Area Forecast Discussion
Issued by NWS Upton, NY

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987 FXUS61 KOKX 201127 AFDOKX Area Forecast Discussion National Weather Service New York NY 727 AM EDT Tue Mar 20 2018 .SYNOPSIS... Arctic high pressure will begin to retreat to the north as a significant low pressure system approaches from the south. The developing coastal system will impact the area through Wednesday, before finally departing to the northeast into the Canadian maritimes on Thursday. High pressure then builds from eastern Canada through the weekend. Another storm system may be possible for early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... With the antecedent dry conditions due to the prevailing Arctic high, the onset of any precipitation expected with the developing coastal storm has once again been delayed until afternoon or evening. Any precipitation will initially be light and largely confined to the southern-most areas - northern NJ, the NYC metro and Long Island. With saturation expected to be slow and precipitation light, any evaporative cooling will be minimal, so precipitation type may initially be rain or perhaps a slushy rain/snow mix as high temperatures will be in the upper 30s to lower 40s. Temperatures are still around 10 degrees below normal for this time of year. && .SHORT TERM /6 PM THIS EVENING THROUGH THURSDAY/... The main story is the developing nor`easter. Winter storm watches have been converted to warnings across all areas except eastern Suffolk County, where the highest uncertainty in precipitation type resides. Steadier precipitation is expected to develop after around midnight from south to north tonight, becoming more intense through the Wednesday morning commute. Areas from about NYC eastward will also need to be monitored for a potential blizzard warning with strong winds around 20-30 mph with gusts up to 45 mph are expected. Primary uncertainty may be with visibilities, as snow ratios will be lower across these areas leading to a wetter, heavier snow. Snow totals have once again been increased. Initially precipitation types will be mixed as strong easterly flow above the surface advects warmer temperatures westward. For now went with a GFS/NAM blend for precipitation types. It should be noted that the NAM tends to be warmer in these warm advection scenarios, which would lead to more significant accumulations of freezing rain for Tuesday night into Wednesday morning, but based on similar events a mix of sleet, wet heavy snow and perhaps minimal accumulations of freezing rain seem more likely, especially closer to the coast. By Wednesday afternoon to evening, the storm intensifies amidst strong diffluence/divergence aloft as the upper low passes just to the south. Given strong convergence at the surface to the northwest of the low and vorticity maximized in a corridor roughly from Connecticut through NYC into NJ, expect the development of a potentially significant snow band, with any mixed precipitation quickly transitioning to heavy snow as a result of dynamic cooling. The heaviest snow rates will be possible around this time, with 1-2"/hr possible. Additionally, temperatures aloft will rapidly cool as the upper low passes just offshore, which may provide enough elevated instability for charge separation, and thundersnow with locally higher rates. As the system ejects to the east-northeast, the snow band(s) will rotate eastward, allowing any remaining areas still in mixed precipitation to transition to all snow as well. For some areas farther east, particularly eastern Long Island and southeast Connecticut, the time frame after 8 pm as the band(s) pull east may be the period of best snowfall potential with the highest rates of accumulation. Also note, that given this is a late season event and temperatures will be somewhat marginal, snowfall is once again expected to be heavy, and combined with strong winds may lead to downed trees and powerlines. High pressure then builds into the area again following the departure of the low pressure system, allowing precipitation to taper off from west to east during the early morning. At least light snow may linger across eastern Long Island and Connecticut through the morning rush, but the more significant accumulations will have ended. Conditions will remain breezy on Thursday, with below normal temperatures continuing. && .LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... Generally dry conditions will continue 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 /12Z TUESDAY THROUGH SATURDAY/... 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 with lowering clouds. 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. MVFR is possible late at night as wintry precipitation moves north. The brunt of the precipitation is currently forecast to develop on Wednesday. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/ KJFK TAF Comments: Timing of gusts may be off an hour or two this afternoon. The afternoon KJFK haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KLGA TAF Comments: Timing of gusts may be off an hour or two this afternoon. The afternoon KLGA haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KEWR TAF Comments: Timing of gusts may be off an hour or two this afternoon. The afternoon KEWR haze potential forecast is GREEN...which implies slant range visibility 7SM or greater outside of cloud. KTEB TAF Comments: Timing of gusts may be off an hour or two this afternoon. KHPN TAF Comments: Timing of gusts may be off an hour or two this afternoon. KISP TAF Comments: Timing of gusts may be off an hour or two this afternoon. .OUTLOOK FOR 12Z WEDNESDAY THROUGH SATURDAY... .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. && .MARINE... As the first of two lows approaches late this afternoon, winds and seas should quickly ramp up, with gales arriving on the western ocean waters by late afternoon, and then out east this evening. They should however take a while to make it to the non-ocean waters, and while there could be a few gusts up to 35 kt at times tonight, the main push for gales on those waters should arrive daytime Wed as the second stronger low intensifies off the Mid Atlantic coast and moves slowly E on Wed. Storm force winds with gusts 50-55 kt expected daytime Wed on all the ocean waters, the out east into Wed evening. Seas on Wed may build as high as 17 ft on the ocean and 7 ft on the central/eastern Sound. Except for morning gales on the outer ocean waters, SCA conds expected on all waters through the day on Thu, then on the ocean Thu night. SCA conds are possible on Sunday mainly on the ocean waters as winds increase with the approach of another low pressure system. && .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 0.75 to 1.50 inches, falling mainly as snow and sleet, so no hydrologic impact expected. && .TIDES/COASTAL FLOODING... Prolonged NE-N flow will push water levels up Tue night into Wed night. Departures of about 2-3 ft for moderate flooding, and 1-1.5 ft are needed for minor flooding for many locations. It now appears that S Nassau should see moderate impacts with the Tue daytime high tide as winds will be stronger initially, with widespread minor impacts mainly along the SW Suffolk south shore bays and in the western Sound. With NE gale to storm force winds going 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, and have manually increased those values for the north shore and bays of eastern Long Island where winds will be strongest and wave action will cause additional runup. 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 multiple-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...Coastal Flood Advisory from noon to 6 PM EDT Wednesday for CTZ009-010. Winter Storm Warning from midnight tonight to 8 AM EDT Thursday for CTZ005>012. Coastal Flood Advisory from 1 AM to 4 AM EDT Wednesday for CTZ009-010. NY...Coastal Flood Advisory from noon to 6 PM EDT Wednesday for NYZ071-078-079-081-177. Winter Storm Watch from late tonight through Thursday morning for NYZ079-081. Winter Storm Warning from midnight tonight to 8 AM EDT Thursday for NYZ067>075-078-080-176>179. Coastal Flood Advisory from 10 AM to 2 PM EDT Wednesday for NYZ074-075-178. Coastal Flood Advisory from 1 AM to 4 AM EDT Wednesday for NYZ071-078-177. Coastal Flood Warning from 9 PM this evening to 2 AM EDT Wednesday for NYZ179. Coastal Flood Warning from 9 AM to 3 PM EDT Wednesday for NYZ080-179. Coastal Flood Advisory from 11 PM this evening to 4 AM EDT Wednesday for NYZ080. NJ...Winter Storm Warning from midnight tonight to 8 AM EDT Thursday for NJZ002-004-006-103>108. Coastal Flood Advisory from 10 AM to 2 PM EDT Wednesday for NJZ006-106-108. MARINE...Gale Warning from midnight tonight to 6 AM EDT Thursday for ANZ330-340-345. Gale Warning from 6 AM to 6 PM EDT Wednesday for ANZ335-338. Storm Warning from 6 AM to 6 PM EDT Wednesday for ANZ353-355. Gale Warning from 3 PM this afternoon to 6 AM EDT Wednesday for ANZ353-355. Storm Warning from 6 AM Wednesday to midnight EDT Wednesday night for ANZ350. Gale Warning from 6 PM this evening to 6 AM EDT Wednesday for ANZ350. && $$ SYNOPSIS...MD NEAR TERM...MD SHORT TERM...MD LONG TERM...Goodman AVIATION...PW MARINE...Goodman HYDROLOGY...MD TIDES/COASTAL FLOODING... EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.