Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 232004
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
304 PM EST Mon Jan 23 2017

.SYNOPSIS...
Strong low pressure off the Mid Atlantic will move northeastward
along the east coast tonight into Tuesday bringing widespread
precipitation to the region. Rain will begin across the Southern
Tier first, then spread northward across the entire area by late
this evening. Precipitation will mix with and change over to snow
tonight with portions of central and northcentral NY receiving
significant snow accumulations. Weak high pressure will then cross
the region Tuesday night into the first part of Wednesday before low
pressure slides north of the region Wednesday night into Thursday
dragging a cold front through the area. Colder air in the wake of
this front will then slowly filter into the region into this weekend
with lake effect snows increasing east of the Lower Great Lakes.

&&

.NEAR TERM /THROUGH TUESDAY/...
...Accumulating wet snow to impact a large portion of western and
north central New York through Tuesday...

WV imagery depicts a large storm system over North Carolina late
this afternoon. This feature will slowly churn northward along the
Mid Atlantic coast tonight...while a highly anomalous easterly flow
in the low levels will pump a wealth of Atlantic moisture back
across our region. The moisture will be lifted by a combination of
divergent upper level flow...significant hgt falls and some weak
H925-70 frontogenetic forcing. While this will virtually guarantee
that all areas will pick up some precipitation...the
challenging portion of the forecast will deal with both
precipitation type and overall QPF.

Initially...the low level thermal profile across the forecast area
will be well above freezing...but as we work through this evening...
the deep easterly flow at cloud level will advect marginally colder
air (H85 temps -1 to -2c) into the region from eastern New York and
parts of New England. This is probably the most common layer that is
used to forecast snow...but in this case the dendritic growth
zone will be found much higher in the alto-stratus layer at 10k ft.
This snow will then `seed` the `warmer` clouds below. At first...the
warmer air below H85 will melt the snow so that mainly rain will be
experienced. As the intensity of the pcpn increases though...the
melting will actually cool the environment (ie. cooling due to
melting). This is the same process that takes place during
evaporational cooling...only one phase higher.

All of that being said...rain that is gradually working its way
northward across Pennsylvania this afternoon will slowly push north
across the western counties this evening. Again...the pcpn will
initially be in the form of rain...then will gradually mix with
sleet and wet snow...starting at higher elevations and working into
the valleys during the overnight. The pcpn will finally reach the
eastern Lake Ontario region after about 03z. Liquid equivalent in
the pcpn tonight is forecast to range from a quarter inch over the
far western counties and north of the Tug Hill...to more than a half
inch over parts of the Srn Tier and Finger Lakes region. This will
translate into as much as 6 inches of heavy wet snow on the hills of
Allegany County and the Bristol Hills by daybreak. This again will
be highly dependent on the temperature profile in the lowest few
thousand feet. Meanwhile...min temps will generally be within a
couple degrees of freezing.

On Tuesday...the vertically stacked storm will be nearly stationary
off the New Jersey coast. Winds aloft will back in the process...and
this will encourage stronger cold advection that will send the whole
column to below freezing. Any mixed precipitation in the morning
will then change to just snow. The most significant accumulating
snow will be during the early morning...then as we work through the
midday and afternoon...substantially weaker forcing and gradual
drying in the mid levels will lead to a tapering off of the snowfall
from west to east. Additional snowfall on Tuesday will range from an
inch or so over the far western counties to about three inches
across the North Country.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY NIGHT/...
The middle portion of the week will be the transition period back to
a more typical winter pattern. Low pressure in the Gulf of Maine
Tuesday night will gradually giveway to a ridge of high pressure
which will extend from the Southeastern states into Western New
York. There may be some lingering snow showers across eastern
portions of the cwa from the departing low with little if any
accumulation. Otherwise, it will be dry with lows ranging from the
mid 20s to lower 30s.

Wednesday will start with a narrow ridge across the region and low
pressure developing near Lake Michigan. 12Z model consensus is
slower with this system, with precipitation expected to mainly hold
off until Wednesday afternoon. The southerly flow will warm areas
south of Lake Ontario into the lower to mid 40s resulting in mostly
rain showers. It will be cooler east of Lake Ontario where some snow
is possible even though any accumulations will be light.

The surface low is forecast to track into Southern Ontario Wednesday
night, with an increasing southwesterly flow. Late Wednesday night
and early Thursday morning it will be windy northeast of the lakes.
The track of the low is favorable for strong winds, however the
system is weakening as it passes by suggesting wind gusts in the 45
mph ballpark.

Otherwise, the system will bring several rounds of precipitation to
the area, with the surface low, upper level trough, and enhanced by
upsloping as the flow becomes WSW on Thursday. Precipitation amounts
should generally be light Wednesday night into Thursday, with the
greatest amounts east of the lakes. Temperatures will gradually cool
with the passage of the mid-level trough, with higher elevations
changing to snow late Wednesday night, and a mix of rain and snow
during the day Thursday. Modest accumulations of a few inches are
possible on the Tug Hill and across higher terrain in the Western
Southern Tier. Amounts should be held down due to the marginally
cold enough temperatures aloft.

By Thursday evening, consensus 850mb temperatures are about -8C,
which is cold enough for all precipitation to change to snow.
Temperatures aloft Thursday night will result in only marginal lake
induced instability, but the flow will be quite moist with upstream
lakes providing some extra moisture. Also, the westerly flow is
favorable for banding and upsloping. Off Lake Ontario, expect lake
snows to focus on the Tug Hill or just south into Northern Cayuga
and Wayne county. Off Lake Erie, suspect that the Chautauqua ridge
will funnel winds with a WSW flow at times.  Lake snows should focus
on the Western Southern Tier and may extend into the Boston Hills.
Expect the lower resolution model guidance available during this
timeframe does not resolve this funneling and is too far south with
its QPF. Moderate snow accumulations are possible off both lakes on
Thursday night. This is the start of what looks to be an active
period for lake effect snow, potentially requiring lake effect
headlines as forecast confidence increases.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Typical mid winter weather will become established again through the
long term period after a several week hiatus. A fundamental pattern
change will take place across North America, with a ridge building
over the west and a longwave trough becoming established over the
eastern half of the continent late in the week through next weekend.

Our area will be in the active region near the base of the longwave
trough, with numerous weak shortwaves moving through the base of the
trough. Each of these shortwaves may produce a few light snow
showers across much of the region, and will also bring subtle
variations to the low level flow, moisture depth, and cold air
structure which will impact ongoing lake effect snow.

The pattern appears favorable for a long period of lake effect snow
starting Thursday night and going right through the entire weekend.
Model guidance is in good agreement initially from Thursday night
through Friday night with mean low level flow from the WNW. This
would target the western Southern Tier off Lake Erie and the
Southern Tug Hill and Oswego County off Lake Ontario, possibly into
portions of northern Cayuga and NE Wayne counties at times. By
Saturday and Sunday model guidance begins to show some spread, with
the GFS maintaining WNW flow while the ECMWF backs flow more to the
WSW. Subtle shortwaves will likely produce some variation in low
level flow, although the details are always uncertain at this time
range.

The airmass is not overly cold by late January standards, with 850mb
temps starting at around -8C Thursday night and dropping to around
-12C over the weekend. Temperatures are fairly cold at 700mb
however, and this combined with deep moisture will allow lake
induced equilibrium levels to rise to near 10K feet. The favorable
instability and longevity of the setup suggest significant
accumulations are possible east of the lakes.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
Northeast flow will continue off Lake Ontario right through the end
of the TAF period.

The abundance of low-level moisture advecting in off of the
southwest end of Lake Ontario will keep MVFR cigs in at KBUF/KIAG
into this evening. Expect intermittent MVFR cigs at KART, with VFR
conditions at KROC/KJHW through early this evening.

A storm system moving up the east coast will then bring a period of
rain, changing over to snow to the region tonight. Expect rain at
the on set this evening, transitioning over to snow for tonight into
Tuesday. Rain will begin across the Southern Tier early this
evening, then overspread the region from south to north, getting
into KART later this evening. Precipitation should transition over
to all snow for most areas by around or a little after midnight.
Expect mainly MVFR flight conditions this evening with the onset of
the rain. After the precipitation transitions over to snow later
tonight expect IFR/LIFR conditions at all terminals right through
tomorrow afternoon. The exception may be at KJHW, where vsby may
come up as snow tapers off very late in the period, however IFR/LIFR
cigs will hang in. The heaviest snow will fall across central and
northcentral NY, where vsbys down to a mile and possibly lower can
be expected.

Outlook...

Tuesday...Areas of MVFR/IFR with wet snow.
Wednesday and Thursday...VFR/MVFR with a chance of rain and snow.
Friday...VFR/MVFR with a chance of snow showers.
Saturday...VFR/MVFR with a chance of snow showers.

&&

.MARINE...
A deep storm system off the Mid Atlantic coast will combine with
high pressure over northern Quebec to produce a fairly strong
east to northeast flow across Lake Ontario tonight into Tuesday. This
will make it quite choppy for the nearshore waters west of Mexico
Bay...so a small craft advisory will remain in effect for those
areas. A SCA is also in effect tonight for the Lower Niagara River.
A weaker surface pressure gradient will be found over Lake Erie.

The aforementioned storm system will push north along the New
England coast later Tuesday and Tuesday night...while a ridge will
advance from the Upper Great Lakes. This will promote relatively
light winds and negligible waves on the Lower Great Lakes through
the middle of the week.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Winter Weather Advisory from 7 PM this evening to 7 PM EST
     Tuesday for NYZ007.
     Winter Storm Warning from 7 PM this evening to 7 PM EST
     Tuesday for NYZ005-006-008.
     Winter Storm Warning until 10 AM EST Tuesday for NYZ004-013-
     014-021.
     Winter Weather Advisory until 10 AM EST Tuesday for NYZ003-011-
     012-020.
MARINE...Small Craft Advisory until 3 AM EST Tuesday for LOZ030.
         Small Craft Advisory until 1 PM EST Tuesday for LOZ042-043.
         Small Craft Advisory from 4 AM to 1 PM EST Tuesday for
         LOZ044.

&&

$$

SYNOPSIS...JM
NEAR TERM...RSH
SHORT TERM...APFFEL
LONG TERM...HITCHCOCK
AVIATION...JM
MARINE...APFFEL



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