Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Apr 09 2025 15:36:55 FOUS30 KWBC 091536 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1136 AM EDT Wed Apr 9 2025 Day 1 Valid 16Z Wed Apr 09 2025 - 12Z Thu Apr 10 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent.=20 Kleebauer Day 2 Valid 12Z Thu Apr 10 2025 - 12Z Fri Apr 11 2025 A broad area of instability is forecast to develop across the Lower Ohio Valley and portions of the Mid-South which shifts into the Southern Appalachians with time. Moisture availability appears sufficient for atmospheric saturation within a cool atmosphere under a digging upper level trough. However, the area of sufficient moisture/moisture plume appears to be on the move. This should limit the overall excessive rainfall/flash flood potential. The probability of rainfall exceeding flash flood guidance may be non- 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- derived instability standpoint down near the VA/NC border, but slantwise convection could be a problem for areas farther north. In theory, hourly rain totals up to 1" with local amounts to 3" are possible. In such situations in the past, the convective band that forms could be quite narrow which makes placement especially difficult to discern at this time horizon. Not helping matters are 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 enough agreement on the heavy rain potential through the various pieces of guidance, when combined with the ingredients above, to maintain the Marginal Risk from continuity. Should model mass fields converge and snow becomes less of a factor, an upgrade to a Slight Risk may be worth considering as some areas of the northern Mid- Atlantic States have quite low flash flood guidance (northern 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!7HKH27qQ0EKkVNeEpo2mYLiWSgIWNZd7yu6xJQ3fvQlB= x06B00o2N5cLMgoShqgMvKyXmxZ6a9sk0ul1AvtsP-Ou8Cw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7HKH27qQ0EKkVNeEpo2mYLiWSgIWNZd7yu6xJQ3fvQlB= x06B00o2N5cLMgoShqgMvKyXmxZ6a9sk0ul1AvtsepWgA6Y$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7HKH27qQ0EKkVNeEpo2mYLiWSgIWNZd7yu6xJQ3fvQlB= x06B00o2N5cLMgoShqgMvKyXmxZ6a9sk0ul1AvtstU8geH4$=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) .