Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 170245 AFDOKX Area Forecast Discussion National Weather Service New York NY 945 PM EST Fri Feb 16 2018 .SYNOPSIS...
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High pressure builds into the region through early Saturday, before sliding off the New England coast Saturday afternoon. A developing low pressure system Saturday will track to the Mid- Atlantic coast Saturday evening and then southeast of Long Island overnight Saturday into early Sunday. Low pressure then departs as high pressure passes to the southeast early next week. A warm front lifts northward, then a cold front passes late in the week.
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&& .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/...
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High, and mid and upper level ridging, is now building to the west. Surface pressure gradient will be weakening along with cold advection. Winds and gusts slowly diminishing, especially after 04Z. Drier air continues to move in quicker than expected with dew points inland into the teens. Clouds have moves south of Long Island and updated sky conditions.
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&& .SHORT TERM /6 AM SATURDAY MORNING THROUGH 6 PM SATURDAY/...
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Saturday will feature ridge axis moving offshore Saturday with a farther northward movement of the upper level jet streak. High pressure at the surface will center across the region Saturday morning before moving off the New England coast in the afternoon. The NW flow decreases with winds eventually swinging around to become E-SE. Daytime mixing will favors a little warm air advection in the afternoon with low level veering of winds. Now the focus will be Saturday night with track, magnitude and precipitation with a coastal low pressure area tracking southeast of Long Island. The upper level jet is shown by models to be quasi-zonal, SW to NE orientation, which will match the track of the low. This low pressure system initially was developing today in SW Pacific and this will have tracked through South Central US through Tennessee Saturday. From there, it heads ENE into the Mid-Atlantic Saturday evening and then deepens off the coast of the Delmarva by late Saturday night. It takes a track to the SE of the 40N/70W benchmark with boundary layer temperatures and the magnitude of deepening of the low crucial to the amounts of snow. The deeper the low, the more dynamic cooling there will be and this goes with the intensity of the snow as well. It`s a positive feedback loop, heavier snow would also force colder air to the surface. The timing of this event Saturday night leans more towards a colder solution compared if this were to occur during the day. Models have trended a little farther south with the low pressure center and without a large pressure gradient between the departing high and the approaching low, lowered snow amounts by around an inch. Also on this side of a slight decrease will be marginal boundary layer conditions at onset of snow. The temperatures will be slightly above freezing and unless the snow is quite intense, it will take an hour or so to decrease the surface temperature to wet bulb temperature. So for first 1-2 hours, snow may not accumulate much. Looking at a general 9pm to 3am timeframe for much of the snow to fall. Still see potential for banding with the snow with 1 inch per hour snowfall amounts possible, so 4-6 inch snow amounts forecast but with locally higher amounts possible. Where this streak of higher amounts occurs is quite uncertain at this time and that is why the winter storm watch was maintained and it can`t be ruled out over Orange and Putnam New York, so watch was expanded to those counties as well. Some higher snow accumulation in the 6 to 8 inch range was shown in the latest SREF but there are members on the lower end showing a few inches as well. Snow rapidly trends down from west to east early Sunday.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/...
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Deep west/southwesterly flow aloft backs as western trough digs and eastern ridge builds early to mid week. The ridge flattens slightly late in the week as heights aloft fall. At the sfc, low pressure departs as high pressure quickly builds and passes offshore. A warm front passes to the north Monday, and the local area remains firmly in the warm sector Monday night through Wednesday. A cold front passes sometime Wednesday night or early Thursday, then high pressure builds to the north Friday. Global models appear to be in general agreement. As for sensible weather, dry conditions are anticipated Sunday, then rain accompanies the warm front Monday. While the area is in the warms sector, hit or miss light rain or drizzle is possible, as is nighttime and morning fog. Some rain may accompany the cold front Wed night, with another warm front approaching Friday. Temps rebound right away Sunday, then rise further Monday through Wednesday. There will be quite a temp difference west of the Hudson river (warmer), and along the south facing coasts (much cooler) as you would expect this time of year. Temperatures cool slightly behind the front Thursday into Friday.
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&& .AVIATION /03Z SATURDAY THROUGH WEDNESDAY/...
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VFR through Saturday afternoon with high pressure building over the terminals. NW winds will continue to gusts 25-30 kt through around 06z. The gusts will gradually weaken and end thereafter. The flow veers to the N-NE Saturday morning and then SE in the afternoon. Speeds will be under 10 kt. Conditions begin to deteriorate after 00z Sunday with an initial rain and snow mix becoming all snow. Forecast snowfall accumulations 00-06z Sunday are 2 to 4 inches. .OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY... .Saturday night...LIFR. Initial rain/snow mix near coast becomes all snow. All snow at KSWF/KHPN. .Sunday...VFR. .Monday...VFR becomes MVFR or IFR in rain and possible fog, especially late. .Tuesday...MVFR or IFR possible in the morning and again at night, otherwise VFR with SW wind G20KT possible. .Wednesday...Mainly VFR. Chance of showers/MVFR.
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&& .MARINE...
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Minor changes to the winds and gusts. Seas, winds and gusts have all increased as cold advection of the waters increased, especially the ocean. Timing on the ending of the SCA on the none ocean waters should still be around 06Z and cold advection and the surface pressure gradient will be diminishing after 04Z. Ocean seas will remain near 5 ft much of tonight. The coastal low tracking southeast of Long Island will be weak in magnitude so winds are not expected to increase much Saturday night, keeping conditions below SCA. NW winds initially Sunday behind low pressure will back around to the south as high pressure builds and passes offshore. The south/southwesterly pressure gradient tightens Monday and Tuesday, and winds will begin to increase during this timeframe. Tranquil seas Sunday and Monday begin to build as the southerly winds pick up in speed. Expect 5 ft seas by Tuesday on the ocean waters.
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&& .HYDROLOGY... Around 0.3 to 0.6 of an inch of liquid equivalent is expected Saturday night with locally higher amounts. With most, if not all of this falling in the form of snow, no hydrologic impacts are expected. No significant hydrologic impacts are expected Sunday through the end of next week. && .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...Winter Storm Watch from Saturday evening through Sunday morning for CTZ005>012. NY...Winter Storm Watch from Saturday evening through Sunday morning for NYZ067>075-078>081-176>179. NJ...Winter Storm Watch from Saturday evening through Sunday morning for NJZ002-004-006-103>108. MARINE...Small Craft Advisory until 1 AM EST Saturday for ANZ330-335- 338-340-345. Small Craft Advisory until 6 AM EST Saturday for ANZ350-353- 355. && $$ SYNOPSIS...JM/PW NEAR TERM...JM/19 SHORT TERM...JM LONG TERM...PW AVIATION...DS MARINE...JM/19/PW HYDROLOGY...JM/PW EQUIPMENT...

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