Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 090938 AFDOKX Area Forecast Discussion National Weather Service New York NY 438 AM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure along the Carolina coast tracks south and east of Long Island today, and off the New England coast tonight. A series of weak fronts will move through the area Sunday into Monday. Another area of low pressure impacts the region on Tuesday and Wednesday. Brief high pressure on Thursday is followed by another late week low pressure system.
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&& .NEAR TERM /THROUGH TONIGHT/...
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Water vapor imagery showing strong shortwave energy diving down the backside of Great Lakes trough early this morning, helping to amplify the longwave trough and begin lifting an intense southern stream shortwave up the coast today. Quiet a bit of convection developing through Florida and off the SE US coast in subtropical moisture feed ahead of these dynamic features. Shortwave energy will continue to race up the coast today, with at least the eastern 1/2 to 2/3rd of CWA under a favorable right rear of 170-180 kt jet dynamics. At the surface, low pressure tracks from the Carolina Coast early this morning to near the 40/70 lat/lon benchmark by this evening. Plenty of radar returns being sense overhead, but dry-low levels are slowing progress of snow onset this morning. Latest NAM and HRRR suggesting that snow will be light and scattered for most of the area through early to mid morning, before picking up in intensity thereafter as trough axis approaches neutral tilt and retrogrades moisture and offshore frontal boundary towards the coast, coupling with increasing large scale lift. This fits what is being seen on radar with steady precip axis still down across the Mid-Atlantic States to just 50 se of Montauk Pt. Thermal profiles would support snow across almost all the areas, outside of some initial mixing with rain at onset for city/LI. Increasingly deep layered lift tapping into a sub-tropical moisture feed offshore should allow for snow to overspread the entire region from SE to NW late this morning into early afternoon, and continuing through this evening for much of the area. The slower snowfall onset is accompanied by a slower tapering of snow tonight due to slower progression of system. W/W/A`s issued this afternoon are still in effect, with only slight changes to snowfall. Models are still indicating some weak mid-level frontogenesis and negative epv aloft across LI/CT which signals potential for some moderate snow banding in the afternoon/evening. This presents a low to moderate probability of 1/2 to 1 inch per hour snow fall rates in the afternoon and evening in vicinity of the warning area. Models have trended farther west with qpf axis, which has increased the likelihood for 3 to 6 inches of snow across much of the advisory area, and 2 to 4 inches of snow into Orange County, where advisory has been expanded into. In terms of ptype, thermal profiles are cold enough for most of the region to be in the form of snow. But an 850-950 hpa warm layer still appears that it could make inroads into far se portions of the area late this aft/eve to result in a mixing with or changeover to rain as low pressure make the closest approach. The reason for this is that northern/southern stream phasing of energies does not start taking place until late today/this evening as the low/mid level low is tracking SE of LI, which does not allow for enough tightening of the thermal gradient to ensure cold air all the way to the coast through the event. Based on an ensemble of thermal profiles, the south fork of LI has the highest probability a period of mixing with rain and sleet this afternoon into evening, possibly working northwest into the north fork of LI and SE New London County in the evening. In terms of QPF, models have wavered a bit from run to run and between each other with QPF but generally still depicting a range of 1/2 to 3/4 of an inch of liquid equivalent across LI/CT (highest east), 4 to 6 tenths across NYC/NJ metro and SW CT/SE NY border, tapering to 1/4 to 4 tenths of an inch for Orange county. With boundary layer and surface temps holding around freezing along the coast during the event, and upper 20s to lower 30s across interior, snow ratios not expected to be too much above 7-10 inches of snow to 1 inch of liquid. In terms of snowfall, based on P-type and dynamics mentioned above, still expecting 5 to 7 inches of snow for SE CT and Eastern LI (except for far SE portions due to mixing). Farther west a solid advisory level snow expected, with 3 to 6 inches to around the NY/NJ metro and the Hudson River. NOTE: Based on the slower onset of steadier snow, marginal surface temps, and diurnal solar insolation, expecting roadway snow accumulation to become more of a concern and hazard starting in the mid to late afternoon through the evening. In term of uncertainty, besides the p-type issue for se areas, the sub- tropical moisture feed and SE US convection could still play a factor in affecting moisture feed into the region. Based on variability from run to run between models and current model spread, snow amount could have to be increased or decreased by 1 to 2 inches from current forecast in spots. Will have to monitor mesoscale models, satellite and radar trend today. Once again accumulating snow will be slightly slower to taper off than what appeared 12 hrs ago, until longwave trough axis begins moving into the region, with snow tapering off across the NYC metro by around midnight and late night across Eastern LI/SE CT.
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&& .SHORT TERM /SUNDAY/...
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Trough axis slides NE Sunday with intensifying low pressure tracking into the Canadian Maritimes. Cyclonic flow should maintain sct to bkn cloud cover and perhaps a few flurries as a second weak shortwave moves through. Blustery day on Sunday with temps likely struggling to get above 40 degrees for city/coast and above mid 30s interior with new snow pack and caa flow.
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&& .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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In general, the extended forecast will be dominated by below normal temperatures and a progressive pattern with a series of low pressure systems moving through the area. Sunday night into Monday will be dry as low pressure quickly departs to the Canadian Maritimes, and high pressure builds in its wake. With strong cold advection, lows Sunday night will only reach into the 20s to near 30, while highs will generally be in the 30s - around 5 degrees below climatological normals. By Tuesday, a strong upper disturbance takes a southward trajectory around a stagnant low centered around the Hudson Bay, with the attendant surface low moving through the Northeast. There is still uncertainty with regard to precipitation type and accumulations, but will continue to monitor. Coastal locations may see a period in the afternoon-evening where snow transitions to rain, while other locations should remain all snow. As the system departs on Wednesday, snow will taper off from west to east and winds will rapidly increase in a tightened pressure gradient and strong cold air advection. Temperatures will be 10-15 degrees below climatological normals into Wednesday night, only slightly moderating on Thursday as brief high pressure moves into the area. By Friday, another disturbance rotates into the Northeast, bringing another chance for snow across the interior and a snow/rain mix elsewhere. Although temperatures will have moderated some near the coast, in general below normal conditions will persist.
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&& .AVIATION /10Z SATURDAY THROUGH WEDNESDAY/...
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Low pressure tracks off the Carolina coast this morning, and into the Canadian Maritimes tonight. Light snow develops from southeast to northwest through 12Z. Please see TAFs for exact timing. Snow becomes heavier thereafter, with the most intense snowfall likely during the middle of the day into the early evening. Snow tapers off after 00Z from west to east. Light and variable winds this morning, with northerly flow generally below 12 kt during the afternoon and evening. Winds veer to the northwest tonight as the low pulls away. Terminal Snowfall Forecast: KJFK: Snowfall rates could reach 1 inch per hour. Around 4-6 inches of total snow forecast. KLGA: Snowfall rates could reach 1 inch per hour. Around 4-6 inches of total snow forecast. KEWR: Snowfall rates could reach 1 inch per hour at times. Around 4- 6 inches of total snow forecast. KTEB: Snowfall rates could reach 1 inch per hour at times. Around 4- 6 inches of total snow forecast. KHPN: Snowfall rates could reach 1 inch per hour at times. Around 4- 6 inches of total snow forecast. KISP: Snowfall rates could exceed 1 inch per hour at times. Around 6-8 inches of total snowfall forecast. .OUTLOOK FOR 06Z SUNDAY THROUGH WEDNESDAY... .Late Tonight...Snow tapers off from west to east. MVFR or lower ceilings. .Sunday...Light snow may linger in the morning near KGON with possible MVFR/IFR, otherwise becoming mainly VFR. NW-W winds G20-25 kt possible. .Sunday night-Monday. Mainly VFR, with MVFR possible in isolated snow showers. WSW-W winds G15-20KT possible. .Monday night-Wednesday...MVFR or lower is possible with a chance of snow. A wintry mix or change over to rain is possible Tuesday afternoon along the south shore of Long Island. SW-W winds G15-25KT possible Monday night-Tuesday. NW winds G20-30KT possible Tuesday night and Wednesday.
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&& .MARINE...
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Pressure gradient tightens today into tonight. Marginal small craft gusts are likely to develop on the ocean this afternoon into tonight, particularly eastern ocean waters. Ocean seas will begin to build above SCA during this time as low pressure passes makes closes approach late Saturday into Saturday night. More widespread SCA gusts expected for all nearshore waters Sunday in wake of departing low pressure, and likely on the ocean well into Monday. Winds will briefly diminish Monday night as high pressure builds across the waters. Another low pressure system is forecast to impact the area waters Tuesday and into Wednesday resulting in the potential for SCA conditions to return. As the low pressure system departs on Wednesday, gale- force winds may be possible. High pressure only briefly builds on Thursday, though with the pressure gradient remaining modest between the departing low and another approaching system, SCA conditions may persist on the ocean waters through late week.
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&& .HYDROLOGY...
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Generally 1/2 to 3/4 of an inch of liquid is expected across LI/CT( highest east), 4 to 6 tenths across NYC/NJ metro to SW CT/SE NY border, decreasing to 1/4 to 4 tenths of an inch for far NW zones through tonight. The precipitation is expected to fall as mainly snow, so no hydrologic impacts are anticipated. No hydrologic impacts are anticipated through the extended period.
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&& .EQUIPMENT... NYC NOAA Weather Radio Station KWO-35 (162.55 MHz) is off the air for an extended period of time. See our headline news on the web at: http:/weather.gov/nyc && .OKX WATCHES/WARNINGS/ADVISORIES...
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CT...Winter Storm Warning until 6 AM EST Sunday for CTZ007-008- 010>012. Winter Weather Advisory until 4 AM EST Sunday for CTZ005-006- 009. NY...Winter Storm Warning until 6 AM EST Sunday for NYZ078>080. Winter Weather Advisory until 1 AM EST Sunday for NYZ067>069. Winter Weather Advisory until 4 AM EST Sunday for NYZ070>075- 176>179. Winter Weather Advisory until 6 AM EST Sunday for NYZ081. NJ...Winter Weather Advisory until 1 AM EST Sunday for NJZ002-004- 103-105-107. Winter Weather Advisory until 4 AM EST Sunday for NJZ006-104- 106-108. MARINE...Small Craft Advisory from 6 AM to 7 PM EST Sunday for ANZ330- 335-338-340-345. Small Craft Advisory from 1 PM this afternoon to 7 AM EST Monday for ANZ350-353-355.
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&& $$ SYNOPSIS...MD/NV NEAR TERM...NV SHORT TERM...NV LONG TERM...MD AVIATION...MD MARINE...MD/NV HYDROLOGY...MD/NV EQUIPMENT...

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