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Flash Flood Guidance
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Issued by NWS
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928 AWUS01 KWNH 121641 FFGMPD WYZ000-UTZ000-NVZ000-122240- Mesoscale Precipitation Discussion 0869 NWS Weather Prediction Center College Park MD 1241 PM EDT Mon Aug 12 2024 Areas affected...Much of Utah...East-central Nevada...Southwest Wyoming... Concerning...Heavy rainfall...Flash flooding likely Valid 121640Z - 122240Z SUMMARY...Stronger than normal FGEN and moisture flux with deeper steering capable of repeating thunderstorms poses potential for scattered flash flooding incident(s) this afternoon. DISCUSSION...GOES-E Visible imagery has seen a narrow banded feature of weak convective elements across central UT with favorable orientation for training, while not currently an issue, it denotes potential this early in the morning for the environment given dynamic set-up and orientation. GOES-E WV shows classic-T shape of mid to upper-level flow regime noting a old elongated but still strong shortwave feature across the Lower Colorado River Valley with some northerly stretching coming into SW UT; favorable anti-cyclonically arched feature across south-central to northeast UT shows the southern stream return flow of the monsoon, while a second polar band of enhanced cirrus denoting broad scale ascent in the favorable ridged entrance to the broad jet across Northeast NV before becoming more flat across western WY, where an exiting stronger mid-level shortwave is analyzed. As such, there is ample broad scale ascent across the UT at this time, stretching into peak early morning period. CIRA LPW denotes favorable channels of 850-700mb flow up the Colorado River Valley into W UT, while 700-500mb layer notes a similar slug in that regime, but also what appears to be a cyclonically curled moisture pocket across eastern NV. Enhanced 700mb Tds of 5-8C are analyzed across N Lincoln and White Pine county. RAP surface analysis aided the detection of a surface effective dry line across south-central NV north toward the UT/ID/NV corner where mid to upper 50s (isolated low 60s) Td rapidly decreases into the 30s and even mid 20s through central NV. FGEN/theta-E gradient alignment is very strong for the Intermountain west. As such total depth TPW is well over .75" across much of the area with 1" through the lower valleys across the NV/UT border and into the Salt Flats of NW UT. Currently, instability continues to build with the increasing insolation across the eastern NV enhanced moisture field with 1000-1500 J/kg still generally capped, though an isolated cell has begun the deeper convective process. This activity is expected to grow in coverage given the favorable DPVA across the central UT terrain but also back across E NV into NW UT with the stronger FGEN/UL Jet ascent pattern. Modest 20-25kts of moisture flux through the Colorado Valley into cloud bases around 800-700mb, should allow for rates of 1-1.5"/hr. Deep layer steering from SW to NNE (slowly converging on the eastern side due steering ridge) will increase potential for repeating. As such a spot or two up to 2-2.5" is possible through 22z, though a greater coverage of 1-2" spots are probable resulting in scattered incidents of flash flooding through early evening. Gallina ...Please see www.wpc.ncep.noaa.gov for graphic product... ATTN...WFO...GJT...LKN...RIW...SLC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 41961078 41750986 41150927 39870916 39200998 38571073 37751121 37101171 37051259 37081340 37701354 38091394 37801472 37901529 38371564 39251518 40661370 41781193