Area Forecast Discussion
Issued by NWS Upton, NY

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351 FXUS61 KOKX 171550 AFDOKX Area Forecast Discussion National Weather Service New York NY 1050 AM EST Fri Nov 17 2017 .SYNOPSIS... Low pressure over southeast Canada will move east today, while high pressure builds from the west. Strengthening low pressure moving northeast across the lower Great Lakes on Saturday will then send a warm front across, followed by a cold frontal passage late Saturday night into Sunday morning. High pressure will then build to the south on Monday. Another frontal system will move through Tuesday night into Wednesday, followed by high pressure for late week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Secondary shot of cold advection with the passage of the upper trough this morning has allowed NW gusts to ramp back up to around 30 kt, or 35 mph. These should be the peak and will slowly diminish through the day. Upper low departs across eastern Canada today with building heights aloft as surface high pressure builds in from the west. It will be a cool day, with highs in the mid and upper 40s. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Ridging both aloft and at the surface moves through the area tonight with diminishing winds. Overnight lows will generally be in the 20s, but in the 30s closer to the coast and the NYC metro. A progressive upper flow will feature another amplifying upper trough approaching the Great Lakes and Ohio Valley states on Saturday. Preference was to go with a deeper, more consolidated surface low tracking across the Midwest and into the Lower Great Lakes by Saturday evening. This combined with high pressure moving off the east coast will result in strengthening southerly winds and gradually deepening moisture. Much of Saturday though should be dry with the exception of western portions of the Lower Hudson Valley and NE NJ, as warm advection ahead of the system encounters a pronounced dry layer in the low and mid levels. The uncertainty lies in how quickly these layers moisten ahead of a strengthening SW low-level jet. Rain could move in faster should this process occur more quickly. Highs will top out in the lower to mid 50s on Saturday, which is closer to seasonable levels. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Strong warm advection will dominate the early portion of Saturday night as a potent short wave and attendant cold front approach from the west. Given a significant low level mass response to the intensifying upper system, the primary uncertainty at this point lies within how much wind can mix to the surface prior to the frontal passage. NAEFS suggests low-level jet speeds significantly stronger than normal for this time of year - in the 90-98th percentile from 850-700 mb. At 925 mb, 50-60kt winds will overspread eastern portions of the forecast area - primarily eastern Long Island and Connecticut. Questions remain on how much will be able to mix to the surface, though given the antecedent dry air mass, there may be a favorable time prior to saturation where these stronger gusts may mix to the surface. As such, if confidence increases there is potential for at least a wind advisory to begin as early as Saturday night. Temperatures Saturday night depend on how quickly the front moves through, though timing is currently more towards morning, so expect that in general with the exception of perhaps up towards Orange County, that temperatures will be above normal. The front moves through Sunday morning from west to east, bringing a brief period of moderate to heavy showers and possibly an isolated thunderstorm. Significant cold advection follows with strong, gusty winds in its wake. Although the previously mentioned low-level jet will have moved east of the area, flow following the deepening, but exiting low will remain strong, with advisory level west-northwest winds possible into Monday night. High temperatures will be somewhat mild, but will likely cool through the afternoon as cold advection continues, falling back to climatological normals Sunday night. Winds will remain strong enough between the exiting low and high pressure developing to the south to prevent lows from falling too far below normals, though if winds decrease faster than forecast, primarily across the lower Hudson Valley, then lows could fall well below freezing. By Monday, winds will remain strong and gusty, though to a lesser extent than Sunday. Given the strong west-northwest flow and a surface trough moving through, there could be enough low-level moisture and instability to support a quick passing shower, or perhaps a snow shower in the lower Hudson Valley. Otherwise, dry and seasonable conditions can be expected through Tuesday as high pressure passes to the south. Less confidence exists for mid-week as another front approaches the area, with Euro and GEFS suggesting at least light precipitation closer to the coast in a brief period of warm advection. As such, left at least slight chance to chance PoPs in the forecast. Dry conditions follow for the remainder of the week in building high pressure. && .AVIATION /16Z FRIDAY THROUGH TUESDAY/... High pressure builds to the west through the day. VFR forecast through the TAF period. W-WNW wind gust to 25-30 KT will gradually subside this afternoon. Wind direction of 310-320 magnetic will waver around or back to the left of 310 magnetic in the afternoon. Winds diminish late tonight and become more southerly into Saturday morning. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/ KJFK TAF Comments: Occasional gusts may be higher than forecast today. The afternoon KJFK haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KLGA TAF Comments: Occasional gusts may be higher than forecast today. The afternoon KLGA haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KEWR TAF Comments: Occasional gusts may be higher than forecast today. The afternoon KEWR haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KTEB TAF Comments: Occasional gusts may be higher than forecast today. KHPN TAF Comments: Occasional gusts may be higher than forecast today. KISP TAF Comments: Occasional gusts may be higher than forecast today. .OUTLOOK FOR 15Z SATURDAY THROUGH TUESDAY... .Saturday...MVFR or lower conditions possible by the afternoon and becoming likely towards evening. Showers at night S-SW winds G25- 35KT. .Sunday...Conditions improve to VFR with showers ending. NW winds G30-40KT. .Monday...VFR. NW G20-25KT. .Tuesday...VFR. SW G20KT. && .MARINE... SCA conditions will end from west to east through this evening as high pressure builds from the west. A gale watch has been issued for the ocean waters starting Saturday afternoon as deepening low pressure tracks into the lower Great Lakes and high pressure retreats over the western Atlantic. Given a strong low-level jet developing prior to the cold frontal passage Saturday night, it is possible that gale to storm force winds will mix down to the ocean waters and possibly the eastern Sound. Gales will likely continue into Sunday night following the cold frontal passage. Expect at least SCA-level conditions thereafter in gusty WNW flow, slowly subsiding through mid week as high pressure passes to the south. && .HYDROLOGY... A band of moderate to locally heavy showers with a strong cold frontal passage late Sat night should cause no more than local nuisance impacts. && .TIDES/COASTAL FLOODING... Storm surge guidance with a strong cold frontal passage may to be on the low side given the strength of southerly winds expected Sat night, and possibly into Sunday morning if the frontal passage continues to trend more slowly. Typically though, S/SW winds are not conducive to building surge. There is a low probability of a surge of 1-2 ft, which if occurred would result in minor coastal flooding with either of the two high tide cycles late Sat into Sunday morning along the south shore back bays of Long Island, and along western Long Island Sound. The most susceptible area to minor coastal impacts with this system may be eastern portions of the Great South Bay and Moriches Bay, due to the SW flow. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...Small Craft Advisory until 3 PM EST this afternoon for ANZ335- 338. Small Craft Advisory until 6 PM EST this evening for ANZ330- 340-345-353-355. Gale Watch from Saturday afternoon through Sunday afternoon for ANZ350-353-355. Small Craft Advisory until 10 PM EST this evening for ANZ350. && $$ SYNOPSIS...MD/DW NEAR TERM...Goodman/DW SHORT TERM...DW LONG TERM...MD AVIATION...BC/JM MARINE...MD/Goodman/DW HYDROLOGY...MD/DW TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.