Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Oct 08 2023 01:06:53 FOUS30 KWBC 080106 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 906 PM EDT Sat Oct 07 2023 Day 1 Valid 01Z Sun Oct 08 2023 - 12Z Sun Oct 08 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER MUCH OF MAINE... 0100 UTC Update -- Have trimmed the western portions of both the Slight and Marginal Risk areas based on the latest observational and guidance trends, keeping the eastern 3/4 of Maine within the Slight Risk area as the remnant low-mid level circulation and more widespread mod-heavy rainfall associated with the remnants of Philippe migrate N-NNW across this region. The remnants of Philippe, along with strong along-stream (Qs) forcing east of the amplified upper trough over the eastern Great Lakes, and strong cross-stream (Qn) forcing within the right-entrance region of the 115-125kt NNW-SSE oriented upper jet streak across western Quebec-far eastern Ontario, will support the widespread mod-heavy rainfall into the early overnight period. The good news however from an excessive rainfall perspective, especially relating to flash flooding, will be the swift progression of the favorable forcing (thus limiting the duration), while the lack of deep-layer instability will cap the rainfall rates to a large degree (as the 18Z HREF shows probabilities of 1+ inch/hr rainfall rates peaking at 50-70%, while probs of 2+ inch/hr rates remain below 15%. Hurley ....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 AND PORTIONS OF WESTERN NEW YORK... ....2000Z Update... Only minor changes to the MRGL across southeast FL to align with the latest 12z HREF blended mean QPF footprint and ensemble bias-corrected QPF. Locally heavy rainfall near the FL coast will create an urban flash flood threat Sunday afternoon. HREF probabilistic output pins a small area with enhanced rates of 2-3"/hr around 50-60% within the population centers between Miami and West Palm Beach. This is right on the cusp of the MRGL potential within that corridor, so there was no reason to deviate from the previous forecast given the signal. An additional D2 MRGL off the east side of Lake Ontario was added after collaboration with the local Buffalo WFO. More on this setup below... ....Western New York... A prolific ULL centered over the northern Ontario Province will help create a fairly dynamic low to mid-level setup for locally heavy lake effect rain bands to stream off Lake Ontario. Steep low to mid-level lapse rates exceeding 7C/km are forecast downwind of the lake thanks to a modest CAA regime on the underside of the UL disturbance. Delta-T's approaching 20-22C are expected Sunday morning and afternoon with regional instability maximum of ~500 J/kg available creating the opportunity for heavy rainfall within a narrow band focused over northern Oswego up into southern Jefferson county. Upslope enhancement across the southern Tug Hill Plateau will generate a focal point for QPF maximum >2" for the day which would fall over areas primed from today's pre-frontal convective scheme. Some thunder is even possible in the setup given the steep lapse rate forecast and instability depth lying between 1000-600mb. The best chance for heavy rainfall will occur between 17-00z as strong PVA around the base of the ULL to the north will rotate overhead creating an enhanced, uni-directional flow and steady long axis band to settle across the aforementioned area. There's some minor deviation on guidance in the exact band placement, but will be generally within the MRGL risk area added this forecast period.=20 Kleebauer ....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/Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6KbbHZFQwUA_uYycMe9YjEigzBTY9FpSPbhqtU4UXK1i= XipblX6ylnpG60gCVmrW5L_cZUD5ivPX7KdKIlKFAVKkl_4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6KbbHZFQwUA_uYycMe9YjEigzBTY9FpSPbhqtU4UXK1i= XipblX6ylnpG60gCVmrW5L_cZUD5ivPX7KdKIlKFjmAGLZs$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6KbbHZFQwUA_uYycMe9YjEigzBTY9FpSPbhqtU4UXK1i= XipblX6ylnpG60gCVmrW5L_cZUD5ivPX7KdKIlKFngUPwLg$=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) .