Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat May 06 2023 00:31:09 FOUS30 KWBC 060031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 830 PM EDT Fri May 05 2023 Day 1 Valid 01Z Sat May 06 2023 - 12Z Sat May 06 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE CENTRAL GULF COAST, CENTRAL TEXAS, NORTH-CENTRAL PLAINS, AND NORTHERN ROCKIES... ....Central Texas... Convection has initiated with CAMs/WoFS in agreement about it drifting eastward into the overnight on fairly light westerly mean deep layer flow. While there is a large hail risk given the rather dry air aloft making for steep lapse rates, the slow motion makes for a flash flood threat. The Marginal Risk was shifted south a bit to include the San Antonio metro area per latest HRRR/WoFS output. ....Southeastern Louisiana... Ongoing linear convection has pushed south of the MS Gulf Shore with the western tail currently approaching the NOLA metro. The Marginal Risk was maintained for far southeastern LA for the risk of slow moving activity backbuilding from the current line along the stationary front near Lake Pontchartrain. Ample gulf moisture and instability make for a threat of excessive rain over this vulnerable metro area. ....South-Central Nebraska to Southern Minnesota... Organized convection in western Neb and discrete super cellular activity over south-central Nebraska are progged to continue moving northeast with their boundaries interacting and leading to some repeating heavy rain through tonight ahead of a shortwave impulse currently moving out of northeast CO. Continued southerly low level flow bringing 1 to 1.25" PW and moderate instability should allow for persistence of the activity. Farther northeast, ongoing activity over southeastern MN continues to push south on a cold pool into the southerly inflow which has maintained slow moving/stalled heavy thunderstorms over southern MN that may expand south into IA. There is a localized flash flood threat there this evening. East in WI there is a lack of MUCAPE and farther north over the Twin Cities the northward drifting activity is with the mean low level flow, so it is less of a heavy rain threat than the activity to the south. ....Northern Rockies... Antecedent conditions and terrain implications continue make for some risk for rapid-onset flash flooding around the Rockies of northern ID through central MT through the rest of this evening. An upper level low over eastern OR will continue to shift anomalously moist air from southeast to northwest over the northern Rockies with some pockets of instability persisting this evening. Jackson Day 2 Valid 12Z Sat May 06 2023 - 12Z Sun May 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER MISSISSIPPI VALLEY AND NORTH-CENTRAL TEXAS... ....20Z Update... ....Lower Mississippi Valley... A Marginal Risk area was introduced in many of the same portions of LA and MS as are under threat this evening. The stationary boundary currently in place will remain parked in the same areas again on Saturday. Afternoon and evening storms are likely to develop along the boundary once again, and move southeast over flood sensitive portions of LA and MS, including Baton Rouge, New Orleans, and Biloxi. The expected convection later today will keep the soils in the area primed for additional flash flooding when the next round of storms move through Saturday. ....North Central Texas... A subtle shortwave disturbance ejecting out of the desert Southwest will race along in the southwesterly flow into north-central Texas Saturday evening. Thunderstorms are expected to break out along the same boundary as today in many of the same areas as today as a result. There's somewhat better agreement in the guidance for Saturday's storms than this evening's, that the focus for convection will be just a hair further north in Texas as compared to today. Unfortunately, this will put the Dallas-Fort Worth Metroplex in more of a target area. Given previous days' rains, expected rain in this region later today, and then yet another round again Saturday, a Marginal Risk was introduced. Just like in the lower Mississippi Valley, the meteorological conditions will be essentially unchanged again tomorrow, which will support many of the same areas seeing renewed showers and thunderstorms again on Saturday. Wegman Day 3 Valid 12Z Sun May 07 2023 - 12Z Mon May 08 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST... ....20Z Update... ....Midwest... Confidence remains low on the area of convection associated with a warm front moving over the Midwest on Sunday. Total forecasted rainfall has not changed much from the previous forecast, but the area under most concern for the potential for isolated flash flooding has shifted a little bit more to the northeast with the latest 12Z cycle of model runs. Thus, the inherited Marginal Risk area was adjusted northeastward, and now extends from central WI southeast to far western MD, and all areas in between. Low FFGs in this area are playing their part in keeping the Marginal Risk area intact, considering the weak signal in the guidance as to how strong any convection that develops in this area will be. Dew points will be around 60, give or take a few degrees across the Marginal Risk area, with a steady southwesterly flow adding atmospheric moisture to the area behind the warm front. Expect healthy instability as well, as MUCAPE values approach 2,000 J/kg Sunday afternoon and evening, so any storms that fire will have a favorable environment to work with. However, inside the broad area encompassed by the Marginal Risk, there remains little confidence as to which areas have a greater threat for flash flooding. Low FFGs in the central Appalachians suggest that area may have the highest threat, but a larger distance from the real forcing further west may negate that. Expect continued adjustments with future updates as the details become more clear. Wegman ....Previous Discussion... Opted to introduce a Marginal Risk area from portions of the Lower Great Lakes into Kentucky where various models have been developing QPF bullseyes anywhere from southern Wisconsin into Kentucky with below average run to run coherency/consistency in placement. In the broad-scale picture...there is initially mid- and upper-level ridging that will be working against much in the way of heavy to potentially excessive rainfall. The various global models latch on to an ill-defined shortwave trough embedded within the broad southwest flow to the west of the ridging axis.=20 Some of the guidance lowers heights in response to the shortwave trough to the point where convection forms underneath difluent flow late Sunday night/early Monday morning. The spread of model spread really limits confidence in any one solution...at various times there have been localized convective bull eyes over Kentucky or Illinois...while the UKMET has favored areas farther north.=20 But the NAM/GFS/ECMWF all develop a convective QPF max of 2 to 4 inch amounts somewhere in the area...felt a Marginal was warranted (even discounting the presumed wet-outlier UKMET). Further adjustments and modifications are expected in subsequent EROs/ERDs. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9WA0Jh6yXlTPt16eC2yoUD-9oYBizJOoMLzTHn_YtlfS= Bmu6vAR0zkOdd3jZAJBPo4uhwJ3S8ZubbXyhcNb6ylFtCtI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9WA0Jh6yXlTPt16eC2yoUD-9oYBizJOoMLzTHn_YtlfS= Bmu6vAR0zkOdd3jZAJBPo4uhwJ3S8ZubbXyhcNb6cq_pICY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9WA0Jh6yXlTPt16eC2yoUD-9oYBizJOoMLzTHn_YtlfS= Bmu6vAR0zkOdd3jZAJBPo4uhwJ3S8ZubbXyhcNb6qMswusM$=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) .