Area Forecast Discussion
Issued by NWS Upton, NY

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512 FXUS61 KOKX 172004 AFDOKX Area Forecast Discussion National Weather Service New York NY 304 PM EST Fri Nov 17 2017 .SYNOPSIS... Low pressure over southeast Canada will continue moving east this afternoon, 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/... Wind gusts are trending down a little as the upper level trough axis has pushed farther east of the region, taking the better cold air advection with it. Heights are building aloft in response as surface high pressure builds in from the west. Clouds are decreasing with the subsidence of this high so sunny conditions are forecast. Wind gusts of mainly now 20-25 KTS are expected to diminish going into this evening. It will be a cool day especially considering the wind, making for apparent temperatures a few degrees cooler than actual temperatures, with highs near the mid and upper 40s for most locations. && .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 FRIDAY/... 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 /20Z FRIDAY THROUGH WEDNESDAY/...
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High pressure builds south of the region and then offshore as a low pressure system approaches during the TAF period. VFR is forecast through early Saturday with high confidence. Chances of MVFR will increase later Saturday afternoon with rain showers approaching so as a result, the confidence in the category forecast will lower Saturday afternoon into Saturday night. Wind gusts mostly 20-25KT, occasionally a few KTS higher but high confidence of gusts continuing through the remainder of this afternoon. Winds diminish tonight and then become more southerly Saturday with speeds picking up again by late Saturday morning into early Saturday afternoon. Wind direction this afternoon is mostly 300-320 magnetic. Wind direction becomes more variable tonight. Wind direction forecast has moderate confidence with observed directions occasionally varying 20-30 degrees from forecast. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/www.weather.gov/zny/n90 KJFK TAF Comments: Gust end time this evening could be 1-2 hours later than TAF indicates. KLGA TAF Comments: Gust end time this evening could be 1-2 hours later than TAF indicates. KEWR TAF Comments: Gust end time this evening could be 1-2 hours later than TAF indicates. KTEB TAF Comments: Gust end time this evening could be 1-2 hours later than TAF indicates. KHPN TAF Comments: Gust end time this evening could be 1 hour earlier than TAF indicates. KISP TAF Comments: Gust end time this evening could be 1 hour later than TAF indicates. .OUTLOOK FOR 20Z SATURDAY THROUGH TUESDAY... .Saturday...MVFR or lower conditions become a greater possibility afternoon into the night as showers move in. LLWS at night, SW flow 40-50KT at 2kft. S-SW winds G25-35KT and occasionally higher especially in any heavier rain. .Sunday...Conditions gradually improve to VFR with showers ending by afternoon. NW winds G30-40KT. Gusts diminish after early Sunday evening, more in the 20-25KT range. .Monday...VFR. W-NW winds G20-30KT. Gusts diminish gradually at night. .Tuesday...Mainly VFR. MVFR possible late Tuesday night with showers. SW winds G15-20KT. .Wednesday...MVFR possible with showers. W-NW winds G15-20KT.
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&& .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...JM MARINE...MD/Goodman/DW HYDROLOGY...MD/DW TIDES/COASTAL FLOODING...

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