Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 302056

Area Forecast Discussion
National Weather Service Buffalo NY
356 PM EST Wed Nov 30 2016

A cold front will advance across the region tonight, bringing rain
showers and eroding the very warm airmass that is in place. A colder
airmass will then follow, Thursday through the end of the week, with
lake effect rain showers east of the lakes Thursday that will mix
with snow Thursday night and through the start of the weekend.


An area of low pressure is located over the Upper Great Lakes this
afternoon, with a cold front slicing across the Ohio Valley and down
through the western Gulf states. Ahead of the cold front a very warm
air mass resides, with our region well into the 50s, and a few
locations reaching the lower 60s briefly.

Within this warm airmass a few rain showers have developed. They are
more spotty across WNY, and more widespread over Lake Ontario and
towards our eastern zones where a weak surface trough resides.

A little feature seen on water vapor imagery over IL/IN this
afternoon will be one of the main players for our tonight period.
This feature, an area of faster wind speeds/shortwave will intercept
a plume of deep moisture over the Ohio Valley late this afternoon.
Upward lift ahead of the shortwave, combined with lift ahead of the
cold front will produce widespread synoptic lift such that a line of
rain showers will form, passing west to east across the region this
evening towards the west, to the early morning hours towards the

There will be an added element of instability, with NAM soundings
suggesting 100 to 300 J/KG of MUCAPE. This instability, combined
with the presence of cooler air aloft with the shortwave steepening
lapse rates over the lingering warm airmass near the surface could
bring isolated thunder tonight, mainly across WNY. Farther to the
east and north, instability is weaker and there will be a lower
chance for thunder, and thus will not mention isolated thunder
across the North Country, and just mention isolated thunder across

After this line of showers/cold front passes across the region
tonight, a much colder airmass will build southeastward across the
Lower Great Lakes. Temperatures at 850 hPa will lower to around -4C
on Thursday which should bring ample instability over a +8 to +10C
lake waters to produce a band of lake effect precipitation.

The lower levels of the atmosphere, and surface temperatures will
remain too warm for snow, such that just plain rain will fall from
these lake plumes later tonight and tomorrow. An initial 240 to 250
wind flow will bring these plumes to around Buffalo (Lake
Erie)/Watertown (Lake Ontario) before settling ever so slightly
southward later Thursday.

Temperature will remain mild this evening ahead of the cold front
with falling temperatures the second half of the night dropping down
into the lower to mid 40s. Under cold air advection aloft highs
tomorrow will not be much warmer than their morning values, with
highs only rebounding a few degrees. This cold air advection will
also create brisk winds tomorrow. Southwest winds will howl near the
lake shores, and especially the northeaster lake shores, with gusts
up to 40 mph. Inland southwest wind gusts will be lower, generally
25 to 35 mph.


Mid level closed low will cross Quebec and morph into an open wave
as it crosses into New England by Friday, eventually re-developing
over the Canadian Maritimes as energy transfers to the coast by
the weekend. There will be a series of minor shortwaves and their
associated surface troughs wrapping into western New York into the
first part of the weekend, resulting in periods of deeper
moisture and lake driven precipitation. Temperatures aloft will
gradually turn colder over this time with 850 mb temperatures
starting the period from -4/-5c, down to -7/-8c by Saturday. Low
level flow starts out westerly, slowly veering to northwest by
Saturday. This seasonably cool and moist cyclonic flow will
providing a favorable background for lake enhanced/effect
precipitation. Outside of the lake driven areas, occasional weak
ascent from passing subtle shortwaves will produce scattered light
showers, with the better coverage of these synoptic showers across
the North Country in closer proximity to better forcing and deeper

Off Lake Erie...

The cooling boundary layer will allow for some wet snow to mix in
across higher terrain Thursday night into Friday, with a probable
change to all wet snow at times for the higher terrain. Expect a
slushy 1-2 inches of accumulation later Thursday night into Friday
morning across the high terrain of the Boston Hills and western
Wyoming County, and the hills of central and northern Chautauqua and
Cattaraugus counties.

Friday night and Saturday, the boundary layer will continue to cool
as boundary layer turn more northwest. This will push the lake
effect precipitation farther south into the Southern Tier. There
may be some additional minor accumulations across higher elevations.
Lake effect precipitation will then slowly diminish during the day
Saturday as a mid level ridge and somewhat drier air begin to build
into the Lower Great Lakes.

Off Lake Ontario...

The boundary layer will undergo modest cooling Thursday night which
will allow wet snow to mix in across the higher terrain of the Tug
Hill and western Adirondacks, with a probable change to all wet snow
late Thursday night through Friday across the highest elevations
where a 1-3 inch slushy accumulation is possible.

Friday night, the boundary layer will continue to cool as boundary
layer turn more northwest. This will push the lake effect
precipitation south and onshore from the Rochester area eastward
into the Finger Lakes. There may be some additional minor
accumulations across higher elevations.

On Saturday northwest flow will continue, with a spray of weak lake
effect showers southeast of the lake from near Rochester to Oswego

Saturday night and Sunday, cool west northwest flow will remain in
place, supporting a few scattered rain and wet snow showers, and
some limited lake effect southeast of the Lakes. Temperatures will
be seasonable, with highs in the lower 40s at lower elevations and
mid to upper 30s across higher terrain.


Model guidance begins to diverge by early next week, with
significant run to run differences noted in both the GFS and ECMWF
over the past few days. A weak trough is forecast to move
northeast across the area Sunday night and early Monday with a few
wet snow and rain showers possible from a combination of weak
ascent from the trough and a broad warm advection regime.

Monday night and Tuesday both the 00Z GFS and ECMWF runs build high
pressure back into the Ohio Valley and Lower Great Lakes. This is in
stark contrast to the previous 12Z/29 run of the ECMWF which brought
a significant low and widespread rain through the area on Tuesday.
The GFS has been a little more consistent, and with that in mind
have gone with a mainly dry forecast for Monday night into Tuesday.
Regardless of the synoptic details, the overall pattern suggests a
return to slightly above normal temperatures by Tuesday.


At 20Z flight conditions were variable. For most areas the
southerly flow is downsloping and keeping CIGS in the VFR/MVFR
category, with the exception the Southern Tier (and JHW) where IFR
CIGS have moved in.

A cold front will cross the region tonight, pushing a line of
showers across the TAF region between 00Z and 08Z. This line of
showers will produce IFR/MVFR flight conditions as CIGS lower.
While visibilities may improve shortly after the rain ends,
lingering moisture in the lower levels may prolong IFR flight
conditions longer.

While low probability, there is enough instability in the lower
levels to enhance the chances for thunder to isolated ahead of the
cold front passage. This confidence will remain far too low to
insert into any TAFs right now, though if it were to occur, the
greatest chance will be south of Lake Ontario.

Later tonight as cold air deepens to the east of the Great Lakes a
band of lake effect rain will form. This activity will nudge
northward Thursday morning, possibly crossing both the KBUF/KART
airfields before settling southward later tomorrow.


Thursday afternoon...MVFR/IFR...with lake effect rain showers
east of the lakes.

Thursday night and Friday...MVFR/IFR with lake effect rain showers
shifting southward and becoming mixed with snow showers.

Saturday and Sunday...Mainly VFR with areas of MVFR/IFR and
rain/snow showers east of the lakes.

Monday...IFR in chance of snow showers, becoming MVFR/VFR as
precipitation transitions to rain showers.


An area of low pressure over the Upper Great Lakes this afternoon
will pass just to the north of Lake Ontario tonight and tomorrow,
while swinging a cold front across the Eastern Great Lakes between
00Z tonight and 12Z tomorrow.

Behind the cold front expect westerly winds to increase, nearing 30
knots on the lake late tonight and through the day tomorrow. These
winds will gradually diminish Friday and Friday night, but based
on the lastest forecast we have extended the small craft headlines
on the lakes as outlined below. Gusty winds will also rush across
the rivers, with no changes to the current small craft headlines.


MARINE...Small Craft Advisory from 7 PM this evening to 6 AM EST
         Saturday for LEZ040-041.
         Small Craft Advisory from 10 PM this evening to 4 AM EST
         Friday for LEZ020.
         Small Craft Advisory from 10 PM this evening to 1 AM
         EST Friday for LOZ030.
         Small Craft Advisory from 10 PM this evening to 6 AM EST
         Saturday for LOZ043.
         Small Craft Advisory from 10 PM this evening to 6 PM EST
         Friday for LOZ042.
         Small Craft Advisory until 6 AM EST Saturday for LOZ044-
         Small Craft Advisory from 4 AM Thursday to 1 AM EST
         Friday for SLZ022-024.



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