr may pose localized 1-1.5" totals and possible flash flooding conditions. DISCUSSION...GOES-W and Regional RADAR mosaic have started to depict increasing convective activity in both coverage and vertical depth with peak heating of the day. The highly anomalous, over 3.5 standard anomaly units from climatology, core of the closed low is bottoming out across SW California with vorticity center at the southern base; this is allowing for an elongation/stretching of the mid-level deformation zone to extend across SW to NE San Bernadino county with increased deep layer convergence. Given steepening lapse rates with cold air advection aloft, surface heating into the 80s is starting to support surface based CAPE values from 500-1000 J/kg to develop over the Southern California Deserts toward the Colorado River Valley near S Nevada. While deep layer moisture is a bit limited with only Tds in the upper 40s, low 50s and total PWat values in the .75 to 1" range. The convergence within the band and orographic ascent confluent across the San Bernadino/Little Bernadino/San Jacinto Ranges supports stronger vertical development and sufficient moisture flux to support .5-1"/hr rates. Limited cell motions given proximity to the closed low/deformation zone will allow for very slow/stationary motion with upstream redevelopment/backbuilding along the deformation band across NE CA. Propagation may also allow cells along the mountain ridges into the Cochella Valley with time. This may allow for spotty .75-1" totals with an isolated 1.5" total remains possible. Given naturally low FFG values less than these hourly rates and potential; an isolated flash flooding incident or two is considered possible across the area through the late afternoon/evening hours. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...PSR...SGX...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 36051537 35651447 35091429 34421518 33621507 33231522 33321587 32881615 32921657 33461673 33991720 34191765 35011753 35581699 36001605