Area Forecast Discussion
Issued by NWS Upton, NY

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000 FXUS61 KOKX 100523 CCA AFDOKX Area Forecast Discussion...CORRECTED National Weather Service New York NY 1223 AM EST Sat Dec 10 2016 .SYNOPSIS... Weak high pressure builds in through Saturday night and then 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. High pressure will then briefly follow 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 /UNTIL 6 AM THIS MORNING/... Forecast remains on track. Pressure gradient has been lowering with a resulting decrease in winds as high pressure continues to build in from the west. Winds are expected to continue lowering overnight. Low temperatures will be well below normal for this time of year, 20s for much of the region. Some upper teens in the outlying areas. Wind chill values will be in the teens region-wide. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... High pressure slowly builds into the region Saturday and Saturday night. A weak shortwave and increasing low-mid level moisture could trigger a few flurries mainly across the far NW portion of the forecast area during the afternoon and evening hours. In general, increasing afternoon clouds, then clouds diminish in the evening. Temperatures through the short term are a blend of MAV/NAM MOS. && .LONG TERM /SUNDAY NIGHT 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 /05Z SATURDAY THROUGH WEDNESDAY/... High pressure builds in from the west through the TAF period. VFR. SCT-BKN CIGS 040-060 on Sat. Scattered snow showers/flurries with brief reductions in cigs/vsby to mvfr/ifr Saturday afternoon. Low prob/sparse coverage of a brief dusting. Gusty WNW-NW winds at NYC Metro terminals decreasing overnight. WNW winds with gusts to 20 kt re-developing Saturday morning for all terminals. Winds diminish Sat Evening. .OUTLOOK FOR 06Z SUNDAY THROUGH WEDNESDAY... .Late Tonight...VFR. .Sunday...Mainly VFR. Chance of light rain/snow near coast and chance snow inland late. .Sunday night and Monday...IFR in light snow Sunday night, changing to rain/snow mix from south to north early Monday morning becoming plain rain along coast thereafter. .Monday night...Rain/snow mix ends becoming VFR. .Tuesday...VFR. .Wednesday...MVFR or IFR possible in chance rain/snow mix. && .MARINE... The SCA remains for the eastern and ocean waters for overnight with these conditions lowering Saturday morning. The Eastern Long Island Bays and Sound are expected to drop below SCA by early Saturday morning while the ocean is expected to subside below SCA by late Saturday morning. Sub-SCA conditions are then expected into Saturday afternoon and night. There is a low chance for SCA conditions to develop Sunday night. Seas on the ocean will build however on Monday as low pressure passes through. Conditions improve on Tuesday with high pressure building in. Low pressure may develop over the Atlantic during the middle of the week, which could bring at least SCA conditions and possibly gales Thursday behind an arctic cold front. && .HYDROLOGY... Up 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 10 AM EST this morning for ANZ350- 353-355. Small Craft Advisory until 6 AM EST early this morning for ANZ330-340. && $$ SYNOPSIS...JC/DW NEAR TERM...JC/JM/JP SHORT TERM...JC LONG TERM...DW AVIATION...NV MARINE...JC/JM/DW HYDROLOGY...JC/DW EQUIPMENT...

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