Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 162335 AFDOKX Area Forecast Discussion National Weather Service New York NY 735 PM EDT Fri Mar 16 2018 .SYNOPSIS... Low pressure over the Canadian Maritimes very slowly moves eastward as high pressure builds from the north and west through Saturday. A cold front passes through Saturday night, with high pressure from South Central Canada then building across the region Monday. That high pressure area then settles into Southeast Canada, just north of the Great Lakes Monday night. A low pressure system will develop in the Southeast and move off the Carolina coast Tuesday, taking a track southeast of Long Island Tuesday night into Wednesday. Low pressure strengthens and moves into the Canadian Maritimes Wednesday night through Thursday. Canadian high pressure then slowly builds in from the west late next week. && .NEAR TERM /UNTIL 6 AM SATURDAY MORNING/... The fcst is on track. A large scale area of low pressure that is anomalously deep for this time of year will remain centered across eastern Canada, with a series of short waves rotating through the northeastern U.S. Confluent flow aloft between the polar jet along the west side of the upper trough and the subtropical jet to the south will allow a surface high to develop southeastward towards the area tonight, maintaining strong northwest flow between the approaching high and the stagnant surface low to the northeast. Amidst strong northwest flow, cold air advection continues to lead to well below normal temperatures, more typical of January than mid March. Lows tonight will generally be in the 20s, with a few 30s possible around the NYC metro. && .SHORT TERM /6 AM SATURDAY MORNING THROUGH SATURDAY NIGHT/... Saturday will mostly be dry as a short wave passes to the north and east early, allowing for a brief period of subsidence. By evening, another short wave rotates southeastward, through New England, though precipitation will be in the form of flurries, at best, mainly for Connecticut. The short wave will allow a surface front to pass to the south, with cold advection following into Saturday night. Already cold temperatures around 5 to 10 degrees below normal will then fall further overnight, with areas outside the NYC metro falling into the teens to lower 20s. Low temperatures will also largely depend on how quickly winds subside as high pressure builds into the area. If winds subside faster than forecast, temperatures could be much colder. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Looking at the large scale, the wave pattern at 500mb exhibits a slowly lifting trough Sunday through Monday night with the height gradient gradually becoming less tight. Then, a series of shortwaves will be moving southeastward and signs of constructive interference or phasing with the waves as ultimately a larger trough develops across the Eastern US Tuesday night into Wednesday. The Friday 00Z ECMWF run showed this but the Friday 12Z run has showed less phasing. The axis of the trough will be along the Eastern Seaboard Wednesday night and then will slowly exit the Eastern Seaboard Thursday into Friday with some ridging behind it. Concerning the jet stream, this is south of the region, and nearly zonal Sunday through early Tuesday. Then, the jet intensifies and becomes more meridional Tuesday into Tuesday night. This continues into Wednesday. Wednesday and Wednesday night seems to be when precipitation would be most intense as there are favorable jet dynamics with the left front quad of jet moving SE of Long Island. Overall, there is apparent a coupled jet structure as well Wednesday into Wednesday night. Although 12Z Friday ECMWF run showed less coupling than previous run. The jet dynamics would favor divergence aloft and deepening of the surface low pressure. Then the jet is well south of the region Wednesday night through Thursday with a large decrease over the local region with 250mb winds. The trough at 250mb exits the region next Friday. The main focus of this time period will be Tuesday through Wednesday night with a coastal storm and its potential impacts. A general model suite conveys the low moving off the coast of the coast of North Carolina in the Tuesday/Tuesday evening timeframe, then passing southeast of Long Island Wednesday before heading towards the Canadian Maritimes Wednesday night into Thursday. Some models are still conveying this as a series of weaker lows. Specifically the 12Z Friday ECMWF run shows the low and its precipitation staying south of the region, in contrast to its previous run. The GFS still shows precip well into the region with the low. A prolonged period of gusty winds and precipitation is in the forecast without much change from previous forecast. For interior areas, this would be a prolonged period of snow while towards the coast, this would be a mix of rain and snow. The precipitation could be heavy at times. For Thursday, the models convey precipitation on the backside of low across the region before all precipitation ends with northerly flow. This would be more in the form or periodic rain and snow showers with the trough lingering across the region. There is the possibility of heavy precipitation in form of rain and snow as well as strong winds for Tuesday through Wednesday night from the coastal storm. The exact details are highly uncertain at this time. The details with timing, magnitude and track of this storm system will be very important in determining when much of the precipitation will fall, how much wintry precipitation there will be, as well as the magnitudes of wind and potential coastal flooding. There is uncertainty and run to run variability contingent upon phasing of shortwaves and coupling of jet structure. Pieces of energy originating from an upper level low currently in the Northwest US and currently near Alaska will play a role as they move southeast this weekend and how the models handle that energy. However, the overall pattern of a trough across the Eastern US remains for mid next week. Drier weather returns Thursday night into next Friday with Canadian high pressure slowly building in from the west. Temperatures will remain below normal. && .AVIATION /23Z FRIDAY THROUGH WEDNESDAY/... Low pressure will continue to spin over eastern Canada through Saturday. High pressure builds in from the west tonight, and slides south of the terminals Saturday. VFR thru the taf period. Gusty WNW flow, around 310 true, with gusts to around 30 kt, will gradually diminish this evening with gusts ending at most terminals between 06Z to 08Z. There is a possibility gusts continue through the night at the NYC metro terminals. WNW winds and gusts increase once again Saturday. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/ KJFK TAF Comments: NW wind gusts to 30kt will be possible through 02Z. KLGA TAF Comments: NW wind gusts to 30kt will be possible through 02Z. KEWR TAF Comments: NW wind gusts to 30kt will be possible through 02Z. KTEB TAF Comments: NW wind gusts to 30kt will be possible through 02Z. KHPN TAF Comments: NW wind gusts to 30kt will be possible through 02Z. KISP TAF Comments: NW wind gusts to 30kt will be possible through 02Z. .OUTLOOK FOR 00Z SUNDAY THROUGH WEDNESDAY... .Saturday night-Monday night...VFR. .Tuesday...VFR...becoming MVFR by the afternoon with rain and snow developing. Gusty E-NE flow develops, 15-25kt. .Tuesday night and Wednesday...MVFR to IFR in snow. Rain and snow possible along the coast Wednesday. Gusty NE wind 20-30kt. && .MARINE... Strong and gusty winds will continue through the night, though with the loss of daytime heating subsequently limiting mixing, gales are expected to gradually subside tonight. The pressure gradient remains tight between an area of low pressure centered north and east of the area and a building high, so SCA-level winds are expected to continue into Saturday. A cold front then moves through Saturday evening, with building high pressure allowing winds and seas to slowly subside overnight. Below SCA conditions are forecast Sunday through Monday night with a weak pressure gradient in place. Seas and winds increase with increase of pressure gradient with developing storm system Tuesday when SCA will be probable. Gales will be probable Tuesday night through Wednesday night, especially for the ocean. && .HYDROLOGY... No significant hydrologic impacts are anticipated through Monday. There is the potential for a period of moderate to heavy precipitation from late Tuesday through Wednesday night. Predictability of any hydrologic impacts is low at this time. && .TIDES/COASTAL FLOODING... There is the potential for a prolonged period of E-NE flow from Tuesday into Wednesday Night. Tidal departures of generally only 1 to 1 1/2 ft are needed for minor flooding for many locations during this time, and 2 to 3 ft for moderate. Depending on the exact track/strength of a coastal low or lows during this time frame, there is the potential for minor to moderate coastal flooding, but predictability on any details is low at timeframe. && .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...None. NJ...None. MARINE...Small Craft Advisory until 6 PM EDT Saturday for ANZ330-335- 338-340-345-355. Gale Warning until 2 AM EDT Saturday for ANZ350-353. && $$ SYNOPSIS...MD/JM NEAR TERM...JMC/MD SHORT TERM...MD LONG TERM...JM AVIATION...BC/19 MARINE...MD/JM HYDROLOGY...MD/JM TIDES/COASTAL FLOODING... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.