Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Apr 01 2023 00:56:04 FOUS30 KWBC 010055 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 855 PM EDT Fri Mar 31 2023 Day 1 Valid 01Z Sat Apr 01 2023 - 12Z Sat Apr 01 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE LOWER MISSISSIPPI/OHIO/TENNESSEE VALLEYS...=20 Continued to shrink portions of the Slight and Marginal Risk areas largely based on trends in late afternoon and early evening radar/satellite imagery. Surface reports and MRMS tended to show higher rainfall rates around the western/southern portion of the Great Lakes...but the storms with the heaviest rates have remained progressive and had not had too much repeat convection or training given the orientation of the storms to the mean flow. Even so...observed rainfall amounts in and around Chicago were close to an inch in the 6-hours ending at 01/00Z suggesting that occurrences of flooding were possible given the flash flood guidance in the area so changes to the on-going Marginal Risk area was minimal.=20 Farther south, changes to the size and placement of the Slight and Marginal Risk areas were based on short-term radar and satellite trend. Convection was more scattered compared with coverage close to the Great Lakes, but locally heavy rainfall was still occurring. And convection tended to be oriented closer to the mean wind which was setting up the potential for some training...but the changes made at 01/01Z were fairly minor over the Tennessee and Ohio Valleys compared with the previous outlook. Bann 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!7xnFIXJCG2kg0woSI_gqZFk3c5tnAHL5kzic4lbCPrUZ= dAEfBVMs7J-SSmQ6Xy63XdgQKequiKRAzOdzUcTEqgUUrfU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7xnFIXJCG2kg0woSI_gqZFk3c5tnAHL5kzic4lbCPrUZ= dAEfBVMs7J-SSmQ6Xy63XdgQKequiKRAzOdzUcTEc-x04K0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7xnFIXJCG2kg0woSI_gqZFk3c5tnAHL5kzic4lbCPrUZ= dAEfBVMs7J-SSmQ6Xy63XdgQKequiKRAzOdzUcTEX4AF6us$=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) .