Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Fri Oct 06 2023 09:14:00 FOUS30 KWBC 060844 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 443 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 Valid 12Z Sat Oct 07 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... The period with the greatest potential for significant flash flooding in the short-term will be on Day 2/Saturday across portions of the Northeast. The longwave trough over the eastern Great Lakes will continue pivoting northeastward as the embedded shortwave trough elongates southeast to northwestward. This in turn will cause the whole longwave trough to become negatively tilted, which will further enhance divergence across New England. A pair of jet streaks will locally enhance speed divergence even more ahead of the trough. The result will be 2 areas of heavy rains. The western of the two will be largely associated with the cold front continuing to track eastward from the Mid-Atlantic into New England from Friday night/Day 1. The enhanced divergence and the slowing of the eastward progress of the entire pattern could result in more numerous instances of flash flooding as repeating storms have a greater likelihood than Day 1 to repeat over the same areas...namely along the Berkshires and the Champlain Valley, following the eastern border of NY with the westernmost New England states. By Saturday night, Vermont may see some wraparound rainfall around the western side of the extratropical remnants of Philippe as they get caught up in the main flow ahead of the trough. By this point the rain shield should be oriented more northwest to southeast from northeast NY across VT. The second area of heavy rains will be associated directly with the remnants of Philippe as they move into Maine Saturday evening through Saturday night. The storm will be fast moving thanks to the aforementioned jet streaks helping the storm move along, but the atmosphere will be moisture-laden as all the tropical moisture gathered by the storm rains out over eastern Maine. Thus, the higher rainfall totals of the 2 areas will likely be over eastern Maine. There are some factors that will help mitigate flash flooding risk. First, the fast movement of the storm itself. Second, individual storms embedded within the larger rain area will also be moving quickly in the 45-65 kt LLJ associated with the storms. Third, the soils in the area are around normal for moisture ahead of the storm. As opposed to being already saturated (worst-case scenario), or bone dry (which results in harder soils promoting runoff early on in the event), starting out with near normal moisture should optimize absorption of the rainfall, at least initially. Of course, 3-5 inches of rain may cause widely scattered instances of flash flooding anyway, but they'll be slower to develop and less extreme overall due to optimal soil conditions going in. One factor that may enhance flash flooding potential locally is the peak of fall foliage across New England, which due to the gusty winds will increase leaf-fall rates, which could quickly clog storm drains and culverts. This may result in more widespread instances of flash flooding. Given the other factors working against flash flooding, think the Slight Risk area remains the optimal forecast, though it's a high end Slight for much of eastern Maine. There remains some uncertainty with where the area in between the heavier rainfall will be. There's general consensus it will include RI, eastern MA, eastern NH, and far western ME, but that is not for sure. This sweet spot in between will likely follow or end up just west of the center of Philippe's circulation. A notable dry slot is on satellite even now associated with Philippe, and the added dynamics/jet streaks should promote the dry slot's persistence near the center. Thus, there's expected to be a local minimum of rainfall, which is highlighted by the much lower risk categories in the area mentioned. Again with some uncertainty this area may shift one way or another over the next 24 hours as the track of the center of circulation becomes more clear. Wegman 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 $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .