Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 222044 CCA AFDOKX Area Forecast Discussion...CORRECTED National Weather Service New York NY 444 PM EDT Sat Jul 22 2017 .SYNOPSIS... A frontal boundary remains to our south tonight as a wave of low pressure tracks along it, this front then sags farther to the south on Sunday after the low passes. Another low then moves along the front, tracking off the mid-Atlantic coast by Monday morning. Still another area of low pressure that moves through the region Monday and Monday night, exits to the east Tuesday. High pressure builds over the northeast late Tuesday through Wednesday, then drifts off the coast Wednesday night. Another wave of low pressure follows for Wednesday night into Friday, with a cold front moving through Friday. High pressure returns Saturday. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/...
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A few isolated showers are possible before 6pm, mainly across far W zones, as the main part of a convective complex passes to the SW, with anvil based precipitation passing mainly to our N. An area of showers and thunderstorms, currently over N Central PA is progged to move into the NYC Metro area 23-01z, then track ESE along/just south of Long Island through around 4z, with scattered mainly showers to the N. This area is associated with a 700-500 hPa shortwave and vorticity maxima that moves through right behind it. Could be some strong to possibly severe storms with this cluster, with gusty winds the main threat. Once this shortwave exits, the overnight hours should be mainly dry, except for possibly some lingering showers (with a slight chance of a rumble of thunder) mainly over Long Island. Will let heat advisory expire at 6pm. Lows tonight should be around 5 degrees above normal. There is a moderate risk for the development of rip currents at ocean beaches through this evening.
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&& .SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/...
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The region should be under locally zonal flow Sunday. With no shortwaves of note forecast to move over the region in this flow, it should be mainly dry. However, this is quite a change from the previous forecast. So to trend things, went with slight chance pops over western zones and mainly dry over eastern zones through late afternoon, then increased pops to chance over far western zones and slight chance throughout elsewhere (reflecting climatological trends for convection). A sharpening 700-500 hPa trough/developing closed low moving into the Great Lakes Sunday night, along with 850 hPa frontogenesis, will produce an increasing threat of showers, with a slight chance of thunderstorms. The pops increase from W to E, to likely throughout by after midnight. Onshore low-level flow should result in a mostly cloudy sky on Sunday, limiting highs to near to slightly below normal levels (mainly around 80-mid 80s). Lows Sunday night should be slightly above normal due to cloud cover. There is a moderate risk for the development of rip currents at ocean beaches Sunday.
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&& .LONG TERM /MONDAY THROUGH SATURDAY/...
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An upper level shortwave and accompanying wave of surface low pressure will be exiting to the east Monday night into early Tuesday. Showers and isolated thunderstorms will be on-going at the beginning of the extended forecast period. Instability and CAPE will be marginal and increase somewhat during Monday, however may still not be sufficient to support widespread convection, so will keep isolated wording. The upper westerly flow will be progressive through the period. A weak ridge builds Monday night into Tuesday, and Wednesday. Then another shortwave, with an embedded closed low, over the Canadian west coast Monday, opens and digs a more significant trough into the eastern states Wednesday night into Thursday with a high amplitude trough, of 1 to 2 standard deviations above normal, digging all the way to northern Florida by Friday. The flow looked to be still progressive with a surface low moving through Thursday and Friday. However, there are hints that the upper low will close off again Friday into Saturday and remain along the northern coast into next weekend. At this time will keep with persistence and the more progressive flow and keep Friday night into Saturday dry as upper ridging builds to the north. Temperatures through the extended period will be near to slightly below seasonal normals.
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&& .AVIATION /20Z SATURDAY THROUGH THURSDAY/... Generally VFR through the TAF period, outside of any showers and thunderstorms that are forecast. Sea breezes have moved through the coastal terminals. A weak cold front will remain nearly stationary just south of the area through tonight. Low pressure will pass just south along this front tonight. Thunderstorms over central and eastern Pennsylvania will continue to be watched as they move eastward tonight. Best timing for these thunderstorms is 01Z-03Z for the NYC terminals. However, there is a PROB30 group to account for more thunderstorms from 04Z-10Z as exact timing after 03Z cannot be determined, but confidence is medium to high that we will see thunderstorms during this time period. Chances are lower, as well as confidence, for eastern terminals such as KISP and KBDR where only VCTS is mentioned from 00Z- 10Z, and there is no mention of thunder for KGON, however an isolated thunderstorm is possible for the same time period. Any showers and thunderstorms have the potential to lower conditions to MVFR or lower. Also, gusts associated with thunderstorms 04Z-10Z have to the potential approach 30 kt. Winds will shift from the south to the northeast tonight, then shift to the southeast on Sunday for most terminals, with speeds generally 10 kt or less. .OUTLOOK FOR 18Z SUNDAY THROUGH THURSDAY... .Sunday afternoon-Monday night...Episodes of SHRA/TSRA with MVFR or lower conditions possible. .Tuesday...An AM shower possible at KGON, otherwise VFR. .Tuesday night-Wednesday night...VFR. .Thursday...Sub-VFR possible in SHRA/TSRA && .MARINE... A light to moderate pressure gradient over the waters around Long Island through Sunday night, will limits sustained winds to 15 KT or less, ocean seas to 4 ft or less, and seas/waves on the non- ocean waters to 1 ft or less through then. Winds, gusts, and seas will remain below small craft levels Monday through Thursday. However, ocean seas may approach 5 feet Wednesday night into Thursday as an increasing southerly flow develops ahead of a cold front. Waves of low pressure passing through the waters will bring a chance of thunderstorms Monday and again Thursday into Thursday night. && .HYDROLOGY... Currently expecting around 1/4-1/2 inch of rainfall across southern portions of the CWA and less than 1/4 of an inch of rainfall elsewhere through Sunday morning. Locally heavy rainfall is possible with any stronger convection tonight, mainly over southern zones. If the locally heavy rainfall occurs, it could result in at most minor flooding of urban and poor drainage areas. An additional 1/2 to 1 inch of rainfall is likely from Sunday night into Monday night, with locally higher amounts possible. There is a small chance for minor flooding or urban and poor drainage areas, if any experience locally heavy rainfall. More rain is possible late Wednesday night through Friday. Significant hydrologic impacts are not expected at this time. && .TIDES/COASTAL FLOODING... With the approach of the new moon (Sun), tides will run high this weekend into early next week. Positive tidal departures of 1/2 to 1 ft will be needed for minor flooding during the night time high tides during this time. Minor flooding is possible starting with the high tide cycle this evening, mainly in the western south shore bays of Long Island. A bit more widespread minor flooding is possible Sunday night with an E/NE flow expected. The threat for minor flooding could continue into Monday, with E/NE flow progged to continue. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...Heat Advisory until 6 PM EDT this evening for NYZ072>075-176- 178. NJ...Heat Advisory until 6 PM EDT this evening for NJZ006-104-106- 108. MARINE...None. && $$ SYNOPSIS...Maloit/MET NEAR TERM...Maloit SHORT TERM...Maloit LONG TERM...MET AVIATION...JP MARINE...Maloit/MET HYDROLOGY...Maloit/MET TIDES/COASTAL FLOODING...

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