Area Forecast Discussion
Issued by NWS Burlington, VT

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481 FXUS61 KBTV 202357 AFDBTV Area Forecast Discussion National Weather Service Burlington VT 757 PM EDT Thu Oct 20 2016 .SYNOPSIS... Strengthening surface low pressure system has started bringing widespread rain to the North Country and will continue to do so through Sunday. Occasionally heavy rainfall is expected, especially this evening into early Friday, across the Saint Lawrence Valley and Northern Adirondacks. As the front pushes east of the area by early next week a return to drier weather and seasonably cooler temperatures is expected. && .NEAR TERM /THROUGH FRIDAY NIGHT/... As of 722 PM EDT Thursday...Going forecast in good shape and just some minor tweaks to lower temperatures based on current conditions. Otherwise...northwest to southeast gradient of precipitation chances looks real good based on radar trends. Rest of forecast untouched. Previous Discussion... The 500mb trough continues to dig south and east over the central USA. The rain has been slightly more widespread, mainly in the St Lawrence Valley and Adirondacks but Western VT is also seeing precip moving through rather quickly from the SW. The main question remains how far north and east it will spread before the deepening trough and surface low riding up the frontal boundary causes area of rain to pivot north and west sometime on Friday. The HRRR continues to have the best handle on the near term precip for the St Lawrence Valley, have adjusted the PoPs and QPF slightly to show the heightened band of precip making it`s way into VT. Strong low level deformation associated with front moves into St Lawrence later this afternoon and into tonight, leading to heavier rainfall for Nrn NY overnight, before weakening on Friday. Another consideration for Friday is sub- tropical system weakening as it heads north far off the Carolina coast, but moisture looks to get absorbed or wrapped into the low affecting the Northeast, and could result in resurgence of heavier precip over eastern VT late Friday into Friday night, per the 00Z ECMWF. This may be dependent on when the 500mb trough becomes more neutral or negatively tilted late Friday. Winds another concern as strong low level jet enters the region this afternoon. Southerly jet of 40-55kts at 850mb stays over the North Country through Friday. This will lead to strong winds across the higher terrain. Best chances for strong winds to mix into the valleys would be early Friday as the precip shield pivots slightly NW, allowing for breaks in precip over VT. Have gusts up to 15-25kts across most of the area on Friday, before the surface low tracks over the area. In summary, cloudy and wet pattern expected throughout the near term with temperatures in the mid 50s to mid 60s today as we remain in the warm sector. Tonight temperatures remain mild in the mid 40s to upper 50s. Forecast max temperatures will be very tricky on Friday as the surface low and associated front traverse across the North Country. Timing of FROPA will dictate wind shift of NW-N and colder air filtering in, especially through the valleys and lower terrain. && .SHORT TERM /SATURDAY THROUGH SATURDAY NIGHT/... As of 406 PM EDT Thursday...Forecast highlight of the period continues to be unsettled conditions and our first mixed precipitation event with the challenge being timing of heaviest QPF and transition from liquid to frozen form. Deep mid/upper level trough shifting out of the Great Lakes Friday night becomes negatively tilted and eventually closes off over the North Country by Saturday morning. Meanwhile at the surface, weak low pressure moving northeast along the frontal wave interacts with a broad area of low pressure currently northeast of the central Bahamas, developing a robust surface low over VT/NH/ME. The result for Friday night is that the best low level deformation remains across northern New York with widespread rains continuing, while points eastward will be affected by a mid-level dry slot with less QPF expected. As the surface low intensifies, the system becomes vertically stacked as it shifts north to the vicinity of Quebec City by days end Saturday. Low/mid level flow shifts to the northwest, but strong low level cold air advection lags a bit compared to previous model runs. This difference in timing amongst the latest hi-res CAMs and global models presents some issues in regards to ptype transition Saturday afternoon as conditions become favorable for upslope precipitation to develop as the 700mb low sits just north of the CWA. Currently I`ve offered a blended approach which brings in colder air Saturday night with the transition from rain to snow mainly occurring above 1500 feet with several inches of snow likely at the highest summit peaks. At the surface below 1500 feet, a dusting to perhaps an inch is possible, with basically nothing in the deeper Champlain and St. Lawrence Valleys. Additionally as a strong low level jet of 30-40kts develops, winds will be increasingly gusty Saturday afternoon and night, gusting to 20-30 mph, mainly across northern New York. && .LONG TERM /SUNDAY THROUGH THURSDAY/... As of 350 PM EDT Thursday...The long term pattern will feature a general mid/upper level trof across the ne conus supporting below normal temperatures and terrain focused precip. Upslope parameters still look very favorable thru 18z Sunday with deep 850 to 500mb moisture and strong northwest flow of 35 to 50 knots...enhancing lift along the northern Dacks and western slopes. Additional qpf amounts will be a tenth to two tenths with highest amounts from Mt Mansfield to Jay Peak. Based on sounding profiles snow levels should be near the bases at 12z around 1500 feet...but lifting above 2000 feet before tapering off by 18z Sunday afternoon. Expecting only a minor additional snow accumulation. Progged 850 mb temps between -3 to -5c support highs only in the 20s mountains to mid/upper 40s warmer valleys. Large scale pattern supports additional free refills of mountain snow showers on Monday into moisture and short wave energy rotates around mid/upper level trof across eastern Canada. The cyclonic flow through all levels will promote favorable upslope flow with terrain focused precip. Have mentioned high chance pops for mountains and slight chance elsewhere for Sunday Night into Weds. Progged 850mb temps stay below 0c and actually get colder during the early part of next week with values between -7c and -9c. Little too early for detailed snowfall amounts and qpf values...but would expect the mountains to stay white for through mid week. Eventually mid/upper level trof lifts and zonal flow aloft develops with temps returning closer to normal...for Thursday. Next system arrives on Thursday night into Friday with additional chances for precip. Overall temps are in the 20s to mid 30s mountains to mid 40s valleys with lows mainly in the lower 20s mountains to lower 30s valleys. && .AVIATION /00Z Friday THROUGH Tuesday/...
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Through 00z Saturday...VFR conditions quickly giving way to MVFR/IFR and even some LIFR during the overnight. Earlier showers have moved out with a much larger rain shield moving into NY and eventually spreading into western VT overnight. it will stay ifr with rain at KMSS and scattered showers at KSLK Friday but will give way to MVFR and some temporary dry air across champlain valley and VT til afternoon then cold front slowly progresses east with north winds/low cigs/drizzle/rain for mvfr- ifr aft 18z. Northeast winds 8-15 kts for KMSS through duration...light variable/SE 5-10kts for rest of area overnight with possibly going south at 10-15 kts for KBTV and KMPV between 12-16z Fri then going north through afternoon. Rather confident of prolonged period of mvfr-ifr and even lifr but lower confidence on exact details/timing. Outlook 00z Saturday through Monday... 00z Saturday through 00z Monday: A prolonged period of unsettled conditions is expected with widespread MVFR/IFR in periods of rain. 00z Monday through 00z Tuesday: A mix of VFR/MVFR...with VFR/IFR possible in showers.
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&& .HYDROLOGY... As of 350 PM EDT Thursday...A widespread and long duration rainfall of 1 to 3 inches with amounts near 4 inches across the St Lawrence Valley from tonight through Sunday. There will be some breaks throughout the event, especially across portions of Vermont. Given most of the region is in moderate to severe drought...we are not anticipating any widespread hydro issues. However...some minor urban and street flooding is possible associated with the heavier rainfall rates on Friday with leaves clogging storm drains. Otherwise...some modest rises in local rivers and streams are likely this weekend...but no widespread flooding is anticipated. The latest guidance from the NERFC brings Ausable River near minor flood stage by Saturday Afternoon. && .BTV WATCHES/WARNINGS/ADVISORIES... VT...None. NY...None. && $$ SYNOPSIS...KGM/MV NEAR TERM...Evenson/MV SHORT TERM...Lahiff LONG TERM...Taber AVIATION...MV/SLW HYDROLOGY...Taber/KGM is the U.S. government's official web portal to all federal, state and local government web resources and services.