Area Forecast Discussion
Issued by NWS Upton, NY

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236 FXUS61 KOKX 190240 AFDOKX Area Forecast Discussion National Weather Service New York NY 1040 PM EDT Sun Mar 18 2018 .SYNOPSIS... A cold front moves across the region tonight. High pressure builds down from southern Canada Monday and Monday night. A couple of coastal lows track from off the mid- Atlantic coast into the Canadian Maritime Provinces from Tuesday into Thursday night. High pressure then builds down from southern Canada from Friday through Saturday. A storm system tracks from the Ohio River Valley to the Mid Atlantic States Saturday night, then to the south of Long Island on Sunday. && .NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
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Clouds upstream have shown signs of expansion the past couple of hours, but per 00z OKX sounding, atmosphere is very dry below H8, so think any snow that falls from the high based stratocu deck will sublimate as it falls to the sfc. Thus, will keep mention of flurries out of the forecast. Otherwise, the forecast for tonight remains on track, with diminishing winds and temperatures falling mainly to the mid teens to mid 20s, except upper 20s to around 30 in the NYC metro.
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&& .SHORT TERM /6 AM MONDAY MORNING THROUGH MONDAY NIGHT/... The upper trough remains across eastern Canada and the northeast Monday and Monday night, meanwhile a ridge remains to the west. The mid and upper level systems remain blocked while a surface high builds in from southern Canada through the period. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Two storm systems could impact the area from Tuesday-Sunday. The first storm system will come in two waves, the first in response to shearing energy emerging a cutoff low over the Rockies that tracks to the south of Long Island Tuesday. At this time it appears any precipitation from this first bit of energy should be relatively light and based on a blended thermal profile and surface wetbulb temperatures, should fall as mainly snow. However, given sun-angle, would expect any accumulation to be limited to mainly grassy surfaces. Generally less than 1 inch of snow is expected during the day on Tuesday, mainly across NYC/Long Island/southern portions of NE NJ. There is generally good agreement among the models with this idea with solid support from the CMC/ECMWF/SREF and the NAM/GFS just offshore (but also with a more sheared out solution to the shortwave than the others, so adjusting for this, you have a reasonable consensus). The second wave with the first storm is in response to a combination of the remainder of the energy associated with the initial closed low over the Rockies and a second piece of energy that comes down from SW Canada (which at one point was also part of the Rockies cutoff). This helps to carve out a larger low-mid level shortwave trough/potential 700 hPa closed low - and hence a stronger coastal low that tracks to the S/E of the 40N/70W bench mark. There is also a general consensus for this idea from the models in a general sense. However, with the NAM/GFS likely too far S/E with initial low, their path for the follow-on low likely also is too far to the S/E, so followed a SREF/CMC/ECMWF blend with the second low. So expect another round of snow to develop later Tuesday night and linger into Wednesday night. Once again thermal profiles and surface wetbulb temperatures support mainly/if not all snow. Still some question of how much and how far to the NW qpf falls with this storm. There is the potential for advisory level snows across mainly the southern 1/2 of the CWA. It should be noted that our snow grids, consistent with duration of WPC produced snow probabilities only go through 00Z Thursday, so do not capture all of the snow expected to fall from this system. There is still uncertainty on the exact strength, timing and track of the storm, so would expect refinements to the forecast with time. Currently the range of solutions range from no snow to a very low chance of a low end warning snowfall across far S portions of the CWA. A mitigating fact to any snow accumulation will be the sun angle this time of year, and even with wet bulbing, could see temperatures near or right above freezing. With low levels drying behind the storm, even with passage of trough axis on Thursday/Thursday night, it should be dry, except for maybe some lingering snow early over far eastern parts of the CWA. Another shortwave pushes through Friday, but once again the dry low levels should preclude any precipitation. Shortwave ridging crosses the area Friday night and Saturday, so it should continue to be dry. The next storm system comes in response to a northern stream trough/developing closed low moving into the Great Lakes/Southern Canada Saturday night and Sunday. At this time there is the potential for mainly snow across northern portions and a rain/snow mix across the S 1/2 of the CWA. Temperatures should be below normal Monday-Sunday. && .AVIATION /03Z MONDAY THROUGH FRIDAY/...
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A weak low will pass just to the SW of the region tonight. High pres extending from central Canada then builds in for Monday. VFR. Light and vrb winds until around 09z becoming N-ENE late around 5kt as low pres passes just to the SW of NYC. N-NW winds around 10 kt then expected for Mon. Directions should remain primarily right of 310 magnetic in NYC on Mon. .OUTLOOK FOR 00Z TUESDAY THROUGH FRIDAY... .Monday night...VFR. .Tuesday...Low confidence forecast through Wednesday. Chance of snow with IFR conditions for the Long Island and NYC Metro terminals. ENE winds G20KT NYC metro/coastal terminals. .Tuesday night...MVFR conditions possible with any lingering snow showers. NE winds G20KT NYC metro/coastal terminals, mainly late. .Wednesday...Snow with IFR conds likely at the Long Island terminals, and may also be possible farther north including all local terminals. NE winds G20-25KT. .Thursday-Friday...VFR. N winds G15-20KT.
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&& .MARINE... Made minor changes this update to reflect the latest trends in observations and guidance. The forecast appears on track. Wind gusts and seas on the ocean waters have diminished. However, there may be an occasional gust to near 25 kt on the eastern, outer waters through the remainder of the afternoon. With wind gusts generally below SCA levels, and seas less then 5 ft, will cancel the SCA for the rest of the afternoon. A weak flow will be across all the forecast waters tonight as a weak low moves through, then high pressure builds to the north Monday into Monday night. Winds and seas will be below SCA levels through Monday night. Small Craft conditions are likely on the coastal ocean waters on Tuesday with sub-Small Craft conditions on the non-ocean waters. The pressure gradient tightens Tuesday night and remains relatively strong over the region into Wednesday night. As a result gale conditions are possible Tuesday night-Wednesday night on the coastal ocean and waters and Small Craft Conditions are probable on the non-ocean waters during this time frame. The threat for gales will be highlighted in the HWO. The pressure gradient then gradually slackens from Thursday through Friday, with Small Craft Advisory (SCA) Conditions lingering on all waters into Thursday and on the coastal ocean waters into and possibly though Thursday night. For now, it appears that sub-SCA conditions should occur on all waters Friday and Friday night. && .HYDROLOGY... It will be mainly dry through Monday night. There is the potential for a prolonged period of precipitation from Tuesday through Wednesday night. At this time, it appears that total amounts should be less than 1 inch of liquid, and with it expected to fall mainly, if not all as snow, no significant hydrologic impacts are expected. && .TIDES/COASTAL FLOODING... There is the potential for a prolonged period of E-NE flow from Monday night into Wednesday Night. Tidal departures of generally only 1 to 1 1/2 ft are needed for minor flooding for many locations Tuesday night through Wednesday night, and 2 to 3 ft for moderate. Depending on the exact track/strength of a coastal low or lows during this time frame, there is the potential for minor to moderate coastal flooding, but predictability on the details remains low. && .EQUIPMENT... NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the air for an extended period of time. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...None. && $$ is the U.S. government's official web portal to all federal, state and local government web resources and services.