Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 17 2023 09:59:01 AWUS01 KWNH 170958 FFGMPD INZ000-WIZ000-ILZ000-171500- Mesoscale Precipitation Discussion 1079 NWS Weather Prediction Center College Park MD 558 AM EDT Sun Sep 17 2023 Areas affected...Chicago metro area, far southeast WI, far northwest IN Concerning...Heavy rainfall...Flash flooding possible Valid 170957Z - 171500Z Summary...Showers with isolated thunderstorms will drift slowly near Lake Michigan through the morning beneath an amplified and vertically stacked trough. Rainfall rates within deepening convection will likely exceed 1"/hr at times, resulting in 1-3" of rain with locally higher amounts. Flash flooding, especially in urban areas, is possible. Discussion...The regional radar mosaic early this morning shows an expanding area of moderate rain with embedded convective elements rotating around the Chicago metro area, with stronger cells developing over Lake Michigan. This rainfall is associated with a wave of low pressure and accompanying frontal structure drifting eastward beneath a positively tilted 500mb trough and closed 700mb low. SBCAPE as analyzed by the SPC RAP has increased to 250-500 J/kg over Lake Michigan, and PWs in the vicinity are 1 - 1.1 inches. Model soundings from the HRRR indicate a deeply saturated column through 400mb with near moist-adiabatic lapse rates. The overlap of this ascent into these favorable thermodynamics is supporting efficient rainfall processes, so despite modest reflectivity, rainfall rates are being estimated via KLOT to be 0.5-1"/hr. As the morning progresses, the surface low over Lake Michigan should gradually pivot southeast dragging the cold front with it. The accompanying 700mb closed low will follow, providing subtly steeper mid-level lapse rates to enhance MUCAPE, while an impressive 850-700mb deformation axis pivots southeast to provide additional ascent into this elevated instability. At the same time, SBCAPE over the lake is progged to expand into northeast IL and northwest IN, driving an even more favorable environment for heavy rainfall. As low level convergence intensifies into this increasing instability both at the surface and aloft, this could result in intensifying rain rates for which the HREF indicates has a 30% chance of exceeding 1"/hr, with 15-min rainfall according to the HRRR possibly reaching 0.25-0.5" at times. This suggests that rainfall will become increasingly convective, with more cells likely embedded within the larger moderate rain area. Storm motions in the vicinity of this system will be quite slow, noted by 0-6km mean winds of just 5-10 kts, with Corfidi vectors aligned anti-parallel to this mean wind collapsing to 5 kts or less. This suggests enhanced training and even backbuilding across Lake Michigan which could lengthen the duration of heavy rain rates, especially near the lake shore convergence between Chicago and Milwaukee. This is reflected by HREF and RRFS exceedance probabilities for 3" reaching 25% and 35%, respectively. Locally higher totals are likely in some areas. Recent rainfall in this area has generally been below normal, but some 7-day rainfall of 150% of normal is noted in AHPS near the lake shore. This recent heavy rain has reduced FFG to as low as 1-1.5"/3hrs, for which the HREF indicates has a 15-20% chance of being exceeded. While this suggests generally an isolated risk, the greatest threat could be across the urban areas where any heavy rain rates atop the more impermeable surfaces could result in rapid runoff and flash flooding. Weiss ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!8zmht4rMHmdrzE0BynUkQ3hZezBZ1InjvThjDXT3h2TfYDykAOps_ug2nQe3frjG86fS= cud9k44oq_CHS-8RW0otHHs$ for graphic product... ATTN...WFO...IWX...LOT...MKX... ATTN...RFC...NCRFC...OHRFC...NWC... LAT...LON 43058806 42628782 42198749 41918725 41598676=20 41258676 40908705 40778738 40578794 40648833=20 40988887 41558904 42198913 42938881=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) .