Subj : MESO: Heavy rain - floodi To : All From : Mike Powell Date : Wed Jun 14 2023 17:21:00 AWUS01 KWNH 142028 FFGMPD COZ000-WYZ000-UTZ000-IDZ000-150230- Mesoscale Precipitation Discussion 0483 NWS Weather Prediction Center College Park MD 427 PM EDT Wed Jun 14 2023 Areas affected...Utah, southwest Wyoming, northwest Colorado Concerning...Heavy rainfall...Flash flooding possible Valid 142030Z - 150230Z Summary...Showers with isolated thunderstorms will expand across the Great Basin and Inter-mountain West this afternoon ahead of a cold front. Rainfall rates in the most intense convection could reach 1"/hr at times, and storms are likely to move very slowly. This will produce 1-2" of rainfall in some areas which could yield isolated instances of flash flooding. Discussion...The GOES-E visible imagery this aftn shows mid/high level clouds eroding, with Cu/Cb developing within cloud breaks. This evolution is a result of increasing ascent downstream of an approaching cold front and associated anomalous mid-level trough digging out of the Pacific Northwest, with an accompanying poleward streaking jet driving downstream upper diffluence. Broad SW flow downstream of the trough is pumping moisture from the Pacific into the region noted by GPS measured PWs as high as 0.7 inches, which is combing with SPC RAP analyzed SBCAPE of 1000-1500 J/kg to fuel the developing convection. Recent estimated rainfall rates from KMTX have exceeded 0.5"/hr, with estimated storm motions generally to the east at around 10 kts, matching the RAP 850-300mb mean wind fields. The high-res models are in generally good agreement that convection will expand in coverage through the aftn as weak shortwaves embedded within the broad SW flow lift atop the region. This subtly enhanced lift within already pronounced ascent will likely yield more significant coverage and intensity as the thermodynamics remain favorable. 0-6km mean winds and generally aligned Corfidi vectors are progged to just be 5-10 kts, indicating the potential for slow moving and short-term training from any of this activity. With rain rates progged by the HREF to be 0.5-1"/hr, and HRRR sub-hourly accumulations forecast as high as 0.25-0.5"/15 minutes, these slow moving storms could produce 1-2" of rainfall in some areas despite the anticipated scattered nature of the convection in the pulse environment. Rainfall during the last 7-14 days has been widely scattered across UT/WY, but in some areas has exceed 150% of normal. Despite this, USGS streamflows are still generally slightly above to well above normal, indicating soils that will be generally more hydrophobic to result in more rapid runoff. 1-hr FFG of 0.75-1 inch has a 20-25% chance of being exceeded according to the HREF, but any flash flooding would be most likely should the heavy rain rates occur atop sensitive terrain features like burn scars or canyons. Weiss ATTN...WFO...GJT...PIH...RIW...SLC... ATTN...RFC...CBRFC...MBRFC...NWRFC...NWC... LAT...LON 42671066 42580918 42200818 41450805 40000873 38900975 38381064 38111198 38011295 38041357 38551390 39761403 41031366 41811288 42541166 = = = --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .