Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 200544 AFDOKX Area Forecast Discussion National Weather Service New York NY 144 AM EDT Tue Mar 20 2018 .SYNOPSIS...
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High pressure builds down from south central Canada today as a coastal low develops off the mid Atlantic coast by morning, then tracks east as it weakens. Another stronger coastal low develops along the mid-Atlantic coast tonight, then lifts northeast to south of Nova Scotia by Thursday morning, then into the Canadian Maritime Provinces through Thursday night. High pressure will then build from southern Canada from Friday through Saturday. Another storm system will may impact the area late in the weekend.
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&& .NEAR TERM /UNTIL 6 AM THIS MORNING/... Forecast remains on track with only minor updates needed to reflect current temperature and dew point trends. Weak northern stream ridging builds in tonight keeping things dry. Will see an increase in high clouds from SW to NE through the night. Lows tonight will be around 5-10 degrees below normal. && .SHORT TERM /6 AM THIS MORNING THROUGH WEDNESDAY NIGHT/... Details are coming in to better focus on a couple of coastal lows that will impact the Tri-State Tuesday-Wednesday night. The first low is in response to a 700-500 hPa shortwave being sheared by a cutoff low over Canadian Maritimes. As a result the low weakens as it exits the mid-Atlantic coast, so only expect light warm advection induced precipitation over mainly the S 1/2 of the area. It does appear that a warm nose works in around/just above 850 hPa so do expect some sleet to mix in with a snow/rain mix (rain mixed in mainly due to March sun angle). Expect any accumulations on Tuesday to be limited to mainly far SW parts of the CWA This warm nose lingers over most of the area into Wednesday morning (except for maybe far NW zones), so do expect sleet to limit accumulations Tuesday night, limiting them to generally less than 1 inch. The second coastal low forms off the mid Atlantic Coast Tuesday night in response to a combination of a shortwave diving SE from the Canadian Plains and another shortwave coming in from the Four Corners region. As the coastal low deepens Wednesday, it taps into colder low level air to the north, coupled with dynamic cooling, should change the precipitation to all snow across the region. There is the potential for a period of heavy wet snow across the region from late Wednesday morning into Wednesday evening. In addition gusty winds of 25 to 40 mph could combine to bring down tree limbs and power lines. The snow tapers off from W to E Wednesday night as the coastal low pulls away to the NE. As a result have issued a winter storm watch for the region from 6pm Tuesday-midnight Wednesday night for NYC/NE NJ/Lower HUdson Valley for 5-11 inches of snow and from 4am Wednesday-6am Thursday for Long Island and S CT for 4 to 9 inches of snow. && .LONG TERM /THURSDAY THROUGH MONDAY/... High pressure builds into the region from southern Canada on Thursday, providing generally dry conditions through the first half of the weekend. Some weak forcing may spark off a few snow showers or flurries late Saturday with an upper level low off the New England Coast, but not enough confidence to put it in the forecast as of yet. However, the bigger story will be the potential for another low pressure system to affect the region for the second half of the weekend. There are significant differences in the model solutions, both in the deterministic and ensemble models, for the second half of the weekend and the potential low pressure system to affect the area. The main differences stem from orientation of a strong 1043 mb surface high pressure over southern Canada Saturday that digs all the way south into Florida and the Southeast US Coast. The 19 12Z ECMWF pinches off a piece of this high Saturday night, giving an area of low pressure developing over the Mid-West a path between the two highs to move toward our region. The 19 12Z GFS does not pinch off this high as notably as the ECMWF, with more of a cold air damming signature noted on Sunday, when the ECMWF pushes the low off the Carolina Coast, with our region on the northern fringes of the precipitation field. The low is then forecast to pass well south of our area and then east out to sea Sunday night. The GFS suppress the low well south and weakens it. This latest model run of the ECMWF is more in line with the GFS. Given forecast uncertainty, will not make much in the way of changes. Proximity of the low to the coast will determine precipitation types. If the ECMWF is to be believed, the track is a bit different to previous storms, at least with its approach to the area, with a more west to east track, then head northeast east, as opposed to development or redevelopment off the Mid-Atlantic coast and heading northeast. However, it will be cold enough for some snow. Temperatures during the long term will average below normal with a persistent northwesterly flow. && .AVIATION /06Z TUESDAY THROUGH SATURDAY/...
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High pressure gives way to a series of lows passing to the south and east of the area, the first this evening, and then another on Wednesday. VFR today and through the evening. Light NE winds this morning gradually ramp up during the day. Gusts to around 20 kt are expected at the coastal terminals in the afternoon and increasing to 25-30 kt into tonight. Inland gusts will be a bit weaker. There is a chance of a light wintry mix developing this evening. The brunt of the precipitation is currently forecast to develop on Wednesday. .OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY... .Late tonight...MVFR conditions possible with a light wintry mix. NE winds G20-30KT NYC metro/coastal terminals. .Wednesday...Snow and sleet with IFR or lower conditions likely. NE winds G20-35KT. .Thursday-Friday...VFR. N winds G20-25KT on Thu. .Saturday...MVFR possible late in chance of rain/snow.
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&& .MARINE... Winds and seas will remain tranquil early tonight as high pressure builds in from the northwest. However, the gradient between low pressure off to the south and west and high pressure to the northwest will increase. Therefore, winds will then gradually begin to strengthen late tonight and into Tuesday morning. This will allow winds and seas to increase to SCA-levels Tuesday afternoon and evening for the ocean and eastern sound, with Gale to storm conditions developing Tuesday night into Wednesday. Have issued a storm watch for the ocean for Tue night into Wed, as E- NE winds rapidly increase in response to the development of the coastal low. Expect gusts mainly 35-45 kt during this time, and peak gusts of 45 to 50 kt may be possible on the outer ocean waters. Gale conditions are expected for the Eastern Sound and the bays of Long Island overall during this timeframe. Gales should then linger on the ocean through much of Wed night. Winds diminish Thursday as low pressure heads away from the coastal waters. SCA winds possible early in the morning for the ocean waters and Peconic and Gardiner`s Bay. Thereafter, winds should remain below 25 kt through Saturday night. Winds increase late Saturday night into Sunday morning as another low pressure system approaches the coastal waters. A period of SCA winds are possible from Sunday morning through Sunday night, mainly on the ocean waters as the low pressure moves toward the region. Waves diminish on Thursday as well, with ocean waters coming down below 5 ft Thursday night. Waves then remain below 5 ft through Saturday, building again Saturday night in response to approaching storm. 5 ft waves possible late Sunday morning. && .HYDROLOGY... It will be mainly dry through tonight. There is the potential for a prolonged period of precipitation from Tuesday through Wednesday night. Total liquid equivalent precip should range from 0.5 to 1.25 inches, falling mainly as snow and sleet, so no hydrologic impact expected. && .TIDES/COASTAL FLOODING... A prolonged NE/N flow will have water levels building Tuesday night into Wednesday. Tidal departures of generally only 1 to 1 1/2 ft are needed for minor flooding for many locations Tuesday night through Wednesday night, and 2 to 3 ft for moderate. At this point, mainly minor impacts are expected during the Tuesday Night high tide as NE winds are just beginning to ramp up to SCA/Gale force. With NE gale to storm force winds continuing into the late Wednesday morning/early afternoon high tide cycle, widespread minor to moderate impacts are likely for the Wed late morning/early afternoon high tide. Latest guidance has trended upward with surge for this tidal cycle. At this point, highest confidence is in the southern bays of Nassau and Western Suffolk county reaching moderate flood thresholds, but this threat may need to be expanded a bit depending on model/surge guidance trends. Another round of minor to locally moderate coastal flooding likely for the Wed night high tide cycle. There is more guidance spread with this tidal cycle, due to competing factors of tidal piling and energetic wave energy build water levels along the coast, versus N/NW gales pushing water away from the coast. Will have to continue to monitor this tidal cycle, as guidance typically has a low bias on the back end of multi tidal cycle surge events. && .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...Winter Storm Watch from late tonight through late Wednesday night for CTZ005>012. NY...Winter Storm Watch from late tonight through late Wednesday night for NYZ078>081-177-179. Winter Storm Watch from this evening through Wednesday evening for NYZ067>075-176-178. Coastal Flood Watch from Wednesday morning through Wednesday afternoon for NYZ080-179. NJ...Winter Storm Watch from this evening through Wednesday evening for NJZ002-004-006-103>108. MARINE...Gale Watch from this evening through late Wednesday night for ANZ330-335-338-340-345. Small Craft Advisory from noon today to 6 PM EDT this evening for ANZ330-340-350-353-355. Storm Watch from this evening through Wednesday evening for ANZ355. Storm Watch from this evening through late Wednesday night for ANZ350-353. && $$ SYNOPSIS...Maloit/JP NEAR TERM...JC/MD/Maloit SHORT TERM...Maloit LONG TERM...JP AVIATION...PW MARINE...JE HYDROLOGY...Maloit/JP TIDES/COASTAL FLOODING... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.