Flash Flood Guidance
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Issued by NWS
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871 AWUS01 KWNH 111847 FFGMPD AZZ000-UTZ000-NVZ000-CAZ000-120040- Mesoscale Precipitation Discussion 0862 NWS Weather Prediction Center College Park MD 246 PM EDT Sun Aug 11 2024 Areas affected...northwestern AZ, southeastern NV and central/southern UT Concerning...Heavy rainfall...Flash flooding likely Valid 111843Z - 120040Z SUMMARY...Thunderstorm coverage will expand over the next few hours leading to scattered flash flood concerns across portions of northwestern AZ, central/southern UT into southeastern NV. Rainfall rates of 1-2 inches in 30-60 minutes are expected. DISCUSSION...Monsoon convection had already initiated as of 1815Z across northern AZ into southwestern UT. A mid-level low off of the Baja Peninsula and ridge over western NM has allowed anomalous moisture to funnel northward into the Desert Southwest with 12Z soundings from VEF and FGZ displaying precipitable water values between 150 to 175 percent of normal along with a lack of appreciable dry air aloft. Northwestern AZ being positioned on the periphery of the Baja low and NM high while placed south of the westerlies in northern UT, was also within a region of weak wind shear with 15 kt or less of tropospheric winds and 10 kt or less deeper layer mean flow, which will support slow moving storms. SPC mesoanalysis data from 18Z showed MLCAPE of 500-1500 centered over northwestern AZ where mostly clear skies were giving way to thunderstorm development along the higher terrain. Expectations are for thunderstorm coverage to increase across northwestern AZ into southern UT as daytime heating continues to expand the coverage of available instability. Activity may also enhance ahead of a well-defined MCV tracking northwestward from southern AZ. Thunderstorm development along the southern end of the Wasatch Range is also expected to continue where mean westerly winds are stronger (~15 kt) but matching similarly to 700 mb wind vectors, which may promote areas of training. Subsequent outflow boundary interactions and storm mergers may allow for convection to merge with isolated convection that develops across adjacent portions of southern NV. Rainfall rates are expected to peak in the 1-2 inch range per 30-60 minutes time, resulting in concerns for flash flooding. The favorable setup for thunderstorms combined with wet antecedent conditions (many areas displaying 200+ percent of average rainfall over the past week) will lead to an increased threat for flash flooding compared to a typical monsoon convective day. The flash flood threat is likely to continue well into the evening hours. Otto ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...FGZ...LKN...PSR...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 39671289 39561146 39041078 38181068 37201085 35921133 34591129 34211168 34021293 34581433 36191588 37341549 38781462 39371379