Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 101231 AFDOKX Area Forecast Discussion National Weather Service New York NY 731 AM EST Sat Dec 10 2016 .SYNOPSIS... A trough approaches today from the north and west and weak high pressure then builds across tonight. The high slides offshore Sunday as low pressure over the Midwest tracks northeast. The low will track across the Great Lakes Sunday night into Monday, sending a warm front through the region. A cold front will follow for Monday afternoon into Monday evening. High pressure will then briefly build in for Tuesday before another southern branch low possibly impacts the area for the mid week period. An arctic cold front then follows behind the system Thursday. && .NEAR TERM /THROUGH TONIGHT/... The forecast is on track this morning. Only slight adjustments were made to temperatures, dewpoints, and cloud coverage to better match observed trends. Today will feature increasing clouds and cold air advection as a shortwave approaches from the west. At the surface, pressure patterns convey an inverted trough approaching. The timing of highest positive vorticity advection and the trough will enable for an isolated snow shower or snow flurries this afternoon into early this evening. Used colder blend of guidance for temperature featuring the ECS/NAM12 with highs maxing out 10-15 degrees below normal. For tonight, the atmosphere will become drier with more subsidence as weak high pressure moves across the region. With this high being weak, going into daybreak Sunday, there will be mid to high level clouds moving in ahead of the next low pressure system. Used a less vast spatial variance with lows consisting of GFS MOS. && .SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/... High pressure slides offshore Sunday with an approaching warm front. The warm front moves across Sunday night into Monday with a subsequent cold front crossing Monday afternoon into early Monday evening. Main upper level shortwave with more amplified pattern and right rear quad of upper level jet moving in Sunday night into Monday. Dry conditions will be in place initially Sunday but the precipitation chances will rapidly increase from afternoon into evening. Initial precip type expected to be snow with cold airmass in place and wet bulb cooling. Then a transition to rain will take place from south to north Sunday night with some freezing rain across the interior. Monday the transition to rain continues for interior locations and the rest of the event is expected to be mainly rain. The rain tapers off Monday evening after the cold frontal passage. A more westerly flow will advect in drier air. Temperatures Sunday used the lower ECS and MAV guidance considering limited vertical mixing. Used raw model temperatures of ECMWF and NAM12 for Sunday night and Monday but for Monday blended it with GFS MOS. GFS MOS was used for Monday Night. Expect non-diurnal trend to temperatures Sunday night with temperatures slowly rising. Potential for interior locations to have 3-5 inches of snow and some locations near interior NE NJ and Lower Hudson Valley could see a period of freezing rain with light ice accumulations possible of under a tenth of an inch. Elsewhere, looking at near 1 inch of snow or less with mostly rain anticipated. Bulk of precip is Sunday night into Monday. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Upper air pattern will feature an anomalously high amplitude ridge over Alaska, which will allow the polar vortex to drop southward into central and eastern Canada by midweek. This will open the door for the coldest airmass of the season for the second half of the week. Prior to the time, an active southern branch of the jet will take several pieces of energy from a shearing upper low dropping southward across western Canada and into the Pacific Northwest quickly eastward. At the surface, this will be reflected as area of low pressure that will track northeast across The Great Lakes and into eastern Canada Sunday night into Monday. A warm front will approach from the southwest during the same time, passing through the region on Monday. This type of fast flow with the low tracking to the northwest of the area is typically not conducive for a significant snowfall event, especially at this time of year due to a return flow developing off of warmer waters. The high is also transient in nature and builds quickly off the coast on Sunday. There are though differences amongst the models in how quickly the cold air erodes over the region Sunday night into Monday morning, with the operational NAM the most aggressive, and the GFS and ECMWF cold enough to support several inches of snowfall across the interior. There is also another potential wrinkle with weak low pressure forming to the south along the warm front Monday morning. This could potentially hold in the cold air a bit longer. The exact timing of when the boundary layer warms sufficiently for a changeover to rain is critical and allows for a fair amount of uncertainty in the forecast. For the time, have taken a multi model ensemble approach, discounting the warm NAM, for a Ptype forecast. This results in a quick changeover to rain at the coast Sunday night, and then a northward transition across the Lower Hudson Valley and interior southern Connecticut near daybreak. A deviation by 1-2 hours with strong overrunning precipitation can have a significant consequence. At this time,the forecast calls for 2 to 4 inches across northern portions of the Lower Hudson Valley and interior southern Connecticut, with less than an inch at the immediate coast. The trailing cold front passes through in the late afternoon/early evening Monday with conditions drying from west to east. Total liquid equivalent looks to be 0.50 to 0.75 inches. High pressure will briefly follow for Tuesday with temperatures just below seasonable levels. Another piece of Pacific shortwave energy races across the country. Global models vary with the amplitude of this feature with the nearly zonal flow in the southern branch of the polar jet. The amount of phasing with the northern branch looks to be the difference. Due to the fast flow, there is likely to be uncertainty in the magnitude of this system the next several days. For now. will run with a chance of rain/snow Wednesday afternoon/night. Arctic air then spills southward on the backside of the system Thursday into Friday with daytime highs possibly not getting above freezing with overnight lows in the single digits to teens. These values are 15 to 20 degrees below normal. To make matters worse, strong NW winds will follow for Thursday producing very cold wind chill values. && .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/... An upper level disturbance will move across today with BKN VFR cigs 040-060 this afternoon and early evening. Scattered snow showers/flurries possible with brief vsby reduction, most likely 18Z-21Z NYC metro and 20Z-23Z KISP. WNW flow left of 310 magnetic just over 10 kt with gusts 15-20 kt also expected. Expect diminishing winds and clearing skies thereafter. .OUTLOOK FOR 12Z SUNDAY THROUGH WEDNESDAY... ***HIGH IMPACT WEATHER LIKELY LATE SUNDAY NIGHT INTO MONDAY MORNING*** .Sunday...Mainly VFR. Chance of light snow in the afternoon with MVFR conditions likely, IFR possible late. .Sunday night...IFR/LIFR conditions developing, with snow mixing with and changing to rain at most terminals. LLWS possible late NYC metros/coast. S winds G25KT late at the NYC metros and KISP. Here are the most likely precip types and snowfall accumulations: KISP/KJFK/KLGA/KBDR/KGON: Snow changing to rain by midnight, with a coating to an inch accumulation. KEWR/KTEB/KHPN: Snow mixing with and changing to rain late, with 1-2 inches accumulation. KSWF: Snow with accumulation 4-5 inches. .Monday...IFR conditions. Snow mixing with/changing to rain in the morning at KSWF, and rain elsewhere. LLWS possible in the morning NYC metros/coast. SW winds G25KT at the NYC metros and KISP, becoming W in the afternoon. .Monday night...Still a chance of rain with MVFR or lower conditions early at KISP/KGON. otherwise VFR. .Tuesday...VFR. .Wednesday...Chance of rain, changing to snow at night. NW winds G20KT after midnight. && .MARINE... Minimal SCA conditions continue on all the ocean waters, with NW winds gusting up to 25 kt and seas as high as 5 ft. This conditions should last through this morning, then gradually subside from west to east this afternoon. There is a chance these conditions could last east of Moriches Inlet into this evening. As a low pressure system approaches Sunday night, SW winds should increase, with SCA conditions becoming likely, and gales possible late at night into Mon morning. As the system passes, moderate W-NW flow should bring at least SCA conditions to all waters on Mon. Ocean seas will take longer to subside, and so SCA conds there should last into Mon night and possibly tue morning out east. Low pressure may develop over the Atlantic during mid week, which could bring at least SCA conditions Wed night behind an arctic cold front. && .HYDROLOGY... A half to an inch of rain and/or liquid equivalent is possible Sunday through Monday. && .EQUIPMENT... New York City NOAA Weather Radio NWR transmitter KWO-35 is experiencing intermittent outages. && .OKX WATCHES/WARNINGS/ADVISORIES... CT...None. NY...None. NJ...None. MARINE...Small Craft Advisory until noon EST today for ANZ355. Small Craft Advisory until 6 PM EST this evening for ANZ350. Small Craft Advisory until 4 PM EST this afternoon for ANZ353. && $$ SYNOPSIS...JM NEAR TERM...JM SHORT TERM...JM LONG TERM...DW AVIATION...Goodman MARINE...Goodman/DW HYDROLOGY...JM/DW EQUIPMENT... is the U.S. government's official web portal to all federal, state and local government web resources and services.