Area Forecast Discussion
Issued by NWS Upton, NY

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704 FXUS61 KOKX 150532 AFDOKX Area Forecast Discussion National Weather Service New York NY 1232 AM EST Fri Dec 15 2017 .SYNOPSIS... High pressure builds into the area tonight then quickly shifts offshore on Friday as an area of low pressure develops off the Mid Atlantic Coast. Low pressure quickly departs eastward on Saturday, with high pressure briefly building in again in its wake. A series of weak low pressure and frontal systems will pass through mid week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Forecast is on track. The pressure gradient will gradually relent tonight as high pressure builds into the region from the west. Strato-cu has quickly dissipated in the subsident environment, with mainly just some jet induced cirrus working in overnight. The result should be favorable radiational cooling overnight, with the remaining snow pack aiding in temperatures across outlying area radiating into the lower teens to single digits, generally about 10 to 15 degrees below climatological normals. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... The persistent blocking pattern will gradually begin to break down on Friday with a subtle eastward shift to a large scale East Coast upper trough, while upstream low pressure flattens a dominant ridge on the West Coast. Phased vorticity maximums will push eastward, allowing a surface low to intensify off the Carolina coast before ejecting eastward. Across the area, temperatures will remain below normal, largely below freezing, with increasing cloud cover through the day as the phased vorticity maximums approach. Although the heaviest snowfall will remain well south of the area, mid-upper level lift/saturation will gradually increase into the evening, with any developing precipitation aloft subsequently saturating low levels enough to produce at least a dusting of snow along the coast. A slight northward shift noted in the 18z guidance, which if trend continues would increase potential for a light measurable snow across at least southern and eastern portions of the Tri- State, but at this point only a 1 in 10 chance of seeing more than an inch of snowfall. Cloud cover decreases again Friday night as the system pushes east of the area, while winds gradually increase as the pressure gradient increases ahead of building high pressure. Low temperatures are already forecast to be well below normal, but could drop further depending on the exact timing of clearing skies and increasing winds. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... A progressive pattern in place across the country for the long term period. High pressure builds to the south on Saturday as a coastal low remains well to the southeast of the area. As the low exists, W- NW winds increase, especially across Long Island and Coastal CT where the low level jet moves across. Below normal temperatures, combined with gusty winds, will keep wind chill values in the low 20s during the day. The forecast becomes more uncertain on Sunday night into Monday as a weak warm front moves towards the region. While models are in good agreement regarding upper level energy, there is still disagreement amongst precipitation amounts. However, with winds shifting to the SW expecting a period of rain/mixed precipitation overnight into Monday. Confidence still remains low based on the differences in model guidance. By Monday temperatures return to near seasonal norms for mid December. Thereafter, models in fairly good agreement with a northern stream trough approaching the region on Tuesday, but differences exist in the amount of interaction with southern stream energy/moisture. Primary low pressure system appears to ride through Quebec/Ontario with trailing cold front approaching the region, but precip coverage amounts will be dependent on the aforementioned southern stream interaction. Thermal profile suggests precip would be a wintry mix transitioning to rain, but predictability at this point is low. Upper trough swings through the NE Tue Night into Wed with a glancing shot of modifying arctic air in the wake of the cold front for midweek. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... High pressure builds in through Friday morning with VFR conditions. Then, weak low pressure moves northeast of the mid- Atlantic coast. This will bring some snow Friday afternoon into Friday evening with associated MVFR/IFR conditions. There is a potential for a light snow accumulation up to 1-2 inches. VFR returns by late Friday night. Wind gusts have subsided. W-NW flow near 10 kt will decrease further overnight before becoming more WSW-W at 5-10 kt on Friday. .OUTLOOK FOR 06Z SATURDAY THROUGH TUESDAY... .FRI NGT...VFR. A few W winds G20KT late. .SAT...VFR. WNW winds G20-25KT daytime. .SUN...VFR most of the time. Chance MVFR in -RA/-SN late at night. .MON...Chance MVFR -RA for coastal terminals, light wintry mix with MVFR/IFR inland early and at night. .TUE...MVFR possible. Slight chance of mainly -RA. && .MARINE... Winds are slower to diminish than guidance is indicating, thus have extended the SCA from Sandy Hook to Fire Island Inlet through 10z. They may come down over the next hour or two though. Winds and seas will fall below SCA levels on the ocean waters by daybreak as high pressure builds into the area. Tranquil conditions will then persist into Friday, before winds and seas increase to SCA levels again Friday night with the departure of a low pressure system to the south and east. These conditions will persist into Saturday night, followed by a better chance of SCA or possible Gales in the wake of a midweek system. && .HYDROLOGY... No significant hydrologic impacts expected. && .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...Small Craft Advisory until 5 AM EST early this morning for ANZ355. Small Craft Advisory until 7 AM EST this morning for ANZ350- 353. && $$

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