Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 07 2023 15:50:58 FOUS30 KWBC 071550 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1150 AM EDT Sun May 07 2023 Day 1 Valid 12Z Sun May 07 2023 - 12Z Mon May 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST AND KENTUCKY/TENNESSEE... 16z update... A Marginal Risk area was raised for southeastern portions of Texas for the potential for heavy rain and isolated flash flooding. For additional details refer to WPC's MPD #0231. The weather pattern across the Upper Midwest/Midwest/Ohio and Tennessee Valleys region today will continue to be messy and active. There will likely be multiple rounds of convection. Given some of the lines will likely bow or dive southeast, the forward speed will be more progressive however the potential to produce 1 to 2.5 inches/hr rain rates will increase the risk for rapid runoff and flooding. Additionally, these segments will more than likely produce outflow boundaries and when/where boundaries interact go either dampen or fuel additional convective activity. The hi-res guidance is indicting 2 possibly 3 rounds of storms to move through the Mid-Mississippi/Ohio/Tennessee Valley during this period and given the potential for a few inches of rain the threat for flooding will be elevated. A Slight Risk was raised for southeast Missouri and parts of Kentucky and Tennessee. The Marginal Risk was expanded as well given to large area where storms will traverse... it spans from eastern Nebraska to far western Maryland and from the Lower Great Lakes to northern Mississippi/Alabama. Some of the newer guidance is suggesting strong to intense storms may blow up over west/central Nebraska/Kansas before moving into Missouri, so the Marginal Risk area may need additional expansions with the next update. Campbell Consensus of 1 to 3 inches across this part of the country persists from the previous run and WPC forecast, with the NAM and UKMET continuing to hint at isolated maxima of 4 to 6 inches. The operational global runs and some of the higher resolution guidance...including the HREF...have started showing about 10 percent probabilities of 1 hour rainfall accumulation in an hour exceeding 1-hour Flash Flood Guidance between 08/03Z and 08/06Z...with and 20+ percent probability of 3-hour rainfall accumulation exceeding the 3-hour Flash Flood Guidance during roughly the same time frame from portions of eastern Iowa into northern Illinois. This corresponds to the when the southwesterly flow at 850 mb ramps up to between 20 and 30 kts. Sided more with the 07/00Z GFS/NAM/UKMET rather than the ECMWF/CMC which sped a QPF max eastward closer to the OH River. Even though the EC/CMC idea can not be ruled out....the latest spaghetti plot of 2 inch contours from the SREF (from both the NMM and ARW) lends more support to solution over Iowa. To cover the possibility that the EC/CMC is right...a broad Marginal risk was maintained to signal the uncertainty rather than telegraphing the idea that the entire area is about to be drenched. Bann Day 2 Valid 12Z Mon May 08 2023 - 12Z Tue May 09 2023 ....THERE IS A MARGINAL RISK LINGERS OVER PORTIONS OF THE MIDWEST ON MONDAY... Maintained the Marginal Risk area pretty much unchanged from the Saturday Day 3 outlook. During the first part of the period...there appears to be enough CAPE lingering from the Day 1 period to help continue the potential for moderate to heavy rainfall. However...Forcing becomes less focused with time suggesting diminishing rainfall rates especially during the latter portion of the forecast period On the other hand...flash flood guidance is low (especially the 1-hour FFG values) in parts of the west facing Appalachians into parts of the Mid-Atlantic region.=20 Spaghetti plots of QPF from the SREF focus this area...although it is the ARW core which is most emphatic for 2+ inch rainfall amounts during the period from Indiana and Kentucky eastward into the West Virginia southward into the northeast corner of Tennessee while the 07/00Z GEFS 2-inch contours were widely scattered from Illinois into Ohio. So will maintain the Marginal in hopes that later runs resolve the fundamental differences that make this a forecast that is below-average in confidence.\ Bann Day 3 Valid 12Z Tue May 09 2023 - 12Z Wed May 10 2023 ....THERE ARE MULTIPLE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL OVER PORTIONS OF THE CENTRAL AND SOUTHERN PLAINS... A surface/low-level front initially draped over the central part of the U.S. will begin to shift northward and southerly low level flow moistens the airmass south of the front with a corresponding increase in instability that could support convection capable of producing heavy to potentially excessive rainfall. Some guidance initiates convection in Texas near the Gulf of Mexico while other pieces of guidance form the convection much farther west near the dryline and where orographics can become a factor. A second focus for storms should extend from Kansas into western Nebraska in proximity to the surface boundary which should be oriented from southeast to northwest by the afternoon. 00Z operational guidance shows potential for 2 to 3 inches amounts...with the Canadian showing some isolated max values over 5 inches with the Texas convection. Started with a Marginal risk until there is more run to run consistency in QPF placement. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9gSNPxT8mpetcs1F9l1IwgY1WQSSkzdccyeaXKSI7FoA= s73zmKSPVGybXquFIs4ZS-N4oBxitxJdN1F323i9-1920fk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9gSNPxT8mpetcs1F9l1IwgY1WQSSkzdccyeaXKSI7FoA= s73zmKSPVGybXquFIs4ZS-N4oBxitxJdN1F323i9MuGekS0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9gSNPxT8mpetcs1F9l1IwgY1WQSSkzdccyeaXKSI7FoA= s73zmKSPVGybXquFIs4ZS-N4oBxitxJdN1F323i9SRl1jMw$=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) .