Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun May 07 2023 00:25:37 FOUS30 KWBC 070025 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 825 PM EDT Sat May 06 2023 Day 1 Valid 01Z Sun May 07 2023 - 12Z Sun May 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHERN PLAINS AND THE MID-MISSISSIPPI VALLEY INTO THE OHIO VALLEY... ....Mid-Mississippi Valley into the Ohio Valley... The supercell over northern MO as of 00z is expected to grow upscale as it moves eastward this evening into tonight. As it grows upscale it should tend to propagate more southeastward (and possibly even southward) with time into the better instability core and aligned with Corfidi Vectors. Most model solutions show the development of a pretty progressive convective line, which would limit the extent of any flash flood risk. However with ~50kts of southwesterly flow at 850mb, and robust upstream instability, do tend to think we will see some backbuilding/training component at times on the south to southwest flank of the growing area of convection across portions of eastern MO and central/southern IL. This would support the potential for rainfall magnitudes locally exceeding model consensus values, and resulting in an isolated flash flood threat. Further north across IL/IN convection should tend to be pretty progressive in nature. Although do note a signal for potential brief training on the northern extent of the instability dome across northern IL/IN...and thus can not rule out a localized flash flood threat here as well. Thus while the heaviest rainfall rate potential is likely further south from eastern MO into central/southern IL...we maintained the northern extent of the Marginal risk into northern IL/IN to account for this secondary area of localized higher rainfall potential. ....Southern Plains... Slow moving convection near/east of the dryline will continue to pose a localized flash flood risk through the evening hours. Cells will eventually propagate eastward off the dryline, but not as quickly as some of the 12z CAMs would have suggested. The slow motions of these supercells combined with some backbuilding on the southwest flank will support the isolated flash flood risk. The risk will extend east to some extent, but probably not too far east of the dryline, and thus we were able to cut back some on the eastern extent of the Marginal risk. Chenard Day 2 Valid 12Z Sun May 07 2023 - 12Z Mon May 08 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE MIDWEST... 21Z update... Consensus of 1 to 3 inches across this part of the country persists from the previous run and WPC forecast, although very isolated areas of 4 to 6 inches can not be ruled out. The latest guidance continued with the small westward shift especially across portions of the Iowa and Missouri -the Marginal Risk area was nudged westward by 1-2 counties on the northwest side. There also continues to be some uncertainty on where the heaviest QPF will fall near the southern bounds, however consensus suggests centering over Kentucky. Only minor reshaping of the Marginal Risk to reflect the trends were made at this time. Campbell Even though there has been some convergence of ideas among the models, the forecast remains below average in confidence in placement of the heavier rainfall amounts/rates and any associated risk of excessive rainfall across the Midwest on Sunday. The trend noted on Friday afternoon of a subtle northward shift in the guidance has continued in the 06/00Z forecast cycle...although the better focus this run has been more northwest than northeast for the larger magnitude of instability and surface low pressure development with a warm front towards the east being the focus for more widespread rainfall.=20 As a result...the inherited Marginal Risk area was nudged westward...although lower FFGs on the eastern periphery of the Marginal Risk area meant smaller adjustments there. A steady southwesterly flow adding atmospheric moisture to the area behind the warm front will help boost MUCAPE values approach 2,000 J/kg Sunday afternoon and evening, so any storms that fire will have a favorable environment to work with. How the different streams interact with each other and the timing of weak shortwave energy that can help trigger and focus convection remains far from clear, Expect continued adjustments with future updates as the details become more clear. Bann Day 3 Valid 12Z Mon May 08 2023 - 12Z Tue May 09 2023 ....THERE IS A MARGINAL RISK LINGERS OVER PORTIONS OF THE MIDWEST ON MONDAY... 21Z update... Very minor adjustments were made to the Marginal Risk area to reflect the latest trends. Still looks like some of the highest totals may focus along the Ohio River as MCSs pass through. The general consensus is for areal averages of 1 to 3 inches while the NAM continues to suggest 4 to 5+ in spots.=20 Campbell The threat for potentially moderate to heavy rainfall...with the risk of excessive rainfall given areas of low flash flood guidance...lingers into the early portion of the Day 3 period across the Midwest. With the mid-level flow becoming more westerly...and even west northwesterly by the end of the period...the magnitude of instability should gradually wane and be shunted southward by later Monday night and early Tuesday. Until that happens...the models were still able to generate some QPF bullseye values where 1000 to 850 mb moisture transport values are 1.5 to 2 standardized anomalies greater than climatology into a region where the precipitable water values are already above normal. NCEP models tended to be more bullish than the ECMWF/UKMET in terms of 24-hour rainfall...with the NAM still able to generate 3 to 5 inch amounts in spots. So kept a Marginal Risk into Day 3. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6bZj2PJnkv0tZeAjH8K9r5yRq876E697cpQaPqzqxwr6= 0AnZdt6z4tnfkZ-5eyMn_fWFmji_M9iCeSqH3pSvd6dK8Yw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6bZj2PJnkv0tZeAjH8K9r5yRq876E697cpQaPqzqxwr6= 0AnZdt6z4tnfkZ-5eyMn_fWFmji_M9iCeSqH3pSvlyJzJN8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6bZj2PJnkv0tZeAjH8K9r5yRq876E697cpQaPqzqxwr6= 0AnZdt6z4tnfkZ-5eyMn_fWFmji_M9iCeSqH3pSvAptbNpo$=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) .