Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 27 2023 20:13:15 FOUS30 KWBC 272013 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 412 PM EDT Wed Sep 27 2023 Day 1 Valid 16Z Wed Sep 27 2023 - 12Z Thu Sep 28 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE OHIO VALLEY... ....16Z Update... The main change this forecast is the upgrade to a Slight Risk across central KY the borders of IN and TN for heavy rain concerns later this evening and overnight. In coordination with the WFO at Louisville, in agreement for a heightened signal for flash flood concerns across much of central KY. More information on the upgrade within the "Ohio Valley" synopsis below... Marginal risk was expanded to include all of South FL, including the FL Keys after coordination with WFO Key West. The premise was to account for heavy rainfall potential being exacerbated with the King tide cycle expected overnight when rainfall will be the heaviest. Local rainfall amounts >5" will be possible across portions of the middle and lower Keys with 12z HREF neighborhood probabilities of 3"/hr rain rates pushing up into the 20-30% range with much higher percentages within the 2"/hr probability. Couple with expected 3 standard deviation above normal PWAT indices across extreme southern FL, this was enough to warrant the extension further south. ....Ohio Valley... An upper-low is currently positioned over northern IL as indicated on UA analysis and WV satellite. The trajectory of the low is forecast to slide eastward with the ULL center situated over northern IN by the end of the period. The slow motion will be a factor in the forecast as limited progression will maintain large scale forcing over generally the same areas within the next 24+ hrs. Forecast soundings within the confines of the ULL show steep lapse rate reflection classic of a mature upper disturbance which will be present downstream as the day evolves. A stationary boundary in-of the the western portion of the Ohio Valley will become a focal point within the convective scheme that is expected this afternoon and evening. Hi-res deterministic is consistent on two origin points of convection today; the first being over eastern KY up through Ohio as area difluence downstream of the mean trough within a core of modest instability will trigger scattered convection across the aforementioned areas. There's some lower grade enhanced rainfall signatures within the precip footprint to allude to localized flood concerns, mainly shown within the 1-2"/hr rain rate, 12z HREF neighborhood probabilities where percentile ranges confirm at least a local threat and within the 5% MRGL risk threshold. The main area of interest is across western and central KY this evening as a stronger vorticity maxima swings through the region this evening along the base of the mean trough. The enhanced ascent will be focused over a stationary front centered from southern IL through KY as noted by latest WPC forecast and forecast thermal gradient within all deterministic guidance. 1000-1500 J/kg of MLCAPE centered near and south of the stationary boundary is plenty sufficient for instability within a corridor of elevated PWATs between 1-1.5 standard deviations above normal. A persistent mean steering flow will present an opportunity for training storms within the axis of best instability creating an environment favorable for flash flooding concerns. 12z HREF mean QPF is now over 2" for portions of central KY with 12z neighborhood probabilities a whopping 30-40% for at least 2" encompassing all of central KY. This rainfall is likely to occur within a window of 6-10 hrs overnight and not over the course of any entire day, thus putting the area within a higher risk than normal. As a result, have included the area with a SLGT risk and was in agreement with the local WFO. ....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. Kleebauer/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... ....2030Z Update... Very little deviation from the previous forecast as the overall synoptic setup is on track and broad scale precip coverage showed little change from the prior forecast issuance. Two main areas of focus for D2 will be across the southern Ohio Valley and the coastal areas of FL. Our upper trough will continue its slow progression through the Great Lakes, northern Ohio Valley through Thursday. Ascent will be maximized across OH/KY/TN with a primary focus over southern KY and TN where remnant boundary from the previous round of convection the overnight period will linger and a backbuilding convective scheme will persist on the front end of the MRGL outlook. An elongated vorticity maxima will continue pivoting over the area of interest with training cells from western KY into northwest TN by Thursday AM. Ensemble bias-corrected QPF and 12z HREF blended mean indicates a cluster of 1-1.25" of precip within a short time frame Thursday (12Z-18Z) which correlates well with the HREF signal for neighborhood probabilities exceeding 30% for at least 2"/3 hrs and a more robust 50-60% for at least 1"/3 hrs. Considering the nature of training precip from the previous D1 period, a MRGL was maintained to match consensus and act as a good pivot point from the slight on D1. The rest of the MRGL will be for scattered convection across eastern KY up through OH where isolated flash flood concerns will persist within a core of elevated PWATs and continued ascent from the upper trough traversing overhead. No change within the Florida MRGL risk as isolated 2-3"/hr rain rates will be possible within any areal convective development as noted by the continued 40-50% neighborhood 3" or greater probabilities Thursday afternoon for much of the state as it resides within an anomalous moisture environment south of a stationary front to the north. Best chances for flooding will occur within urban environments along the coast where runoff is most likely and sea breeze convergence occurs during diurnal storm patterns. Kleebauer ....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.=20 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.=20 Bann Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8gsLgFbYNxCvfxsll-dJxNRNlJeQp1mbt-JuceqOYtR_= uFCQWxXCfPMGE-vpaQkcoliLwp2V1sBJTwUJ-E-AeZh26DM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8gsLgFbYNxCvfxsll-dJxNRNlJeQp1mbt-JuceqOYtR_= uFCQWxXCfPMGE-vpaQkcoliLwp2V1sBJTwUJ-E-AsddNJpI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8gsLgFbYNxCvfxsll-dJxNRNlJeQp1mbt-JuceqOYtR_= uFCQWxXCfPMGE-vpaQkcoliLwp2V1sBJTwUJ-E-A3ShcnZM$=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) .