Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 131409 AFDOKX Area Forecast Discussion National Weather Service New York NY 909 AM EST Wed Dec 13 2017 .SYNOPSIS... Strong low pressure moves into the Canadian Maritimes today. A weaker low pressure system tracks south of the area late tonight and then moves offshore Thursday, followed by high pressure briefly Thursday night into Friday. Low pressure passes southeast of the region Friday night into Saturday, followed by a second wave of low pressure late Sunday and Monday. High pressure briefly builds over the region Monday night, followed by another chance of unsettled weather for mid- week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Strong low pressure will track into the Canadian Maritimes today as high pressure builds across the southeast states. An arctic airmass is already in place as observed by the 12z OKX RAOB. The sounding observed an 850 mb temperature of -17C, which is about a degree higher than the min observed value of -18.2C for December 13 per SPC Sounding Climatology page, which dates back to 1957. Anomalous upper level trough and closed low swing across this morning, and then lift into the Canadian Maritimes this afternoon and evening with brief height rises ahead of another strong vort max swinging across the Great Lakes. Forecast soundings show mixing to 4-5kft, with winds around 40 kt at this layer. Strong pressure gradient between the departing low and high pressure to the south will lead to gusts 35 to 40 mph today. A few locations could see wind advisory criteria met, but do not think it will be widespread enough for the issuance of an advisory. Have lowered high temperatures several degrees with this update. Highs will be in the middle and upper 20s for most locations away from the coast, with highs around 30 across Long Island, NYC metro, and coastal Connecticut. These temperatures combined with the wind will make it feel like the single digits this morning and teens this afternoon. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Winds are forecast to diminish tonight ahead of approaching clipper low. Mostly clear conditions to start the night will give way to increasing clouds ahead of the low. Temperatures fall into the teens to low to mid 20s. Strong vort max swings around the mean upper trough tonight with just enough lift to develop light snow overnight from west to east. Strong lift from the vort max should be enough to overcome limited moisture. Cold atmosphere in place should lead to a higher ratio, dry light snowfall into the early morning hours. Current mesoscale guidance indicates slightly better moisture near the coast, so amounts could be a bit higher there. Overall less than inch expected inland to around an inch at the coast. This will be refined once the full 12z suite of model data is available. The light snow ends during just after sunrise with high pressure building in behind the passing low. High temperatures on Thursday will be in the upper 20s to mid 30s. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... The long term period starts off with weak high pressure over the region Thursday night into Friday. 13/00z forecast guidance indicating a shortwave/upper level low diving out of Canada and moving over the region Friday night into Saturday. As it does, a surface low develops, passing well offshore. Thinking at this point, that most of the region stays dry, but can not rule out a slight chance of some snowflakes across the twin forks of LI and eastern CT. Weak high pressure and ridging then briefly builds back over the region Saturday night into Sunday, allowing temperatures to rebound a bit back to more seasonable levels. Another weak wave of low pressure slides across region late Sunday through Monday. Both the ECMWF and GFS capture this system, however the ECMWF is a bit more robust with its precipitation. Also, there are some slight timing issues with just how quickly the system moves into and out of the region. With the warmer temperatures in place, do expect precipitation to be either rain or a rain/snow mix along the coast, with temperatures cold enough for some light snow inland. Temperatures do warm enough on Monday for precipitation to become rain everywhere. Dry conditions then expected Monday night into the first part of Tuesday. Additional light precipitation is then possible Tuesday afternoon into Tuesday night as another shortwave slides across the region. A few left over rain or snow showers will be possible on Wednesday, however for now will keep conditions dry as weak high pressure starts to build southwest of the region. && .AVIATION /14Z WEDNESDAY THROUGH SUNDAY/... VFR today as low pressure tracks into the Canadian Maritimes. MVFR conditions or lower are possible tonight in snow as another area of low pressure approaches from the west, generally after 07Z. Less than an inch of accumulations is possible during this time frame. As for winds, W winds of 20 to 25 kt true, with gusts 35 to 40 kt. There could be some occasional gusts over 40 kt. Best timing for the strongest gusts is 13Z to 21Z. Thereafter, gusts should slowly diminish through the rest of the TAF period. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component fcsts, can be found at: http:/www.weather.gov/zny/n90 KJFK TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z-21Z. The afternoon KJFK haze potential forecast is GREEN, which implies slant range visibility 7SM or greater outside of cloud. KLGA TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z-21Z. The afternoon KLGA haze potential forecast is GREEN, which implies slant range visibility 7SM or greater outside of cloud. KEWR TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z- 21Z. The afternoon KEWR haze potential forecast is GREEN, which implies slant range visibility 7SM or greater outside of cloud. KTEB TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z- 21Z. KHPN TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z-21Z. KISP TAF Comments: Occasional gusts over 40 kt possible, mainly from 13Z-21Z. .OUTLOOK FOR 12Z THURSDAY THROUGH SUNDAY... .Thursday morning...MVFR/IFR in snow, mainly overnight Wednesday night through early Thursday morning. W winds 15-20 kt G25-30 kt Wednesday evening. W winds diminish overnight Wednesday. .Thursday afternoon-Friday...Mainly VFR. NW wind G15-20kt into early Thursday night, diminishing thereafter. .Friday night...VFR .Saturday-Saturday night...VFR. NW winds G20-25kt during day, G15-20KT during evening. .Sunday...MVFR or lower possible in rain/snow. && .MARINE... Gale warning continues across all waters through tonight, with NY Harbor, Western LI Sound and the South Shore Bays falling below gales by this evening. Small craft advisory conditions are then expected tonight as the next system approaches. Some leftover 5-6 ft seas continue Thursday night on portions of the ocean waters, then conditions fall below SCA levels Friday morning. Low pressure, well south and east of the region intensifies Friday night into Saturday as it moves northeast off the coastal waters. Tightening pressure gradient and building seas will most likely yield Small Craft Advisories, with possible Gales for the ocean waters late Friday night and Saturday. Winds and seas then fall below SCA levels on Sunday. A series of weak wave of low pressure move across the area waters Monday-Wednesday of next week. As each of these systems moves across the area waters, Wind and seas may increase to SCA levels. && .HYDROLOGY... No significant hydrologic impacts expected. && .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...Gale Warning until 6 PM EST this evening for ANZ335-338-345. Gale Warning until 6 AM EST Thursday for ANZ330-340-350-353- 355. && $$ SYNOPSIS...BC/Fig/DS NEAR TERM...DS SHORT TERM...Fig/DS LONG TERM...BC AVIATION...JP MARINE...BC/Fig HYDROLOGY...BC/Fig EQUIPMENT...

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