Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Apr 10 2025 00:03:33 FOUS30 KWBC 100003 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 803 PM EDT Wed Apr 9 2025 Day 1 Valid 01Z Thu Apr 10 2025 - 12Z Thu Apr 10 2025 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 2 Valid 12Z Thu Apr 10 2025 - 12Z Fri Apr 11 2025 20Z Update: A digging shortwave trough will slide through the Ohio Valley, gaining amplitude as it maneuvers southeast into the southern Mid Atlantic. The mean flow will align as such that primary vorticity advection will traverse the same corridor over the course of the period, interacting with a modestly unstable environment in-of the Ohio and Tennessee Valleys during the afternoon. The bad news is the energy advection regime will allow for focused mid and upper- level ascent to initiate scattered showers and thunderstorms across IL, propagating southeast through areas that are still recovering from the recent flooding over western KY and TN. The good news is the moisture source available is significantly lacking due to meridional flow being much more prevalent for points east of the affected areas. This will limit the overall precip output with a lack of better hourly rates that are more conducive to flash flood prospects. The threat will also be fairly progressive as the trailing energy will slide through very quickly leading to a low probability of any one storm training and all cells quickly in and out of any given locations. Areal average QPF is closer to 0.25-0.5" with perhaps a few locations seeing close to 1" in total during the period of impact. This should curb flash flood potential below the necessary threshold for a risk area, in agreement with the current first guess fields that take into account current FFG indices. The probability of rainfall exceeding flash flood guidance is less than 5 percent. Kleebauer 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... 20Z Update: The overall synoptic evolution has not changed much within the past 24 hrs with the discussion below from the previous MRGL risk issuance still valid. The biggest change for this forecast was the extension of the MRGL risk further west into the southern Blue Ridge due to expectation of elevated convection with mean storm motions leaning very slow due to proxy under the influence of a closed upper level reflection centered overhead. The overall environment is conducive for low-topped convection over the western half of VA due to the influence of the amplifying mid and upper-level pattern as forecast soundings depict a significant low to mid-level lapse rate structure within a marginally favorable moisture field as PWAT indices hover around +1 deviations overall. The area of concern is also favored for flash flooding due to the complexity of the terrain within the southern bounds of the Blue Ridge. The combination of favorable upper level dynamics and slower storm motions could yield a better prospect of training that could induce localized flooding. The remainder of the forecast was fairly minor in terms of changes with the only other area of focus for change was the trimming of the MRGL risk into southern New England as guidance remains modest in the overall QPF output and likely not sufficient for excessive flooding prospects. Plus, a lack of even elevated instability that would be customary for a stronger precip output is noticeably missing during this period, another indicator for staying just below the MRGL threshold. The main area of concern will continue to focus along the VA and MD Piedmont where the FGEN and deeper moisture align within a formidable left-exit region dynamic. Kleebauer ...Previous Discussion.. 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!7vIqjnYOcpk-tUQTefzNUh_q3ZEspzbYCtPF9JXlEDs7= mwd7-3owvrxxjagQb0buhWcrNd5u-_r6Jx9hAxUwhN52L0E$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7vIqjnYOcpk-tUQTefzNUh_q3ZEspzbYCtPF9JXlEDs7= mwd7-3owvrxxjagQb0buhWcrNd5u-_r6Jx9hAxUwr816waY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7vIqjnYOcpk-tUQTefzNUh_q3ZEspzbYCtPF9JXlEDs7= mwd7-3owvrxxjagQb0buhWcrNd5u-_r6Jx9hAxUw_Hto-dk$=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) .