Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 240516 AFDOKX Area Forecast Discussion National Weather Service New York NY 116 AM EDT Wed May 24 2017 .SYNOPSIS... A low pressure system tracks southeast of Long Island late tonight into Wednesday morning. Weak high pressure builds in later Wednesday, followed by a slow moving low pressure system affecting the area Thursday into Friday. Weak high pressure returns for Saturday. Another frontal system affects the area for the latter half of the holiday weekend. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Minor adjustments made to reflect current temperature, dew point and wind trends. Prolonged chances of light rain this morning across Long Island and eastern Connecticut. Previous discussion follows. Anomalously large and deep upper trough and associated upper low digs into the the Central US tonight...with jet streak and shortwave riding up the coast late tonight into Wednesday morning. In response...southern low pressure tracks to the Mid- Atlantic coast tonight and then quickly near of just se of the 40/70 lat/lon Wed morning. Still quite a bit of spread in terms of areal coverage of rainfall between models. A rather tight gradient of heavier rainfall from frontogenetic zone to the N/NW of the low...which should mainly stay se of LI. With region in briefly favorable left front quad of approaching upper jet streak...have leaned towards higher likelihood of light rain for city/coast late tonight/early Wed morning...with decreasing chances for measurable rain to the North and West. Gusty N/NE winds develop along the coast late tonight and continue into Wed morning in response tot he low as well. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM THURSDAY/... Anomalously large and deep Central US upper trough and associated upper low gradually slides east into the Ohio/Tennessee River Valleys Wed into Wed night. Initial shortwave/jet streak and resultant coastal low pressure departs Wed aft...with brief high pressure building in Wed aft/eve. Any rain pushes east early Wed morning...with gusty N/NE winds Wed morning gradually subsiding into the afternoon. At least partial afternoon clearing should allow temps to rise to near seasonable levels...upper 60s/lower 70s. Models in good agreement with increasing potential for rain development late Wed Night into Thu morning ahead of approaching deep closed low with deep layered lift of sub- tropical moisture tap. Easterly winds will be increasing as well...with tightening gradient between warm front/low pressure to the S/SW and Canadian Maritimes high. See TIDES/COASTAL FLOODING for associated impacts. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Unsettled pattern for much of the long term. High amplitude upper level trough over the eastern two-thirds of the country, with its axis stretching from the Central Great Lakes region to the Gulf Coast Thursday morning will push east and become negatively tilted through the day. This is indicative of a mature system. At the surface, a warm front will approach from the south. However, this front looks to remain south of the region, but should provide a lifting mechanism. A moist airmass, with dewpoints rising into the upper 50s, with some 60 degree dewpoints not out of the question, will already be in place. As the upper low with the system moves closer to the area, cooler air aloft will provide some elevated instability. MUCAPE values of up to 500 J/kg noted in some of the models. A period of moderate to briefly heavy rain is possible Thursday morning. Right now, just some general thunderstorms are possible. Although precipitation fields are not currently indicating a high rainfall event for Thursday with flash flooding, there is a low probability, given local climatology. The potential for embedded convection, PWATS near 1.50", and low pressure moving into the area from the west all point to this potential. Although, winds will be east, a shift more to the southeast on Thursday would be another ingredient favoring excess rainfall. Again, would be more confident in flash flooding if QPF was higher and occurring later in the warm season, but for now, minor urban and poor drainage flooding seems most likely. Surface low pressure moves over the area Thursday night, then heads northeast, into the Gulf of Maine Friday morning. Light showers will continue for Friday on the back side of the low. Weak high pressure builds in for Saturday, with another frontal system impacting the area Sunday into Memorial Day. && .AVIATION /05Z WEDNESDAY THROUGH SUNDAY/...
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A wave of low pressure moves off the mid Atlantic coast, moving south of Long Island overnight through Wednesday morning, and well east during the afternoon. Weak high pressure builds in once again behind the low Wednesday afternoon. This high weakens Wednesday evening as another, deeper, low approaches from the west. VFR conditions are expected through much of the forecast period. There is a low chance of MVFR ceilings and visibilities developing briefly toward 12Z along the coast, then improving as the rain ends from west to east. MVFR conditions are expected at KISP after 07Z and MVFR conditions will be possible at KGON. Ceilings fall below VFR late in the 30 hour TAF sites as the next low pressure system approaches. Light and variable winds become NE less than 10 KT after 06Z, then increasing to around 10 KT with gusts up to 20 KT possible Wednesday morning. Winds will become E late in the day as the next low approaches. There may be a brief period of light northerly winds 17Z to 20Z. .OUTLOOK FOR 06Z THURSDAY THROUGH SUNDAY... .Wednesday night-Friday night...MVFR, IFR possible. Slight chance of thunderstorms Thursday with E winds G20KT. .Saturday...VFR. .Sunday...MVFR/IFR developing.
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&& .MARINE... Winds and seas are expected to remain below small craft levels through tonight. Overnight, an initially weak pressure gradient will tighten as an area of low pressure moves up the coast. As such, marginal NE SCA winds and seas are likely to develop on the ocean late tonight into Wednesday morning. Winds will become light by Wednesday afternoon...but seas may remain elevated on the ocean through Wednesday night. Low pressure will approach the waters Wed Night into Thursday Morning with easterly SCA winds likely developing across all waters...with a low probability for gale gusts. Winds likely begin to diminish late Thu into Thu eve. Ocean seas building to 5 to 7 ft Thursday and remain Friday, then diminish to less than 5 ft for Saturday and the remainder of the weekend. && .HYDROLOGY... 0.75" to 1.00" of rain is currently forecast from Thursday through the day on Friday. Higher amounts are possible in any thunderstorms that develop on Thursday. The potential for flash flooding is low given local climatology (see Long Term section) and the most likely scenario is minor urban and small stream flooding given antecedent wet conditions. Another round of light to moderate rainfall is possible for beginning of next week. && .TIDES/COASTAL FLOODING... Increasing threat for successive rounds of minor coastal flooding for the Wed Night, Thu Morning and Thu Night high tide cycles. Greatest threat for widespread minor and potentially moderate coastal flooding appears to be with the Thursday eve/night high tide cycle as a slow moving low pressure system approaches the region...and with less than 1/2 ft of surge needed in many places to reach minor flood thresholds and 1 to 2 ft for moderate. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...Small Craft Advisory from 6 AM this morning to 8 AM EDT Thursday for ANZ350-353-355. && $$ SYNOPSIS...JP/NV NEAR TERM...JC/MD/NV SHORT TERM...NV LONG TERM...JP AVIATION...BC/MET MARINE...JP/NV HYDROLOGY...JP/NV TIDES/COASTAL FLOODING...

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