Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 172121 AFDOKX Area Forecast Discussion National Weather Service New York NY 421 PM EST Fri Nov 17 2017 .SYNOPSIS...
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Low pressure over the Canadian maritimes will move farther away tonight, allowing high pressure to the west to move across tonight into early Saturday. A warm front will lift to the north later Saturday, then a strong cold front will approach Saturday night, and sweep sweep eastward Sunday morning as strong low pressure passes to the north. A few weather disturbances are possible later next week.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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Winds have held on a little longer into the afternoon as is often the case, but should quickly diminish this evening with loss of daytime heating. Fairly good radiational cooling conditions overnight should lead to a wide range of low temperatures, from the upper 30s in NYC, to the 20s in most outlying areas, and even some teens in the Long Island Pine Barrens and some inland valley locations especially over interior eastern CT.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/...
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Main concern will be an intensifying low moving into the Ohio Valley Sat morning, then across the lower Great Lakes Sat night and into the St. Lawrence Valley in Sunday. An associated warm front will lift north of the area daytime Sat, with some showers moving into the area from NYC north/west in the afternoon. The pressure gradient between the departing high and the approaching low will quickly tighten up, with S-SW winds gusting to 25-30 mph in most places, and as high as 35 mph across Long Island. Main impacts ahead of and following a strong cold trailing the low expected Sat night into Sunday, with S-SW winds increasing further and a band of moderate to heavy showers sweeping across with the front overnight. Models show fairly good low level mixing with max boundary layer winds from 40-55 kt, highest across eastern Long Island and SE coastal CT. Do not think the full complement of these winds will mix down to the surface, but gusts up to 35 mph are likely inland, 40-45 mph in NYC metro and along most of the coastline, and 50-55 mph across eastern Long Island. Confidence level in seeing winds over 60 mph is not quite high enough to warrant a high wind watch for eastern coastal sections, but this could change with later forecasts. Will have to watch band of moderate to heavy showers with the cold front itself for best low level momentum transfer on the western fringe of the low level jet. Winds could abate somewhat right after cold fropa, then should pick up from mid morning into the afternoon after the passage of a strong mid level vort max, via strong downward momentum transfer of W-NW 40-kt H9 winds to the surface, with gusts up to or just over 45 mph likely during this time frame.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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A broad area of high pressure will build into the region Monday and Tuesday. This will produce fair weather. SuperBlend was used for temperatures except for Monday night, where MEX was used to better capture radiational cooling. Wednesday remains a bit of a question. A cold front will be approaching from the west. In addition, there remains the suggestion that some semblance of a coastal low will develop. The 12Z GFS is way off the Southeast coast and not a factor. The 12Z ECMWF is trending west and now produces a solid swath of rain across the area. The forecast has been trended towards the ECMWF, with low chances for precipitation included for all areas. The rain would help to suppress high temperatures as well. The cooler SuperBlend was used as a result. The cold front comes through Wednesday night. The ECMWF is much different than the GFS, which pours the cold air into the region. The ECMWF keeps the coldest air locked to the north of the area, most likely due to the dynamics of the strengthening offshore low. A blend of the models was used, however depending on how the pattern ultimately develops, Thanksgiving temperatures could end up being roughly plus or minus 5 degrees from the current forecast for highs. All solutions are dry for the day however, followed by a continuation of the dry weather on Friday. The GFS does spin up a low along the subtropical jet and tracks it east of the area on Friday. The ECMWF spins up something similar on Saturday. This feature will need to be watched for any westward trend.
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&& .AVIATION /21Z 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 20-25 kt, occasionally a few kt 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. ...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 40-50 kt at 2 kft AGL. 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 20-25 kt 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...
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SCA conditions will end from west to east through this evening as high pressure builds from the west. A Storm Watch has been issued for the eastern ocean waters for late Sat night into early Sunday morning, for frequent gusts up to 50 kt in S-SW flow ahead of an approaching strong cold front. Gale warnings have been issued elsewhere, beginning Sat afternoon on the western ocean waters, and elsewhere Sat evening, with S-SW winds increasing to 40-45 kt Sat night. W-NW gales should continue after the cold frontal passage into at least Sunday afternoon, and possibly into Sunday night especially on the ocean. SCA conds will follow into Mon, and are still possible mainly on the ocean into Tue. Conditions may improve to below advy criteria on Wed.
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&& .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...
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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 the high tide cycle Sat morning or Sunday morning along the south shore back bays of Long Island. The most susceptible area to minor coastal impacts may be eastern portions of the Great South Bay and Moriches Bay with the high tide cycle Sunday morning, due to strong SW flow.
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&& .OKX WATCHES/WARNINGS/ADVISORIES...
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CT...None. NY...None. NJ...None. MARINE...Gale Warning from 6 PM Saturday to 6 PM EST Sunday for ANZ330- 335-338-340-345. Small Craft Advisory until 6 PM EST this evening for ANZ330- 340-345-353-355. Storm Watch from late Saturday night through Sunday morning for ANZ350-353. Gale Warning from 1 PM Saturday to midnight EST Saturday night for ANZ350-353. Gale Warning from 1 PM Saturday to 6 PM EST Sunday for ANZ355. Small Craft Advisory until 10 PM EST this evening for ANZ350.
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&& $$ SYNOPSIS...JMC/Goodman NEAR TERM...Goodman SHORT TERM...Goodman LONG TERM...JMC AVIATION...JM MARINE...JMC/Goodman HYDROLOGY...JMC/Goodman TIDES/COASTAL FLOODING...Goodman

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