Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 082346 AFDOKX Area Forecast Discussion National Weather Service New York NY 646 PM EST Fri Dec 8 2017 .SYNOPSIS... An area of low pressure begins to organize along the southeast coast tonight. The low tracks south and east of Long Island on Saturday and off the New England coast on Saturday night. High pressure will return Sunday into Monday. Another area of low pressure impacts the region on Tuesday and Wednesday, followed by high pressure Thursday. Another low pressure system approaches for the end of the week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... Models in good agreement with strong shortwave energy diving down the backside of Great Lakes trough, helping to amplify this trough and begin lifting an intense southern stream shortwave up the coast. This will also intensify a phased SW upper jet 165-175kt west of the region. At the surface, low pressure over the Gulf of Mexico will track up off the SE US coast by Saturday morning. Increasing lift and moisture due to this scenario will result in lowering and thickening clouds, with increasing potential for light precipitation development from SE to NW along the coastal plain after midnight. Initially quite a bit of dry air to overcome, but with a southern stream energy feed and subtropical moisture connection have leaned towards a quicker saturation and slightly quicker precip development along the coastal plain late tonight into Saturday morning. Already observing snow accums in Delaware. Thermal profiles would support snow across almost all the areas, outside of some initial mixing with rain. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Models continue in good agreement with northern stream shortwave energy diving down through the Great Lakes into Ohio Valley on Sat, and then pivoting into the NE Sat night. The resultant amplifying Eastern US trough will send a southern stream shortwave up the coast on Saturday, with at least the eastern 1/2 to 2/3rd of CWA under a favorable right rear of 165-175 kt jet dynamics. At the surface, low pressure tracks from the SE US coast Sat morning to near the 40/70 lat/lon benchmark by Sat Evening. This deep lift tapping into a sub-tropical moisture feed offshore should allow for snow to overspread the entire region from SE to NW late tonight into Saturday morning, becoming steadier later Saturday morning. Models also indicating some weak mid-level frontogenesis and even some 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. Meanwhile, NW zones will be on the edge of frontogenetic and jet forcing, so there will be subsidence and dry air to combat, resulting in uncertainty on the expanse of accumulating snowfall across areas NW of NYC. 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 appears that it could make inroads into far se portions of the area Sat 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 Sat/Sat 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. At this point, based on an ensemble of thermal profiles, the south fork of LI appears to have the highest probability of seeing the warm nose impinge and result in a period of mixing Sat afternoon into evening. In terms of QPF, models have trended higher with QPF across he region, likely better sampling the 3-4+ std PWAT sub-tropical convection riding up the warm conveyor belt. Generally 1/2 to 3/4 of an inch of across LI/CT, highest east, 3 to 5 tenths across NYC/NJ metro and SW CT/SE NY border, tapering to 1/4 inch for far nw zones. 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, confidence in 6 inches or more of snow for SE CT and Eastern LI (except for the south fork) is high enough to upgrade to warning. Farther east a solid advisory level snow expected, with 3 to 6 inches to at least the Hudson River. The caveat in terms of snowfall amounts will be that snow accumulation on pavements may be limited until solar insolation decreases in the mid to late afternoon through the evening due to marginal surface temps. In term of uncertainty, the subtropical moisture feed and SE US convection strengthening Atlantic ridging are two players that could either increase QPF across the area or result in a slight westward shift in the storm. This would have the implication of increasing potential for warning level snow westward towards the Hudson River, Advisory level snows in Orange County. On the other end this would also increase potential for mixing and reduce snow amounts for the Eastern LI and SE CT. These are low chance probs at this time, and w ill have to monitor mesoscale models, satellite and radar trend tonight. Snow should taper from NW to SE Sat evening/night as shortwave energy slides NE of the region and low pressure departs into the Gulf of Maine. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Snow showers expected Sunday morning as a low pressure system lifts into the Canadian Maritimes on Sunday. A tight pressure gradient develops on Sunday resulting in a gusty westerly flow with high temperatures in the mid to upper 30s and overnight temperatures in the low to mid 20s. Weak high pressure passes through the local area Monday with temperatures slightly moderating but, remaining below normal for this time of year. A strong upper level low will approach from the Ohio Valley region, impacting the local area Tuesday through Wednesday. Due to much uncertainty at this time with QPF and/or snow, have continued with the chance of snow, possibly mixed with rain at the coast Tuesday into Tuesday night. High pressure then follows for Thursday ahead of another clipper low and reinforcing cold shot at the end of the week. Temperatures Tuesday and through the end of the week will remain below seasonable levels. && .AVIATION /00Z SATURDAY THROUGH WEDNESDAY/... Low pressure strengthens off the Mid-Atlantic coast tonight, then passes to the south of Long Island Saturday afternoon. VFR through at least 6Z. 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. West winds become light and variable tonight. N-NE winds around 10KT at city/Long Island terminals and KGON and less than 10KT elsewhere develop on Saturday. ...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: Snowfall rates could reach 1 inch per hour on Saturday. Around 4 inches of total snow forecast. KLGA TAF Comments: Snowfall rates could reach 1 inch per hour on Saturday. Around 4 inches of total snow forecast. KEWR TAF Comments: Snowfall rates could reach 1 inch per hour at times on Saturday. Around 4 inches of total snow forecast. KTEB TAF Comments: Snowfall rates could reach 1 inch per hour at times on Saturday. Around 4 inches of total snow forecast. KHPN TAF Comments: Snowfall rates could reach 1 inch per hour at times on Saturday. Around 4 inches of total snow forecast. KISP TAF Comments: Snowfall rates could exceed 1 inch per hour at times on Saturday. 4-6 inches of total snowfall forecast. .OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY... .Saturday night...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. && .MARINE... A relaxed pressure gradient will remain over the waters through tonight. The pressure gradient tightens Saturday into Saturday night. Marginal small craft gusts are likely on the ocean Saturday Afternoon into Saturday night, particularly eastern ocean waters. Ocean seas will begin to build above SCA Saturday afternoon as low pressure passes makes closes approach late Saturday into Saturday night. SCA conditions will continue Sunday into Monday as a low pressure system moves into the Canadian Maritimes. Winds will gradually diminish through Monday as high pressure builds across the area waters. Another low pressure system is forecast to impact the area waters on Tuesday and into Wednesday resulting in the potential for SCA conditions to return. High pressure builds across the area waters on Thursday with seas falling slowly below SCA conditions. && .HYDROLOGY... Generally 1/2 to 3/4 of an inch of liquid is expected across LI/CT( highest east), 4 to 5 tenths across NYC/NJ metro to SW CT/SE NY border, tapering to 1/4 inch for far NW zones; Saturday into Saturday night. 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... CT...Winter Storm Warning from 4 AM Saturday to 6 AM EST Sunday for CTZ007-008-010>012. Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday for CTZ005-006-009. NY...Winter Storm Warning from 4 AM Saturday to 6 AM EST Sunday for NYZ078>080. Winter Weather Advisory from 4 AM Saturday to 1 AM EST Sunday for NYZ068-069. Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday for NYZ070>075-176>179. Winter Weather Advisory from 4 AM Saturday to 6 AM EST Sunday for NYZ081. NJ...Winter Weather Advisory from 4 AM Saturday to 1 AM EST Sunday for NJZ002-004-103-105-107. Winter Weather Advisory from 4 AM Saturday to 4 AM EST Sunday for NJZ006-104-106-108. MARINE...Small Craft Advisory from 1 PM Saturday to 7 AM EST Sunday for ANZ350-353-355. && $$ SYNOPSIS...Fig/NV NEAR TERM...NV/PW SHORT TERM...NV LONG TERM...Fig AVIATION...JMC/PFM MARINE...Fig/NV/PW HYDROLOGY...Fig/NV EQUIPMENT...

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