Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Oct 07 2023 16:06:46 FOUS30 KWBC 071606 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1205 PM EDT Sat Oct 07 2023 Day 1 Valid 16Z Sat Oct 07 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE NORTHEAST... ....16Z Update... Primary focus of the D1 remains in the Northeast U.S with two distinct regions of interest being along and ahead of the cold front and over ME where remnants of what was Tropical Storm Philippe move inland later this afternoon and evening. The signal for elevated FF potential continues across eastern NY into western VT where convergent flow extending from the Mid-Atlantic continues to propagate eastward along and ahead of a cold front currently analyzed over eastern PA up through central NY state. Local observed rates between 1-2"/hr have been documented across the Hudson Valley extending into NY North Country with the primary hourly rate settled between 0.25-0.5" outside the enhanced areas. 12z HREF mean QPF was steadfast across NY state into VT/NH with the axis of heaviest QPF still along the Green Mtns in VT where deep convergence signatures over the terrain provide the best localized enhancement scheme over the western side of the Northeast US ERO placement. Now through about 21z is the primary time frame for impacts for any FF potential with a rapid down-scaling of potential as we march into the evening as the best low-mid level forcing vacates behind the frontal boundary as it traverses the area. An extension of the MRGL was collaborated with the local Buffalo FO due to expected heavy lake effect increasingly likely off the eastern side of Ontario with guidance becoming a bit more robust in the overall QPF footprint within a zone from Oswego/Southern Jefferson counties which would align within the southern Tug Hill area. Over ME, expected heavy rainfall over the northern half of the state will begin late this afternoon and evening as increasing easterlies advect tropical moisture off the Atlantic over the coastal plain of New England. There's a better signal for a heavy axis of rainfall between Rockland up to about 15-20 miles northeast of Bar Harbor and points north. Through collaboration with the local Gray/Caribou WFO's, maintained a SLGT risk for the area due to a persistent signal for rates settling between 1-2"/hr for the short 3-6 hr time frame of heaviest rainfall. 12z HREF probabilities for 3"/3-hrs was between 30-40% within the aforementioned corridor, but the deterrent for upping the risk was the lack of enhanced rates exceeding 2"/hr where probabilities were only a low 10-20% for a short period before disappearing for the remainder of the storm. The forward propagation of the heaviest precip is a positive for limiting the potential as well as the 6 hr QPF forecast indicates a rapid onset and rapid decline thereafter of the heavy precip shield, so the time frame of interest will be within a window of 00-09z for much of any flooding threat before quickly shifting in Canada. Thus, have maintained the SLGT risk with more of a higher-end slight by definition as local impacts could still be significant, but short-lived. A MRGL risk was added for the southeast FL coast due to sea breeze convergence signals for locally heavy rainfall over the urban corridor north of Miami. More on this threat below... Kleebauer ....Northeast U.S... 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 .....Southeast Florida... Local QPF maximum stemming from area convective development along the sea breeze will enhance flash flooding concerns within the urban corridor focused north of Miami up through West Palm Beach. 12z HREF probabilities indicate some higher rates between 2-3"/hr increasingly likely within that narrow zone along the coast which could cause some local flooding concerns for a short time this afternoon. This is a lower end threat, but still lies within that MRGL flash flood threat window, so made the addition with aid from local hi-res deterministic and ensemble probabilities for higher rain rates. Kleebauer 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!51O4fmGzVQVwYsY58cPThmiXnTJHPCnABEVgBCDWxeHI= G45EdVtTEBWycre44XDkrFAyvd-qw11QGmSX2dWQg8bs_bc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!51O4fmGzVQVwYsY58cPThmiXnTJHPCnABEVgBCDWxeHI= G45EdVtTEBWycre44XDkrFAyvd-qw11QGmSX2dWQ0lbzCLo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!51O4fmGzVQVwYsY58cPThmiXnTJHPCnABEVgBCDWxeHI= G45EdVtTEBWycre44XDkrFAyvd-qw11QGmSX2dWQJG8HjuM$=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) .