Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 240910

Area Forecast Discussion
National Weather Service Buffalo NY
510 AM EDT Mon Oct 24 2016

Cold northwesterly flow will result in cloudy skies and below
average temperatures into Wednesday. A low pressure system tracking
out of the Midwest is then expected to cross the lower Great Lakes
Thursday, bringing widespread rainfall. Another round of rainfall
will be possible this weekend, as another low pressure system drops
across the region.


Cold cyclonic flow across the Great Lakes is generating plenty of
cloud cover downwind of the lakes this morning, as evidenced by
regional satellite imagery. Expect the cloudy and cool conditions to
persist through tonight, as the forecast area will remain positioned
between a slow-moving upper level low over eastern Quebec and
upstream ridging over the Great Plains, placing Western and North-
Central New York under cold northwesterly flow on the front side of
the ridge. With 850mb temps falling to around -5C by tonight,
equilibrium levels are expected to climb towards 10kft, suggesting a
potential for a few lake effect showers developing from this
afternoon into tonight...particularly across areas aligned with
Georgian Bay and Lake Huron on 315 degree flow. This would include
the Rochester metro and the Chautauqua Ridge.

Temperature-wise, the ongoing cold air advection across the forecast
area means that temperatures will climb little from present readings
today, with temperatures remaining in the upper 40s to lower 50s
throughout the day. The cold advection will be more noticeable
tonight, as readings fall into the 30s, with lower 30s across the
North County.


For the first 48 hours of this period...our weather will continue to
be dominated by persistent upper level troughing...the core of which
will only slowly drift from eastern Quebec Province to the Canadian
Maritimes. This feature will feed a continued northwesterly to northerly
flow of colder Canadian air across our region...which will result in
temperatures averaging solidly below normal right through midweek. More
specifically...expect daytime highs mostly in the lower to mid 40s both
Tuesday and Wednesday...while nighttime lows mostly ranging through the
30s Monday night will drop off into the upper 20s and lower 30s Tuesday
night...likely resulting in a killing freeze in most areas where the
growing season has not already ended.

Precipitation-wise...the persistent northwesterly to northerly flow
of Canadian air across our region will also lead to the potential for
some lake effect precipitation southeast and south of the lakes. This
stated...the combination of the shortening fetch...a fairly low (5-6 kft)
capping inversion...and less than impressive moisture below this level
should all help to keep any such activity fairly limited/scattered in
nature. As such...have kept PoPs confined to the 20-40 range...with the
highest values southeast of Lake Erie Monday night and early Tuesday...
where and when moisture will be a little bit more favorable. Ptype
with the lake effect precip will be predominantly rain showers by day...
mixing with or changing to snow each night/early morning as the boundary
layer cools. Any lake effect activity lingering into Tuesday night will
come to an end during the day on surface-based ridging
settles directly overhead along with increasing large-scale subsidence
and shear.

Wednesday night the axis of the surface ridge will grudgingly drift
east into New England...while a developing storm system over the Upper
Midwest slowly pushes into the western Great Lakes. The developing warm
air advection regime out ahead of this next system will initially drive
a west to east increase in cloud cover during the night...before leading
to the possibility of some showers across far western New York late.
While the bulk of these should be in the form of plain rain given the
strengthening warm air advection regime...interior sections of the
Southern Tier may well remain cold enough to support a brief mix at
the onset of the precipitation. Given the still somewhat-distant time
frame...have elected to keep this in the form of a general rain-snow
shower mix for now...though the degree of warming currently portrayed
aloft would ultimately be supportive of other precipitation types.

On Thursday...the surface low will continue to slowly deepen while
lifting northeastward into Southern Ontario...while pushing its
attendant warm front across areas south of Lake Ontario. Coupled
with height falls aloft and deepening synoptic-scale moisture...this
will lead to a likelihood of precipitation spreading across our
region from southwest to northeast...with the continued warm air
advection regime supporting rain showers as the predominant precip
type. This stated...high temperatures will remain a bit below average
thanks to thicker cloud cover and increasing precipitation coverage...
with readings generally remaining confined to between 45 and 50.


A progressive low amplitude flow will remain in place across the
country during this period. The center piece for this pattern will
be a 100kt H25 jet straddling the Canadian border....which will
effectively block any anomalously cold air to the north. This will
encourage temperatures across the Lower Great Lakes to average
within a couple degrees of normal.

As we open this period Thursday night...a compact area of low
pressure over Southern Ontario will start to push northeast away
from the region. In the process...this initial storm system will
start to transfer a bulk of its energy to a coastal low off New
England. We can anticipate rain showers as a result of this
scenario...particularly over the Eastern Lake Ontario region.

The negatively tilted H5 trough will deepen enough to generate a
stacked low in the vcnty of northern New England/Maine on Friday...
while weak sfc based ridging will extend from the Upper Great Lakes
to the Ohio Valley. This will place our region within a cyclonic
flow of moist chilly air so that scattered rain showers can be
expected. A minimal lake response should enhance the coverage and
possibly the intensity of these showers southeast of Lake Ontario.

The aforementioned weak ridging is forecast to push east across the
Lower Great Lakes Friday night and Saturday. This will enable our
weather to improve somewhat...but with low confidence in the
guidance and significant differences between the various
ensembles...will carry slight chc pops.

On Sunday...the weak ridging will exit across New England while the
next southern stream shortwave is forecast to approach from the Ohio
Valley. This would support the next round of precipitation...
although as mentioned...confidence will be lower than normal due to
significant discrepancies in the ensemble packages.


Persistent cold cyclonic northwesterly  flow across the Great Lakes
will continue to generate plenty of VFR cigs across the forecast
area through the TAF period, and a few lake effect rain showers will
be possible at times across the far western Southern Tier, as well
across areas south of Lake Ontario. Cigs may drop to MVFR in these
showers, but otherwise cigs will remain VFR elsewhere through the
TAF period.


Tuesday and Tuesday Night...Mainly VFR.
Thursday...MVFR/IFR with rain showers likely.
Friday...MVFR with a chance of lake effect showers.


After a brief relative lull overnight, northwesterly flow will
freshen across the lower Great Lakes today as cold advection
strengthens and the gradient tightens between low pressure drifting
into the Gulf of Saint Lawrence and high pressure over the central
Great Lakes. Small craft advisories remain in effect, as
northwesterly winds will freshen to 15 to 25 knots, building waves
along the south shores of the lakes.

The brisk northwesterly flow is expected to persist into Tuesday
evening, at which point a broad surface ridge extending from a high
centered over James Bay progresses across the lower Great Lakes,
bringing another short-lived respite from small-craft advisory
conditions. Expect easterly flow to develop Wednesday and freshen
into Thursday, as another low pressure system tracks out of the
Midwest and across the lower Great Lakes.

High pressure will briefly ridge across the lake on later Tuesday
but another fast moving storm system will arrive Wednesday night
into Thursday.


MARINE...Small Craft Advisory until 9 AM EDT Tuesday for LEZ040-041.
         Small Craft Advisory until 6 PM EDT Tuesday for
         Small Craft Advisory until 9 AM EDT Tuesday for LOZ045.



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