Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 31 2023 00:55:15 FOUS30 KWBC 310055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 854 PM EDT Thu Mar 30 2023 Day 1 Valid 01Z Fri Mar 31 2023 - 12Z Fri Mar 31 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PARTS OF THE UPPER MIDWEST... Continued to trim some territory from the Marginal Risk area in the Upper Midwest given the proximity of wintery precipitation especially in northern Wisconsin. 00Z surface observations still show dewpoints high enough to support rain remained well south of an east-west warm front...but close enough to support several run of CAM guidance for some enhanced rainfall amounts immediately north of the warm front in a region of frontogenesis and warm advection in the low levels. The latest HRRR runs show at least some hint at some precipitation amounts over 0.5 inches late evening or overnight where precipitation type is expected to be in the form of rain. More issues could arise if the warm front ends up being farther north and brings an added component of snow melt. At 00Z...a complex area of low pressure was beginning to organize over the Western High Plains as a vigorous deep-layer trough makes its way out of the Great Basin and crosses the Rockies, 35 to 50 kt low level winds over Nebraska and Iowa will continue to advance northward and ride over the surface warm front...helping transport enough instability and boost precipitable water values where rainfall rates could support isolated excessive runoff. Bann 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 MID-SOUTH AND LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS... ....2030Z Update... The overall meteorological pattern described below still looks on track for this update cycle, with the potential for quick-hitting storms but with high rain rates, as initially possibly more discrete convection is likely to organize by evening/night into a cold frontal squall line. This squall line should be mainly progressive farther north but with the most potential for the front to "lay down" a bit, becoming more southwest to northeast or even west-east oriented rather than south to north, across the Mid-South (southern Tennessee into northern Mississippi and Alabama). This could lead to more training of convection, and indeed the highest probabilities for elements like QPF to exceed thresholds of an inch, QPF to exceed FFG, etc. in the first run of the HREF to go through this full period are also maximized there. Thus the Slight Risk was maintained across these regions, though it was trimmed back on the northwestern side per the newer model guidance and WPC QPF. Additionally, the northwestern side of the Marginal Risk was shaved off due to QPF being predominately snow/ice. See the previous discussion for more details on the pattern. Tate ....Previous Discussion... 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 Valid 12Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST... ....2030Z Update... No notable changes were needed to the day 3 ERO. The tail end of the cold front and associated convection along/ahead of it could cause isolated flash flooding issues for parts of the Southeast, parts of which have above average soil moisture in place. See the below discussion for more details. Tate ....Previous Discussion... Heading into Saturday morning, deep convective activity is expected to be most robust along the southern edge of a departing robust mid-level short wave (moving from the Great Lakes into New England, only lifting slightly in the process). The best overlap of available moisture and instability looks to be from south-central AL into GA/SC and the northern FL Panhandle. Some 1-2"+ totals may occur over a relatively short period, with the FL Panhandle and surroundings most likely to experience localized training along the tail end of a cold front. The rapid movement of the convection will likely limit the potential for flash flooding to isolated/localized areas (though relatively wet antecedent conditions are noted, per NASA SPoRT-LIS 0-100 cm moisture anomalies as high as the 90th percentile across a portions of the region, which may necessitate a future targeted Slight Risk upgrade). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4l4YXWoByaMIzJYtM--NbZhyWczbN3BcMiJLMEm0Mg3H= k8flFU7GB3qMRluLhMHwqcotWO3TttbSZhFbUBAUhY0u5c0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4l4YXWoByaMIzJYtM--NbZhyWczbN3BcMiJLMEm0Mg3H= k8flFU7GB3qMRluLhMHwqcotWO3TttbSZhFbUBAUc9_Bzeo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4l4YXWoByaMIzJYtM--NbZhyWczbN3BcMiJLMEm0Mg3H= k8flFU7GB3qMRluLhMHwqcotWO3TttbSZhFbUBAUL3EoHSU$=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) .