Area Forecast Discussion
Issued by NWS Upton, NY

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217 FXUS61 KOKX 232102 CCA AFDOKX Area Forecast Discussion...CORRECTED National Weather Service New York NY 502 PM EDT Sun Jul 23 2017 .SYNOPSIS... A frontal boundary remains to our south tonight. Low pressure then moves along the front, tracking off the mid-Atlantic coast Monday. High pressure builds over the northeast late Tuesday through Wednesday, then drifts off the coast Wednesday night. Another wave of low pressure follows for Thursday into Saturday. High pressure returns Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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A stationary front will remain to the south of the CWA tonight. A weak area of low pressure develops along this frontal boundary and slowly tracks northeast, situated near the New York City metro area late tonight into early Monday morning. This will be the focus for showers to start off with, but a more widespread rain will develop as the low approaches. There is a period of heavy rainfall possible, generally from 2 am to 8 am for New York City and points west and from 4 am to 10 am for points east. All the deterministic models have increased QPF from their previous runs, and in particular, the 12Z GFS had twice as much QPF from 2 am to 8 am Monday. There is also the possibility of 1 to 2 inch rainfall rates in the metro area as per the NCAR ensemble, which shows a 50-70% chance of seeing rainfall rates greater than 1" at some point overnight, from 4 am to 6 am, just before the morning rush. Thereafter, a period of moderate rain is still possible. Also of note, the 12Z ECMWF had a bulls eye of over 2" of rainfall just north of the city. Therefore, a Flash Flood Watch was issued for all of New York City, western Long Island, northeast New Jersey, and portions of the Lower Hudson Valley. The rest of the CWA has a chance of seeing flash flooding, however, there is more uncertainty with where the axis of heavy rain will be for these areas, so did not issue a watch for these areas. This will need to be looked at overnight. SPC has placed portions of New York City and northeast New Jersey in a marginal risk for severe thunderstorm through 8 am Monday. However, stability does not look impressive with this low, and did not mention any severe, though an isolated severe is not out of the question. There is a moderate risk for the development of rip currents at ocean beaches through this evening.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/...
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Low pressure will pull away from the area Monday afternoon. This will allow rainfall to taper off from west to east from Monday morning into the afternoon. However, chances for rain continue as the upper level trough remains over the area through Monday night, with just a slight chance for the entire area from 3 am onward Monday night. Rainfall amounts will not be nearly as high as for Monday morning. SPC has placed much of the CWA, except southwest Connecticut in a marginal risk for severe thunderstorm Monday into Monday night. However, stability does not look impressive with this low, and did not mention any severe, though an isolated severe is not out of the question.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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The extended forecast mainly remains consistent with the previous forecast with the exception that precipitation is expected remain into Saturday as was hinted at by the ECMWF late Saturday, as an upper trough is slower to move east. The northern stream flow along the US/Canadian border remains nearly zonal and progressive with embedded shortwaves moving through the flow. One shortwave will be exiting the northeastern coast, along with the accompanying surface low, Tuesday. CAPE and instability remain limited so will continue with showers. A weak ridge axis builds into southern Canada from the southern ridge over the southwest and moving into the plain states mid week. Another shortwave was coming onshore of the Canadian Pacific coast as seem on the upper level water vapor loop. And these system has been picked up by the models. This shortwave open and tracks quickly across southern Canada then digs and large trough along the eastern United States late in the week into the weekend as a western north Atlantic ridge builds and blocks the eastern trough. This will keep waves of low pressure tracking along a frontal boundary Thursday into Saturday before the last one exits later Saturday. So now have chances of precipitation through Friday into Saturday. The upper trough weakens Sunday as surface high pressure builds to the north resulting in a dry Sunday. Temperatures through the extended period will be near seasonal normals, except for being below normal Tuesday.
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&& .AVIATION /20Z SUNDAY THROUGH FRIDAY/...
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A stationary front will remain to our south into Monday morning, with a wave of low pressure moving along it towards the region tonight, then passes to the south into Monday afternoon. Conditions become MVFR at all terminals by late evening/early morning. Showers also overspread the area, mainly after midnight. in addition, an isolated rumble of thunder cannot be ruled out during the overnight hours as well, with the best chance from the city terminals on N/W. IFR conditions are probable by around sunrise, then should improve back to MVFR by mid morning. Winds become E throughout by early morning at around 10-15kt. Gusts to around 15-20kt are probable overnight for CT/Long Island Terminals/KLGA and briefly at KJFK. Widespread 15-20kt gusts are probable throughout by mid Monday morning, as the winds back to the NE. .OUTLOOK FOR 18Z MONDAY THROUGH THURSDAY... .Monday afternoon-Tuesday...MVFR likely. IFR, NE-E winds G15-25KT and isolated-Scattered showers are possible. .Tuesday night...MVFR likely. IFR and NE winds G15-25KT are possible. .Wednesday-Thursday...VFR. There is a chance of showers and possibly a thunderstorm mainly to the N/W of city terminals Thursday afternoon. .Thursday night-Friday...MVFR or lower conditions and isolated to scattered showers and thunderstorms are possible.
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&& .MARINE...
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SCA waves of 5 ft are possible over the ocean waters tonight, generally after 10 pm as a persistent easterly flow allows waves to build. Winds gust of up to 30 kt are possible over the ocean after 2 am, and the Sound, Peconic Bay, and Gardiner`s Bays after 7 am Monday morning as a weak area of low pressure develops over a stalled frontal boundary and heads northeast, passing south and east of Long Island into Monday morning. Winds should diminish below 25 kt over most waters by the afternoon, except for eastern areas, which may remain above 25 kt through the day Monday. Waves remain above 5 ft through Monday night. A persistent easterly flow will be on going Tuesday, with winds and gusts below small craft advisory levels. However, ocean seas will be at small craft levels. The easterly flow continues into Wednesday and then gradually diminishes and becomes southeasterly as high pressure moves well east of the forecast waters. Ocean seas are expected to subside below 5 feet during Wednesday. Then winds and seas remain below small craft levels Wednesday night into Friday.
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&& .HYDROLOGY... Heavy rainfall is possible late tonight into Monday morning for the entire area. Generally, 1 to 1.5 inches is forecast, with locally higher amounts possible in any thunderstorms. A Flash Flood Watch has been issued for portions of the area (see above) for this threat. In particular, the New York City area could see a period of heavy rainfall late tonight into early Monday morning, leading to urban flooding. Additional rainfall is possible Thursday through Friday night. Hydrologic impacts are not expected at this time. && .TIDES/COASTAL FLOODING...
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With the new phase of the moon today, tides are running high. Positive tidal departures of 1/2 to 1 ft are needed for minor flooding during the night time high tide. With a SE to E flow expected, and looking at what occurred last night and with this mornings high tide, have continued the coastal flood advisory for western LI sound, the south shore bays of LI, and lower NY Harbor for the evening and night time high tides (will add E Union with this update). A few locations could reach or slightly exceed moderate benchmarks across the south shore bays of Nassau and Queens. The threat for minor flooding could continue into Monday (possibly the morning and likely the night time high tide), with E/NE flow progged to continue.
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&& .OKX WATCHES/WARNINGS/ADVISORIES... CT...Coastal Flood Advisory from 11 PM this evening to 2 AM EDT Monday for CTZ009. NY...Flash Flood Watch from midnight EDT tonight through Monday morning for NYZ067-069-071>075-078-080-176>179. Coastal Flood Advisory from 11 PM this evening to 2 AM EDT Monday for NYZ071-073-176-177. Coastal Flood Advisory from 8 PM to 11 PM EDT this evening for NYZ074-075-178-179. NJ...Flash Flood Watch from midnight EDT tonight through Monday morning for NJZ002-004-006-103>108. Coastal Flood Advisory from 8 PM to 11 PM EDT this evening for NJZ108. MARINE...Small Craft Advisory from 7 AM to 2 PM EDT Monday for ANZ330- 335-340. Small Craft Advisory from 10 PM this evening to 6 PM EDT Monday for ANZ350-353-355. && $$ SYNOPSIS...JP/MET NEAR TERM...JP/PW SHORT TERM...JP LONG TERM...MET AVIATION...Maloit MARINE...JP/MET HYDROLOGY...JP/MET TIDES/COASTAL FLOODING...

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