Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Oct 06 2023 08:28:47 FOUS30 KWBC 060828 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 427 AM EDT Fri Oct 06 2023 Day 1 Valid 12Z Fri Oct 06 2023 - 12Z Sat Oct 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHERN MID-ATLANTIC... ....Northern Mid-Atlantic... A strong digging high-amplitude upper level trough over the Great Lakes will shift eastward into the northern Mid-Atlantic today. The leading strong surface cold front will draw warm, moist air from the Southeast northward up the Mid-Atlantic and into New England ahead of the trough. T.S. Philippe will contribute additional moisture and warmth to the leading air mass, further enhancing the gradient. The unimpressive rain shield moving into the Appalachians currently will fill in and become enhanced over the Marginal Risk area late this afternoon through tonight as a strong shortwave trough and associated vorticity rounds the base of the trough and begins moving northeast ahead of the trough. The negative tilt to the resultant trough will further enhance divergence aloft, resulting in heavier rain. Initially through tonight the moisture availability will be somewhat limited as the front won't have the benefit of Philippe's moisture until Day 2/Saturday. Nonetheless PWATs to 1.25 inches with impressive upper level dynamics should allow for convective rainfall ahead of the main cold front. The front will slowly push east, but the convection developing along the front will be rapidly translating NNE-ward. Thus, the threat for flash flooding will only be where multiple rounds of heavy rain repeat over the same areas. Given the recent dry conditions over much of PA/NY away from the NYC metro, think any flash flooding will be isolated. That said, now that the area is well into the autumn season, gusty winds ahead of the front will result in more leaf debris from the trees in the area, which could quickly block storm drains and culverts, resulting in a higher likelihood of the development of flood sensitive areas. Nonetheless, think most of the rain will be beneficial for the region, and it's only where repeating rounds of storms cause multiple inches of rain where there's that isolated flash flooding threat. CAMs guidance suggests most of the heaviest rains will be after sunset this evening over northeastern NY. The Marginal Risk includes the NYC metro since that area remains much more vulnerable due to favorable antecedent conditions after last week's flooding rains. However, this vulnerability should be offset somewhat by the fact that the main forcing, and thus heaviest rains, should be further north into upstate NY. Instability and better moisture availability in the strong southerly flow could result in localized stronger cells and locally heavy rains in the NYC metro. There is high confidence that this will not be anything close to a repeat of the last event, as the storms will be fast moving and scattered in nature, with significant dry time in between any storms. Wegman Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 Valid 12Z Sun Oct 08 2023 - 12Z Mon Oct 09 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7C61LB1j0HkK8qxMMOt_mLh13aFdvgb6JcWCyuEFOiQR= -i5UfP1KAo1N7dixye1fWXrBivVsoNxfp9_oK4QsDEj6cVQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7C61LB1j0HkK8qxMMOt_mLh13aFdvgb6JcWCyuEFOiQR= -i5UfP1KAo1N7dixye1fWXrBivVsoNxfp9_oK4QspUfx6y4$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7C61LB1j0HkK8qxMMOt_mLh13aFdvgb6JcWCyuEFOiQR= -i5UfP1KAo1N7dixye1fWXrBivVsoNxfp9_oK4Qs643h6_8$=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) .