Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Aug 28 2023 20:04:44 FOUS30 KWBC 282004 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 404 PM EDT Mon Aug 28 2023 Day 1 Valid 16Z Mon Aug 28 2023 - 12Z Tue Aug 29 2023 ....A SLIGHT RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE SOUTHEAST TO THE CENTRAL MID-ATLANTIC... ....West Virginia... Early morning heavy rain in central WV that developed along a trough along the western flank of the Appalachians has diminished. Additional rains today are forecast to be generally light to moderate, though a remnant MCV could allow a very localized area of heavy rain, including over central WV. A Marginal Risk is maintained for this area of the western flank into southwest PA. ....Southeast U.S into the Central Mid-Atlantic... Southerly low level flow over a stalled front extending from southeast Louisiana into central AL/GA and up the Carolina coast will continue to support areas of heavy rainfall through tonight. The corridor of higher theta-e located along the boundary and weak steering flow will provide an axis in which rainfall rates exceed 2 inches per hour from the most intense convection. Main areas of heavy rainfall featured in the 12Z HREF continue to be southeast VA, where ongoing heavy clusters of thunderstorms are ongoing, the southern Appalachians, and just interior of the central Gulf Coast. Worked with the Marginal and Slight Risk areas to fit this forecast better with the Slight still connecting the southern Apps and the southern Mid-Atlantic across NC where boundary layer moisture convergence is still progged this evening by the RAP. Also, the surge of moisture and some instability into the Chesapeake Bay region continues overnight where the 12Z ARW2/HRRR produce heavy/slow moving activity late tonight. As a result the Slight Risk was expanded north into southern MD and up the DelMarVa a bit. Confidence seems greater that insufficient instability should keep heavy rains from the Philly metro, so less of that area is in the Marginal Risk. ....Florida Peninsula... Diurnally forced, slow moving heavy activity across central and southern portions of the FL Peninsula in the surge of tropical moisture ahead of Idalia warrants an expansion of the Marginal Risk across these areas. PWs in excess of 2.25" are progged over much of the FL peninsula by 00Z, so localized excessive rain seems probable as supported by 12Z HREF CAMs. Outer bands may yet reach the western coast south from Tampa Bay by 12Z, so those areas are still included in the Marginal. ....Colorado through West Texas.... Afternoon and evening convection is expected once again from Colorado into West Texas on Monday downstream of a strong ridge axis. Flow aloft increases as it veers a bit from northwesterly to north-northwesterly as ridge axis approaches. This should help focus activity closer to the eastern slopes of the southern Rockies than yesterday. A localized excessive rainfall event is anticipated. The heaviest rain (1-2") fell yesterday over the Caprock along the TX/NM border, with most guidance generally keeping activity west of there today, so the need for a targeted Slight Risk seems limited at this time. Jackson/Bann Day 2 Valid 12Z Tue Aug 29 2023 - 12Z Wed Aug 30 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR A PORTION OF NORTH FLORIDA ALONG AND NEAR THE TRACK OF IDALIA... ....Florida into Georgia... Still some uncertainty with the exact track and timing of Idalia. Despite the 12Z GFS/ECMWF being a bit slower and farther west than before, having a Moderate Risk along the Nature Coast is reasonable. The core of Idalia should be pushing inland by 12Z Wednesday. Considering that the NHC forecast has Idalia a well developed hurricane when it makes landfall...intense rainfall rates are expected near the core of the storm and from rain bands ahead (and generally east) of the storm pose a threat of flash flooding. The Slight Risk was expanded east a bit into south-central FL given consensus among 12Z HREF CAM guidance on the eastern extent of the outer bands. Since minor forecast changes make notable rainfall threat changes, check with the National Hurricane Center and local offices for latest information on Idalia. ....Southeast through Mid-Atlantic into New England... A surge of tropical moisture ahead of Idalia overspreads the Southeast through Tuesday night while existing moisture over the Carolinas/southern VA/Gulf Stream up into New England. A persistent frontal boundary from the central Gulf Coast through the Carolinas lifts north into the Mid-Atlantic on Tuesday and into New England Tuesday night. Convergence of tropical moisture along the front may lead to a predecessor type rainfall event per conceptual model. 12Z CAMs were not resolving any particular focus, but there is decent agreement on heavy rain from northern Georgia through western North Carolina where the Slight Risk is maintained/expanded into southwest VA which is where heavy rain is occurring this afternoon/tonight. 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 central Pennsylvania. Later Tuesday into the overnight, moisture stream northward into New England ahead of a shortwave trough approaching from the west. The Marginal Risk is further expanded inland over the northern Mid-Atlantic and southern New England. ....Pacific Northwest.... A strong, negatively tilted upper trough pushes into the Pacific Northwest Tuesday with a progressive shortwave maxima rounding the base of the trough from OR to ID. 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 is over this area which features notably steep terrain and low flash flood thresholds. The Marginal Risk is maintained. Jackson/Bann Day 3 Valid 12Z Wed Aug 30 2023 - 12Z Thu Aug 31 2023 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM NORTH FLORIDA NORTHWARD INTO NORTH CAROLINAS AS IDALIA TRACKS ACROSS THE REGION... ....Florida to the Carolinas... Heavy rainfall will spread northward and eastward in association with Idalia. Given its strength at the beginning of the period, a rather traditional rainfall distribution is expected with intense rainfall rates near the core of the storm and from rain bands feeding into the system and any trough interaction to the left of the track. There are considerable track differences among 12Z global guidance with the 12Z GFS/UKMET farther west and the 12Z ECMWF/CMC farther east. All solutions lead to considerable excessive rainfall risk with generally 4 to 8 inches along the track. Confidence is greatest for North/Big Bend of FL into GA with a likelihood of the eastern Carolinas significantly impacted. Placement of the Slight and Moderate Risks is based on the official NHC track forecast, so please refer to latest statements from the National Hurricane Center for updated Idalia information. ....Pacific Northwest.... A potent, leading shortwave low on a negatively tilted upper trough pushes northeast through western Montana Wednesday. Locally heavy rain from Tuesday night continues Wednesday until the wave lifts in to Canada. Locally moderate to heavy rainfall during the day may still result in run off problems, especially in areas of steep terrain and low flash flood thresholds, so the Marginal Risk is maintained. Jackson/Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7ZIKviqeXmAVo3LpXvraM3uNSyZPj8d1SA1fm-VXzZ04= Kos2zu2EaAJzoARW7geZvT8yAqFJN46WkMXZ4EeSbgl9hSM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7ZIKviqeXmAVo3LpXvraM3uNSyZPj8d1SA1fm-VXzZ04= Kos2zu2EaAJzoARW7geZvT8yAqFJN46WkMXZ4EeS0UYDSPY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7ZIKviqeXmAVo3LpXvraM3uNSyZPj8d1SA1fm-VXzZ04= Kos2zu2EaAJzoARW7geZvT8yAqFJN46WkMXZ4EeSu4G-R7c$=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) .