Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Aug 27 2023 08:24:43 FOUS30 KWBC 270824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Sun Aug 27 2023 Day 1 Valid 12Z Sun Aug 27 2023 - 12Z Mon Aug 28 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT ACROSS PORTIONS OF THE SOUTHEAST, LOWER MID ATLANTIC, AND THE TENNESSEE VALLEY... ....Parts of the Southeast into the Lower Mid Atlantic Region... Synoptic setup remains favorable across the Carolinas into southern Virginia down into northeast Georgia. Enhancement of an upper jet over the northern Mid Atlantic and Northeast will provide increasing ascent across the area mentioned above. Anomalously moist environment centered over the Carolinas provides a footprint for enhanced rainfall risks, especially located along a stalled frontal boundary bisecting eastern NC down through central SC and GA. MLCAPE breaching 2500 J/kg will be common across much of the Lower Mid Atlantic and Southeast US providing one of the last key ingredients to the setup. 00z HREF neighborhood probability of greater than 2-3" of rainfall remains fairly robust across much of eastern NC down into northeast SC with local maxima approaching 5+" in spots. Deterministic guidance still doesn't have a perfectly defined area of where the heaviest rain will occur, but the threat is certainly there for somewhere within the coverage area to receive very high amounts of rainfall capable of locally significant flash flooding. The slight was also expanded to the west to include the western NC mountains through the SC Piedmont due to increasing signals for locally heavy rainfall mid-afternoon with 00z HREF probability of 2"/hr rainfall rates now over 25% for portions of the higher terrain. Hi-res deterministic and ensemble guidance has been consistent the past few runs of initiation over the Appalachians leading to the best chance for flooding to occur mid to late afternoon before shifting focus to the eastern half of the state(s). ....Western Tennessee into Southeast Missouri and Southwest Kentucky... A potent mid-level vorticity maxima is analyzed over Missouri this evening with a slow push to the southeast, generating scattered convection across southeast Missouri into northeast Arkansas and southern Illinois. Recent hi-res guidance trends point to a regional maximum in convective coverage and anticipated heavy rainfall rates thanks to ascent within a PW anomaly around 2 standard deviations above normal. 00z HREF probability of greater than 2"/hr of rainfall is focused over northwest Tennessee into southwest Kentucky with 24 hr totals ranging from 3-6" across guidance. The local enhancement is likely in part due to the ascent focused over a surface trough analyzed from northern Mississippi north into Kentucky. With the signal across all hi-resolution deterministic and ensemble mean(s), elected to introduce a slight for the aforementioned area to account for enhanced flash flood concerns beginning this morning, through the afternoon.=20 ....Rockies and High Plains... Scattered convection is anticipated across the Rockies down into the southern high plains of Texas this afternoon incorporated along an axis of elevated theta-e on the eastern flank of a western mid-level ridge. Marginal instability on the order of 500-1000 J/kg with greater instability over west Texas will aid in diurnally driven convection beginning after 18z. NW'ly flow will help propagate convection towards the south and east across the Front Range of CO down through eastern NM and the Caprock of TX. Locally heavy rainfall will allow for a chance of flash flooding, especially within any larger towns or cities where local FFG's are generally lower.=20 Kleebauer 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 LOWER MID ATLANTIC... ....Southeast U.S into the Lower Mid Atlantic... Stalled frontal boundary across the southeast U.S will be the focal point for a corridor of heavy rainfall during peak diurnal convection on Monday afternoon and evening. Deep moisture field present over all the southeast will maintain a formidable environment for heavy rain potential stemming from convective development along the front as low-level convergence signature within a corridor of higher theta-e located along our boundary will provide an axis for flood potential given expected rainfall rates exceeding 2-3"/hr in the heaviest cells. Weak steering flow as noted by forecast bufr soundings across the southeast will lead to slow-moving or stationary pulse thunderstorms capable of producing very heavy rainfall in a short period of time. Both hi-res ensemble and global deterministic guidance indicate local precip maxima across the Piedmont of South Carolina down through northern Georgia, leading to a slight in aforementioned area. A secondary max is positioned over the Gulf Coast across southeast Louisiana, east into southern Alabama. Given the higher FFG's in the general corridor stated, have opted to maintain just a marginal for the setup, but some localized flooding will be possible given the forecasted rain rates after convective initiation Monday afternoon.=20 Farther south...the Marginal risk area across west and southwest portions of the Florida peninsula was maintained as the current tropical depression begins to make its way northward and ensemble guidance indicates some convection on the outer periphery of the main circulation moving inland from the Gulf with enhanced rainfall rates. Highest potential will be along the Gulf coast of Florida where the urban corridors are more pronounced with lower FFG's compared to further inland. ....Southwest U.S.... More convective development is anticipated across the Rockies down into the high plains of Texas and New Mexico. Ridge center is a bit further to the west compared to the previous period, leading to a bit of a western shift in the main axis of the marginal. Look for diurnally driven convection within a corridor of instability and moist boundary layer environment to create heavy rainfall potential and localized flash flood threat across the above area. Kleebauer Day 3 Valid 12Z Tue Aug 29 2023 - 12Z Wed Aug 30 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF FLORIDA, SOUTHEAST ALABAMA, AND SOUTHERN GEORGIA... ....Western Florida and Panhandle up through Southeast Alabama and Southern Georgia... Increasing moisture field expected across the southeast U.S due to an incoming tropical cyclone located over the eastern Gulf. Highly anomalous PWATs (3-5 standard deviations above normal) associated with the expected system will create a favorable environment for very heavy rainfall within a corridor where the storm is expected to make landfall. Highest totals are expected to be located on the eastern flank of the circulation with outer bands forecast to move through all of Florida with higher coverage on the western coast, closer to the eastern side of the cyclone. Current National Hurricane Center forecast is for this storm to move up into the Florida Panhandle, coinciding with the center of the slight risk through the Florida Big Bend up into southern Georgia. 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. Any shift in the track of the cyclone will cause shifts in the main axis of heavy rainfall with adjustments to the excessive rainfall risk likely, so be sure to check with the National Hurricane Center for the latest forecast updates on the track of the eventual named system. ....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. A marginal risk remains over the region. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7kxK4TtlDMdssb-lRppUmDV8p9oNNYTUt8pGGZmI6DgO= HiAmicBLnRNwoQq9mgV2SA3qM14D2ts5WYZEcf_gBkho4Hg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7kxK4TtlDMdssb-lRppUmDV8p9oNNYTUt8pGGZmI6DgO= HiAmicBLnRNwoQq9mgV2SA3qM14D2ts5WYZEcf_gOTn96fs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7kxK4TtlDMdssb-lRppUmDV8p9oNNYTUt8pGGZmI6DgO= HiAmicBLnRNwoQq9mgV2SA3qM14D2ts5WYZEcf_gUbALW9g$=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) .