Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 230847 AFDOKX Area Forecast Discussion National Weather Service New York NY 447 AM EDT Mon Oct 23 2017 .SYNOPSIS...
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Strong high pressure continues to pull away from the mainland today. A slow moving frontal system approaches Monday night, impacting the region Tuesday into Wednesday with strong winds gusts and potentially heavy rain. This frontal system slowly moves east Wednesday. Higher pressure builds in for late Wednesday into Thursday, then moves offshore Friday and Saturday. A cold front impacts the region Saturday night and Sunday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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Fog/stratus has developed across the area early this morning due to increasing low level moisture, light to calm sfc winds and mostly clear skies. Fog is mainly confined to areas east of NYC and southern CT and is locally dense at times. Have issued an SPS to account for this. Vsbys have been fluctuating up and down, so no dense fog advsy yet, but could be issued in the next hour or two if 1/4 mile vsbys become more widespread. Fog will burn off by mid morning, although expect partly to mostly cloudy skies with stratocu development. Dry weather is expected through the day with limited atmospheric moisture. Highs remain above normal levels topping out in the upper 60s to lower 70s across the region.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/...
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NWP guidance is in good agreement through Tue night. As strong high pres over the western Atlantic continues to depart, vigorous PAC shortwave/jet energy over the Central US this evening will continue to the Gulf coast by Tuesday Night. The resultant -3 to -4 standard deviation deep eastern US longwave trough will then tilt towards the NE US for Midweek. with a full latitude trough developing across the eastern US. The associated frontal system will approach the region tonight, and then track very slowly across the region Tue/Tue night in the longitudinally oriented steering flow. Trend with this system has been slower based on the upper pattern and dprog/dt. Low level moisture does begin to increase tonight, although with dry midlevels only expect DZ. QPF fields for tonight appear overdone with this. Moisture and forcing increases Tue as the frontal system approaches, presenting the following potential hazards Tue/Tue night: Heavy Rain: The slow movement of the front, deep lift (potentially right rear of 150+ kt jet streak), weak instability, and interaction with an increasingly moist and tropical airmass (+3 to 4 std) signal potential for multiple bands of heavy rain as weak waves lift north along the front. Lower resolution ensembles continue to range between 1 1/2 to 3 inches of rain. Higher res NAM is indicating the potential for a localized swath of 3-5 inches. This looks to be a credible threat based on synoptic pattern, but location/timing is very much uncertain based on front and frontal wave timing and location. Would also like to see additional support of this in other hi-res models. See hydrology section for associated flooding threat. Strong Winds: The other potential hazard is for strong winds during the Tue into Tue Night period as a 50-60 kt 950 hpa LLJ moves overhead Tue afternoon into Tue night. NAM/GFS BUFKIT soundings indicating a 12-15 hr period of southerly winds gusts of 30 to 40 mph, with potential for any low topped convective line/s ahead of the cold front being able to drag down localized 45-55 mph wind gusts to the surface. SPC has placed western portions of the area in a slight risk with damaging wind gusts the main threat, although a isolated tornado cannot be ruled out.
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&& .LONG TERM /WEDNESDAY THROUGH SUNDAY/...
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A full latitude longwave northern stream trough will become slightly negative during Wednesday and then weaken and lift to the northeast Wednesday night into Thursday. Meanwhile a surface cold front will be crossing the region early Wednesday with a line of showers and embedded thunderstorms expected to be ongoing. At the beginning of Wednesday this line is expected to be in the vicinity of central Connecticut and into western Long Island. With surface based instability and strong winds aloft of 30 to 45 KT, some stronger winds in the heavier showers and thunderstorms will mix to the surface and will continue with the enhanced wording of heavy rain and gusty winds. An impulse moving into the negatively tilted trough Wednesday night may bring a few sprinkles. Will leave probabilities below slight chance as forcing becomes much more limited behind the cold front. The trough axis moves across Thursday and well northeast of the region Thursday night. The front moves slowly farther east of the region Thursday into Thursday night. High pressure then moves in on Friday and quickly goes offshore, shifting eastward heading into the start of next weekend before another frontal system approaches for next Sunday. Weak ridging builds aloft late Wednesday night into Thursday night before weakening as the next northern stream trough digs into the central United States and becomes fully amplified. This trough moves slowly to the east Friday night into the upcoming weekend. Forcing with the cold front will be much weaken than with the early week system, however, there will still be a chance of precipitation Saturday night into Monday.
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&& .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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High pressure moves east of the area today. A slow moving cold front approaches tonight. Patchy fog and stratus has developed mainly outside of the NYC terminals. Some of the cigs/vsbys have fallen to LIFR or less. Fog and low stratus gradually improve this morning, mainly between 12- 14z. Then, during the mid-late morning and afternoon hours, generally VFR conditions are anticipated, but patchy MVFR ceilings are possible. Patchy drizzle may be possible this evening, then showers become more widespread from west to east after 06z with MVFR/IFR cigs. Light SE winds tonight remain SE and increase Monday. Speeds of 5 to 10 kt after 13-14Z increasing to 10 to 15 kt in the afternoon. A late day gust or two to 20 kt is possible, especially near the coast (NYC and LI terminals). Wind increase and gust Monday night to around 20kt. ...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: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. The afternoon KJFK haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KLGA TAF Comments: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. The afternoon KLGA haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KEWR TAF Comments: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. The afternoon KEWR haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KTEB TAF Comments: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. KHPN TAF Comments: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. KISP TAF Comments: Amendments may be needed for changing flight categories today. Cigs may remain MVFR longer than forecast. .OUTLOOK FOR 06Z TUESDAY THROUGH FRIDAY... .Mon night-Tue...Potential period of S/SE G30-40 KT with LLWS SSW 45-55kt@2kft for coastal terminals. MVFR/IFR SHRA continues with low prob/sparse TSRA Tue. .Tue night-Wed...IFR possibly continues in SHRA Tue night, low prob/sparse TSRA early evening, improving to MVFR and possibly VFR by late Wed. .Wed night-Thu...VFR, with iso-sct MVFR in SHRA. .Fri...VFR.
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&& .MARINE...
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Quiet conditions on all waters today as high pressure moves further out to sea. The pressure gradient will begin to tighten tonight as low pressure strengthens over the Great Lakes. SCA conds are expected to develop tonight with the potential for marginal gale force gusts Tue into Tue night on all waters. Have issued a gale watch for this potential, although the higher winds may be more convectively correlated. Winds subside late Tuesday night into Wednesday morning in the wake of the frontal system. Winds and gusts are then forecast to remain below small craft advisory levels through Friday night as high pressure builds into the waters. However, ocean seas are expected to remain at or above 5 feet Wednesday into Friday night while slowly subsiding.
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&& .HYDROLOGY...
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A widespread 2 to 3 inches of rain is likely Tuesday into Wednesday morning, with locally 3 to 5 inches. Concern is that much of this rain could fall in a 6 hr period in slow moving and/or training bands of low topped convection/heavy rain, but the predictability on location of where this occurs is low. Despite antecedent dry conditions, these potential rainfall rates present an areawide potential for minor urban and poor drainage, with a localized threat for flash flooding. In addition, if the heavier rain amounts occur over Northeastern New Jersey and the Lower Hudson Valley, rapid rises in flashy small streams and minor flood impacts to adjacent areas would be plausible. Heavy rain threat diminishes Wednesday afternoon, with drying conditions expected Thursday night through Saturday.
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&& .OKX WATCHES/WARNINGS/ADVISORIES...
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CT...None. NY...None. NJ...None. MARINE...Gale Watch from Tuesday morning through late Tuesday night for ANZ330-335-338-340-345-350-353-355. Small Craft Advisory from 2 AM to 6 AM EDT Tuesday for ANZ330- 335-338-340-345. Small Craft Advisory from 8 PM this evening to 6 AM EDT Tuesday for ANZ350-353-355.
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