Area Forecast Discussion
Issued by NWS Upton, NY

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315 FXUS61 KOKX 201809 AFDOKX Area Forecast Discussion National Weather Service New York NY 109 PM EST Fri Jan 20 2017 .SYNOPSIS... An upper level disturbance weakens as it moves across tonight. High pressure briefly returns on Saturday. Deepening low pressure moving across the southern states on Sunday will reach the southern Mid Atlantic region by Monday morning, while high pressure builds southeast into the Canadian Maritimes. The low will intensify into a significant coastal storm as it moves up the coast toward the area Monday through Tuesday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Minimal changes made to the forecast to reflect current temperature/dew point trends. 17Z radar imagery showing drizzle/light rain farther northeast than any hi-res or deterministic model prediction. 12Z RAOB from OKX shows very dry air above the inversion still in place, so expect any precipitation that reaches the ground to be very light. Despite the dry air, have adjusted chances of precipitation upward this morning to account for any light rain that may fall before the steadier precipitation later today as moisture slowly increases. Subsidence inversion around 925 mb continues to lock about a 1 kft layer of moisture in the form of stratus. Main challenge this morning is if some of these clouds erode or continue to hang on. Trends have pointing to the clouds hanging on into the morning as depicted by thermodynamic profiles on the last several runs of the RAP and HRRR. Latest forecast calls for mostly cloudy to overcast skies this morning. The inversion may begin to erode a bit by afternoon. However, middle and high level clouds will be increasing ahead of the approaching southern stream shortwave energy. This will lead to a mostly cloudy sky condition this afternoon. A few breaks in the clouds should help temperatures rise into the middle and upper 40s this afternoon. Best lift associated with the southern stream shortwave energy arrives between 3 and 6pm across NYC metro, NE NJ, and Lower Hudson Valley. Have increased pops to categorical here with a fairly sharp gradient of lower further east as best lift and moisture lie in a narrow corridor. High probability of measurable precipitation on the latest ensembles and deterministic models supports categorical pops. Temperatures warm enough for plain rain, which will be on the light side. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Band of light rain progresses east this evening. Lift weakens as the shortwave weakens and gets trapped under buildings heights aloft. Pops diminish from west to east through night. A few hundredths to a tenth of inch total forecast with this system. Low level moisture lingers through the night and with weak flow some patchy fog is possible. Temperatures will not be able to fall too much due to linger clouds with lows forecast above freezing across the CWA. High pressure returns on Saturday. Mostly cloudy conditions continue in the morning, but should see enough breaks in the afternoon for some sun. With ridging building overhead, the airmass is quite warm. Highs should reach the lower and middle 50s. The only negating factor to seeing these temps would be if clouds hang on longer through the day, which some of the higher res model profiles indicate as a possibility. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Focus is on a storm forecast to take shape as strong Pacific jet energy about to come onshore the California coast closes off over the southern Plains Saturday night, then intensifies on Sunday and moves up the coast on Monday, as downstream blocking via a full latitude ridge over the western Atlantic and a closed low east of Labrador prevent its escape out to sea. One leading low could approach the Mid Atlantic region on Sunday and bring us some light rain, then as the closed low becomes negatively tilted over the Southeast states, the primary low should quickly intensify as it rides up the coast. As this low moves up the coast, a sharp pressure gradient between high pressure moving southeast from Quebec toward the Canadian Maritimes will produce strong winds along the coast and in the higher interior elevations, and cold air moving southward with the high looks to make it into the area from late Sunday night into Monday, with a period of wet snow across the interior before warmer air moves in aloft. A surface low track closer to the coast than the ensemble means, and slower than operational models, is preferred attm. Potential for strong, possibly damaging easterly winds, continues to increase. EPS probabilities of sustained 40 mph winds and 60 mph wind gusts continue to remain high for the coastline and now also for the highest interior elevations, and its probabilities of 45-mph wind gusts are over 50 percent over most of the area. Also, both GFS and ECMWF MOS continue to predict increasing potential for sustained winds over 30 mph Mon night into Tue morning, perhaps as high as 40 mph at the peak of the storm across Long Island/coastal CT/NYC metro. A prolonged moderate rainfall of 1-3 inches remains likely as the storm maintains a tropical moisture connection to the Caribbean and Eastern Pacific. The rain could become heavy Mon afternoon/evening especially east of NYC, and maybe even throughout the region if the low tracks just a little farther west. With colder air from the Canadian high filtering down into the region, a period of wet snow is likely across the interior from late Sunday night into Monday before warmer air moves in aloft ahead of the low. Amounts should be light, but but as with several other events this season, could be substantially higher in the hills of Orange/Putnam/Rockland, and also farther north in most areas just to the north. Gradually improving conditions Tue afternoon/night as the low passes to the northeast. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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A wave of low pressure will move across the area tonight with high pressure returning for Saturday. MVFR cigs through the remainder of the afternoon with rain arriving. Rain continues through the evening with IFR conditions developing. Localized LIFR is possible. Some forecast guidance is hinting at residual low level moisture, which could result in areas of fog early Saturday morning. Winds remain light through the period at less than 10 kt. Wind directions will be Southeast, then South overnight. Some locations may become variable overnight. Winds then become Westerly on Saturday. ...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: Timing of lower CIGs may be off by an hour or two. KLGA TAF Comments: Timing of lower CIGs may be off by an hour or two. KEWR TAF Comments: Timing of lower CIGs may be off by an hour or two. KTEB TAF Comments: Timing of lower CIGs may be off by an hour or two. KHPN TAF Comments: Timing of lower CIGs may be off by an hour or two. KISP TAF Comments: Timing of lower CIGs may be off by an hour or two. .OUTLOOK FOR 18Z SATURDAY THROUGH WEDNESDAY... .Saturday afternoon/night...MVFR or lower possible early Saturday morning, otherwise VFR. .Sunday-Sunday Night...MVFR or lower in light rain, then moderate rain at night. NE winds G20-30KT by late day, and G30-40KT later at night. .Monday-Monday Night...IFR conditions. Snow and rain for the interior including KSWF, and moderate-heavy rain elsewhere. ENE winds 25-30KT with G40-45KT. .Tuesday-Tuesday night...Rain tapering off in the afternoon/evening with improving ceilings. NW winds G25-35KT, highest in the morning. .Wednesday...VFR.
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&& .MARINE... Seas will remain below SCA levels through Saturday night under a relaxed pressure gradient. An intensifying storm moving up the coast early next week will likely bring easterly gales to all waters from Sunday night into Monday night. Storm force wind gusts may be possible on the ocean and the eastern Sound/bays. If these winds come to fruition, seas as high as 15-20 ft could occur on the ocean waters, and as high as 6-9 ft on the eastern Sound. && .HYDROLOGY... A coastal storm could bring rainfall of 1-3 inches early next week. Most likely time frame of heaviest rainfall is Monday afternoon/night. If the higher end of these rainfall amounts occur, minor areal flooding would be likely. && .TIDES/COASTAL FLOODING... A strong coastal storm may cause widespread minor and locally moderate coastal flooding with any of the high tide cycles from Mon morning into Tue morning. Stevens Tech NYHOPS ensemble guidance continues to show potential for a 3-4 ft surge on Monday close to the times of high tide Mon evening. Since the amount of surge needed to produce flooding is large, and since astronomical tides with the evening high tide cycle are about 1/2 foot lower than those of the morning cycles, exact timing of the strongest winds will be critical to the eventual outcome. Because of the timing uncertainty, right now it is wise to talk in terms closer to that worst case scenario as is often done with tropical systems. Approximate surge needed to cause flooding (in ft). Please refer to our Web page at http:/www.weather.gov/okx/coastal_flooding for exact details: Mon/Tue Morning Minor Moderate Long Island South Shore Back Bays: 1.5 to 2.5 2.5 to 3 Elsewhere: 2.5 to 3 3 to 4 Mon Evening Minor Moderate Long Island South Shore Back Bays: 2 to 3 3 to 3.5 Elsewhere: 3 to 3.5 3.5 to 4.5 The other concern will be for significant beach erosion from long period easterly swells of 10-15 or more ft from Mon into Tue. Localized washovers could also occur if higher total water levels are realized. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...None. && $$ SYNOPSIS...Goodman/DS NEAR TERM...DS SHORT TERM...DS LONG TERM...Goodman/NV AVIATION...BC MARINE...Goodman/DS/NV HYDROLOGY...Goodman/DS/NV TIDES/COASTAL FLOODING...

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