Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 270235 AFDOKX Area Forecast Discussion National Weather Service New York NY 1035 PM EDT Wed Apr 26 2017 .SYNOPSIS... Weak low pressure slowly departs tonight and dissipates on Thursday with weak high pressure building in. A cold front approaches late Thursday night and moves into the region Friday but will also weaken with time. A cold front will slowly approach from the north late Friday night into Saturday, and pass through Saturday evening. High pressure will briefly build in its wake from southeast Canada on Sunday. A warm front will approach Sunday night and lift north on Monday, followed by a trailing cold front late Monday in to Monday night. Weak high pressure will pass to the south from Tuesday into Wednesday. && .NEAR TERM /UNTIL 6 AM THURSDAY MORNING/... Forecast is on track with only minor adjustments to T/Td to better reflect current conds and trends. Otherwise...the main story tonight will be the low clouds and fog. The upper level low and surface low both translate east of the region, getting into the vicinity of coastal New England by early Thursday. With atmospheric flow staying below 15kt below 10kft and a low level inversion at around 1-2 kft, the low level moisture stays in place. Most susceptible areas for fog will be where relatively more rain has fallen, Eastern Long Island and Southeast Connecticut where just a little cooling this evening with light easterly flow will help advect and further develop the fog, and it will eventually become dense. Dense fog advisory for Eastern Long Island and New London County Connecticut was issued for late this evening into early Thursday morning. Other areas farther west will also get fog but not as confident in widespread dense fog attm. Took relatively warmer MAV guidance for lows, upper 40s to upper 50s. && .SHORT TERM /6 AM THURSDAY MORNING THROUGH FRIDAY/... Fog burns off in the morning with daytime heating and as more westerly flow develops in the atmosphere. The upper level low moves farther east of the region as a surface low dissipates. More of a return W-SW flow develops in the region thereafter. Models agree on a substantially warmer day Thursday, getting well into the 60s for a majority of the region for highs. Weak high pressure will briefly reside in the region with dry conditions during the day and through the first half of Thursday night. The next low pressure system and associated upper level trough will be approaching late Thursday night.The upper level flow remains SW and the parent low with the system moves northward Friday within Southeast Canada while its cold front moves in and dissipates with time. There will initially be enough vertical lift late Thursday night into early Friday along with some elevated instability to give the region some showers and thunderstorms. However, this looks to be brief without producing much rain. Outside of the rain with the flow ahead of the cold front turning more southerly, the region will see a return of at least patchy fog Thursday night into early Friday with more onshore flow and inherent moisture advection. Dry conditions return later Friday. The overall shift will be one of a gradually warming airmass. Temperatures trend several degrees warmer for both lows Thursday night and highs on Friday. && .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Big picture shows an amplified patter developing over the eastern half of the lower 48, with a building upper ridge over the East and a closed low moving from the Plains states through the Midwest and into southeast Canada. At first the area will be under zonal flow aloft between the ridge to the south and an upper trough moving across southeast Canada, that will send a back door cold front toward the area late Friday night into Saturday. A shortwave embedded in the flow aloft could trigger a shower or tstm late Fri night or early Sat morning, then Sat should be a partly sunny to mostly cloudy day with temps on the warm side, with highs 80-85 from NYC north/west and in the 70s across Long Island and southern CT. Low levels look capped just above the boundary layer until late day Sat, so do not expect any precip until late day Sat with the cold fropa, and only and isolated shower or tstm at most. Cooler air returns for Sat night into Sunday night, with lows in the 40s and 50s, and highs in the 60s to near 70. As the flow aloft amplifies, a leading warm front well in advance of the surface low over the Midwest will approach, and move through some time on Monday, The front should lift through NYC metro and points north/west by afternoon, nut could take longer across Long Island and southern CT. Then as upper ridging moves east, cold front should pass through late day Monday into Monday night, with showers and a few tstms. Do not expect strong convection with the cold fropa attm, but a faster timing per 12Z ECMWF could yield a brief window for this to occur late day Monday from NYC north/west. Dry wx with near to slightly above seasonable temps expected for Tue into Wed, with the weakening low passing well north, weak surface high pressure passing to the south. Zonal flow aloft will also become re-established by Wed, that could lead to late week precip chances just beyond the forecast period. && .AVIATION /03Z THURSDAY THROUGH MONDAY/... Low pressure will continue to pull away to the east tonight. MVFR to start but will deteriorate tonight to IFR/LIFR with fog and low ceilings. Winds become light and variable this evening. Improvement to IFR to start the day on Thursday, with MVFR likely during the afternoon. .OUTLOOK FOR 18Z THURSDAY THROUGH SUNDAY... .Thursday aftn...MVFR. .Thursday night...MVFR with chc IFR. .Friday...Chance showers/thunderstorms/MVFR AM, VFR PM. .Saturday...Mainly VFR. .Sunday...Mainly VFR. && .MARINE... No changes to current forecast. Dense fog advsy is in effect for all waters through 14z Thu due to a weak pressure gradient and abundant low level moisture. The fog is expected to gradually dissipate Thursday mid to late morning before returning again Thursday night into early Friday. Winds will stay below SCA through Friday but the higher ocean seas will remain through Thursday night and perhaps into Friday as well. Non-ocean waters stay below SCA. Ocean seas are more marginal for SCA Friday, at near 5 ft, so left end time of 6am Friday for SCA for hazardous seas on the ocean. Ocean seas above 5 ft should linger into Fri night, and perhaps into part of Sat into Sat night, depending partly on the strength of SW flow ahead of a back door cold front that looks to move through Sat night. Advy level conditions are also possible on the ocean Mon afternoon and night after a warm frontal passage and before/just after a cold frontal passage, with S-SW flow gusting up to 25 kt and seas building to 5-7 ft per combo of WaveWatch and wind/wave climatology. && .HYDROLOGY... No significant rainfall expected through Friday. Rain amounts through Friday are generally expected to remain below a quarter of an inch. However, there may be isolated brief heavy downpours in any thunderstorms that develop. Combo of a warm frontal passage late Sunday night into Mon morning, and a cold frontal passage late day Mon into Mon night, could bring precip amounts of 1/2 to 1 inch, highest NW of NYC. Localized higher amts possible that could cause typical nuisance ponding. && .TIDES/COASTAL FLOODING... Due to high astronomical tides and an easterly swell the next couple of days, water levels may approach minor coastal flood benchmarks, in particular across the south shore back bays of western LI. Only 1/3 to 3/4 of a foot of surge is needed for minor flooding. In addition, the elevated water levels combined with a prolonged period of 5-9 ft breaking surf tonight into Thu will result in beach erosion issues and may cause some localized dune toe erosion during the high tides. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...Dense Fog Advisory until 8 AM EDT Thursday for CTZ008-012. NY...Dense Fog Advisory until 8 AM EDT Thursday for NYZ079-081. NJ...None. MARINE...Dense Fog Advisory until 10 AM EDT Thursday for ANZ330-335-338- 340-345-350-353-355. Small Craft Advisory for hazardous seas until 6 AM EDT Friday for ANZ350-353-355. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.