Area Forecast Discussion
Issued by NWS Upton, NY

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915 FXUS61 KOKX 140315 AFDOKX Area Forecast Discussion National Weather Service New York NY 1015 PM EST Wed Dec 13 2017 .SYNOPSIS...
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Strong low pressure continues moving into the Canadian Maritimes tonight as a Clipper low approaches from the west. This low quickly moves south of Long Island Thursday morning and offshore Thursday afternoon. High pressure builds from the west Thursday night. A series of weak low pressure and frontal systems will pass through Friday through early next week, with high pressure building between each.
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&& .NEAR TERM /THROUGH THURSDAY/...
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Minor updates were made to reflect the latest conditions. Onset of light snow across the area looks to be toward midnight across the western half of the forecast area, overspreading the remainder of the area during the early morning hours. Closest reports of snow at this time are across central PA and northeast MD. Unseasonably cold weather will continue with our next accumulating snow occurring late tonight into Thursday morning. A winter weather advisory remains in effect for Long Island beginning 3 AM and ending at 10 AM on Thursday. A special weather statement has been issued elsewhere, which highlights lower snow accumulation. Some impacts to the morning commute are anticipated, especially in the advisory area as temperatures during the early morning hours will be in the upper teens inland and lower to middle 20s closer to the coast. Winds will be subsiding this evening has the pressure gradient weakens between low pressure systems. Mostly clear skies will start the night, but will quickly become mostly cloudy to overcast as the clipper low fast approaches from the west. Already beginning to see light echoes in western PA as warm advection commences there. This warm advection is progged to to start in the middle levels after 03z west of the Hudson River and 05-06z east of the Hudson River. Light snow will overspread the region from west to east with this warm advection. Main vort max associated with the clipper will swing around the base of the mean upper trough and enhance lift, especially 09z-12z. The surface low translates south of the area during this time, which could help to enhance snowfall on Long Island. These features quickly translate east as the upper trough, vort max, and surface low move offshore 13-15z. Lift all also be maximized in the dendritic growth zone between 750 and 900 mb from around 09z to 13z across eastern Long Island. Models have continued to come into better agreement with the above scenario with the biggest uncertainty lies within the mesoscale details, mainly across Long Island. Latest thinking is for around a tenth of an inch of liquid across the interior, increasing to around two tenths across Long Island. There may be a shadow effect over NYC and NE NJ as the surface low moves downslope of the higher terrain to the west. For snowfall accumulation, 2 to 3 inches are forecast in the advisory area across Long Island with 1 to 2 inches elsewhere. There could be locally higher amounts with any enhancement or banding on Long Island, so it would not be surprising if a few spots reach closer to 4 inches. SREF plumes, NAM, NAM-3km, and HRRR all support this forecast thinking. The snow should be a higher ratio, powdery snow, potentially around 15:1 near the coast and 17-18:1 across the interior. The snow ends on the east end around 15z with skies quickly clearing from west to east. It will become breezy as high pressure builds in from the west Thursday afternoon. Highs well below normal in the upper 20s and lower 30s are forecast.
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&& .SHORT TERM /THURSDAY NIGHT/... High pressure settles over the Tri-State Thursday night. With light winds, mostly clear skies, cold airmass, and some snowpack on the ground, temperatures should bottom out in the lower teens inland and middle and upper teens elsewhere. && .LONG TERM /FRIDAY THROUGH WEDNESDAY/... First upper shortwave in a progressive pattern dives across the Great Lakes region Friday. Sfc low passes across the Great Lakes region as offshore low develops well southeast of the area late Friday and Friday night out ahead of the shortwave energy. Some light snow is possible, with the bulk of the associated QPF remaining offshore, and to our NW, closer to the inland parent low. This shortwave passes Saturday as ridge builds, and remains in control through much of the weekend. Thereafter, the forecast is much more uncertain. Downstream shortwave begins to make eastward progresses as ridge flattens. Weak PVA noted in fast zonal flow approaching area Sunday night, which could trigger some light precip. Then, operational ECMWF remains a fast and stronger outlier as this mid west shortwave moves east. This would result in higher precip chances Monday when compared to GFS and Canadian NH. For Tuesday, northern stream shortwave approaches, with GFS picking up the mid west shortwave as well, about 24 hours after ECMWF. All in all, minimal chance for precip is forecast Monday through Tuesday, and possibly into Wednesday, but still plenty of time to iron out the details. Chilly temperatures Friday and Saturday gradually warm closer to normal as zonal flow or weak ridging develops. && .AVIATION /03Z THURSDAY THROUGH MONDAY/...
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VFR initially but MVFR/IFR in snow overnight into mid morning Thursday with clipper low passage just south of Long Island. VFR then returns for Thursday afternoon into Thursday evening. Snow accumulation is expected to be 1-3 inches overnight into mid morning Thursday. W winds diminishing with still some gusts to 20kt. Expecting gusts to drop off overnight. Winds will back to SW ahead of clipper low overnight and then return to westerly Thursday behind the low with gusts redeveloping in the late morning and afternoon to 20-25kt. Wind direction could become quite variable early Thursday with low center in close proximity. .OUTLOOK FOR 03Z FRIDAY THROUGH MONDAY... .Thursday night-Friday...VFR. .Friday night...VFR. W winds G15-20KT develop towards early Saturday morning. .Saturday-Saturday night...VFR. WNW winds G20-25KT daytime and G15- 20KT evening. .Sunday...Mainly VFR. .Sunday night and Monday...MVFR or lower flight cat possible in snow/rain.
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&& .MARINE...
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All Gales have been converted to SCA, which will come to an end across Eastern LI Sound and Peconic and Gardiners Bays during the early morning hours. Seas though will stay up on the ocean through at least Thursday. Gusts should increase again on Thursday behind departing Clipper low. SCA gusts are possible on the ocean. On Friday, high pressure builds and passes east. Light winds and tranquil seas are expected as a result. Low pressure develops Friday night just east of the waters and deepens as it departs Saturday, with increasing westerly winds, and building seas expected behind it. Winds abate and seas subside as high pressure builds Saturday night through Sunday. Some uncertainty in the forecast exists for Monday as a frontal system could impact the waters. However, will forecast sub SCA winds this time frame as this scenario remains in question. Seas remain under 5 ft.
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&& .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...
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CT...None. NY...Winter Weather Advisory from 3 AM to 10 AM EST Thursday for NYZ078>081-177-179. NJ...None. MARINE...Small Craft Advisory until 3 AM EST Thursday for ANZ330-340. Small Craft Advisory until 6 PM EST Thursday for ANZ350-353- 355.
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&& $$ SYNOPSIS...DS/PW NEAR TERM...DS/DW SHORT TERM...DS LONG TERM...PW AVIATION...JM MARINE...DS/DW/PW HYDROLOGY...DS/PW EQUIPMENT...//

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