Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 200648 CCA
AFDBUF

Area Forecast Discussion...CORRECTED
National Weather Service Buffalo NY
148 AM EST Sat Jan 20 2018

.SYNOPSIS...
Breezy conditions will persist through Saturday as our region
remains sandwiched between a storm system passing across
southern Canada, and a broad area of high pressure across the
Southeast. This storm system may bring a few light snow showers
or pockets of freezing drizzle to the North Country Saturday and
Saturday night. There will be additional chances for light
precipitation later Sunday and Sunday night as a warm front
lifts across the region...a warm front that will also bring
surface temperatures into the 40s for Monday.

&&

.NEAR TERM /THROUGH TODAY/...
A tight pressure gradient will persist across the eastern Great
Lakes tonight. Sustained southwest winds 10-20 mph will keep the
region from decoupling and therefore temperatures will stay mild
overnight. Low temperatures will range in the upper 20s in the North
Country to the low 30s across Western NY.

Low pressure will move across James Bay this evening. Light
snow will move to the north of Lake Ontario tonight and may
brush the St. Lawrence River and Jefferson and Lewis counties.
Little to no accumulation is expected. Further south...clouds
will increase as low pressure moves by to the north and warm air
advection continues aloft.

Warming in the lower levels will form an inversion...such that a 60
knot LLJ will remain above the boundary layer. However, an
increasing pressure gradient between the two surface systems will
maintain breezy conditions tonight and tomorrow...with occasional
gusts 40 - 45 mph across along the Lake Erie shoreline, the northern
Niagara Frontier and western Monroe County, and 25 to 40 mph
elsewhere.

Tomorrow will mainly be dry, with clouds thickening across the
region as low level moisture increases. Westerly flow east of Lake
Ontario will bring an upslope component...such that this saturated
lower level may form patchy freezing drizzle or a flurry of snow.
Confidence remains low at this point in time.

Temperatures tonight will remain fairly steady with the southwest
wind flow. Highs tomorrow will near 40F for many across the region.

&&

.SHORT TERM /TONIGHT THROUGH TUESDAY NIGHT/...
Saturday night modest surface based ridging will maintain largely
dry weather across our area...though skies will be mostly cloudy
owing to plentiful low level moisture that will become increasingly
trapped beneath a subsidence inversion. Any precip chances will be
confined to the northeastern and southwestern ends of our area...
with a dissipating backdoor cold front and upslope flow potentially
leading to some patchy light snow showers or freezing drizzle across
the North Country and Saint Lawrence Valley...and the western
Southern Tier possibly seeing some very spotty drizzle/freezing
drizzle owing to additional upsloping there. Have thus maintained
slight chance to low-end chance PoPs in these areas...with the night
otherwise expected to be dry. Meanwhile...low temps will range from
the upper 20s to lower 30s.

On Sunday diurnal warming will help to eliminate any spotty leftover
freezing drizzle during the morning...with just some spotty drizzle
or very light rain sprinkles then possible south of Lake Ontario
during the afternoon owing to continued plentiful low level moisture...
and a warm front that will slowly be extending eastward into our
region from the Central Great Lakes. Otherwise...somewhat above
normal temperatures will continue...with high temperatures ranging
from the mid to upper 30s east of Lake Ontario to the lower 40s
elsewhere.

Sunday night and Monday low pressure over Kansas will track
northeastward to around the Wisconsin-Illinois border...with its
attendant warm front first extending further eastward into our
region Sunday night...before slowly lifting northward toward Lake
Ontario during the course of Monday. Slowly increasing warm air
advection/isentropic lift and deepening moisture out ahead of the
warm front will bring slowly increasing chances for some spotty
light precipitation Sunday night...followed by a better likelihood
for some precip on Monday as the warm front slowly lifts northward
across areas south of Lake Ontario. Projected model soundings and
statistical guidance both suggest that ptype during this timeframe
will initially feature a rain/snow/sleet mix Sunday night...with
this then going over to mostly rain during Monday as the atmospheric
column continues to warm.

During Monday...the various guidance packages continue to disagree
on how quickly/exactly how far the warm frontal boundary lifts northward
across areas south of Lake Ontario...with the 12Z/19 GFS/GEM remaining
on the faster/more aggressive side of the guidance envelope...while
the ECMWF remains a bit slower/more suppressed overall. Exactly how
quickly/how far north the front pushes will have big implications on
both precipitation coverage and temperatures south of Lake Ontario
Monday afternoon...with the faster solutions supporting the potential
for a relatively dry period with temperatures pushing into the 50s...
while the slower solutions keep things wetter and notably cooler. Given
the resultant forecast uncertainty/bust potential...for now have opted
to take a middle ground with the temperature forecast for Monday.

Monday night and Tuesday the surface low will track northeastward
across the central Great Lakes and into southern Quebec...and in the
process will finish pushing the warm front across our area Monday
night...followed by its trailing cold front on Tuesday. Plentiful
lift and moisture accompanying the warm and cold frontal passages
will translate into fairly high probabilities for widespread
precipitation during this period...and as such have continued to
push PoPs higher into the categorical range. With a mild airmass
in place...the majority of this will just fall as plain rain Monday
night into Tuesday morning...before mixing with and/or changing to
wet snow Tuesday afternoon as cold advection sets up following the
cold frontal passage. The one possible exception to this evolution
in ptype will be across the North Country and Saint Lawrence Valley...
where the initial lingering northeasterly flow/colder low level air
out ahead of the cold front could lead to a brief wintry mix Monday
night...before the warm front passes through and forces a changeover
to all rain by Tuesday morning.

As has been pointed out previously...the combination of the above
rainfall and increased snowmelt from higher temperatures and dewpoints
may eventually bring some flood concerns by later Monday and Monday
night. While this warm-up is not as dramatic as that of last weekend
and overall rain amounts are also likely to be lower given the fast
passage of the cold front...there may still be some potential for
ice jam flooding in the late Monday-Monday night-Tuesday time frame.

In the wake of the cold front...a westerly flow of colder air will
continue to deepen across the area Tuesday night. Coupled with a
trailing region of wraparound moisture behind the departing low...
this will bring some scattered snow showers to the area...with some
lake/orographically-driven snows also likely to set up east of Lake
Ontario as the cold air deepens. Meanwhile off Lake Erie the lake
response may be weaker owing to the presence of continued ice cover...
so have gone with somewhat lower PoPs there to account for this.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
A low amplitude...Pacific air dominated pattern over the Lower 48 at
the start of this period will experience significant amplification
as we head towards the end of the week. The impetus for this
evolving pattern will be a series of strong shortwaves that will
slam into the West Coast and lead to broad troughing over the
western half of the country. This in turn will promote a burgeoning
progressive ridge over the Ohio Valley and Great Lakes regions that
will eventually pump another period of very mild weather into our
region for next weekend (just outside of the scope of this
forecast). Breaking this down on a day to day basis...

We will begin this period with temperatures trending lower...as a
deep cyclone exiting across the Canadian maritimes Wednesday and
Wednesday night will circulate a fresh batch of relatively cold air
across the Lower Great Lakes. While the bulk of the synoptic
moisture from this storm will be stripped away from our region by
Wednesday morning...there will be enough low level moisture around
to support some minor accumulations of lake effect snow...especially
southeast of Lake Ontario. Have raised pops to likely for sites
southeast of both lakes for Wednesday...with high chc pops now in
place for the same areas Wednesday night. It will be partly to
mostly cloudy outside of these lake snows.

As ridging starts to build across the Upper Great Lakes on
Thursday...warm advection will begin over our region with
significant drying taking place between H925 and 850. This will
bring an end to the nuisance lake snows by early afternoon.

The axis of a large surface high will cross our forecast area
Thursday night and Friday...during which time steady warm advection
will push our H85 temperatures into positive territory. This will
set the stage for notably nicer weather across our region...as
temperatures should climb back into the 40s across the western
counties for Friday afternoon.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions will prevail across the bulk of the forecast area for
much of the forecast period, with mainly high clouds crossing the
region. The one exception will be at KART, where enhanced low-
level moisture associated with an upper level disturbance passing
to the north is expected to bring a period of MVFR cigs between
12-20Z.

Otherwise, the main story overnight will continue to be strong
southwesterly winds gusting 20-35kts, as stronger winds just off the
deck mix down to the surface. A strong inversion around 2kft will
shield the surface from the strongest winds associated with a 60kt
LLJ, however this will result in a pronounced area of LLWS near this
inversion, around 2kft. The core of the LLJ should move off to the
SE, with winds aloft, and thus the LLWS threat, diminishing after
12Z.

Increasing low-level moisture associated with an approaching warm
front will begin to filter into the western Southern Tier Saturday
night. As a result, MVFR cigs are expected to develop at KJHW late
in the TAF period.

Outlook...

Sunday through Sunday night...Widespread MVFR ceilings persisting
through Sunday. A chance of rain/snow late.
Monday and Monday night...MVFR/IFR with rain showers developing.
Tuesday...MVFR/IFR with rain showers changing to snow showers before
ending...then areas of lake effect snow also developing east of the
lakes Tuesday night.
Wednesday...VFR/MVFR with a chance of snow showers.
Thursday...Mainly VFR.

&&

.MARINE...
A moderate to strong pressure gradient between an area of low
pressure crossing Southern Canada and surface high pressure over the
Southeast will maintain breezy conditions on the Lakes, and the
Upper Niagara River. A small craft advisory will remain in effect
for the Lakes, as well as the Upper Niagara River. High pressure
will reach the Eastern Great Lakes region Sunday that will relax the
winds and allow for wave heights to diminish.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 10 PM EST this evening for LEZ040-
         041.
         Small Craft Advisory until 1 PM EST this afternoon for
         LEZ020.
         Small Craft Advisory until 4 AM EST Sunday for
         LOZ042>045.

&&

$$

SYNOPSIS...THOMAS
NEAR TERM...HSK/THOMAS
SHORT TERM...JJR
LONG TERM...RSH
AVIATION...WOOD
MARINE...THOMAS



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