Area Forecast Discussion
Issued by NWS Upton, NY

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769 FXUS61 KOKX 241941 AFDOKX Area Forecast Discussion National Weather Service New York NY 341 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/... 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. && .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 /20Z MONDAY THROUGH SATURDAY/... Low pressure slowly moves into the Mid-Atlantic coastline by the end of the TAF period. VFR prevails until late tonight when MVFR and IFR will develop in response to rainfall becoming more continuous with lowering stratus. Expecting IFR to continue for Tuesday as rain continues. Winds initially are E-SE but will back to a more NE flow tonight into Tuesday, becoming gusty late tonight and during the day Tuesday. Winds maximize Tuesday with speeds 15-20 kt and gusts 25-30 kt. NY Metro Enhanced Aviation Weather Support... Detailed information, including hourly TAF wind component forecasts, can be found at: http:/ KJFK TAF Comments: Timing could be off with start time of MVFR/IFR by 1-3 hours. Could see VFR go right down to IFR within one hour late tonight. KLGA TAF Comments: Timing could be off with start time of MVFR/IFR by 1-3 hours. Could see VFR go right down to IFR within one hour late tonight. KEWR TAF Comments: Timing could be off with start time of MVFR/IFR by 1-3 hours. Could see VFR go right down to IFR within one hour late tonight. KTEB TAF Comments: Timing could be off with start time of MVFR/IFR by 2-4 hours. Could see VFR go right down to IFR within one hour late tonight. KHPN TAF Comments: Timing could be off with start time of MVFR/IFR by 1-3 hours. Could see VFR go right down to IFR within one hour late tonight. LIFR possible late tonight into Tuesday. KISP TAF Comments: Timing could be off with start time of MVFR/IFR by 2-4 hours. Could see VFR go right down to IFR within one hour late tonight. .OUTLOOK FOR 18Z TUESDAY THROUGH SATURDAY... .Tuesday-Tuesday Night...IFR/LIFR in rain. NE gusts 25-30 KT near coast. Gusts drop off Tuesday night. .Wednesday-Wednesday Night...MVFR/IFR during the day and then mainly IFR east of city terminals Wednesday night with patchy fog development. .Thursday...MVFR/IFR to start with any fog. Otherwise mainly VFR. .Friday...Chance of MVFR in showers. .Saturday...VFR. && .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... There is the potential for widespread minor coastal flooding with the Tuesday evening high tide cycle. The threat will be along the southern bays of western Long Island/Queens/Brooklyn, western Long Island Sound and NY Harbor. There is even a chance of locally moderate coastal flooding along the south shore bays of Nassau County. This is in response to a low pressure system moving northward along the eastern seaboard. Only 1/2 to 1 foot of surge is needed to reach minor flood thresholds Tuesday evening. Coastal Flood Advisories might need to be issued tonight. Elsewhere, minor coastal flooding threshold might be touched along parts of coastal New Haven County. Thereafter, minor widespread coastal flooding will be possible with the high tide cycles during Wednesday morning and night primarily across the south shore bays of western Long Island and the lower half of NY Harbor. && .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 from 8 PM this evening to 6 AM EDT Wednesday for ANZ350-353-355. && $$ SYNOPSIS...JC/Maloit NEAR TERM...JC SHORT TERM...JC LONG TERM...Maloit AVIATION...JM MARINE...JC HYDROLOGY...JC/Maloit TIDES/COASTAL FLOODING...JC is the U.S. government's official web portal to all federal, state and local government web resources and services.