Area Forecast Discussion
Issued by NWS Buffalo, NY

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102
FXUS61 KBUF 301842
AFDBUF

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

.SYNOPSIS...
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.

&&

.NEAR TERM /THROUGH THURSDAY/...
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
east.

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
WNY.

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.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/...
Thursday night through Friday a mid level closed low and
associated surface low will move slowly east across Quebec,
eventually re- developing over the Canadian Maritimes as energy
transfers to a coastal low later Friday. This system will maintain
a seasonably cool and moist cyclonic flow across the Great Lakes
through the late week period, providing a favorable synoptic scale
environment for lake effect precipitation. Outside of lake effect
areas, occasional weak ascent from passing subtle shortwaves will
produce a few widely scattered light showers, with the better
coverage of these synoptic showers across the North Country in
closer proximity to better forcing and deeper moisture.

850mb temps will drop to around -5C on Friday, with moist low
levels and favorable lapse rates up to around 700mb contributing
to lake induced equilibrium levels of 8-10k feet through the
period.

Off Lake Erie...

Thursday night boundary layer flow is forecast to undergo subtle
veering to a more westerly direction, focusing most of the lake
effect into Southern Erie/Wyoming counties and portions of
Chautauqua and Cattaraugus counties. This westerly flow will
continue through Friday, keeping lake effect precipitation in the
same general area.

The boundary layer will slowly cool and allow some wet snow to mix
in across higher terrain later Thursday night into Friday, with a
probable change to all wet snow at times for terrain above about
1500 feet MSL. 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. Lake effect rain
totals will be decent as well, with some areas seeing up to three
quarters of an inch of rain including the Buffalo Southtowns.

A mid level shortwave will cross the Lower Great Lakes Friday Night.
Behind this shortwave boundary layer winds will veer northwesterly.
This will push lake effect farther south into the Southern Tier and
NW PA, and may also allow a weak Lake Huron connection to develop.
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. 850mb temps drop a little more to around
-7C during this period, which will support more mix with wet snow
across lower elevations and all snow across higher terrain. There
may be some additional minor accumulations across higher
elevations.

Off Lake Ontario...

Some modest cooling Thursday night 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 at times late Thursday night
through Friday. Expect 2-3 inches of slushy accumulation above about
1500 feet MSL. Lake effect rain totals will also be decent, with
around three quarters of an inch in some locations east of the lake.

Friday evening a mid level shortwave will cross the lake. The
increase in large scale ascent and stronger low level convergence
associated with a passing surface trough will likely increase the
coverage and intensity of lake effect at the east end of the lake.
Overnight boundary layer winds will veer to the northwest, pushing a
band of lake effect precipitation south and onshore from the
Rochester area eastward into the Finger Lakes and Central NY. On
Saturday northwest flow will continue, with a spray of weak lake
effect showers southeast of the lake from near Rochester to Oswego
County.

During Friday evening precip type will stay rain at lower elevations
and some wet snow across higher terrain. Overnight into Saturday
850mb temps dropping to around -7C will allow a few wet flakes to
mix in across lower elevations as well. Expect little to no
accumulation at lower elevations, with some additional minor amounts
across higher terrain.

&&

.LONG TERM /SATURDAY NIGHT THROUGH WEDNESDAY/...
Saturday night through Sunday, cool WNW 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.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
At 18Z VFR flight conditions were common across the region, with
just a few pockets of MVFR within rain across eastern zones.

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.

Outlook...

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.

&&

.MARINE...
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. Gusty
winds will also rush across the rivers, such that Small Craft
Advisories that are presently in effect will continue.

Continued bursts of colder air will continue brisk winds and choppy
waters through the end of the week, such that the end time for
these SCA may need to be extended.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory from 7 PM this evening to 10 PM EST
         Friday 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 10 PM EST
         Friday for LOZ043.
         Small Craft Advisory from 10 PM this evening to 1 PM EST
         Friday for LOZ042.
         Small Craft Advisory until 10 PM EST Friday for LOZ044-045.
         Small Craft Advisory from 4 AM Thursday to 1 AM EST
         Friday for SLZ022-024.

&&

$$

SYNOPSIS...THOMAS
NEAR TERM...THOMAS
SHORT TERM...HITCHCOCK
LONG TERM...HITCHCOCK
AVIATION...THOMAS
MARINE...THOMAS



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