Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 240901 AFDOKX Area Forecast Discussion National Weather Service New York NY 501 AM EDT Tue Oct 24 2017 .SYNOPSIS...
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A slow moving cold front approaches the region through tonight. The front slowly moves across Wednesday and exits east of the region Wednesday night. A cold front just east of the area Thursday becomes stationary as a wave of low pressure tracks northward along the front. The low moves into the Canadian Maritime Thursday night as high pressure builds into the region. High pressure dominates into Saturday. A cold front then merges with a developing low along the southeastern coast Saturday night into Monday.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The upper level trough deepens to the Southeast with likewise a huge dip in the jet stream. The will strengthen a parent low in the Great Lakes as well as and its associated cold front. The front approaches the region as the parent low tracks into Quebec. The high precipitable water of 1.8 to 1.9 from all the warm and moist air advection will be above the 90th percentile for this time of year according to OKX sounding climatology. The low level jet will be a main reason for this with magnitude of 50-60 kt at 2kft. Instability will be increasing as well in the low levels from this. In terms of weather and potential hazards, rain showers become more expansive in coverage today with rain showers becoming heavy by this afternoon and into tonight out east. There will also be a chance of thunderstorms. The winds will be increasing out of the south, with higher winds potentially with the downward momentum transport via heavy rain or thunderstorms. Low level jet and surface southerly winds maximized most along the coast where gusts of 40 to 50 mph are forecast so areal extent of wind advisory was not changed. Inland zones have a lesser chance of getting 40 to 50 mph gusts and models reflect this lower magnitude in their wind gust fields as well. SPC has placed much of the region in a slight risk for severe thunderstorms, mainly due to the potential for damaging winds with a very low percentage probability of a tornado.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH WEDNESDAY NIGHT/...
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The front continues to approach tonight and then slowly moves across Wednesday, exiting Wednesday night. Meanwhile, the region will still be to the east of the very deep upper level trough with high precipitable water over the region until late Wednesday. Wind advisory still will be in effect much of tonight for Central and Eastern Long Island and Southern CT while to the west it ends at midnight. The heavy rain and potential thunderstorms east of NYC will still keep that downward momentum transfer going to bring down the low level jet at times. Again there will be a slight chance for winds to be severe as indicated in the slight risk area from SPC. Late at night, the intensity of rain and chances of thunderstorms for Southern CT and Long Island lowers with the forecast decrease in instability. Wednesday will have lighter winds but still some steady rain mainly across Long Island and Southern CT. Weather dries out Wednesday night with a light west to northwest flow taking place behind the cold front.
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&& .LONG TERM /THURSDAY THROUGH MONDAY/...
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A negatively tilted upper trough will be moving through the region Thursday with the axis moving to the northeast Thursday evening. Meanwhile, at the surface a cold front just east of Long Island and extending south, offshore of the coast, will have a wave of low pressure developing and moving along the front Thursday into Thursday night. There are some spatial differences with the low, with the GFS slightly farther to the east than the ECMWF. Now looking like there will be a chance of some light precipitation, so introduced slight to low chance probabilities for Thursday into Thursday evening. Weak upper ridging builds Thursday night and moves through during Friday. Then Friday night into Saturday the ridge builds and strengthens across the Western Atlantic as another northern stream longwave trough moves through the northern plains and into the upper midwest while digging into the southern plains and Gulf coast region. All signals still indicting that the full latitude trough will be slow to progress eastward with the Atlantic ridge in place. Also a wave pf low pressure will be moving out of the Gulf of Mexico and move through the eastern portion of the trough, and then merge with a cold front and low pressure moving into from the Ohio Valley. So will have a prolonged period of probabilities for rain early Sunday through Monday, with the coastal low expected to track inland and then move north through the eastern states. There remains uncertainty with the location and timing of the low, and the potential for any heavy precipitation with this system.
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&& .AVIATION /09Z TUESDAY THROUGH SATURDAY/...
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A slow moving cold front will approach the region today and crosses the region tonight into Wednesday morning. MVFR/IFR cigs have returned this morning along with some scattered showers. The showers become more widespread from west to east after 12z. With the approach of a low level jet, showers with embedded thunder are likely today. The rain will be heavy at times accompanied by strong wind gusts, especially near the coast. Compression and low level wind shear are likely, mainly SSW 45-55kt@2kft for coastal terminals. SE winds have been increasing this morning with gusts becoming more frequent around 25kt. Some gusts 30-40kt possible after 12z today, especially the NYC terminals. ...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 are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. The afternoon KJFK haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KLGA TAF Comments: Amendments are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. The afternoon KLGA haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KEWR TAF Comments: Amendments are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. The afternoon KEWR haze potential forecast is YELLOW...which implies slant range visibility 4-6SM outside of cloud. KTEB TAF Comments: Amendments are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. KHPN TAF Comments: Amendments are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. KISP TAF Comments: Amendments are likely for changing flight categories in rain and possible embedded thunderstorms. Highest winds gusts expected this afternoon and evening. .OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY... .Tue night-Wed...IFR possibly continues in SHRA Tue night, low prob/sparse TSRA early evening, improving to MVFR and possibly VFR by midday Wed. .Wed night-Thu...VFR, with iso-sct MVFR in SHRA. .Fri and Sat...VFR.
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&& .MARINE...
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A strong cold front approaching will increase the southerly fetch with a strong low level jet making for eventual gale force winds as heavy rain develops and also with the chance of thunderstorms. The gales are expected to be frequent enough so a gale warning is in effect for all waters through much of this evening, remaining for eastern ocean waters, Long Island Sound and Bays all night. SCA criteria seas on the ocean linger behind the cold front Wednesday into Wednesday night but winds overall will become less than SCA and much lighter as the pressure gradient decreases. Small craft ocean seas Thursday will continue to slowly subside as low pressure moves into the Canadian Maritime and high pressure builds to the south. During Thursday evening ocean seas will likely fall below 5 feet and then remain below advisory levels until late Saturday. Winds and gusts are forecast to remain below small craft advisory levels Thursday through Saturday night. Then as a low deepens along a stalled front offshore Sunday a south to southeast flow will be increasing with gusts reaching small craft advisory across all the forecast waters. In additions, ocean seas will be building with small craft seas possible late Saturday night into Sunday.
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&& .HYDROLOGY...
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A widespread 2 to 3 inches of rain is likely through Wednesday evening with locally up to 4 inches. 6hr rainfall shown by models maxes out near 1-1.5 inches but locally higher amounts than that are possible. The location of where this would occur is of low confidence. Despite antecedent dry conditions, these potential rainfall rates present an area wide potential for minor urban and poor drainage, with a localized threat for flash flooding. In addition, if the heavier rain amounts occur over Northeastern New Jersey and the Lower Hudson Valley, rapid rises in flashy small streams and minor flood impacts to adjacent areas would be plausible. Heavy rain threat diminishes Wednesday into Wednesday night with a drying trend late Wednesday night. However, there is a chance of light rainfall Thursday but no hydrologic impacts are expected. The next chance for widespread rainfall across the area will be Sunday and Monday. A period of heavy rainfall is possible, however, timing and location are uncertain at this time.
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&& .OKX WATCHES/WARNINGS/ADVISORIES...
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CT...Wind Advisory from 8 AM this morning to 2 AM EDT Wednesday for CTZ007>012. NY...Wind Advisory from 8 AM this morning to 2 AM EDT Wednesday for NYZ078>081. Wind Advisory from 8 AM this morning to midnight EDT tonight for NYZ071>075-176>179. NJ...None. MARINE...Gale Warning until 6 AM EDT Wednesday for ANZ330-335-340-345- 350-353. Gale Warning until 10 PM EDT this evening for ANZ338-355.
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