Area Forecast Discussion
Issued by NWS Upton, NY

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253 FXUS61 KOKX 091710 AFDOKX Area Forecast Discussion National Weather Service New York NY 1210 PM EST Sat Dec 9 2017 .SYNOPSIS...
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Low pressure along the Carolina coast tracks south and east of Long Island this afternoon, 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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Minor changes were made with this update to reflect the latest trends in observations and guidance. The forecast appears on track. 12Z RAOBS show NAM has fairly well diagnosed upper jet structure, capturing 195KT maximum observed at KCAR and 175KT AT KALY at 250 hPa. So feel confident region will experience right rear quadrant of upper jet, with associated enhancement of upward vertical velocities. This coupled with developing frontogenesis associated with development of coastal front just off of Long Island coast, should support development of banding over eastern Long Island/SE CT. 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. As a result, feel current warning/advisory structure makes sense. Do note slight increase in qpf in latest guidance over eastern zones, but want to make sure trends continue before adjusting snow amounts. Otherwise...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 lift intense southern stream shortwave up the coast today. Quite a bit of convection developing through Florida and off the SE and Mid Atlantic US coast in subtropical moisture feed ahead of these dynamic features, meanwhile while a broad deformation snow band exists from AL/GA up through the Central appalachians. Snow will increase in coverage/intensity late morning into early afternoon as large scale lift increases and a bit of the offshore sub- tropical moisture feed advects towards the coast. Thermal profiles would support snow across almost all the areas, outside of some initial mixing with rain at onset for city/LI. 00z models 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. 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 issues for se areas, the offshore convection could be playing a factor in affecting magnitude of offshore 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, along with the slower snow onset, accumulating snow appears to be slightly slower to taper off than what appeared 12 hrs ago; not until the longwave trough axis begins moving into the region tonight. Accumulating snow expected to taper off across the NYC metro by around midnight, and late night across Eastern LI/SE CT.
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&& .SHORT TERM /SUNDAY/... 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. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... 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. && .AVIATION /18Z SATURDAY THROUGH THURSDAY/... Low pressure tracks off the NC/VA coast this morning, and into the Canadian Maritimes tonight. Snow will continue to spread into the remaining terminals by 16-17z. VFR/MVFR conds in snow for the first hour, then lowering to IFR within 2-3 hours of snow onset. Heaviest snowfall still expected mid- day early evening. Snow tapers off after 00Z from west to east. North-northeast flow generally below 12 kt is expected 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 12Z SUNDAY THROUGH WEDNESDAY... .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. && .MARINE...
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Minor changes were made with this update to reflect the latest trends in observations and guidance. The forecast appears on track. 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... 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. && .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 until 7 AM EST Monday for ANZ350-353-355.
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&& $$ SYNOPSIS...NV/MD NEAR TERM...Maloit/NV SHORT TERM...NV LONG TERM...MD AVIATION...JC/MD MARINE...Maloit/NV/MD HYDROLOGY...NV/MD EQUIPMENT...

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