Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 212321

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

A cold front will cross the region tonight, with rain showers along
and ahead of the front mixing with and changing to snow before
diminishing early Wednesday morning across Western New York, and by
early afternoon across the eastern Lake Ontario region. 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 Wednesday into
Wednesday night. Expect some snow and rain showers to return for
Thanksgiving Day northeast of both lakes.


Cloud cover will rapidly thicken and lower from west to east this
evening as a cold front approaches the area. An area of lake
enhanced rain will develop northeast of Lake Erie initially out
ahead of the cold front around mid evening, after 8-9PM. The main
area of rain associated with the cold front itself will then merge
with lake enhanced band late this evening and plow from west to east
across the area overnight. The front will become somewhat anabatic
in nature overnight, with precip lingering behind the surface cold
front due to strengthening DPVA and upper level jet dynamics aligned
parallel to the front. This will allow the rain to change to wet
snow just prior to tapering off. No accumulation is expected across
lower elevations, with minor accumulations of less than 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
today, 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.


VFR conditions will continue for a few more hours through mid to
late evening before a cold front arrives. Rain will overspread the
area from west to east as the cold front moves across the region,
with the steadiest rain found east and northeast of Lakes Erie and
Ontario with some lake enhancement along the front. CIGS/VSBY will
deteriorate to MVFR after the rain develops. Colder air pouring into
the region will allow the rain to mix with, and change to wet snow
before ending, especially across higher terrain. Expect a few hours
of IFR CIGS and MVFR to IFR VSBY just behind the front late tonight
as precipitation continues and low levels become saturated beneath
the frontal inversion.

Wednesday morning the rain and wet snow associated with the cold
front will quickly end from west to east. Scattered lake effect snow
showers will continue through midday southeast of the lakes, but
these will be very light with just spotty/brief MVFR. CIGS will run
MVFR lower elevations and IFR higher terrain early, and improve to
VFR lower elevations and MVFR higher terrain by midday.


Thursday...Mainly VFR, with a chance of localized MVFR in scattered
snow and rain showers 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 this evening ahead of an
approaching cold front, before veering to northwesterly overnight
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.


MARINE...Small Craft Advisory until 1 PM EST Wednesday for LEZ040-041.
         Small Craft Advisory until 7 PM EST Wednesday for



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