Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 131818 AFDOKX Area Forecast Discussion National Weather Service New York NY 118 PM 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/... There have been a few reports of flurries as there is just enough moisture around 4-5kft. As the surface low lifts further north, drier air should continue to work in from the west. This is already being seen on visible satellite imagery across eastern PA and NE NJ. 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 lifts 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 gusts up to 45 mph, but not widespread enough for 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 /18Z WEDNESDAY THROUGH MONDAY/...
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VFR into this evening as low pressure well to the northeast and its associated BKN045-050 cigs pull away, then IFR vsby in light snow expected tonight into mid morning Thu as low pressure passes just to the south. W winds 20-25 kt with gusts around 35 kt should continue until around 22Z-23Z, gradually diminish this evening, and then back WSW before snow starts. Winds should become mainly light and variable overnight with the arrival of the snow, then after the low passes (close to 11Z-12Z Thu NYC metro) become NW and gradually increase, with gusts close to 20 kt likely at the NYC metros by late morning. ...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: Snow accumulation 1-2 inches after midnight tonight into mid morning Thu. KLGA TAF Comments: Snow accumulation 1-2 inches after midnight tonight into mid morning Thu. KEWR TAF Comments: Snow accumulation 1-2 inches after midnight tonight into mid morning Thu. KTEB TAF Comments: Snow accumulation 1-2 inches after midnight tonight into mid morning Thu. KHPN TAF Comments: Snow accumulation 1-2 inches late tonight into mid morning Thu. KISP TAF Comments: Snow accumulation 2-3 inches late tonight into mid morning Thu. .OUTLOOK FOR 18Z THURSDAY THROUGH MONDAY... .Thursday afternoon and night...VFR with gusty NW winds, just right of 310 magnetic, diminishing during the evening. .Friday and Friday night...VFR. .Saturday-Saturday night...VFR. WNW winds G25KT daytime and G15-20KT evening. .Sunday...VFR. .Sunday night and Monday...MVFR or lower flight cat possible in snow/rain.
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&& .MARINE... Gale warning continues across all waters through tonight, with NY Harbor, Western LI Sound and the South Shore Bays likely falling below gales by this evening. Small craft advisory conditions are then expected tonight with a weaker pressure gradient. 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... MARINE...BC/Fig HYDROLOGY...BC/Fig EQUIPMENT...

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