Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Aug 04 2023 15:22:31 AWUS01 KWNH 041522 FFGMPD SDZ000-NDZ000-MTZ000-WYZ000-042130- Mesoscale Precipitation Discussion 0858 NWS Weather Prediction Center College Park MD 1121 AM EDT Fri Aug 04 2023 Areas affected...eastern Montana, western Dakotas Concerning...Heavy rainfall...Flash flooding likely Valid 041530Z - 042130Z Summary...Showers and thunderstorms will expand around a stacked low pressure drifting eastward today. Rainfall rates will rise to 1-2"/hr and then train slowly to produce 2-4" of rain with locally higher amounts. Flash flooding is likely. Discussion...The GOES-E visible imagery this morning shows an impressive low pressure lifting slowly across northwest SD. Although the satellite indicates widespread cloud cover north of the low, MUCAPE as analyzed by the SPC RAP is 500-1500 J/kg, with a finger of SBCAPE above 500 J/kg developing within clearing northeast of the surface low. The vertically stacked low pressure to drive some height falls and associated PVA is combining with at least modest upper diffluence within the RRQ of a jet streak to the north to provide impressive deep layer ascent, resulting in the increasingly expansive shield of rain and embedded convective elements. PWs on the 12Z U/A soundings were above record values at KBIS, and well above the 90th percentile at KUNR, with a warm cloud depth of almost 10,000 ft suggesting efficient warm rain processes. Rainfall rates have been estimated this morning above 0.5"/hr, and these will likely steadily increase through the aftn. The evolution this afternoon is almost that of a fall system, but with the addition of summertime moisture and instability, and this could result in a very impressive rainfall event today. Low/Mid level flow out of the SE downstream of the closed low will advect an anomalous theta-e ridge northward which will lift into a TROWAL over the Dakotas. Within this moisture plume, 700mb mixing ratios are at near record values, which will wring out as widespread moderate to heavy rain during periods of isentropic ascent. Additionally, an impressive mid-level deformation axis will pivot near the ND/MT/SD borders, into which moisture transport vectors converge to drive even more impressive ascent. The overlap of this strong deep layer ascent into the favorable thermodynamics will result in widespread heavy rain, with rates at times reaching 2"/hr within pockets of greater instability. This could result in a bi-modal rainfall distribution. The first area of greater flash flood risk will be along and east of the low as it moves northeast. Here, SBCAPE may rise to as much as 2000 J/kg to support 1-2"/hr rain rates, and even greater short-duration rates noted by HRRR 15-min rainfall reaching 0.75". 0-6km mean wind wil be generally SW to NE at 15-20 kts, but increasingly anti-parallel Corfidi vectors suggest efficient backbuilding to prolong the rainfall. This could result in stripes of 2-4" of rain with locally higher amounts. This will overwhelm compromised soils noted by low FFG and high USGS streamflow anomalies, resulting in instances of flash flooding. The other potentially higher risk area will be in the vicinity of the deformation axis from the Canadian border to the Black Hills. Here, although instability will be more muted due to cloud cover, the pronounced TROWAL and convergence of moisture transport vectors will allow for persistent moderate to at times heavy rain with rates reaching 1"/hr. There could also be some enhancement into the Black Hills as flow upslopes into the NE side beneath this deformation. This will likely produce 1-3" of rain with locally higher amounts which will also be falling atop primed soils. This will likely cause at least isolated instances of flash flooding. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!_oNdtAMB8pmeU_EG6cwtOEHPyCbxPxaO_hMsQ423jxuInPZ94moXz4btpgcIlcm-duiL= KvzKGDmy2TpmkFbOSHtfNn0$ for graphic product... ATTN...WFO...ABR...BIS...BYZ...GGW...UNR... ATTN...RFC...MBRFC...NCRFC...NWC... LAT...LON 49000263 48730199 48340141 47500066 46180023=20 44779989 44049997 44020048 44290131 44540196=20 44510254 44170293 43740319 43810376 44210449=20 44810471 45830495 46800505 47450509 48050484=20 48460456 48910365=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) .