Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 120254 AFDOKX Area Forecast Discussion National Weather Service New York NY 954 PM EST Mon Dec 11 2017 .SYNOPSIS... Low pressure continues to approach overnight and this passes to the north Tuesday. This intensifying low pressure system will move into the Canadian Maritimes Tuesday night into Wednesday night. A weak Alberta Clipper passes to the south on Thursday, followed by a low pressure system passing off the coast Friday night into Saturday. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Adjustments were made again to hourly temperatures, dewpoints, and cloud coverage to better match observed trends but the forecast overall remains on track. Min temperatures were slightly adjusted as well. To note about the low levels, a lot of dry air was evident on the 00Z OKX sounding between 850mb and 600mb. This layer will take some time to moisten so echoes on Doppler Radar are likely virga with any precipitation making it to the ground holding off until late overnight into daybreak Tuesday. Shortwave tracks across IL and OH toward the region just ahead of vigorous shortwave diving out of Canada toward the western Great Lakes region. At the sfc, two low pressure centers approach, passing across the Great Lakes region toward western NY State by Tuesday morning. A warm front/WAA out ahead of the lows/upper shortwaves will result in an increase in clouds, and precipitation chances increase overnight. Cold air initially across the northeast will warm as the night progresses. Sfc temperature trends starting to level off with temperatures in many locations holding steady. Expect the lows to have pretty much already been established. Temperatures are still expected to slowly rise overnight as south flow begins to pick up. At this time, precip looks to hold off until overnight per most operational models solutions, and many GEFS members. Across the interior, a few hours of snow is possible, with very low chances toward the coast for either a wintry mix or plain rain. Timing and placement will be the key as low levels warm ahead of the lows. && .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... Upper trough closes off, as sfc low deepens across NY State and into New England during this time frame. QPF looks light, with lower amounts or even trace amounts near the coast, and higher totals as you head north across the interior. Would expect interior portions of southern CT, and the lower Hudson Valley of southeast NY to observe higher qpf up to a 1/4 of an inch liquid equivalent. Early in the morning, a snow or rain snow mix will quickly give way to plain rain as WAA continues, and the lower levels of the atmosphere warm. With increasing southerly flow, temperatures warm through the 40s, and coastal locations should warm to around 50. Will lean toward the warmer guidance, and would not be surprised to see some temps slightly higher than guidance. Any precip tapers off as the day progresses, and drier air moves in behind the low/front by evening thanks to NW flow. With the upper low in the vicinity, a few snow showers are possible overnight, but this should be isolated. Gusty NW flow and CAA will commence. Temps fall to the upper teens/20s across the region. Quite cold. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... Wednesday begins with an upper low over the Northeast and a deepening surface low entering SE Canada. There could be some snow showers over eastern LI and SE CT in the morning before the upper low shifts east and overall lift diminishes. Dry, cold and windy otherwise. 850mb temperatures will fall to around 15C, but partial sunshine and a strong downsloping wind should somewhat temper the cold air mixing down from aloft. Highs expected to range from the upper 20s well inland to mid 30s for coastal sections. Wind chills start in the single digits in the morning and average around 20 in the afternoon. For now it appears that wind gusts should fall short of advisory levels, but probably not by much, so it`s possible that an advisory would be needed as the event draws closer. Some coastal spots might even meet criteria based on sustained winds. Winds subside Wednesday night and low temperatures will average 5-10 degrees below normal. A clipper low is progged to pass south of us on Thursday. Models show some track and timing differences as well as available moisture. Snow from this system may however stretch north into our region, so will introduce low chances for this into the forecast for southern sections. Like most clippers, this has the potential for only light accumulations. The active weather pattern continues on Friday with an upper trough axis digging into the Ohio Valley and models showing low pressure strengthening off the Mid-Atlantic Coast. The trough becomes negatively tilted Friday night, but not before the low is pushed SE of the 40N/70W benchmark. Upper jet winds/streaks are not currently progged in a position to help expand the PCPN shield NW towards us as they did with the storm this past weekend. Shortwaves embedded within the longwave upper trough however may help spread PCPN closer to us. Looks like this would be an all-snow event with best chances of snow Friday afternoon and night. Too early to go with anything higher than chance PoPs for this potential event. A high pressure ridge follows for Saturday with dry weather, then another low moves through the vicinity of the southern Great Lakes/Ohio Valley regions on Sunday. Global models disagree on its evolution and track, but will go with a dry forecast through Sunday. A better chance of PCPN across the entire region appears to be Sunday night or Monday as the low shifts east. && .AVIATION /03Z TUESDAY THROUGH SATURDAY/... Low pressure through the Great Lakes and Ohio Valley will bring a warm front through the terminals late overnight into early Tuesday morning. As the low moves into southern Canada and northern New England Tuesday a cold front will move through during the afternoon. VFR overnight. MVFR ceilings and visibilities develop early Tuesday morning as precipitation, light rain, becomes steady, the exception is KSWF where a period of rain and snow is likely. There is now more confidence in rain at the terminals as warm air moves in ahead of the cold front. After the cold frontal passage snow showers will be possible, even at the coastal terminals. Southerly winds develop behind the warm front, then shift to the west to northwest behind the cold front, and become gusty. There is the potential for winds to gust as high as 30 KT, especially Tuesday evening. .OUTLOOK FOR 00Z WEDNESDAY THROUGH SATURDAY... .Tuesday night-Wednesday...Mainly VFR, with a chance of MVFR in snow showers. W-NW winds G20-35KT probable. .Wednesday night-Thursday night...VFR. W-SW winds G15-25KT possible. .Friday...MVFR possible with snow. W-NW winds G15-20KT possible. .Saturday...VFR. NW winds G15-20KT possible. && .MARINE... Seas and winds are expected to remain below SCA criteria tonight with a weak pressure gradient in place. Deepening low pressure approaches from the west late tonight and Tuesday, increasing the pressure gradient between the low center and the high center out into the Atlantic. South winds increase, and SCA conditions return during the day Tuesday all waters. Cannot rule out a gale gust or two during the day, but as a front approaches, winds may temporarily diminish. By nighttime Tuesday, westerly winds pick up behind the deepening low as it passes north across New England. Gale Watch remains in effect for the ocean waters beginning Tuesday night for increasing winds. The SCA continues for non ocean waters, with wind gusts in the 25 to 30 kt range. A gale watch remains in effect for all waters Wednesday and Wednesday night with a strong NW flow behind low pressure strengthening as it heads towards the Canadian Maritimes. SCA conds will likely continue into Thursday morning for most of the waters. Sub-SCA conds finally reach all waters by Thursday night with the pressure gradient weakening. This relatively tranquil period should continue through Friday. A coastal storm may then impact the waters Friday night into Saturday with at least SCA conditions developing. && .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. See our headline news on the web at: http:/weather.gov/nyc The Riverhead NOAA Weather Radio Station WXM-80 (162.475 MHz) is currently out of service. NWS technicians are working with Verizon to restore service. An estimated time of return to service is not currently known. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...Gale Watch from Wednesday morning through late Wednesday night for ANZ330-335-338-340-345. Small Craft Advisory from 6 AM Tuesday to 6 AM EST Wednesday for ANZ330-335-338-340-345. Gale Watch from Tuesday evening through late Wednesday night for ANZ350-353-355. Small Craft Advisory from 6 AM to 6 PM EST Tuesday for ANZ350- 353-355. && $$ SYNOPSIS...JC/PW NEAR TERM...JM/PW SHORT TERM...PW LONG TERM...JC AVIATION...19 MARINE...JC/JM/PW HYDROLOGY...JC/PW EQUIPMENT...

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