Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon May 08 2023 01:01:06 FOUS30 KWBC 080101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Sun May 07 2023 Day 1 Valid 01Z Mon May 08 2023 - 12Z Mon May 08 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF IOWA AND ILLINOIS... A couple main areas of concern heading into the overnight hours...one across portions of central TN into western KY, and another over portions of IA and IL. Convection over TN/KY has taken on a northwest to southeast orientation...a setup that can often result in impactful flash flooding across this portion of the country. Going into tonight the environmental ingredients do favor some continued backbuilding of cells into this region, so periodic training convection is possible through 12z. Thus far we have not really seen quite enough alignment of cells for flash flooding...and there is still some uncertainty on how things evolve overnight. Even if activity stays more scattered in nature and never quite aligns into a classic backbuilding/training scenario...the continued periodic heavy rainfall should be enough to gradually saturate soils and result in an isolated to scattered flash flood risk. However there does remain the possibility that lower level convergence tightens some as we head into the overnight hours...resulting in a more focused area of training cells and a more significant flash flood threat. The second area of concern over IA and IL is already seeing heavy rainfall this evening, with additional rounds expected as we go through the overnight hours. A mid level wave and upper jet streak will result in an expansion of convection over southeast NE into southwest IA. This activity will then expand eastward near the warm front across IA. The expectation is that eventually this activity will grow upscale into a fairly progressive MCS by later tonight. However in the formation phase we are likely to see some cell merging occur as cells develop ahead of the main line along/near the aforementioned warm front. Isolated to scattered flash flooding is possible over far southeast NE into southern IA with this activity. More numerous flooding impacts are possible from southeast IA into portions of central IL...where we have upgraded to a Moderate risk. Portions of this area have seen heavy rainfall this afternoon from slow moving cells...and other parts of the area are experiencing heavy rainfall now as training convection has developed near the warm front. Unfortunately strong upstream forcing will continue to move into this area through the overnight hours. Thus the expectation is that convection will continue through the night, or at a minimum another round of heavy rainfall will move through later tonight after a brief break. The combination of saturating ground conditions and the expectation of another convective round suggests flash flood coverage should continue to increase through tonight, some of which could be locally significant in nature. Isolated flash flooding will continue to be a threat over north central TX for a couple more hours, but should decrease thereafter. The risk is likely peaking now, or is even just past peak at this point. Chenard 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... 21Z update... Convection will be ongoing from the Day 1 period and there are additional rounds of convection expected from the Midwest to eastern Ohio/Tennessee Valleys. Much of this part of the country has 3 hr FFG in the 1 to 3 inch range, however there are a few isolated locations as low as 0.25 inch. The latest guidance depicts 1 to 3 inches over much of the same areas as Day 1. A Marginal Risk was already in effect for this part of the country. The northwest boundary was trimmed southward across northern Illinois and Indiana while the western boundary was expanded westward to west-central Missouri. Campbell 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... 21Z update... The latest guidance has lessened the spread somewhat on the NW-SE QPF axis that is expected to setup from western Nebraska to western Missouri/eastern Kansas. The Marginal Risk area was reshaped to reflect the latest WPC forecast and QPF trends and also shifted a bit southward out of the Sand Hills. Storms during the Day 1 period across Texas will help precondition soils. Most of the guidance show scattered convection across central and eastern portions of Texas. Heavy downpours may be possible with any of the storms, thus increasing the threat for isolated flash flooding. No changes were made at this time for the Marginal Risk over Texas. Campbell 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!-bCOsOk8gygqzm21FMChCRrKC8_r5oZGuuqXsj7b5Ed2= 2Dgf110o-MlgYv3fOIHVJzebgR8WTuD2e6lld1-N5FySlRk$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-bCOsOk8gygqzm21FMChCRrKC8_r5oZGuuqXsj7b5Ed2= 2Dgf110o-MlgYv3fOIHVJzebgR8WTuD2e6lld1-N6aPmEHw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-bCOsOk8gygqzm21FMChCRrKC8_r5oZGuuqXsj7b5Ed2= 2Dgf110o-MlgYv3fOIHVJzebgR8WTuD2e6lld1-NuiWPEmI$=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) .