Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 172029 AFDOKX Area Forecast Discussion National Weather Service New York NY 329 PM EST Sun Dec 17 2017 .SYNOPSIS...
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A warm front will approach tonight. Weak low pressure will then track along the front near or over Long Island on Monday. A cold front approaches on Tuesday, passes through the region Tuesday Night, with high pressure then building in for the mid to late week. A frontal system will likely affect the region for the weekend.
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&& .NEAR TERM /THROUGH MONDAY/...
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Canadian high pressure was nosing southward into the eastern half of the forecast area this afternoon. A large dewpoint gradient exists as a result of this, with dewpoints near 8 across eastern Connecticut to around 25 in New Jersey. Areas of rain were evident over northern Ohio in the vicinity of an approaching warm front. The radar composite indicated the atmosphere was saturating southwest of the forecast area. There features are expected to combine to produce a light precipitation event over the area from late tonight into early Monday. The thermal structure of the airmass suggests snow at the start then sleet and finally freezing rain and rain. A Winter Weather Advisory has been issued for areas where light freezing rain is expected, as even trace amounts can be hazardous and require an advisory. Along the south shore of Long Island and into the New York City area, a developing light onshore flow looks to allow for the transition to skip the freezing rain and go right to rain. These areas were therefore excluded from the advisory. There are a couple of ways the forecast can go wrong. The first is that precipitation just doesn`t materialize, as there is not a lot of strong lift with the system. The second is that the precipitation ends before the warm nose allows for the transition to freezing rain. The greatest threat for this occurring is along and west of the Hudson River where the precipitation should end quicker, and across eastern Connecticut, where the cold air may just remain entrenched. Any precipitation ends Monday morning, with temperatures rising into the upper 30s and 40s as heights increase and southwest flow develops.
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&& .SHORT TERM /MONDAY NIGHT/...
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Westerly flow and lingering clouds will serve to limit low temperatures. With a lack of deep moisture and lift, the forecast has been kept dry. The Superblend was used for temperatures in order to account for the better low level mixing due to the wind.
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&& .LONG TERM /TUESDAY THROUGH SUNDAY/...
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Models continue in good agreement with a broad but shallow northern stream trough approaching the NE on Tuesday and sliding through New England on Wednesday. At the surface, the primary low pressure/s will track through Quebec/Ontario with a trailing cold front approaching the region Tue, and crossing Tue Night. Models have continued dry with this frontal passage for the local region with the best forcing remaining north, and confluent upper flow over the region. The one thing to watch is late Monday Night into early Tuesday morning, the NAM is hinting at some stratus/drizzle with low- level saturation in the waa pattern. At this point, a low prob of occurrence, but if anything develops there would be a threat for light freezing rain across interior as cold air will have a tough time scouring out until daytime in this setup. Upper trough swings through the NE Tue Night into Wed with a glancing shot of modifying arctic air in the wake of the cold front for Wed. Upper flow flattens for Thursday, and then becomes ridged heading into Friday ahead of developing Western/Central Us troughing. At the surface, high pressure builds in from the west Wed into Thu, with dry and seasonably cold conditions. Models continue in agreement with a pattern shift to a more longitudinally oriented upper flow heading into the weekend, as several pieces of northern stream energy continue to dig a deep longwave trough into the Western/Central US. This may bring the coldest air of the season into the Central US and Rockies US by around Christmas. The noticeable trend versus 24 hours, is that the troughing has shifted a bit westward, with strong Western Atlantic/East Coast ridging. There is still agreement on energy across the 4 corners are mid-eek shearing towards the NE for the weekend, resulting in a a frontal system affecting the region for the weekend. If this trend holds, unseasonable warmth would be expected up the East Coast for this Christmas weekend. Quite a bit of model divergence thereafter on how this troughing and arctic air moves eastward for next week.
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&& .AVIATION /21Z SUNDAY THROUGH FRIDAY/...
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High pressure over New England will move east into tonight as a warm front approaches from the SW. The warm front gradually moves north of the area on Monday. VFR through this evening. Ceilings will then lower to MVFR and possibly IFR overnight. The exact timing of these conditions may be off by 1-3 hours from the current forecast. There will be also be a light wintry mix, especially after 06z. This should taper off from west to east 11z to 14z. Winds will continue to become light and variable at all terminals by this evening. Winds may briefly become WNW Monday morning before backing towards to the SW in the afternoon. .OUTLOOK FOR 18Z MONDAY THROUGH FRIDAY... .Monday Afternoon...Mainly VFR with a low chance for MVFR. .Tuesday...MVFR possible in the afternoon. SW G20-2KT possible. .Wednesday-Thursday...VFR. NW winds G20-30KT probable Wednesday. .Friday...MVFR possible in chance of showers.
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&& .MARINE...
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Winds and seas will remain below Small Craft Advisory levels through Monday as weak low pressure tracks in the vicinity of Long Island. Westerly flow may increase to 25 kt by late Monday night, especially on the ocean. Moderate potential for SCA conditions developing Tue in strengthening SW flow and building ocean seas ahead of approaching cold front. Widespread SCA expected Tuesday Night through Wednesday with tight pressure gradient and caa in wake of cold front. Marginal gale gusts are possible Tuesday night into Wednesday. Conditions then should gradually fall back below SCA Wed night into into Thu, with relatively tranquil conds continuing into Fri as high pressure builds towards the waters. Next chance of SCA appears to be ahead of an approaching frontal system for the weekend.
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&& .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...
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CT...Winter Weather Advisory from 10 PM this evening to 7 AM EST Monday for CTZ005-006-009-010. Winter Weather Advisory from midnight tonight to 9 AM EST Monday for CTZ007-008-011-012. NY...Winter Weather Advisory from 10 PM this evening to 7 AM EST Monday for NYZ067>071-078-079-177. NJ...Winter Weather Advisory from 10 PM this evening to 7 AM EST Monday for NJZ002-004-103>107. MARINE...None.
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&& $$ SYNOPSIS...JMC/NV NEAR TERM...JMC SHORT TERM...JMC LONG TERM...NV AVIATION...DS MARINE...JMC/NV HYDROLOGY...JMC/NV EQUIPMENT...

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