Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 09 2025 07:49:07 FOUS30 KWBC 090748 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 348 AM EDT Wed Apr 9 2025 Day 1 Valid 12Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025 At the moment...heavy rain potential near the Florida coast=20 appears to be mostly offshore, except perhaps Cape Canaveral where there is a non-zero chance of heavy rainfall within convective banding due to onshore flow advecting in instability from the=20 nearby Atlantic. However, precipitable water values appear low for atmospheric saturation, even taking into account the cooler air=20 mass in place. Overall, any heavy rain related issues would be=20 isolated at best and the probability of rainfall exceeding flash=20 flood guidance is considered to be less than 5 percent. Roth Day 2 Valid 12Z Thu Apr 10 2025 - 12Z Fri Apr 11 2025 A broad area of instability is forecast to develop across the=20 Lower Ohio Valley and portions of the Mid-South which shifts into=20 the Southern Appalachians with time. Moisture availability appears=20 sufficient for atmospheric saturation within a cool atmosphere=20 under a digging upper level trough. However, the area of sufficient moisture/moisture plume appears to be on the move. This should=20 limit the overall excessive rainfall/flash flood potential. The=20 probability of rainfall exceeding flash flood guidance may be non-=20 zero, but it is considered to be less than 5 percent. Roth Day 3 Valid 12Z Fri Apr 11 2025 - 12Z Sat Apr 12 2025 ....A MARGINAL RISK OF EXCESSIVE RAINFALL EXISTS FOR PORTIONS OF THE MID-ATLANTIC STATES AND SOUTHERN NEW ENGLAND... A deep layer cyclone slows down as it moves across the Mid-Atlantic States on Friday. The 1000-500 hPa thickness values are quite low across the region, so one would only need 0.75-1" precipitable water values to saturate the atmosphere. When combined with slow- moving low- to mid-level frontogenesis across PA in particular, heavy rainfall with possible areas of snow through dynamic cooling could ensue. MU CAPE is only sufficient for issues from a vertically-=20 derived instability standpoint down near the VA/NC border, but=20 slantwise convection could be a problem for areas farther north. In theory, hourly rain totals up to 1" with local amounts to 3" are=20 possible. In such situations in the past, the convective band that=20 forms could be quite narrow which makes placement especially=20 difficult to discern at this time horizon. Not helping matters are=20 mass field differences between the guidance as the mid- to upper- level low is attempting to close off as the upstream ridge is beginning to falter in strength, which would normally side against a more southerly solution like the 00z NAM. Even so, there is=20 enough agreement on the heavy rain potential through the various=20 pieces of guidance, when combined with the ingredients above, to=20 maintain the Marginal Risk from continuity. Should model mass=20 fields converge and snow becomes less of a factor, an upgrade to a=20 Slight Risk may be worth considering as some areas of the northern=20 Mid- Atlantic States have quite low flash flood guidance (northern=20 NJ, west- central PA, and the Southern Tier of NY). Roth Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7KKDUqSxhDhKhyzL2N3RU_wOcrrQ-l5A9SPnAReJ45jG= bG3Uw3fEtJ48E95gDwnpaiQE_5Nwc39pHyrLs1NNIDip5G0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7KKDUqSxhDhKhyzL2N3RU_wOcrrQ-l5A9SPnAReJ45jG= bG3Uw3fEtJ48E95gDwnpaiQE_5Nwc39pHyrLs1NNhj6TYSQ$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7KKDUqSxhDhKhyzL2N3RU_wOcrrQ-l5A9SPnAReJ45jG= bG3Uw3fEtJ48E95gDwnpaiQE_5Nwc39pHyrLs1NNB0Y0cm0$=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) .