Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Aug 22 2024 20:14:26 AWUS01 KWNH 222014 FFGMPD AZZ000-230200- Mesoscale Precipitation Discussion 0921 NWS Weather Prediction Center College Park MD 413 PM EDT Thu Aug 22 2024 Areas affected...Southeast through Central Arizona Concerning...Heavy rainfall...Flash flooding possible Valid 222013Z - 230200Z Summary...Slow moving showers and thunderstorms will continue to expand and lift northward through the afternoon. Rainfall rates in excess of 1"/hr may produce 0.5 to 0.75 inches of rain in less than 1 hour, possibly resulting in instances of flash flooding. Discussion...The GOES-E WV imagery this afternoon indicates a weak shortwave lifting northward out of the Sonoran Desert and into southern Arizona. Downstream of this impulse, convection is blossoming in response to the accompanying ascent (aided by weak upper diffluence), and in the presence of impressive thermodynamics characterized by PWs of around 1.5 inches, above the 90th percentile for the date, and MLCAPE of 1000-1500 J/kg. As the shortwave pivots slowly northward, it will result in pinched SE flow to its east. This will drive locally enhanced convergence as 850-700mb winds merge into weak winds to the NW, while concurrently surging more robust thermodynamics northward. The overlap of this convergence into the higher PW/CAPE will likely cause an expansion of thunderstorms through the afternoon, especially when aided by the modest PVA and height falls. Rainfall rates within deepening convection have a 20-30% chance of exceeding 1"/hr according to HREF neighborhood probabilities, which could result in sub-hourly rainfall of 0.5-0.75 inches as reflected by UA WRF hourly, and HRRR 15-min, rainfall accumulations. Concerning to the flash flood risk is the slow motion of these storms as well. Although 0-6km mean winds are likely to remain from the south at 15-20 kts, Corfidi vectors aligned to the mean winds are progged to collapse to just 5 kts, also originating from the south. This suggests that storms will regenerate within the greater thermodynamics over southern AZ and lift northward, resulting in weak net motion to lengthen the duration of these intense rain rates. Where storms repeat or train the longest, HREF neighborhood probabilities suggest 1 to as much as 3 inches of rain is possible. These storms will be moving across sensitive terrain features and antecedent moist soils noted by NASA SPoRT 0-10cm RSM that is 90-98% in some areas. 1-hr FFG in these moist soils is only 0.5-0.75 inches, which has a 20-30% chance of exceedance according to the HREF. This indicates at least an isolated flash flood risk anywhere across the region through the evening, but the potential will be slightly higher should any training occur across urban areas or more vulnerable terrain features. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!-5Fss7y01ISHyRJITrhUw0hziORDRUjFrwW3TOY9Rlyo9F-9GSHwPweqMGDgDZ3O5eda= XdgBM854aNWv2Mv1Zd54IGY$ for graphic product... ATTN...WFO...FGZ...PSR...TWC... ATTN...RFC...CBRFC...NWC... LAT...LON 35471093 35441010 34770948 33690937 32500931=20 31790934 31390958 31241009 31241058 31621143=20 32581165 33451173 34581236 34921245 35141219=20 35411151=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .