Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Aug 29 2023 08:19:57 FOUS30 KWBC 290819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Tue Aug 29 2023 Day 1 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... Little change made to the placement of the Moderate Risk as Idalia approaches the Florida coast late Tuesday night/early Wednesday morning. Took advantage of the Excessive Rainfall Outlook being probabilistic and nudged the western boundary of the Marginal and Slight risk areas a bit westward to account for some of the track/timing uncertainty still being shown by the models...while maintaining the Moderate risk area along the path of the NHC forecast track. Considering that the NHC forecast has Idalia as a major hurricane at landfall...intense rainfall rates are expected near the core of the storm and from rain bands ahead (and generally east) of the storm which pose a threat of flash flooding. Since minor forecast changes can 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 deep moisture will spread north and east that overspreads the Southeast U.S. through the upcoming night while existing moisture over the Carolinas/southern Virginia spreads into parts of New England. The potential for heavy to excessive rainfall develops as a frontal boundary initially from the central Gulf Coast through the Carolinas lifts northward into the Mid-Atlantic today and into New England later tonight.=20 Convergence of tropical moisture along the front may lead to a predecessor type rainfall event per conceptual model. The 00Z suite of convective allowing models still struggled to resolve 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 tonight will be one focus for storms capable of producing high rainfall rates. Expanded the Slight risk to cover areas where flash flood guidance lowered considerably by storms that resulted in flash flooding on Monday...and where frontogenesis is forecast by the models to be co-located with anomalously high precipitable water values over/near the terrain of the Appalachians. By late this evening...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. There could even be a risk of excessive rainfall farther up the coast of Maine as precipitable waters surge to 2 inches or more and sets the stage for some warm rain processes. ....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. Bann Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9bIKOllYAR2nJGgZ2tBJ7ubJyUjN81pxZDl7OnAb235s= a0dlYmRw-wYzJLvY3thamCDS2chEuPPnXT1GUdH5tB7OfLU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9bIKOllYAR2nJGgZ2tBJ7ubJyUjN81pxZDl7OnAb235s= a0dlYmRw-wYzJLvY3thamCDS2chEuPPnXT1GUdH5PucmzLI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9bIKOllYAR2nJGgZ2tBJ7ubJyUjN81pxZDl7OnAb235s= a0dlYmRw-wYzJLvY3thamCDS2chEuPPnXT1GUdH5CTBbevA$=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) .