Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Wed Sep 27 2023 09:51:00 FOUS30 KWBC 270839 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 439 AM EDT Wed Sep 27 2023 Day 1 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... A slow moving mid- and upper-low will continue to make its way eastward from the Mid-Mississippi Valley early this morning into Indiana or far southern Michigan by late this coming night. Weak and difficult to time shortwave energy...but significant in helping trigger and focus convection...will be rotating around the low as it tracks eastward during the period. South to southwest low level winds will help draw deeper moisture back into the area with precipitable water values increasing to around 1.25 inches within the outlook area ahead of the approaching dynamics. Deterministic guidance still shows a ribbon of elevated MLCAPE between 1000-1500 J/kg. The 27/00Z HREF mean showed another increase in QPF along an east to west axis across parts of Kentucky on Wednesday evening...which looks plausible for a region of training on the south side of the low. As a result...expanded the Marginal risk a bit south and westward here. Overall...though...the not many changes were needed to the previous Day 2 ERO as it becomes the current Day 1 ERO. ....Southeast U.S.... A near carbon-copy outlook of recent days given 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 2 Valid 12Z Thu Sep 28 2023 - 12Z Fri Sep 29 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF FLORIDA AND OHIO VALLEY... ....Ohio Valley... Maintained the Marginal Risk area that was introduced on Tuesday afternoon with only a few adjustments. The latest runs of numerical guidance continue to show increasing ascent ahead of an approaching longwave trough over the northern Midwest/Ohio Valley with some overlap with areas expected to get locally heavy rainfall on Day 1. Global deterministic guidance became much more aggressive compared to previous runs in terms of QPF...with local 1-2+" totals focused within the region...and that largely persisted into the most recent model runs. The HREF probabilities that are available...through the first 12-hours of the Day 2 period during this outlook cycle...tends to focus higher amounts in the southern Ohio Valley closer to the better instability and better precipitable water values. Made a southward nudge/expansion as a result. ....Florida... Locally heavy rainfall across the coastal areas of central and south FL will pose a non-zero risk for flash flooding within a tropical environment entrenched over the Sunshine state. Best convergence is still being depicted over the adjacent waters, but elevated instability across both coasts will pose a threat for enhanced rainfall rates where thunderstorm activity is prevalent. The model consensus is that most activity will be off-shore...but the model consensus was for more inland convection than shown by earlier runs. Bann Day 3 Valid 12Z Fri Sep 29 2023 - 12Z Sat Sep 30 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE UPPER MIDWEST....THE FLORIDA PENINSULA AND PARTS OF THE MID-ATLANTIC REGION INTO SOUTHERN NEW ENGLAND... ....Upper Midwest... Locally heavy rainfall will break out across parts of the Upper Midwest on Friday as surface low pressure develops along the front and heads northeastward. South to southwest winds around 30 kts develops at 850 mb...resulting in increasing moisture during the day with precipitable water values increasing to between 1.25 inches and 1.50 inches over parts of Minnesota by late afternoon. That pattern will also support strong thetae advection helping to support locally heavy rainfall rates. Given how progressive the storms should be...will maintain a Marginal at this point,. ....Mid-Atlantic Region into Southern New England... Low pressure developing off the Mid-Atlantic coast will be close enough for some enhanced rainfall rates within the deformation zone that scrapes the coastline during the day. By Friday afternoon or evening...the rain shield should be approaching the southern New England coast with heavier rates not expected until later at night. Given that part of the area has had wet antecedent conditions...will keep the previously issued Marginal. ....The Florida Peninsula... A cold front attached to the area of low pressure moving northward off the Mid-Atlantic region mentioned above will provide a focus for additional convection capable of producing locally heavy rainfall that results in isolated flooding...with at least a small chance that the heavy rain occurs in areas that received a dousing in the previous two or three days. Precipitable water values will be ranging from 2 to 2.25 inches with persistent flow of moisture from the south and east being drawn towards the front and a stream of mid-level vorticity tracking from southwest to northeast throughout the day and into the evening to help support the storms and their rainfall rates. Bann $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .