Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 290430 AFDOKX Area Forecast Discussion National Weather Service New York NY 1230 AM EDT SUN MAY 29 2016 .SYNOPSIS... Bermuda high pressure persists through Sunday. The high weakens and moves farther out into the Atlantic Sunday Night into Monday. A cold front is forecast to cross the area Monday night into Tuesday with Canadian high pressure building in behind it from the north. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Only minor changes made to the near term grids, mainly to dew points across Long Island as they are running well below expected values. Dew points have mixed out into the upper 40s and lower 50s Long Island where better mixing has persisted. Otherwise, the forecast remains on track. Bermuda high pressure still in place tonight. Lows will be mild and well into the 60s. Not confident in patchy fog development late tonight, but if any location completely decouples patchy fog would be possible. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM MONDAY/... Once again with the Bermuda high in place, the winds will be out of the south to southwest. Another warm and humid day is expected. 850mb temperatures are forecast to be nearly the same to slightly less than Saturday`s values. In terms of convection, the upper level ridge axis will be sliding east of the region, allowing for a lowering of heights Sunday Afternoon into Sunday Evening. This will aid dynamic lift judging by today and how convection is developing north and west of NYC. For Sunday Afternoon into Sunday Evening, heights are even lower, about 30-40 meters lower at 500mb. The scattered coverage will extend farther south and east across NYC into Western Long Island. This further increases Sunday Night with a continuance of lowering heights aloft. There will therefore be less subsidence and more lift as a shortwave approaches riding along the NW edge of the upper level ridge, the axis of which will be over the open Atlantic. Instability for thunderstorms will be mostly near where there is higher CAPE, mainly north and west of NYC so greater chances of thunderstorms will be there. Used warmer MET guidance for interior and a blend of MAV/MET for the coastal areas for highs on Sunday. The southerly flow continues at the surface, moistening the low levels further. There will be a potential for heavy rain especially late Sunday Night through Monday as precipitable waters increase to near 2 inches as there will be some tropical moisture entrainment into the region as well. The bulk of the rain as conveyed by numerical weather prediction QPF fields looks to be 06Z to 18Z Monday. Rain looks to linger Monday Afternoon but the high precipitable water axis will be moving across Eastern Long Island and Southeast Connecticut, keeping more of the relatively heavier rain potential in those locations with lighter rain to the west. Outside of any heavier rain, Sunday Night into Monday will have some patchy fog lingering as well. Used the warmer MAV guidance for lows on Sunday Night. Looking at the vertical wind profiles through the atmosphere, there is a deep layer of south to southwest flow. The jet comes into play early Monday Morning through much of Monday Afternoon with a right rear quad between of 50 to 70 kt across the region, lighter part of the range towards the coast and higher part of the range inland. This will allow for widespread and potentially heavy rainfall. The uniformity of flow from low to upper levels will allow for training of cells as well. Instability will be less and elevated so for thunderstorms, will just have chance of thunderstorms. These aforementioned factors signal a potential predecessor rain event but there is uncertainty on where the heavy band of rain sets up. The confidence at this time of where exactly the heavy rain occurs is low so chance for flooding is at this time around 30 percent. Used the GMOS for Monday high temperatures. There is a moderate risk of rip current development at Atlantic ocean beaches on Sunday. && .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Models have come into somewhat better agreement with the overall H5 pattern across north america although timing and amplitude differences still become apparent by the middle of the week which is impacting the progression of a cold front late in the week. Approaching shortwave trough flattens as it moves into the northeast Monday night and appears to keep the remnant low over the Carolinas from being picked up by the northern branch of the jet stream. The 12z EC is slower than the GFS moving the cold front and pcpn out of the area Mon night and even lingers it in srn zones into Tue morning. Have kept chc pops in during the eve...but pcpn may linger longer than forecast...especially e of nyc...if the front is slower to move across or even gets hung up over the area. In fact...the EC slowly tracks the low up the eastern seaboard while the GFS keeps it nearly stationary over the Carolinas through the middle of the week until its finally picked up by the next trough. While the current forecast is dry tue through thu...this may change depending on the eventual upper pattern which does have differences by this time and is leading to varying solns. A pre-frontal trough on fri ahead of the approaching upper trough and associated sfc cold front should trigger shower and tstm activity mainly from NYC and areas n and w on fri and across the area into sat as the front moves through. && .AVIATION /06Z SUNDAY THROUGH THURSDAY/... High pressure will remain nearly stationary across the western Atlantic through Sunday with a thermal trough across the interior in the afternoon. Convection has dissipated across the area with no additional activity forecast overnight. There is a low chance of afternoon/evening convection Sunday, primarily north and west of the NYC terminals. Winds may be variable in and around the NYC terminals early due to the dissipated convection, otherwise S/SW flow this evening diminishes to less than 10 kt. KISP and KJFK may experience an occasional gusts up to 20 kt early as the coastal jet weakens. Persistence type of forecast Sunday with seabreezes once again developing with local enhancements in the afternoon. MVFR fog is possible late tonight at KSWF, with a low chance across the remainder of the non-NYC terminals. .OUTLOOK FOR 00Z Monday through Thursday... .Sunday night-Monday...MVFR or lower likely after midnight. Showers/thunderstorms could produce locally heavy rainfall. .Monday night-Tuesday night...Becoming VFR Monday Night. .Wednesday and Thursday...VFR. && .MARINE... Through Monday, the pressure gradient will be weak and diffuse. This will allow for winds and seas to stay below sca thresholds. Have capped seas at 4 ft Mon night into Tue as both wavewatch and swan appeared high with no real swell component present and a relatively light sly flow. otherwise...sub-advsy conds are expected through the remainder of the forecast period. && .HYDROLOGY... No significant widespread precipitation is expected through Sunday. For Sunday Night through Monday Evening...widespread rainfall is forecast. While rainfall totals are forecast to range from around 1 to 1.5 inches, some locally higher amounts are quite possible with a chance of flooding. The main type of flooding would be that of urban and small streams. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...None. && $$ SYNOPSIS...24/JM NEAR TERM...24/JM/DS SHORT TERM...JM LONG TERM...JM AVIATION...DW MARINE...24/JM HYDROLOGY...24/JM is the U.S. government's official web portal to all federal, state and local government web resources and services.