Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Oct 19 2023 20:08:00 FOUS30 KWBC 192007 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 407 PM EDT Thu Oct 19 2023 Day 1 Valid 16Z Thu Oct 19 2023 - 12Z Fri Oct 20 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Otto Day 2 Valid 12Z Fri Oct 20 2023 - 12Z Sat Oct 21 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. ....Northeast... Precipitable water values will be increasing ahead of an eastward advancing upper trough over the Great Lakes/Midwest on Friday with values forecast to locally exceed 1.2 inches from the northern Mid-Atlantic coast into southern New England. A zone of 30 to 50 kt of southerly flow at 850 mb will focus from New Jersey into and across NYC/Long Island into southern New England beneath areas of upper divergence and diffluence associated with an increasingly negatively tilted trough axis aloft. Modest moisture values and forecasts of only weak instability (~500 J/kg or less MUCAPE) should limit rainfall rates across the Northeast. However, the potential will exist for localized rainfall rates near 1 in/hr and highly localized 2 inch per 3 hour totals. While the threat for exceedance of FFG is non-zero, it is considered too low to warrant a risk area for the Day 2 outlook. Areas of heavy rain will continue through Friday night into Saturday across the Northeast, with a Marginal Risk forecast for the Day 3 outlook. Otto Day 3 Valid 12Z Sat Oct 21 2023 - 12Z Sun Oct 22 2023 .....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF NORTHEASTERN NEW YORK INTO NEW ENGLAND... Areas of heavy rain should be ongoing across the Northeast Saturday morning as mid-level shortwave energy moving through the middle of a longwave trough over the eastern U.S. becomes negatively tilted by Saturday morning, with the axis extending from western New York to off of the northern Mid-Atlantic coast. At the surface, a minimum of two surface lows are forecast to be in place 12Z Saturday in the vicinity of the Northeast. The main/primary low with attached occluded/cold fronts is expected to be just south of southern Nova Scotia while a secondary low with an attached secondary cold front is expected to be near/south of Long Island. While the main plume of moisture (2+ inch PWATs) will be well offshore ahead of the primary cold front, anomalous moisture (+1 to +2 standardized anomalies) with 1.0 to 1.3 inch PWATs should wrap back to the west into New England. Through 00Z Sunday, while the details of the evolution are a bit unclear, consolidation into a single surface low is expected to occur toward the Gulf of Maine with an inverted surface trough extending northwestward/westward into New England. Weak instability is forecast by the model consensus to be in place over southern New England from 12-00Z where some slow movement of heavy rain will be possible near the surface low/inverted trough in the vicinity of the southern New England coast, just ahead of the negatively tilted uper trough axis swinging across the region. However, limited instability should favor rainfall intensities that are below area FFG values. For the remainder of New England, 10 to 20 kt (locally higher) 850 mb winds from the southeast will focus into New England ahead of the developing surface low with some upslope component helping to enhance rainfall intensity. As the consolidated surface-850 mb low moves away from the region into the Canadian Maritimes through Saturday night, convergence along an attached low level inverted trough axis pivoting across portions of New England, beneath an evolving deformation zone, is expected to support slow moving areas of heavy rain. While a lack of instability should keep 1 hour rainfall rates within this regime low (no more than 0.5 in/hr is expected), the accumulation of rainfall over the 24 hour period could amount to localized 2 to 4 inch totals across northeastern New York into Vermont, New Hampshire, Maine and perhaps Massachusetts. The 12Z GEFS/EC/CMC ensembles show that northern Maine has relatively lower probabilities of exceeding 2 inches of rainfall for the 24 hour period compared to the rest of New England, but northern Maine also has the highest soil moisture values (via NASA SPoRT) which may leave the region somewhat more vulnerable to runoff. The Marginal Risk area from continuity appears to be in good shape and remains the same for this update, especially given some of the lingering uncertainty in forecast details. Otto Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8l4t0LyLKe6csk_PMDheZoyu_MZuorJ2CnzqI-nWeSLh= 3EaRfnVJtEVVn8x7utffo3I64phe5WsqABc_dIp_mPhza94$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8l4t0LyLKe6csk_PMDheZoyu_MZuorJ2CnzqI-nWeSLh= 3EaRfnVJtEVVn8x7utffo3I64phe5WsqABc_dIp_x8ygpHY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8l4t0LyLKe6csk_PMDheZoyu_MZuorJ2CnzqI-nWeSLh= 3EaRfnVJtEVVn8x7utffo3I64phe5WsqABc_dIp_kr_k_Cc$=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. 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