Area Forecast Discussion
Issued by NWS Upton, NY

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590 FXUS61 KOKX 181834 AFDOKX Area Forecast Discussion National Weather Service New York NY 234 PM EDT Fri Aug 18 2017 .SYNOPSIS... A cold front will move slowly east across the area late tonight, then stall just east of Long Island on Saturday. High pressure will follow Saturday night through Tuesday. A cold front will pass across the area Wednesday night followed by high pressure. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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...SEVERE THUNDERSTORM WATCH IN EFFECT FOR PORTIONS OF NORTHEAST NJ...SOUTHEAST NY...NYC...WESTERN LONG ISLAND AND WESTERN CT THROUGH 9 PM... Damaging wind gusts are possible with any thunderstorms that develop this afternoon into the evening as they approach from the west. In addition to the severe weather threat, the threat of flash flooding remains high due to a combination of earlier rainfall of up to 4 inches in some areas, anomalously high dew points and precipitable water values up to 2.2 inches. As such, a Flash Flood Watch was issued for NE NJ, the Metro NJ/NYC area, including Rockland, Westchester, and Nassau Counties in NY and Southern Fairfield County in CT until 11 pm this evening with the expectation that showers and thunderstorms will increase ahead of a cold front. Satellite imagery shows a SW/NE dry slot advecting NE from VA/MD across SE PA, that should advect NE across the Lower Hudson Valley. This will increase sfc based and low lvl instability resulting in the development of organized convection mainly aft 20z this afternoon from SW to NE. There is a moderate risk of rip currents at ocean beaches, potentially becoming high this afternoon.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH SATURDAY NIGHT/... Models at times can struggle with the precipitation forecast with the departure of large mid level ridges above 580dm. So, forecast will adjust for this in situation where there is more forcing and precip is too low and vice versa. The upper level trough will be approaching the region tonight. The height falls become more remarkable late tonight into Saturday morning and this is when the trough moves eastward a greater distance. The region will be getting closer to the right entrance region of this jet. The trough amplifies and shortens its wavelength during the day Saturday. 500mb heights lower 20-30 m from early morning until early evening. Vorticity still increasing with height tonight and Sat looking at 925, 850, and 700mb levels. Still have a threat of gusty winds and heavy rain with thunderstorms tonight. High precipitable waters near 2 inches still will be in place as will the vertical wind shear. A cold front will continue to approach tonight. The cold front moves into the region late tonight into Saturday but does not totally clear the region until Saturday night. However with aforementioned dynamics, pops still in place for parts of the CWA during Sat evening with possibility for isolated shower activity. West to Northwest flow behind the cold front later Saturday night and continued ridging will promote mainly dry conditions. && .LONG TERM /SUNDAY THROUGH FRIDAY/... Shortwave trough pivots northeast of the region on Sunday, with generally zonal upper flow over the region for early to mid of next week, between southern ridging and a closed low moving into Hudson Bay. At the surface, high pressure builds into the region Sunday, and then gradually sinks south and east of the region Monday into Tuesday. The result will be dry and seasonably warm conditions on Sunday, giving way to increasing heat and humidity Mon into Tue. Perhaps some afternoon shower/tstm activity along a lee trough Tue afternoon depending on any energy sliding through the upper flow aloft, but low predictability at this point. Decent model agreement with shortwave energy digging into the Great lakes Tue into Tue Night and then towards the NE US on Wed. An associated frontal system will bring the next chance for organized shower and tstm activity late Tue into Wed. && .AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
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A cold front slowly approaches region late this afternoon. MVFR/IFR/lIFR ceilings continue in the wake of earlier showers and thunderstorms. Ceiling conditions have been slow to improve but some minor improvement is still possible before another round of showers and thunderstorms arrives around 20-21z. Thunderstorms are likely this evening until about 01z across the NYC terminals. Gusty winds and heavy rain possible in any thunderstorm. Southerly winds will increase this afternoon, mainly between 10 to 15 kts. Gusts to 25 kt possible along the coast. Higher gusts possible with any thunderstorm. For tonight, fog/stratus with IFR or lower conds may linger through the overnight and especially early morning Saturday. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/www.weather.gov/zny/n90 KJFK TAF Comments: Amendments possible for timing and coverage of showers and thunderstorms this this afternoon and evening. KLGA TAF Comments: Amendments possible for timing and coverage of showers and thunderstorms this afternoon and evening. KEWR TAF Comments: Amendments possible for timing and coverage of showers and thunderstorms this afternoon and evening. KTEB TAF Comments: Amendments possible for timing and coverage of showers and thunderstorms this afternoon and evening. KHPN TAF Comments: Amendments possible for timing and coverage of showers and thunderstorms this afternoon and evening. .OUTLOOK FOR 00Z SATURDAY THROUGH WEDNESDAY... .Saturday-Monday...VFR. light NW wind becoming SW by Late Sunday and Monday. .Tuesday...Isolated showers and thunderstorms possible with MVFR or lower conditions...otherwise VFR. Light southwest wind. .Wednesday...Scattered showers and thunderstorms with MVFR possible. Gusty southwest wind to 20 kt.
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&& .MARINE... SCA on ocean today through tonight with sub SCA on other waters. Overall sub SCA conditions expected Sat and Sat night with offshore flow developing behind cold front. Winds and seas are expected to remain blw sca lvls on all waters Sun thru Wed as high pressure builds across the waters. && .HYDROLOGY...
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Heavy showers and thunderstorms resulted in 1 to 4 inches of rain this morning from the Metro NJ/NYC area east across Long Island. Another round of heavy showers and thunderstorms will be possible this evening ahead of the cold front. Rainfall accumulations are expected to be around a half an inch to an inch, with locally higher amounts possible in thunderstorms. It still remains difficult to pinpoint where heavier showers and thunderstorms will develop. There is a chance of flash flooding with the more likely flooding hazard being minor flooding of poor drainage and urban areas especially tonight with places that received heavy rain during the morning. Impacts from potential showers and thunderstorms Tuesday into Wednesday are uncertain at this point.
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&& .OKX WATCHES/WARNINGS/ADVISORIES... CT...Flash Flood Watch until 11 PM EDT this evening for CTZ009. NY...Flash Flood Watch until 11 PM EDT this evening for NYZ069>075- 176>179. NJ...Flash Flood Watch until 11 PM EDT this evening for NJZ002-004- 006-103>108. MARINE...Small Craft Advisory until 6 AM EDT Saturday for ANZ350-353- 355. && $$ SYNOPSIS...GC/JM/NV NEAR TERM...GC SHORT TERM...GC/JM LONG TERM...NV AVIATION...IRD MARINE...GC/JM/NV HYDROLOGY...GC/JM/NV

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