Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 20 2023 21:44:57 AWUS01 KWNH 202144 FFGMPD AZZ000-UTZ000-NVZ000-CAZ000-210343- Mesoscale Precipitation Discussion 0953 NWS Weather Prediction Center College Park MD 543 PM EDT Sun Aug 20 2023 Areas affected...Central and Northern AZ...Southwest UT...Southeast NV Concerning...Heavy rainfall...Flash flooding possible Valid 202143Z - 210343Z SUMMARY...Scattered bands of heavy showers and thunderstorms are expected to develop over the next couple of hours across central and northern AZ which will likely expand into southwest UT heading through the balance of the afternoon and into the evening hours. Areas of flash flooding will be possible and the highly sensitive slot canyon areas will need to be closely monitored. DISCUSSION...The afternoon GOES-W visible satellite imagery shows an expanding CU/TCU field across central and northern AZ with convective initiation imminent across these areas as sufficient levels of solar insolation and plenty of low-level moisture results has resulted in a rather steady increase in instability. Overall, the instability is still rather modest with SBCAPE values as high as 1000 J/kg, but some additional uptick in instability is expected over the next couple of hours and this coupled with increasingly strong wind fields/shear around the eastern flank of Tropical Storm Hilary will begin to favor an increase in convection. Additional support for convective development is expected in association with smaller scale differential heating boundaries that have set up given the deeper layer cloud deck associated with Hilary just off to the west over adjacent areas of the Lower CO River Valley. Over the next several hours heading into the evening time frame, there will be stronger forcing/convergence noted in the 850/700 mb layer from areas of central AZ up through southwest UT and southeast NV as Hilary rapidly lifts north across southern CA and stronger wind fields pivot around the northeast flank of Hilary's circulation. Speed convergence and strong southeast moisture flux will favor convection that will tend to become aligned in relatively linear, but broken bands, and the individual convective cells will be capable of producing very heavy rainfall rates. Analysis of the last few runs of the experimental WoFS suggests some of the stronger cells that do develop will be capable of producing as much as a 0.50" to 1" of rain in as little as 15 minutes, and the 18Z HREF does show some probabilities locally for some 1" to 1.5" totals in an hour going through 03Z. Given the concerns for some repeating cell activity, and the upslope flow locally over rugged terrain, some storm total amounts may potentially reach upwards of 2 to 2.5 inches by mid-evening. Expect there to be concerns for some flash flooding as a result, and the area slot canyons will need to be closely monitored for enhanced impacts which will include Zion National Park. Orrison ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!56s49PEyQ9-n3tNZWJS30ZQcCu2YztEbz8vbHL52cvszoVUYHvaTb6HzYAnMB27wATnR= nJXksmIxhPt5p0p82SlHS3g$ for graphic product... ATTN...WFO...FGZ...GJT...LKN...PSR...SLC...TWC...VEF... ATTN...RFC...CBRFC...CNRFC...NWC... LAT...LON 38821343 38531200 37731111 35811042 34700989=20 33520992 33271148 33351294 33851422 35311466=20 36111454 36861491 37621559 38221614 38711494=20 =20=20=20=20=20=20=20=20=20=20=20=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .