Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 26 2023 15:54:50 FOUS30 KWBC 261554 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1154 AM EDT Tue Sep 26 2023 Day 1 Valid 16Z Tue Sep 26 2023 - 12Z Wed Sep 27 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF FLORIDA AND THE MIDWEST... ....16Z Update... There is very little change to the previous D1 ERO. The overall synoptic evolution is still on track with the ULL progression over the Midwest and the surface trough positioning across the Gulf Coast. The main mesoscale factor(s) will continue to be outflow and sea breeze propagation within the afternoon and early evening time frame across FL. 12z sounding from KJAX showed a tall, skinny CAPE signature within a zone of very high PWATs running between 2.1-2.2" which is within the 1.5 deviations above normal climatologically. The primary target will be within the urban corridor stretching from Savannah down through Jacksonville into northern Daytona Beach. Weak ridging to the north will shift winds near the coast out of the northeast with sea breeze progression inland after 18z. Surface trough to the east, in tandem with a weak shortwave traversing overhead has created a conglomeration of heavy rainfall within the Panhandle thanks to a solid convergence zone from Tallahassee and points southwest. Expecting the convective energy remnants to shift eastward and interact with the sea breeze located over northeast FL which will generate a second area of very heavy rainfall within the aforementioned corridor. 12z HREF neighborhood probabilities for both 2" and 3"/hr are incredibly high within the northeast FL Panhandle up into southern GA with 2"/hr signals exceeding 60-70% for a large chunk of the area and 3"/hr potential between 40-50% across Jacksonville and surrounding suburbs. Local FFG's are still high considering FL standards, but even so the risk will be higher within that corridor today given the expected interaction of convergence within a zone of deep tropical moisture and modest instability. Urban flooding will the primary focus due to the runoff potential, but even some areas within the coastal plain will have an opportunity for flash flooding concerns if training storms becomes an issue. Only adjustment made for FL was to bring in the western fringes of the MRGL to match current radar trends and 12z HREF blended mean of anything >1.5". Upper low over the Midwest will continue to pivot southeast with convective development underneath the closed circulation and within the difluent area downstream of the main low. PWATs are elevated with 12z sounding out of KDVN indicating moisture extending pretty solidly through the boundary layer, up towards the tropopause. Strong ascent under the ULL will generate plenty of lift within a zone of modest instability to create a scattered heavy rainfall risk within the QPF footprint. Hi-res deterministic was consistent in the axis of where the heaviest rain would fall, even extending a little more into northwest IN and southwestern corner of MI. Considering the urbanized area surrounding Lake Michigan, this is the primary area of flash flooding potential, but thunderstorms across northern IL over into northern IN will create isolated flash flood risks as well thanks to hard soils from very low soil moisture (0-4%) as depicted by the latest NASA SPoRT soil moisture analysis. 12z HREF neighborhood probability of 2"/3 hrs indicates areas of 15-20% with a max of 30-40% near the Quad Cities later this afternoon. This was sufficient for maintenance of the MRGL risk with little need to deviate from the previous forecast.=20 Kleebauer=20=20 ....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 result...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!-M1Jy4LXz3-ZPsVnP8EfZjq_gMcWbqtOxB1QVA9BEopB= 1OjVvT4WIpNEV6R8kBFWFmGSF09mg--sP8woE51tvloRRGs$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-M1Jy4LXz3-ZPsVnP8EfZjq_gMcWbqtOxB1QVA9BEopB= 1OjVvT4WIpNEV6R8kBFWFmGSF09mg--sP8woE51t8U3NE3c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-M1Jy4LXz3-ZPsVnP8EfZjq_gMcWbqtOxB1QVA9BEopB= 1OjVvT4WIpNEV6R8kBFWFmGSF09mg--sP8woE51te2nTQu4$=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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