Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 212133

Area Forecast Discussion
National Weather Service Buffalo NY
433 PM EST Tue Nov 21 2017

The brisk southwesterly flow of mild air will continue through early
this evening, before quickly coming to an end late this evening into
the overnight as a strong cold front sweeps through the region. Rain
showers along and ahead of the front will mix with and change to
snow before diminishing early tomorrow morning across western New
York, and by early afternoon across the northcountry. In the wake of
the front, building high pressure will then bring a return to colder
weather for midweek, with just some nuisance-type lake effect snow
showers possible downwind of the lakes at times later Wednesday into
Wednesday night. Expect some snow and rain showers to return for
Thanksgiving Day northeast of both lakes.


Heading into this evening, loss of daytime mixing will allow surface
winds to subside some as the sun sets, although the tight pressure
gradient in place ahead of the front will keep breezy conditions in
place through later this evening. Expect cloud cover to rapidly
thicken and lower as the cold front approaches the area. An area of
lake enhanced rain will develop northeast of Lake Erie initially out
ahead of the cold front early this evening. The main area of rain
associated with the cold front itself will then merge with lake
enhanced band later this evening and plow from west to east across
the area overnight. As colder air wraps in rapidly on the backside
of the front, rain will change to snow with some minor accumulations
possible, ranging from a coating across the lower terrain, to around
an inch across the higher terrain. Low temperatures by morning will
bottom out in the upper 20s to around freezing across the region.

As colder air continues to pour across the region Wednesday, there
will be a limited lake response early in the day southeast of Lake
Erie, with just some light nuisance snow showers found across the
Western Southern Tier through the first half of the morning, before
much drier air moves in and inversion heights crash. Off Lake
Ontario there will be a little more activity east and southeast of
the lake with a bit more available synoptic moisture aiding with
lake enhancement/upslope. That said, this will generally be light
with another inch possible by early afternoon across the higher
terrain before this activity falls totally apart by Wednesday
afternoon as drier air and crashing inversion heights move into this
region as well. Winds will be noticeably lighter and out of the
northwest. Temperatures will be some 15-20F degrees colder than
yesterday, with highs generally in the lower to mid 30s.


The axis of a large, elongated surface based ridge will cross our
forecast area Wednesday night. While this will provide our region
with fair dry weather, cirrus streaming in from the west should
temper an otherwise favorable environment for radiational cooling.
The only thing we will have to watch for will be some very minor
lake snow showers that could develop northeast of the lakes towards
daybreak. A low cap of about 5k ft will severely limit any real
development though.

On Thanksgiving, the surface high will push east and weak mid-level
wave will cross the forecast area ahead of a stronger shortwave
moving over the upper Great Lakes. The combination of weak synoptic
lift with height falls aloft, will help spawn a few nuisance lake
enhanced showers northeast of Lakes Erie and Ontario. This will
start as either lake cloud cover or light precipitation Thursday
morning, but pick up some into Thursday afternoon and evening as the
better moisture and ascent arrives ahead of the shortwave. This will
result in some light rain and snow showers near Niagara Falls and
Buffalo, as well as near Watertown. As the shortwave passes, the
flow will turn more westerly, with some upslope and lake enhancement
over the Tug Hill. Less than an inch, if any, snow accumulation is
expected NE of Lake Erie, meanwhile, 1 to 3 inches of snow are
possible east of Lake Ontario, with the highest amounts over the Tug

As a flat shortwave ridge pushes across the Lower Great Lakes on
Friday, it will force the remnants of the stalled frontal boundary
back into Canada. This will also establish another brief warm up,
as a strengthening south-southwest flow ahead of an approaching
Pacific based cold front will boost H85 temps from the minus single
digits c to around 5c. Our temperatures will respond by climbing
into the 40s in most areas by Friday afternoon. The moderating
temperatures will be accompanied by at least partial sunshine.

Conditions will then deteriorate somewhat Friday night, as a
vigorous shortwave will push the first of two frontal boundaries
across our region. The first one, a Pacific based cold front,
will push through our region late Friday night. This feature should
generate some mixed rain and snow showers, mainly over the western
counties after midnight.

Saturday will be somewhat unsettled across our region, but it will
be far from a wash out. In fact, there may be more hours of pcpn
free weather than those with mixed rain and snow showers. The
Pacific based cool front will pass to our east during the morning,
and this should leave a lull in the synoptic forcing for the
afternoon. As the cold air behind the first front deepens though,
any residual rain showers should mix with/change to wet snow showers.


A trailing blast of arctic air behind Saturday`s cold front
continues to look like it should support a period of decent upslope
and lake effect snow showers. However, 21/12z guidance hints that
the duration may be a bit shorter than previous model runs have
indicated. Both GFS and EC are now not as cold (850mb temps down to
-10C verses -15C in previous runs) and both bring in drier air much
quicker (Sunday afternoon) than previous model runs. Even so, there
is still low-medium confidence for at least a 12-18 hour period of
significant lake effect and upslope snows as the airmass briefly
turns colder behind the front. Northwest flow will continue to focus
the highest POPs southeast of the lakes which could still impact
late night/early morning travelers headed home from Thanksgiving.
Have cut back POPs Sunday afternoon owing to the fast arrival of the
drier air and expanding surface ridge building in from the Ohio
Valley. Still way too early for any snow totals but this potential
event will continue to be watch closely.

Sunday night and Monday are likely to then be mainly dry with the
surface ridge expanded across much of western and central NY. The
12z GEM, however shows a weak surface wave dropping south across
Lake Ontario so have left slight/low chance POPs to partially cover
this outside chance Sunday night with a low chance for some weak
lake effect trailing this Monday but again should be mainly dry
based on the 12z runs. Tuesday seems also likely to be dry but if
models are too slow on handling the the next front the GFS brings
in Tuesday night then there could be some precip arriving earlier.
Slight chance POPs have been forecast for Tuesday with possible
timing issues as is common 7 days out.

Surface temperatures will run well below normal with the forecasted
blast of arctic air. Highs Sunday and likely also Monday will only
run within a few degrees of freezing. Tuesday we may see a return
of 40s with a potential for warm advection ahead of a storm system
moving into the western Great Lakes.


Widespread VFR conditions will continue right through early this
evening, with just some patchy thin/high cirrus for the most part.
Stronger winds will subside this evening as we lose daytime mixing
toward sunset, however a tight pressure gradient out ahead of the
approaching cold front will keep breezy conditions in place through
this evening.

Later this evening, rain showers associated with the cold front will
press from west to east across the area, eventually changing over to
snow showers late tonight as colder air wraps in on the back side of
the front. Mainly MVFR cigs/vsbys are expected across the lower
terrain, with some IFR conditions possible across the higher terrain

Wednesday, expect mainly MVFR conditions across the western New York
lake plains in the morning, with mainly MVFR (localized IFR)
ceilings across the higher terrain of the Western Southern Tier.
Expect MVFR (localized IFR) ceilings to hang in through at least the
end of the TAF period across the Eastern Lake Ontario region with
just some nuisance snow showers slowly diminishing by early


Wednesday...Mainly VFR across western New York lake plains, with
MVFR/localized IFR in scattered snow showers southeast of the lakes.
Thursday...Mainly VFR, with a chance of localized MVFR in scattered
snow showers north-northeast of the lakes.
Thursday night...A chance of snow showers/MVFR across the North
Country, otherwise mainly VFR.
Friday...Mainly VFR.
Saturday...VFR/MVFR with rain showers likely.
Sunday...Mainly MVFR, with chance of localized IFR in lake effect
snow showers southeast of the lakes.


Brisk southwesterlies will continue into early this evening ahead of
an approaching cold front, before veering to northwesterly tonight
following the frontal passage, then slowly diminishing from west to
east during Wednesday as high pressure builds across the region.
This will keep Small Craft Advisories in place as outlined below.

A bit further down the road, it appears like that another round of
Small Craft Advisories will be needed again for Thursday afternoon
and night as the pressure gradient tightens around a cold front
passing by just to our north.


NY...Wind Advisory until 5 PM EST this afternoon for NYZ001-010.
MARINE...Small Craft Advisory until 7 PM EST this evening for LEZ020.
         Small Craft Advisory until 1 PM EST Wednesday for LEZ040-
         Small Craft Advisory until 7 PM EST this evening for
         Small Craft Advisory until 7 PM EST Wednesday for



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