Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 07 2023 08:19:38 FOUS30 KWBC 070819 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Sat Oct 07 2023 Day 1 Valid 12Z Sat Oct 07 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... A highly amplified trough over the Great Lakes will become negatively tilted is it shifts into the Northeast by tonight. The increased upper level divergence east of the trough will allow a low, the extratropical remnants of Philippe, to intensify as it moves into New England and eastern Canada. Two areas of heavy rain will develop as a result. First, the aforementioned low associated with Philippe will direct a 95 kt LLJ off the Atlantic into Maine. This extreme LLJ will advect abundant moisture into eastern Maine, resulting in very heavy rain. Expect a wide swath of 3 to 5 inches of rain across much of eastern Maine as the low lifts through. The storm will be fast moving thanks to the aforementioned jet streaks, but the atmosphere will be moisture-laden as all the tropical moisture gathered by the storm rains out 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. The second area will be further west over eastern NY and far western New England from the NYC metro/western CT through VT. While there will be significantly less atmospheric moisture at any one time over the area than over Maine, since the whole pattern will be pivoting around this area the rain will persist for much longer. The result will be areas where rainfall totals will exceed 4 inches, but the continued favorable dynamics from the upper levels will allow for the steady rains to continue unabated. NASA Sport imagery shows that drier than normal soils are present over northeast NY and much of VT, while wetter than normal soils remain over southeast and downstate NY and western CT. With a bit better potential for more rainfall further north, the result will be substantially the same chances of flash flooding all along the NY/New England border. For the NYC metro, a number of CAMs suggested the potential for stationary heavy showers and thunderstorms to develop over/near NYC for much of the overnight through the morning, a scenario that at least so far has not materialized. While the threat hasn't been removed entirely, with the front over PA/upstate NY still off to the west needing to cross through later this morning, it has certainly diminished a bit, as the rainfall associated with the front itself should be increasing in forward/eastward speed. Will continue to monitor this area through the morning, but a decrease in the risk area is probable should the dry conditions continue. Wegman Day 2 Valid 12Z Sun Oct 08 2023 - 12Z Mon Oct 09 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE ATLANTIC COAST OF SOUTHEAST FLORIDA... A front currently sweeping across the Southeast will stall out over central FL by the start of the Day 2/Sunday period. The front will act as a conveyor belt for deep Gulf moisture to ride northeastward along and south of the frontal interface. Low level easterly flow will create an area of frictional convergence along the coast in the Miami-Ft. Lauderdale area. CAMs guidance shows that there's potential for strong thunderstorms that may linger over those urban areas, resulting in multiple inches of rain in a short period over a flood-sensitive area. Thus, a Marginal Risk area was introduced with this update. Wegman Day 3 Valid 12Z Mon Oct 09 2023 - 12Z Tue Oct 10 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!_1NKPtI46t7WKFK1Q0wBYanu7B70mSsHVk30nwELxiBi= MpK_zT5fCR3tK1cDCVQQJY0kikJuC0P22OVAs4CmCpEzPRA$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_1NKPtI46t7WKFK1Q0wBYanu7B70mSsHVk30nwELxiBi= MpK_zT5fCR3tK1cDCVQQJY0kikJuC0P22OVAs4CmNXAEJWI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_1NKPtI46t7WKFK1Q0wBYanu7B70mSsHVk30nwELxiBi= MpK_zT5fCR3tK1cDCVQQJY0kikJuC0P22OVAs4CmayROT1s$=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) .