Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 31 2023 20:21:30 FOUS30 KWBC 312021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Fri Mar 31 2023 Day 1 Valid 16Z 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...=20 The main change to the Slight Risk was to extend it a bit eastward into Kentucky, which seemed necessary based on potential for FFG exceedance there per the HREF and higher totals in the 12Z suite of hi-res guidance. Considered an extension into West Virginia as well where FFG is lower and 24 hour QPF totals could be higher overall--but rain rates are likely to be lower there with lower instability, with the QPF occurring over multiple rounds. But did extend the Marginal eastward in West Virginia for the potential of multiple rounds priming soils that eventually exceed FFG. Farther north, also saw a good signal for exceedance of FFG in parts of the Upper Mississippi Valley, around the intersection of southern MN/northern IA/southern WI. The surface low pressure system is likely to pivot there, allowing for convection to perhaps stall somewhat before stratiform rain could come through and then could change to wintry precipitation. In coordination with the MPX/DMX/ARX offices, this is not expected to cause notable flash flooding impacts, as those areas are no longer snow covered. So kept the risk at Marginal at this point--it did have to be expanded southwest a bit from the previous issuance. See the previous discussion for more details on the synoptic pattern. Tate ....Previous Discussion... A deep layer cyclone rapidly strengthens as it moves through Iowa from Friday afternoon/evening and then weakens as it moves across portions of WI and MI Friday night into Saturday. The relatively swift progression of its attendant cold front would limit the excessive rainfall potential over the northern OH Valley and western Great Lakes. Farther south, there will be a better chance of cell training along outflow-generated effective fronts (as mid-level capping across the region should be negligible to zero) oriented quasi-parallel to the deep-layer west-southwest flow. Where instability is plentiful and the dynamics are most impressive, deep convection is expected to rapidly develop probably around local noon with quasi-discrete supercells initially expected to the preferred storm mode. Some of the strongest activity is anticipated to be across the Slight Risk area, where hourly totals of 2-2.5" and local amounts of 3-5" are possible (despite the recent downturn in QPF amounts across the available guidance). The evolution of the moisture field indicates the possibility of two skirmishes of showers and thunderstorms, which could lead to a longer duration heavy rain threat overall. Low-level moisture transport is expected to be quite impressive, as a strong low-level jet (50-70 kts at 850 mb) ushers in precipitable water values of 1.2-1.8" (above the 90th percentile for the bulk of the MS/OH/TN Valleys). Farther to the north in the Midwest and Great Lakes region, hourly rain totals will be less impressive (perhaps as high as 1.5" at times Friday afternoon and evening), 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).=20 Farther south of the Slight Risk area (into portions of the Deep South), flash flood guidance is generally higher, the dynamics of the system are less impressive, and there's a greater chance for mid-level capping when looking at 700 hPa temperatures. The 00z HREF and coordination with JKL and MRX led to some contraction of the Slight and Marginal Risk areas across eastern KY and northeast TN which appear to be relatively far removed from the best instability.=20=20=20 =20 Roth Day 2 Valid 12Z Sat Apr 01 2023 - 12Z Sun Apr 02 2023 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHEAST...=20 No notable changes were needed to the existing Marginal Risk. This still appears to be a low-end flash flooding threat for convection in the first half of the period that could cause brief high rainfall rates before the front clears offshore. See the previous discussion for more. Tate ....Previous Discussion... Deep convective activity is expected to be most robust but generally progressive Saturday morning along the southern edge of a departing deep layer cyclone moving from the Great Lakes into New England. 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).=20 =20 Roth Day 3 Valid 12Z Sun Apr 02 2023 - 12Z Mon Apr 03 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FROM THE ARKLATEX THROUGH THE ARKLAMISS... The Slight Risk was shifted a bit southward from the previous issuance, which follows the updated WPC QPF. There remains some spread in the model guidance for this time period on the axis of heaviest QPF. GFS/UKMET/GEFS mean amounts and GEFS probabilities for QPF of 1" are farther north than the 12Z CMC and 00Z EC/ECens suite--though the 12Z deterministic ECMWF came back north a bit to a middle ground position that matches the WPC QPF well. Most models indicate convective initiation in northeastern Texas but then have the storms propagating various degrees from straight east. Propagation with some southerly component would make the most sense along the instability gradient, and the 12Z CMC GEM regional seemed to have a reasonable handle on this. In any case, the risk area may shift around with time as the models hopefully converge. See the previous discussion for more details on the pattern. Tate ....Previous Discussion... A southern stream shortwave interacting with a Gulf Coast warm front along with sufficient moisture/instability is expected to produce locally heavy rainfall from eastern Texas into the Lower Mississippi Valley. Its general progression should keep amounts from becoming particularly extreme. Outside of the 00z NAM, there was pretty good agreement on a zone of low- to mid-level frontogenesis tracking roughly between the ArkLaTex and through the ArkLaMiss into central MS. Instability and low-level inflow/effective bulk shear should be sufficient for convective organization, though the 00z NAM and 21z SREF mean/CAPE spread were not sold on this area. Hourly rain totals up to 2" with local amounts in the 2-4" range are expected in this region, which should have received some heavy rainfall Friday and Friday night and saturated soils to some degree by the time this round occurs, hence the upgrade to the Slight Risk. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6HFNPiXNeAHCufZmBs3ikR30qPhh3ZP3ACHLcMOnoB80= 4JZjIsRRbdyPR8DFpcmq9EiM6WmNZFFBiWv1HW0DWu69aUc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6HFNPiXNeAHCufZmBs3ikR30qPhh3ZP3ACHLcMOnoB80= 4JZjIsRRbdyPR8DFpcmq9EiM6WmNZFFBiWv1HW0DzKDR5Cs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6HFNPiXNeAHCufZmBs3ikR30qPhh3ZP3ACHLcMOnoB80= 4JZjIsRRbdyPR8DFpcmq9EiM6WmNZFFBiWv1HW0DjteMoiQ$=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) .