Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Burlington Vermont
757 PM EDT Thu Oct 20 2016
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.
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 Vermont 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 quantitative precipitation forecast
slightly to show the heightened band of precip making it's way
Strong low level deformation associated with front moves into St
Lawrence later this afternoon and into tonight, leading to
heavier rainfall for nrn New York 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 Vermont late Friday into Friday night, per the 00z European model (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 Vermont. 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 frontal passage will dictate wind shift of northwest-north
and colder air filtering in, especially through the valleys and
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
quantitative precipitation forecast 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 quantitative precipitation forecast
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 County Warning Area. 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 weds...as 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/...
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 New York and
eventually spreading into western Vermont 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 Vermont 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/southeast 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
00z Monday through 00z tuesday: a mix of VFR/MVFR...with VFR/IFR
possible in showers.
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.