Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 30 2023 08:34:32 FOUS30 KWBC 300834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 433 AM EDT Thu Mar 30 2023 Day 1 Valid 12Z Thu Mar 30 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... A vigorous deep-layer trough traversing the Great Basin and Southwest today will lift across the central-southern Rockies and High Plains tonight and early Friday. Increasingly difluent upper-levels across the Upper Midwest will make for a more favorable environment for deep-layer ascent, particularly later tonight as the the models depict an enhanced right-entrance region of a departing northern stream jet streak (of 130+ kts). A rapid increase in low- to mid-layer moisture transport from the western Gulf of Mexico is expected, as 850-700 mb moisture flux anomalies climb quickly to between 4 and 5 standard deviations above normal tonight into early Friday (on the heels of robust SSW 850 mb flow of 50-60 kts). On the nose of that strong flow, enough instability (500-1000 J/kg of MUCAPE) and convergence will materialize to support deep convection that may become surface-based along the warm front across northeastern Nebraska and vicinity this evening before migrating east-northeastward into a slightly more stable airmass. The orientation of convection could allow for spotty areas of convective training, with FFG thresholds generally around 1"/hr supporting isolated potential for excessive runoff. The main limitation to a higher excessive rainfall threat is the transient upper flow pattern, with the progressive closed 500 mb low and associated surface frontal features (along with relatively dry antecedent conditions, per NASA SPoRT-LIS 0-40 cm soil moisture anomalies at or below the 10th percentile for much of the region). Such a pattern, one without the benefit of more prolonged low-mid level frontogenesis, would limit the risk of cell training (though that starts to become more of a factor going into Day 2). Churchill/Hurley/Cook Day 2 Valid 12Z Fri Mar 31 2023 - 12Z Sat Apr 01 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE ARK-LA-TEX, MID-SOUTH, AND LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS... The guidance remains in fairly good agreement with the timing of the compact mid-upper level trough traversing the Central Plains into the Upper Midwest from Friday afternoon/evening into early Saturday. The relatively swift progression of this feature will limit the excessive rainfall potential over the northern OH Valley and western Great Lakes; however, farther south, under a more confluent, zonal mid-upper flow, there will be a better chance of cell training along outflow-generated effective fronts oriented quasi-parallel to the deep-layer westerly flow. Where instability is plentiful and the dynamics are most impressive, deep convection is expected to rapidly develop with quasi-discrete supercells initially expected to the preferred storm mode. Some of the strongest activity is anticipated to be across the newly introduced Slight Risk area, where hourly rates of 1-2"+/hr are possible (with these hourly rates largely driving the flash flood threat, as the bulk of the forecast precipitation is expected over a period of 3-6 hours. Low-level moisture transport is expected to be quite impressive, as a large low-level jet (50-70 kts at 850 mb) ushers in precipitable water values of 1.2-1.8 inches (above the 90th percentile for the bulk of the MS/OH/TN Valleys). Farther to the north in the Mid-West and Great Lakes region, rainfall rates will be less impressive (perhaps as high as 1.5"/hr at times), but the prolonged nature of the rainfall may lead to some higher areal average totals (closer to ~2", though localized totals will likely be higher to the south). The Slight may need to be expanded northward in future cycles, which may be partially dependent on how rainfall totals evolve late on Day 1. Farther south of the new Slight Risk (into portions of the Deep South), flash flood guidance is generally higher and the dynamics of the system are less impressive (so an expansion of the Slight Risk southward is less likely). Churchill/Hurley Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9KODfc-Tue3vUJLWSSgVSQkTcyUDnwXNftl7_wtlbvyX= tMNXD2KpOeH-IsS00Q02OBSMFJgO-BqrVjFaxgNbQdBgvLM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9KODfc-Tue3vUJLWSSgVSQkTcyUDnwXNftl7_wtlbvyX= tMNXD2KpOeH-IsS00Q02OBSMFJgO-BqrVjFaxgNboJ_yZnY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9KODfc-Tue3vUJLWSSgVSQkTcyUDnwXNftl7_wtlbvyX= tMNXD2KpOeH-IsS00Q02OBSMFJgO-BqrVjFaxgNbgj-okcc$=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) .