Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 250302 AFDOKX Area Forecast Discussion National Weather Service New York NY 1102 PM EDT Mon Apr 24 2017 .SYNOPSIS... Low pressure slowly tracks up along the eastern seaboard over the next few days. It will eventually track from south of Long Island to off of Nova Scotia by Friday morning. A weakening frontal boundary then pushes into the region by Saturday, then stalls out nearby into early next week. && .NEAR TERM /UNTIL 6 AM TUESDAY MORNING/...
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Some minor adjustments to the forecast with this update. Light returns on radar continue to stream north. Lower and middle levels are still very dry, so anything that does reach the ground will likely be in the form of sprinkles. Lowered POPs somewhat given the coverage on radar and latest guidance. However, there is an area of more widespread precipitation well to the south of Long Island that should make its way onshore between 11 pm and 2 am. Again, this should only be light rain showers as the column is still rather dry in the lower levels, but enough moistening should occur for more widespread light rain to move onshore. Low pressure over the Carolinas slowly moves up the coast tonight. Levels below 850mb will be dry to start, so any rain that manages to fall before midnight will likely be light. PoPs increase overnight with deepening moisture and a weak low level jet approaching, but again not expecting any rain to be heavy with some NVA progged below 700mb during this time. Will go with PoPs increasing to likely for most areas by daybreak.
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&& .SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY/... Low pressure continues to move up the east coast during Tuesday. Moisture increases with a theta-e ridge shifting in from the south and PWATs increasing to 1.25 to 1.50 inches. Isentropic and shortwave lift will help push the likelihood of rainfall northward through the rest of the CWA. Further lift will then be supplied by another low level jet by late in the day. Breezy conditions develop, especially along the coast with a tightening pressure gradient increasing easterly winds. High temperatures will be below normal. With a stacked system, the coastal storm will slowly drift up the Atlantic Seaboard Tuesday night and Wednesday while weakening as it does so. Rain remains likely Tuesday night before chances lower through the day on Wednesday. Models differ regarding the placement of the axis of greatest qpf. With the low stacked, prefer a more eastward track for the surface low - which would be closer to the consensus track of the 500 mb low. This places the strongest lift over the eastern zones. Will therefore continue with the idea from the previous forecast and RFCs that the higher amounts of rain are more likely to fall somewhere over the eastern half of the Tri-State Area. No thunderstorms are anticipated as elevated instability is progged to remain to our south, and it appears that the entire CWA will fall short of flash flood and headwater guidance flooding for this event. See the hydrology section below for more details. Went closer to the cooler side of guidance for Wednesday`s high temperatures. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... A gradually filling cutoff low tracks from south of Long Island Wednesday night to off of Nova Scotia by Friday morning. The result should be spotty rain Wednesday evening, tapering of to showers and patchy fog by Thursday morning. For now have gone dry for the bulk of Thursday and Thursday night, but cannot rule out a stray shower on the backside of the departing low during this time frame. Given abundant low level moisture and an inversion forecast to set up around 850 hPa, do expect some cloud cover, for now going partly sunny/mostly cloudy however do have possibility of partly cloudy (optimistic) to overcast (pessimistic). As a result have decreased the diurnal range in this time frame. A 700-500 hPa shortwave passes on Friday, with isolated to scattered showers and thunderstorms (best chance basically area to the N/W of NYC - where highest CAPES are expected). Ridging builds in from the South From Friday night into Sunday - however should also have a frontal boundary stalled out near the area. The models differ on the strength and orientation of the ridge, so the amount of precipitation. if any, received during this time frame is in question. It will be a balance between increasing subsidence under the building ridge and vs. lift with the front - for now going dry Friday night/Saturday then slight chance pops Saturday night-Sunday. For now have chance pops Sunday night/Monday as shortwaves approach ahead of a full latitude trough (with a cutoff low at its head). This might be a little on the aggressive side, especially if ECMWF idea of a stronger ridge plays out. Temperatures Wednesday night through Thursday are a blend of MOS and NAM 2-meter temperatures (with a mix down from 975-950 hPa blended in for highs on Thursday) and should be around 10 degrees above normal then 5-10 degrees above normal respectively. Temperatures Thursday night-Monday were based on the Superblend, and are forecast to remain above normal. && .AVIATION /02Z TUESDAY THROUGH SATURDAY/...
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Low pressure slowly moves up the Mid-Atlantic coastline through Tuesday night. MVFR and IFR cigs expected to gradually develop in bands of steadier light rain late tonight. Vsby will likely be slower to come down...perhaps not until Tuesday morning. Expecting IFR/LIFR cigs Tuesday...with vsby potentially lowering to these levels as well...as rain continues. E/NE winds increase tonight flow tonight...maximizing Tuesday with speeds 15-25 kt and gusts 25-30 kt. Highest sustained winds for coastal terminals. .OUTLOOK FOR 00Z WEDNESDAY THROUGH SATURDAY... .Tuesday Night...IFR/LIFR in potentially heavy rain. NE gusts 20 to 25 KT. Winds drop off late Tuesday night. LLWS possible Tuesday evening with SE winds 45-50 kt at 2kft. .Wednesday-Wednesday Night...IFR in rain. Potential for IFR/LIFR in fog Wednesday night. .Thursday...MVFR/IFR to start...with gradual improvement to VFR. .Friday...Chance of MVFR in showers. .Saturday...VFR.
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&& .MARINE... Have started the SCA on all waters a few hours earlier tonight than what was previously posted with the expectation that winds will increase more quickly than the previous forecast. Will still go with the SCA ending Tuesday night after collaboration with the surrounding offices, and although there could be a few gusts to gale force on the ocean waters at some point Tuesday or Tues day evening, confidence to go with a gale warning was not high enough at this time. A relatively relaxed pressure gradient over the region Wednesday night through Saturday will limit winds to 15 KT or less during this time frame. Sub-small craft conditions are expected on the non-ocean waters as a result Wednesday night through Saturday. On the coastal ocean waters, swells from the departing coastal low will keep seas above SCA levels over at least part of each coastal water zone Wednesday night through Friday. Seas should fall below 5 ft on all coastal ocean zones by Saturday. && .HYDROLOGY... Total rainfall of 3/4 to 2 inches of rainfall is expected tonight through Wednesday. The highest amounts are most likely to occur across eastern Long Island and southeastern Connecticut where the best chances of minor flooding of poor drainage and low lying areas will be. The heaviest rainfall probably occurs during Tuesday night. No significant (0.5 inches or more) widespread rainfall is forecast from Wednesday night through at least Sunday. && .TIDES/COASTAL FLOODING...
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In response to a low pressure system moving northward along the eastern seaboard, minor coastal flooding (1 to locally 2 ft above ground) is likely with the Tuesday evening high tide cycle for vulnerable coastal communities along the southern bays of western Long Island/Queens/Brooklyn, western Long Island Sound and portions of lower NY/NJ Harbor. Localized moderate coastal flooding ( 2 ft above ground) is possible along the south shore bays of Nassau County. Only 1/2 to 1 foot of surge is needed to reach minor flood thresholds Tuesday evening. Minor coastal flooding thresholds will be approached in the same areas once again with the lower Wed Am high tide, and possibly briefly exceeded along the southern bays of Western Long Island. More widespread minor coastal flooding is possible with the higher Wednesday night high tide cycle...with minor inundation likely along the southern bays of LI. Only 1/3 to 3/4 of a foot of surge is needed to reach minor flood thresholds Wed night. In addition...the elevated water levels combined with a prolonged period of 5 to 9 ft breaking surf Tue into Thu will result in beach erosion issues and may cause some localized dune toe erosion during the Tue Night into Wed Night high tides.
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&& .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...Small Craft Advisory from 2 AM to 8 PM EDT Tuesday for ANZ330- 335-338-340-345. Small Craft Advisory until 6 AM EDT Wednesday for ANZ350-353- 355. && $$ SYNOPSIS...JC/Maloit NEAR TERM...JP SHORT TERM...JC LONG TERM...Maloit AVIATION...NV MARINE...JC HYDROLOGY...JC/Maloit TIDES/COASTAL FLOODING...NV

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