Subj : MESO: Nws Weather Prediction Center College Park Md To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 19 2023 05:17:04 AWUS01 KWNH 190517 FFGMPD TNZ000-KYZ000-ILZ000-MOZ000-ARZ000-191200- Mesoscale Precipitation Discussion 0761 NWS Weather Prediction Center College Park MD 116 AM EDT Wed Jul 19 2023 Areas affected...much of MO...southern IL...western KY...portions of West and Middle TN Concerning...Heavy rainfall...Flash flooding likely Valid 190600Z - 191200Z Summary...Areas of significant flash flooding are likely to develop overnight with localized totals of 3-5"+ expected by dawn. Locally catastrophic flash flooding is possible, particularly in the vicinity of Paducah (including surrounding portions of MO/IL/TN). Discussion...Convection has begun to initiate in the vicinity of the MO/IL/KY Tristate area, very near where the consensus of the 00z HREF guidance indicated (and shortly after a line of cumulus became evident via GOES-East infrared imagery, likely near an old outflow boundary from earlier convection). While a surface front has been analyzed by WPC about 50 miles north of where initiation has occurred, the convection is likely a bit elevated, as it is coming about from strong boundary layer moisture convergence via isentropic ascent along (and near) the 310 Kelvin surface (with flow increasing to 30-40 kts). This serves as a much better proxy of the low-level jet (as compared to a isobaric surface of 850 mb), and convection is likely to proliferate upstream into central MO along this convergence axis. The mesoscale environment across this broad convergence axis is characterized by an ML CAPE gradient of 500-2000 J/kg (increasing to 1000-2500 J/kg), precipitable water values that are rapidly increasing to 1.8-2.3 inches (near the max moving average, per SGF and BNA sounding climatology), and strong deep layer shear of 40-50 kts. There is increasing concerns for significant to potentially catastrophic flash flooding developing overnight and into the early morning in association with training convection, as the strengthening low-level jet looks to supply ample moisture transport to support long-lived deep convection in an increasingly unstable and exceedingly moist tropospheric column. As expected, the 00z HREF depicts a concerning evolution, with the probability-matched mean QPF indicating 3-5"+ localized totals in the MO/IL/KY Tristate area (and edging into portions of northern West and Middle TN) for the 6-hr period ending at 12z. The associated 40-km neighborhood exceedance probabilities for 3" and 5" thresholds indicate a maxima of 30-40% and 5-10%, respectively. While probabilities are a bit lower for the 3" threshold to the west across much of central MO (generally 20-30%), very interestingly the 5" probabilities are actually higher just south of Jefferson City (with a maxima of 15-25%). This is due to a bi-modal distribution amongst the HREF members, with the most intense members (the ARW2 and the NAM-nest) having the best QPF and training axis farther upstream. Given the current observational trends, this scenario seems less likely.. and the probability-matched mean QPF accurately depicts this discrepancy, only indicating 2-3" localized totals in the vicinity of the Jefferson City. Even still, this amount of rainfall will be enough to cause at least scattered instances of flash flooding across central MO, as Flash Flood Guidance (FFGs) are indicated to be as low as 1.5-2.0" (over 3-hr) across the region. Downstream into the Tri-State area, FFGs are generally 2.0-3.0" (over 3-hr). Despite the fair amount of spread in the 00z CAM guidance (and the likely underdone QPF amounts from the non-ARW2/NAM-nest members for the primary area of concern), scattered to numerous flash floods are considered likely along the majority of the expected training axis (from central MO south-southeast into north-central portions of West and Middle TN). The highest confidence for flash flooding exists across the aforementioned Tristate area where repeating cells with an increasingly linear orientation are already developing. With MRMS already indicating 15-min totals approaching 1". Hourly accumulations will quickly ramp up to 2-3"+ where storms train most efficiently, and this could very well result in localized totals of 5" or greater before dawn. Therefore isolated to scattered instances of significant (i.e. life threatening) flash flooding are considered likely, and there is the potential for locally catastrophic flash flooding should these short-term totals of 5"+ be realized. While convection should be coming to an end across central MO by 12z, the HREF indicates that backbuilding of storms may continue in the Tristate area (and into downstream areas of TN) where subsequent MPDs will likely be needed. Churchill ....Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!= DZ3fjg!7DHa51qlzO2Dixy-N4FRE5BG03W3knMVZed6QvuDv5h03FcfRdbcXvrLEiVRrSfmK5kI= mWr76uaQBKKDW3ooMUTLq-s$ for graphic product... ATTN...WFO...EAX...LMK...LSX...LZK...MEG...OHX...PAH...SGF... ATTN...RFC...LMRFC...MBRFC...NCRFC...OHRFC...NWC... 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