Area Forecast Discussion
Issued by NWS Upton, NY

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558 FXUS61 KOKX 150346 AFDOKX Area Forecast Discussion National Weather Service New York NY 1146 PM EDT Mon Aug 14 2017 .SYNOPSIS... A frontal boundary south of Long Island will slowly move northward tonight into Tuesday. Meanwhile a weak cold front will also be approaching the region from Southeast Canada Tuesday and will move across Tuesday night. High pressure builds across the area Wednesday then moves offshore by Thursday. A warm front moves northward Thursday night, while a slow-moving cold front approaches from the west through the latter part of the week. The front finally passes to the east by the weekend with high pressure building in its wake. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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Forecast is generally on track, although have made some minor adjustments to the database this evening. Showers across central NJ will move across the NYC metro between 04z and 06z and continue across Long Island through the overnight hours as a weak upper level shortwave and accompanying entrance region of an upper level jet moves through the region. Coverage has been decreasing somewhat over the past hour, but expect some increase as the jet streak moves into the area. Also lowered lows a few degrees, mainly inland as some locations had already hit their forecasted low. Otherwise...synoptic map shows a frontal boundary extending from Virginia through the Delmarva and into the Western Atlantic. High pressure is well offshore and weakening. A weak frontal system will be moving through the Great Lakes.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH TUESDAY NIGHT/... The region will still be under the right entrance region of the jet streak Tuesday. Upper level low pushes farther southward through Quebec. Mid levels show an approaching trough from the west during the day, moving across at night. At the surface, the frontal boundary across the Western Atlantic slightly moves farther northward, closer to the local region. Tropical cyclone Gert will be south of this front moving northeast far away well out into the Atlantic. However, long period swells will be emanating out from the center of Gert, starting to arrive late Tuesday. To the west across the Great Lakes, the frontal system and its associated cold front will slowly approach the region during the day. The cold front will move across Tuesday night. Rain shower chances will be higher earlier in the day with more SE low level flow to keep moisture levels higher near the ground. Rain showers chances lower thereafter and by the afternoon, there will still be a slight chance of showers and thunderstorms as well, mainly across the western half of the region. In the mid levels, the local region is conveyed to be in between the shortwaves. The system to the west is weakening and the forecast models show a negative vorticity advection trend. Therefore, the cold front looks to move through dry Tuesday night. The jet streak is exiting northeast of the region. Outside of a slight chance of showers and thunderstorms across northern extremes of the Lower Hudson Valley, the forecast is dry. Thunderstorms are a slight chance and are being introduced into the forecast Tuesday into early Tuesday night because instability is expected to be a little more due to daytime heating. For temperatures, MAV/MET blend was used for high temperatures as both solutions had relatively similar temperatures. The temperatures reach the upper 70s to lower 80s for highs, slightly cooler than the previous day. A combination of MAV/MET/NAM12/GMOS was used for lows at night Tuesday night, ranging from the low 60s to near 70. A high risk of rip currents continues at Suffolk County ocean beaches on Tuesday. A moderate risk of rip currents for ocean beaches in Nassau and NYC should prevail Tuesday morning, but long period swells from tropical cyclone Gert are expected to begin impacting the ocean waters Tuesday afternoon which should result in a high risk during the afternoon and evening hours. && .LONG TERM /WEDNESDAY THROUGH MONDAY/... The primary concern for mid week will be at ocean beaches as southerly to southeasterly swell continues to the north of Tropical Cyclone Gert. This may create hazardous conditions with an enhanced risk of rip currents. Apart from the dangerous surf, high pressure will build across the area Wednesday into Thursday. Initially the offshore flow on Wednesday will lead to subtle downsloping, aiding in above normal temperatures across the area. As the high continues to build southeastward overnight, lower dew points will advect into the area allowing temperatures to fall close to or slightly below normal values in favorable radiational cooling. By Thursday the high begins to shift offshore leading to a return flow and increasing humidity from south to north once again. Aloft, a short wave of low pressure will slowly approach from the west which may gradually increase shower and thunderstorm chances by evening, continuing into Friday as multiple disturbances rotate around the low. The attendant cold front finally reaches the area on Saturday with shower and thunderstorm chances gradually ending from west to east as the front moves through late in the day. Humidity will decrease into the early week as high pressure builds back into the area. && .AVIATION /04Z TUESDAY THROUGH SATURDAY/...
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A series of weak waves of low pressure will track east, along a warm front to the south of the region, tonight through Tuesday. Latest high-res model guidance indicating showers across southeastern PA working into the NYC/NJ terminals after midnight, and working east through the terminals through Tue morning. Additional shower activity possible Tue aft. Increasing likelihood for MVFR stratus late tonight into Tue morning, with IFR potential for eastern terminals. MVFR cigs possible to remain through the day, especially for eastern terminals. Light and variable winds overnight, becoming SE under 10 kt Tuesday morning. .OUTLOOK FOR 00Z WEDNESDAY THROUGH SATURDAY... .Tuesday night...MVFR or lower conds possible in stratus/fog, particularly for eastern terminals. .Wednesday and Wednesday night...VFR. .Thursday...MVFR conds possible from NYC metro west, with chance of showers. .Thursday night through Saturday...MVFR or lower flight cat possible, with chance of showers and tstms. Best chance for tstms in the afternoon.
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&& .MARINE... Sub SCA conditions are forecast tonight with a weak pressure gradient remaining in place. However, Tuesday while starting likely below SCA with a weak pressure gradient, the long period swells from distant far offshore tropical cyclone Gert will allow for building seas up to near 4 ft. There is a chance ocean seas could reach 5 ft by late Tuesday afternoon. SCA chances are higher Tuesday night when modeled long period swell comes more into the waters. Due to uncertainty in start time and 5-6 ft seas being mostly in the 3rd period, did not issue SCA at this time. Swells from tropical cyclone Gert will maintain SCA level seas through mid week. The official NHC forecast places the system well east of the area by Thursday, which combined with a weakening pressure gradient as high pressure builds in should allow seas to slowly subside Thursday. Despite a frontal system approaching late week, the pressure gradient will remain weak keeping sub-SCA conditions on the waters through the weekend. && .HYDROLOGY... No hydrologic problems are anticipated through Tuesday night. Rainfall expected to be less than a quarter inch. A slow moving-frontal system may bring periods of heavy rainfall towards late week. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...High Rip Current Risk through Tuesday evening for NYZ080-081. NJ...None. MARINE...None. && $$

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