Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 250538 AFDOKX Area Forecast Discussion National Weather Service New York NY 138 AM EDT Tue Apr 25 2017 .SYNOPSIS... Low pressure slowly tracks up along the eastern seaboard through Wednesday. The low eventually tracks from south and east of Long Island Wednesday night then to off to 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 THIS MORNING/... Radar returns south of the Long Island have continued to increase over the last few hours. However, lower levels are still quite dry so as the precipitation moves further north there continues to be weakening of echoes. These returns are associated with increasing warm advection ahead of low pressure over the Carolinas. Have adjusted PoPs for the remainder of the early morning hours. The low levels should moisten just before day break for the light rain to become more widespread, especially across the southern half of the region. Not confident in going with likely PoPs before 6 am as the dry air seems to be winning the battle and slowing the timing down for seeing a more widespread measurable rain. Where light rain does occur, amounts will be about a tenth of an inch or less through day break. Rainfall amounts will be on the light side, with about a tenth of an inch or less through day break and much of this will come after 3 am when the low levels finally moisten. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM 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 /06Z TUESDAY THROUGH SATURDAY/...
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Low pressure slowly moves up the Mid-Atlantic coastline today and tonight. MVFR and IFR cigs expected to gradually develop and prevail in bands of light rain early this morning. Visibility will take some time to lower, possibly late morning. Expecting IFR/LIFR ceilings this afternoon as rain becomes steadier and moderate to heavy. The low conditions continue early tonight. E/NE winds increase this morning, with speeds 15-25 kt and gusts 25-30 kt after 12-14Z. Highest sustained winds for coastal terminals. Winds diminish somewhat tonight. LLWS possible this evening with SE winds 45-50 kt at 2kft. .OUTLOOK FOR 06Z WEDNESDAY THROUGH SATURDAY... .Late tonight...IFR/LIFR in rain. NE gusts 20 to 25 KT. Winds diminish further. .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/thunderstorms. .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 Today 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 until 8 PM EDT this evening for ANZ330- 335-338-340-345. Small Craft Advisory until 6 AM EDT Wednesday for ANZ350-353- 355. && $$ SYNOPSIS...JC/Maloit NEAR TERM...JP/DS SHORT TERM...JC LONG TERM...Maloit AVIATION...PW MARINE...JC HYDROLOGY...JC/Maloit TIDES/COASTAL FLOODING... is the U.S. government's official web portal to all federal, state and local government web resources and services.