Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Feb 21 2023 15:46:57 FOUS30 KWBC 211546 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1046 AM EST Tue Feb 21 2023 Day 1 Valid 16Z Tue Feb 21 2023 - 12Z Wed Feb 22 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Local radar imagery over Mississippi and Alabama this morning showed a couple of narrow axes of higher reflectivity, oriented west to east, correlating to MRMS-derived and at least one gauge confirmed (Wunderground) hourly rainfall total in excess of 1 inch. Cloud tops on infrared imagery and 12Z soundings from JAN and BMX indicated the showers were largely warm rain based with little lightning driven by shallow instability predominantly below 500 mb. Low level convergence near 925 mb was helping to focus these showers. Short term training could conceivably support 2 inches of rain in 1 to 2 hours, which given above average soil moisture across Mississippi and Alabama, could result in very localized flooding, including any overlap with any urban areas. This threat is too low to include a risk area on the outlook and the showers/localized thunderstorms are expected to translate eastward into Georgia (drier soils) by late afternoon or early evening, coinciding with weakening low level winds and convergence. Otto Day 2 Valid 12Z Wed Feb 22 2023 - 12Z Thu Feb 23 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEASTERN MISSOURI TO SOUTHWESTERN MICHIGAN... A strong shortwave trough will eject northeastward from the southern Plains (initially over west Texas at 12Z Wednesday) northeastward toward the Midwest and southern Great Lakes through 00Z Thursday. Meanwhile at the surface, one or two weak low pressure areas will organize along a stout baroclinic zone extending from northern Missouri through northern Illinois and southern Michigan. This boundary will likely only move slowly for most of the day, but begin to retreat as a warm front after 06Z as strong low-level cyclogenesis occurs over northern Missouri/Iowa late. The combination of lift associated with the ejecting shortwave, stationary low-level boundary, strong low-level moist advection (with upper 50s F dewpoints and 1.3 inch PW values in the open warm sector) will result in scattered showers and thunderstorms initially over western Illinois and vicinity (near the mid-Mississippi River Valley). These storms will likely evolve into a cluster that will migrate east-northeastward along the low-level boundary (where convergence will be maximized), potentially reaching southwestern Michigan in the evening. Model depictions of the axis of convection (parallel to both low-level/initating boundaries and southwesterly flow aloft) suggest that localized areas of training are probable especially across the northern half of Illinois during the day time. The degree of surface-based destabilization is in question, and the greatest concentration of convection is likely to focus along the boundary where inhibition will be weakest (and forcing strongest). 1-3 inch rainfall amounts are expected within this axis (locally higher), which will overlap areas of ~1 inch FFG thresholds and dry/barren soils supporting efficient runoff. These factors all support the ongoing Slight risk especially in Illinois. One potential caveat for the northern extent of flash flood risk involves 1) the northern extent of the surface boundary and 2) the presence of sub-freezing temperatures collocated with some of the convection - especially closer to the Illinois/Wisconsin/Iowa border area and vicinity. Should these boundaries end up farther south than depicted in current model guidance, the attendant flash flood threat will lessen due to lower runoff and freezing precipitation on trees and exposed surfaces. Farther east into Indiana, uncertainty exists with respect to convective evolution overnight and it appears that the risk of training storms should be less widespread/more localized compared to points farther northwest. The Slight Risk for this area has been nudged westward to account for this uncertainty but may need to be re-introduced later pending convective trends and ultimately the placement of any surface boundaries across the region. Lastly, more isolated convective training potential will exist in a few spots across northeastern Oklahoma and Missouri in the early morning hours (12-18Z) and very late in the period across southern Michigan and northern Ohio. The Marginal risk area remains in place to address lower-end threats in these areas, although trends will be monitored for a southwestern extension of the Slight risk in Missouri especially if models/CAMs trend stronger/more numerous with early-day convection in that area. Cook Day 3 Valid 12Z Thu Feb 23 2023 - 12Z Fri Feb 24 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Cook Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8Ov4fbrfJGAaW2OKI_hRz9gmn9edREa1W7eFo3iJfMCw= nlqWCwUGllSipbVKia4qO4VqS1i5M03DAaX0TgZMeXKTrZI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8Ov4fbrfJGAaW2OKI_hRz9gmn9edREa1W7eFo3iJfMCw= nlqWCwUGllSipbVKia4qO4VqS1i5M03DAaX0TgZM039boQ0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8Ov4fbrfJGAaW2OKI_hRz9gmn9edREa1W7eFo3iJfMCw= nlqWCwUGllSipbVKia4qO4VqS1i5M03DAaX0TgZMyKuhMYo$=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) .