Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 26 2023 08:31:47 FOUS30 KWBC 260831 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 430 AM EDT Tue Sep 26 2023 Day 1 Valid 12Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF FLORIDA AND THE MIDWEST... ....Florida... Given the 2 to 2.25 inch precipitable water values over Florida...a quasi-stationary surface front draped across the region and a mid- and upper-level trough over the Gulf of Mexico...the most active convection that forms should be able to take advantage of nearly 2000 J per kg of MLCAPE to produce some 2 or 3 inch per hour rainfall rates. With those kind of rainfall rates...instances of flash or urban flooding from this afternoon into early this evening. The previously issued Marginal Risk still appears to capture the area well. ....Midwest... Areas of moderate to locally heavy rainfall will continue to spread south- and eastward today in association with an upper level closed low. As a resuit...maintained the Marginal risk area from parts of the Upper Mississippi Valley to the western/southern Great Lakes. The above-mentioned low pressure system will continue to funnel 850mb moisture flux on the eastern flank of its circulation with PWS rising to around 1.25". MLCAPE is expected to range between 500-1,000 J/kg and mean 1000-500mb RH values are likely to average >80%. It is the slow progression of the low that could allow for training convection and elevated rainfall amounts given slower storm motions. The 26/00Z HREF still showed probabilities for 3-hr QPF > 3-hr FFGs as high as 40-45% in parts of northern IL this afternoon. Some soils are a little more sensitive than others, particularly north-central IL where AHPS 7-day rainfall totals have been as high as 400-600% of normal. Given these reasons, have chosen to introduce a Marginal Risk for parts of the western Great Lakes region this forecast cycle. Bann Day 2 Valid 12Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND OHIO VALLEY... ....Ohio Valley... Maintained the Marginal Risk over portions of the Ohio Valley as a slow moving cutoff low meanders eastward on Tuesday. Afternoon and evening convection embedded within an airmass characterized with PWATs around 1.25 inches will develop ahead of/east of the cutoff low. The slow movement of the forcing will support slow-moving/training storms. As mentioned before...even though the area has been much drier than normal, the storms will have the potential to cause high enough rainfall rates and/or locally heavy rain amounts over an urban center to make flash flooding possible, albeit in isolated instances. ....Southeast... A near carbon-copy forecast from today as there will still be a pool of 2-2.5" PWs to go along with a quasi-stationary front draped over Florida and an upper trough over the Gulf of Mexico. Wednesday features a little more instability across the Gulf side of the Florida rather than the Atlantic for now, maintained the focus for Excessive Rainfall from southwest Florida on north to southern Georgia and southern Alabama. As much as 500-1,000 J/kg of MLCAPE will be present across central and northern Florida, and with such a tropical air-mass in place, rainfall rates could approach 3"/hr in the strongest storms. A Marginal Risk for Excessive Rainfall remains in place as urbanized communities and poor drainage areas are most vulnerable to possible flash flooding. Bann Day 3 Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FLORIDA... Once again...a moist airmass will be in place across the Florida peninsula with a quasi-stationary surface front and some dynamics aloft helping to trigger late day and early evening convection over portions of the peninsula. The guidance tends to show most the convection off-shore. However...thinking is that there should be enough instability to support locally intense rainfall rates which leads to isolated flooding in urbanized communities and poor drainage areas.=20 Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!49qe5Z8gX8XzgYovIEd7YA6NQ7CfaApjnFW1OIK_GN4x= 98xu1oLdTDPsojqaUTQHM333mDxpqqpOBChERp0YgYEpGMU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!49qe5Z8gX8XzgYovIEd7YA6NQ7CfaApjnFW1OIK_GN4x= 98xu1oLdTDPsojqaUTQHM333mDxpqqpOBChERp0YFAA6mhA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!49qe5Z8gX8XzgYovIEd7YA6NQ7CfaApjnFW1OIK_GN4x= 98xu1oLdTDPsojqaUTQHM333mDxpqqpOBChERp0Yu4lp6No$=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. 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