Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Apr 27 2025 22:51:57 AWUS01 KWNH 272251 FFGMPD MTZ000-WYZ000-280430- Mesoscale Precipitation Discussion 0186 NWS Weather Prediction Center College Park MD 650 PM EDT Sun Apr 27 2025 Areas affected...Northwest WY...South-central MT... Concerning...Heavy rainfall...Flash flooding possible Valid 272250Z - 280430Z SUMMARY...Increasing convective activity along/south of sharp stationary deformation zone will allow for slowly increasing rainfall rates and localized totals up to 2" through early overnight period. Rain over snow pack will further increase runoff and the potential for flooding conditions. DISCUSSION...An anomalous deep closed low across the Great Basin has a strong vorticity center rotating along the northeastern quadrant across the north-central Rockies of WY and ID. Both GOES WV suites show a highly divergent mid to upper-level pattern across southern MT enhancing the deformation shear axis from central ID across SW to central MT. A similarly sharp 500-1000 thickness ridge extending across SE MT to the northern High Plains, suggestive of very slow propagation vectors and convergent deep layer steering flow along the downshear side of the deep upper-low in the area of concern. Strong dynamic ascent will need to overcome limited instability warm/moist air in the lower profile and utilize slant-wise ascent toward the deformation zone. However, a ribbon of low to mid-level moisture has been trying to bleed through the terrain of south-central MT into northwest WY with Tds into the low 40s supporting .25 to .5" low level PWat values.=20 As the main forcing lifts north an 850-700mb cyclone is deepening across northern WY which has further strengthened low level northeasterly flow convergent with southerly flow along/ahead of the synoptic cold front starting to press through W WY/E UT at this time. So solid moisture flux will continue from the east and deeper source throughout the evening into the early overnight period. Already, the deep layer convergence is utilizing available moisture with convective towers seen breaking through the cirrus canopy across south-central MT with increasing lightning noted. Given the stationary deformation zone and increased convergence, cell motions will be limited or even stationary with potential of increasing upstream mergers throughout the evening. So while intensity of .25-.5"/hr is expected (with some HREF probability nearing 50% for 1"/hr rates, driven mainly by the NAM-Nest) these rates are falling in proximity to some warming snow pack that will also add to the runoff and potential flooding concerns through the evening with localized totals of 1-2" possible. Even where snow has melted, NASA SPoRT 0-40cm soil saturation values are running well above normal and over 70-80% across areas north of YNP. As such, localized flooding is considered possible through the evening into early overnight hours. Gallina ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_7trUiFzDZpdFHVCPwZIM83ppYryUyQRRhtOIDLZs72pgPSCQ5aGJYYx-4XZC61TNV4I= ZVaYMRSxOdfLAyZLOEcaukY$ for graphic product... ATTN...WFO...BYZ...RIW...TFX... ATTN...RFC...MBRFC...NWC... LAT...LON 46790966 46640866 46060778 45300753 44850810=20 44120844 43910862 43860911 44090974 44481043=20 44781126 44911207 45341225 45801200 46321153=20 46631078=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) .