Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 28 2023 00:31:26 FOUS30 KWBC 280031 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 830 PM EDT Sun Aug 27 2023 Day 1 Valid 01Z Mon Aug 28 2023 - 12Z Mon Aug 28 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHEAST... ....Southeast into the Tennessee Valley... Favorable forcing for ascent provided by an upper jet positioned over the Mid-Atlantic/Northeast and a frontal boundary in the vicinity across the Carolinas combined with a very moist airmass (PWs > 2") and remaining instability will continue the slow moving, highly efficient rain producing thunderstorms across portions of the Southeast through tonight. The strongest cores and deepest convection are expected over the next several hours but there's enough forcing to maintain numerous/widespread showers/storms past 06Z. Latest HREF probabilities favor 1" to locally 2" hourly totals will be possible in the strongest cores with some signal for localized 3-5" totals remaining possible across portions of NC through tonight. The Slight Risk was adjusted/confined to the latest model trends and radar imagery. Further west into portions of the Tennessee Valley, convection is beginning to wane somewhat with the lack of deeper instability present. Given the surface trough and remnant MCV in the vicinity plus the moist environment however, additional showers and thunderstorms will be possible though threat of heavy rainfall seems to have diminished enough to drop the Slight Risk across Tennessee but keep the Marginal Risk. ....Rockies and High Plains... Ongoing convection has generally moved off the terrain into the Front Range of CO and will continue to slide southeastward through portions of eastern New Mexico and the Texas Panhandle overnight. Locally heavy rainfall will remain possible and may lead to a few instances of flash flooding for the most vulnerable locations. Taylor Day 2 Valid 12Z Mon Aug 28 2023 - 12Z Tue Aug 29 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHEAST AND SOUTHERN MID-ATLANTIC... ....Southeast U.S into Southern Mid-Atlantic... Stalled frontal boundary across the southeast U.S will again be the focal point for a corridor of heavy rainfall during peak diurnal convection on Monday afternoon and evening. The corridor of higher theta-e located along the boundary and weak steering flow will provide an axis for rainfall rates exceeding 2-3"/hr in the heaviest cells. 12Z HREF CAM consensus is for two general areas of precip maxima; the southern Appalachians to Upland portions of South Carolina down through northern Georgia, and southeast Virginia and northeast North Carolina. Given the frontal boundary between the two with the aforementioned axis of moisture as well as Day 1 activity throughout, thought it prudent to expand the Slight Risk through Hampton Roads. The frontal zone continues southwest to the central Gulf Coast where similar heavy rain is anticipated, but given the typical higher flash flood guidance of the Deep South, maintained the Marginal Risk for southern AL/MS/southeast LA. ....Florida Peninsula... Constricted the Marginal Risk area to parts of west and southwest Florida peninsula coast as 12Z consensus for leading/outer bands of Idalia Monday night is generally limited to this more sensitive urban corridor from Tampa Bay through Naples. Monday afternoon activity across the rest of the southern peninsula as tropical moisture ahead of Idalia spreads north will need to be monitored, but as of now any excessive rain concern is rather isolated given the recent generally dry weather and high flash flood guidance. ....Colorado through West Texas.... Diurnally driven convective development downstream of a strong ridge axis is anticipated Monday afternoon into the overnight across the Colorado and New Mexico Rockies and onto the central/southern high plains, as well as west Texas. Flow aloft increases as it veers a bit from NWly to NNWly on approach of the eastward ridge axis which will help focus activity closer to the eastern slopes of the southern Rockies than activity this afternoon/tonight. A localized Slight Risk may be warranted should a focus area on Monday repeat over areas of heavy rain through tonight. Otherwise, a localized excessive rain risk is anticipated given the PW anomaly is near +1.5 sigma. Jackson Day 3 Valid 12Z Tue Aug 29 2023 - 12Z Wed Aug 30 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FROM MUCH OF THE FLORIDA GULF COAST INTO SOUTHERN GEORGIA, AS WELL AS PORTIONS OF NORTHERN GEORGIA INTO CENTRAL NORTH CAROLINA... ....Florida into Georgia... Surge of tropical moisture into the Southeast U.S. expected ahead of Idalia Tuesday through Tuesday night. Highly anomalous PWATs (3 to 4 standard deviations above normal) associated with the system will create a favorable environment for very heavy rainfall ahead of where the storm. Highest totals are expected to be located on the eastern flank and immediately ahead of the circulation. 12Z consensus is a bit east of prior guidance, extending the length of the western coast of Florida through the Florida Big Bend. Forward speed of the cyclone is expected to be on the faster side as it moves northward ahead of a mean trough pushing through the eastern half of the U.S. The western side of the Slight Risk was trimmed a bit given decent confidence in current forecast of precip to 12Z Wednesday. The forward speed forecast of Idalia continue to increase, so an earlier arrival would warrant upgraded excessive rain outlooks earlier, so be sure to check with the National Hurricane Center for the latest forecast updates on Idalia. ....Southeast through Mid-Atlantic... The aforementioned surge of tropical moisture ahead of Idalia is concerning given the persistence of the frontal boundary from the central Gulf Coast through the Carolinas as well as lifting north into the Mid-Atlantic Tuesday. Convergence of this tropical moisture along the front over northern GA through central/western SC into central NC may lead to a predecessor type rainfall event. Furthermore, heavy rain is expected through this areas tonight and Monday, making it more sensitive to heavy rain. A separate Slight Risk area is drawn to cover this risk with the understanding that as we get more into the CAM time range, focusing/upgrading the risk level may become necessary. Farther north, the surge of higher moisture up the Mid-Atlantic in time for the Tuesday diurnal cycle ahead of the cold front expected to move into/through the area Tuesday night warrants an expansion of the Marginal Risk up through the Philadelphia metro area which is where the overlap of greater instability and moisture has its northward extent. Should the moisture/instability surge make it farther north, areas farther north would need a Marginal Risk. ....Pacific Northwest.... A strong, negatively tilted upper trough is expected to push into the Pacific northwest Tuesday with a progressive shortwave maxima rounding the base of the mean trough. Enhanced mid-level forcing generated from strong positive vorticity advection across the Cascades into northern Idaho will generate a rapidly developing area of precipitation within a corridor of modest instability. Despite fast storm motion expected within enhanced mean flow, heavy rainfall will be likely over the aforementioned area within a region of notably steep terrain and low flash flood thresholds. The Marginal Risk is maintained. Jackson Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!769ckl_R2SR96Zlk_FqnVfic4lfIFVOJjRXj699tA6fj= LD65vezSq5tksuJ5DXaFeRkLMfKnDYGTo32p9ryL2GAiOF8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!769ckl_R2SR96Zlk_FqnVfic4lfIFVOJjRXj699tA6fj= LD65vezSq5tksuJ5DXaFeRkLMfKnDYGTo32p9ryLP14UYCo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!769ckl_R2SR96Zlk_FqnVfic4lfIFVOJjRXj699tA6fj= LD65vezSq5tksuJ5DXaFeRkLMfKnDYGTo32p9ryLDy-8qQ4$=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) .