Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Apr 08 2025 00:27:56 FOUS30 KWBC 080026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 826 PM EDT Mon Apr 7 2025 Day 1 Valid 01Z Tue Apr 08 2025 - 12Z Tue Apr 08 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS EASTERN NORTH CAROLINA INTO FAR SOUTHEAST VIRGINIA... Convection will move across eastern NC through the evening hours posing a localized minor flood risk. Strengthening low level convergence and some southwest to northeast training should result in total rainfall amounts generally in the 1-3" range, with hourly rainfall as high as 1.5". Given the high FFG over this region, it=20 seems unlikely that the forecast rainfall will exceed FFG. However 2-3" of total rain and 1.5" in an hour could be enough to result=20 in localized minor flooding in susceptible urban or low lying=20 areas. Chenard Day 2 Valid 12Z Tue Apr 08 2025 - 12Z Wed Apr 09 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS IN SOUTHERN FLORIDA... ....1930 UTC Update... Based on the 12Z guidance, particularly the HREF and its suite of especially the exceedance probabilities, have trimmed the Marginal Risk a little to include far southeast FL from Homestead to West Palm Beach. This given the anticipated frontal progression and the fact that the bulk of the heaviest rainfall after 12Z Tue will be east towards the northern Bahamas. 12Z model QPFs follow suit, especially the CAMs, as the 12Z HREF probabilities of QPF greater than 3" in 24 hours (12Z Tue-12Z Wed) now shows only a sliver of 30-40% right along the coast along and north of Pompano Beach. ....0800 UTC Discussion... Deep southwest flow is expected ahead of an advancing cold front across the FL Peninsula. Precipitable water values of 1.75-2" are forecast to reside here, and ML CAPE should maximize in the 2000-3000 J/kg range per NAM guidance. Low- level inflow and effective bulk shear are sufficient for convective organization. Whether or not a flash flood risk fully materializes depends upon whether or not an East Coast seabreeze front can develop and focus convection, which could anchor convection in the large urban area of South FL as it is unlikely to progress inland within this synoptic pattern. In this sort of atmosphere, hourly rain totals to 2.5" and overall totals to 5" are possible. The 00z HREF does have some non- zero probabilities of 5"+ totals across sections of the northern Keys, while the 3"+ probabilities are more prominent along the Gold Coast. Considering the urban environment, ingredients available, and mesoscale guidance signal, added a Marginal Risk to South FL and the Upper Keys as a precaution as isolated to widely scattered flash flooding appears possible. Roth Day 3 Valid 12Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Southern Illinois... Conceptually, there is a non-zero threat of heavy rainfall in and near southern IL Wednesday evening as precipitable water values rise near to above 1" within a region with 1000-500 hPa thickness near 5550 meters, implying a completely saturated troposphere. MU CAPE is expected to rise above 250 J/kg per NAM guidance, which tends to be under-forecast at this time range. The area will still be attempting to dry out from recent heavy rainfall/ saturated soils. However, there is only about a six hour window for heavy rain concerns to materialize near the parent cyclone's associated frontal boundary before it moves through. There is the potential for hourly rain totals to maximize near 1.5". The guidance is this time range QPF-wise is underwhelming, so no risk area was depicted. However, this may change as we move closer in time into the high resolution guidance window. Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!90v_mV7W8y1xdJIQnpp6qM-XErgmCDODq4APCjfePQ0g= osVYfbCZlFTpi4TvH5GE-x-lSjPi_zyWA80quhlHDvzDzww$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!90v_mV7W8y1xdJIQnpp6qM-XErgmCDODq4APCjfePQ0g= osVYfbCZlFTpi4TvH5GE-x-lSjPi_zyWA80quhlHNRZqzhw$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!90v_mV7W8y1xdJIQnpp6qM-XErgmCDODq4APCjfePQ0g= osVYfbCZlFTpi4TvH5GE-x-lSjPi_zyWA80quhlHARipc3o$=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 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .