Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 151448 AFDOKX Area Forecast Discussion National Weather Service New York NY 948 AM EST Fri Dec 15 2017 .SYNOPSIS... High pressure shifts offshore this morning as low pressure develops along the Middle Atlantic coast. This low will strengthen as it tracks northeast through the evening and continues out to sea tonight. High pressure then builds in through Sunday. A series of weak low pressure and frontal systems will pass through mid week. && .NEAR TERM /THROUGH TONIGHT/... Accumulating snow is expected during the evening commute. Winter weather advisory remains in effect for eastern Suffolk County this evening for 2 to 3 inches of snow. Have issued a Special Weather Statement elsewhere to account for lighter snow amounts, but still potential impacts to the evening commute. No other changes to headlines have been made at this time, but will continue to assess new 12z data. Everything appears on track for a quick, accumulating snowfall beginning this afternoon and continuing into this evening. Already seeing divergence increase aloft as a 170-180 kt jet streak moves across the Middle Atlantic. Our region will be located in the favored left exit region for synoptic lift today. There will also be some interaction/phasing of the northern and southern stream which combined with the upper jet dynamics will intensify a surface low over the Carolinas as it shifts towards the Middle Atlantic and then south of Long Island this evening. Middle level forcing is quite impressive for a short time in the dendritic growth zone this evening, especially just south of Long Island. Already seeing some indications in the last HRRR Simulated Reflectivity for banded snow, so will have to monitor trends for locally higher snowfall amounts at least across Long Island. Snow will blossom rapidly from west to east this afternoon and move east this evening, ending around 7-9pm in NYC and around midnight on the east end of Long Island. Liquid equivalent amounts around 0.05 inches inland and close to 0.20 inches are forecast. Thermodynamic profiles do not show much liquid water saturation aloft for riming, so think snow will be a dry, high ratio type similar to what we observed Thursday morning. Ratios may be 15-18:1. Surface temperatures should drop into the 20s once the snow begins. Did not make any changes to snow forecast at this time, but it is possible slightly higher amounts could added to portions of the coastal region. Around 1 to 2 inches forecast for much of the area with 2 to 3 across eastern Long Island. Snow tapers off from W to E between 00z and 06z tonight with clearing and increasing winds as the pressure gradient tightens between the strengthening departing low, and strong high pressure over the SE US. Another chilly night with lows in the ranging from upper teens well inland to mid 20s at the coast. && .SHORT TERM /SATURDAY/... Mean trough axis shifts offshore on Sat with gradual rising heights aloft. Could see some lake/ocean effect streamers come close to the forecast area, although high res guidance is not indicating we will be impacted by these. Right now, just expect an increase in cloud cover during the day. Otherwise, a breezy westerly flow and weak WAA should help temps rebound into the 30s. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Fairly good agreement with the operational models in the long term. Generally, zonal flow in the upper levels through the middle of next week, allowing for a progressive pattern. At the surface, high pressure moves overhead during the day Sunday and then pushes offshore Sunday night. This allow a warm front to approach and lift north of the area from Sunday night through Monday morning. Light overrunning precipitation will occur during this time frame. Models differ somewhat on timing. Uncertainty will also lie in precipitation type. Best chances for frozen would be inland areas, with a rain/snow mix along the coast changing to plain rain late Sunday night. The rain/snow line will move north Monday morning, with plain rain expected everywhere by Monday afternoon. Little, if any accumulation is expected with QPF amounts of only a few hundredths of an inch. Surface cold front then approaches Monday night and pushes through Tuesday. Warmer temperatures expected on Monday and Tuesday with warm air advection continuing out ahead of the front in the warm sector of the frontal system. Temperatures will be slightly above normal on Monday, and a few degrees warmer on Tuesday. Cooler temperatures for Wednesday and Thursday. Conditions will become breezy for Tuesday night into Wednesday as the pressure gradient tightens between the parent low from the cold front that tracks into the Canadian maritimes and the high pressure building in from the southwest. && .AVIATION /15Z FRIDAY THROUGH TUESDAY/... ***SNOW EXPECTED FOR EVENING PUSH*** High pressure slides east, giving way to an area of low pressure passing south of the region this evening. VFR this morning. Clouds gradually lower and thicken late this morning and afternoon. Snow may initially start around 18Z, becoming steadier by around 20Z. Conditions fall to MVFR then quickly to IFR with steadier snow late this afternoon and evening. Expect a widespread 1-2 inches at most locations, with 2-3 possible at KISP. Snow ends and conditions return to VFR between 03z-06z. Light and variable winds this morning become westerly at 5-10 kt this afternoon. Winds increase tonight with some gusts to 20kt after midnight. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/www.weather.gov/zny/n90 KJFK TAF Comments: Best chance of snowfall expected between 20z-02z with 1-2 inches of accumulation. The afternoon KJFK haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KLGA TAF Comments: Best chance of snowfall expected between 20z-02z with 1-2 inches of accumulation. The afternoon KLGA haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KEWR TAF Comments: Best chance of snowfall expected between 20z-02z with around 1-2 inches of accumulation. The afternoon KEWR haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KTEB TAF Comments: Best chance of snowfall expected between 20z-02z with around 1-2 inches of accumulation. KHPN TAF Comments: Best chance of snowfall expected between 21z-02z with around 1-2 inches of accumulation. KISP TAF Comments: Best chance of snowfall expected between 20z-02z with around 2-3 inches of accumulation. .OUTLOOK FOR 12Z SATURDAY THROUGH TUESDAY... .SAT...VFR. WNW winds G20-30KT daytime. .SUN...VFR during the day. Chance MVFR in -RA/-SN towards midnight. .MON...Low chance MVFR in -RA for coastal terminals, light wintry mix inland. .TUE...MVFR possible. Slight chance of light -RA. && .MARINE... A brief period of tranquil conds on all waters today as high pressure over the NE moves offshore. The pressure gradient will tighten tonight as low pres passes S and E of the waters. SCA conds are expected to develop around midnight and continue through Sat, with the potential for gale force gusts on the ocean waters late tonight and into Sat. Have issued a gale watch for this potential, although it currently looks marginal. Could also have a few hours of 35 kt on eastern LI Sound during the day Sat, but didn`t have the confidence to include in the watch. Winds and waves diminish Saturday night as high pressure builds in. Waves come down below 5 ft across all ocean zones late Saturday night. Waves then build to above SCA criteria late Tuesday into Tuesday night as a cold front moves through and winds increase behind the cold front. Waves build to 5 to 9 ft on the ocean by Wednesday and diminish thereafter through Thursday. Windy conditions expected late Tuesday through Wednesday and at least part of Wednesday night as the pressure gradient increases. SCA winds likely across all waters through this time period. Gales are possible during this time frame as well, but there is still uncertainty this far out. && .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...Winter Weather Advisory from 1 PM this afternoon to 1 AM EST Saturday for NYZ079-081. NJ...None. MARINE...Small Craft Advisory from 11 PM this evening to 6 PM EST Saturday for ANZ330-335-338-340-345. Gale Watch from late tonight through Saturday afternoon for ANZ350-353. Gale Watch from late tonight through Saturday afternoon for ANZ355. && $$ SYNOPSIS...JP/DS NEAR TERM...DS SHORT TERM...24 LONG TERM...JP AVIATION...BC/MD MARINE...24/JP HYDROLOGY...24/JP EQUIPMENT...

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