Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 17 2025 17:55:12 AWUS01 KWNH 171755 FFGMPD AZZ000-UTZ000-NVZ000-172353- Mesoscale Precipitation Discussion 0728 NWS Weather Prediction Center College Park MD 154 PM EDT Thu Jul 17 2025 Areas affected...Southwest Concerning...Heavy rainfall...Flash flooding likely Valid 171753Z - 172353Z Summary...Convective initiation is underway over portions of Northwest Arizona and Southern Utah. Expanding coverage of slow moving storms with max rainfall rates exceeding 1"/hr will lead to scattered flash flooding this afternoon. Discussion...Recent GOES Day Cloud Phase RGB and LightningCast data depict the first signs of glaciation and lightning activity in a handful of storms developing over Northwest Arizona and Southern Utah. These cells are forming on the warm side of a differential heating zone, downstream of a pair of compact MCVs which have generally been lifting north this morning. Recent mesoanalysis data on the warm side of the boundary highlights an increasingly unstable, uncapped atmosphere characterized by 1000-1500 J/kg of SBCAPE to support expanding cell coverage and intensity in the presence of the upstream MCVs, heating boundary, and terrain. Over the next several hours, short term RAP forecasts suggest upwards of 2000 J/kg of SBCAPE (over 1000 J/kg MLCAPE) could materialize -- which is noteworthy as impactful flash flood events in the region tend to feature at least 1000 J/kg of SBCAPE. Overlapping this buoyancy is a steady influx of precipitable water in the 1-1.7" range, well over the 90th percentile for much of the region. In all, this will support efficient, slow moving updrafts (10 kt storm motions) with hourly rainfall rates exceeding 1"/hr -- as suggested by the 12z HREF. As additional storms form this afternoon, both the HREF and REFS neighborhood probabilities depict a 30-70% chance of 6 HR QPF exceeding the 10 year ARI in the highlighted area, with embedded 20-30% probabilities of 100 year ARI exceedance focused over Northwest Arizona and Southwest Utah. This suggests that scattered instances of flash flooding are likely this afternoon as this activity expands in coverage and intensity. Considerable to significant flash flooding impacts are possible, especially over sensitive burn scars, dry washes, and slot canyons. Asherman ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-zTsaI2zTx4COxWeSeA64hbog6Nt5IWNgBAib0JjVQ8Ht57gMx9B5UDFWq2bnt2P9ObS= wKDYo2FJh206klXcxf0dIaM$ for graphic product... ATTN...WFO...FGZ...SLC...VEF... ATTN...RFC...RSA...STR...NWC... LAT...LON 38581192 37831152 36161280 35061330 34941420=20 36501592 37321410 38371312=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 3.28-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .