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Area forecast discussion
National Weather Service Buffalo New York
1033 PM EDT Mon Oct 23 2017

Synopsis...
a robust cold front will plow across our region tonight
with strong gusty winds and a period of steady rain marking its
passage. Cooler air in its wake will then encourage temperatures to
finally return to autumn like levels by mid week.

&&

Near term /through Tuesday/...
after several days of salubrious weather...a very dynamic storm
system will track by to our west tonight and explosively deepen by
more than 20mb as it will make its way to the confluence of the
upper Great Lakes. The strengthening storm system will swing a robust
surface occlusion through our region in the process...and this will
result in a period of showers tonight along with some strong gusty
winds. Accordingly...wind advisories are in place for the western
southern tier...the higher terrain of The Finger lakes region...and
the north country for winds that could gust to near 50 mph. Looking
more closely at the forecast details...

A phasing of a split flow over the eastern half of the country will
take place tonight...as a robust shortwave within the northern branch
will dive south to the Ohio Valley. This will carve out a full latitude
longwave trough that will help to eject a storm system already found
over that region. As the mature southern stream system ejects to the
north...it will push a strong surface occlusion across our forecast
area. Strong h925-70 frontogentic forcing associated with this boundary
will be accompanied by some lift under the right rear entrance region
of a cyclonic 110kt h25 jet over southern Ontario...providing plenty of
forcing within an airmass that will include precipitable water values in the vcnty of
1.5 inches. As a result showers will become more widespread across far
wny through midnight...before pushing east across the remainder of the
area overnight in concert with the eastward advance of the surface
occlusion. At this point...it still appears that most areas should see
a general 3-5 hours Worth of fairly persistent showers...some of which
could also feature some embedded convection and attendant moderate to
heavy rain.

The precipitation tonight will be found within a deep southerly
flow...one that will favor some training while also retarding the
eastward progression of the main swath of showers. While this will
encourage a soaking rainfall across the southern tier...the resulting
downsloping will tend to eat away at rainfall amounts across the lake
plains. As the occlusion pushes east during the course of the night...
a distinct dry slot will start to work across the region. This will
bring an end to the widespread rain over the far western counties
overnight...while the steady pcpn will be in the process of working
across the north country.

The other potential issue for tonight will be the risk for some
strong gusty winds. A 60-70 knot low level jet will precede the sfc
occlusion and will likely precede the steadier rain. A portion of
these strong winds should end up mixing to the surface...with the
greatest risk areas lying across our higher terrain...and also
possibly in regions of favored downslope flow along the Lake Erie
shoreline...north-northwest facing slopes of the Tug Hill...and in
the northwest to southeast oriented Black River valley. While the
orientation of the low level jet and strength of the corresponding
inversion are not exactly ideal for a classic downslope wind event...
there is still little doubt that it will be windy...with the highest
winds probably more likely to mix down across the higher elevations.
Either way...a Wind Advisory is in effect as outlined below to cover
both of these possible scenarios.

Tuesday...all of the forcing associated with the initial storm
system will exit across the St Lawrence Valley while a wealth of mid
level dry air will spread across the entirety of our forecast area.
This will bring an end to the remaining steady rain that will be
found over eastern sections and should greatly limit any possible
lake effect rain that tries to get going north/northeast of Lake Erie.
850 mb temps will drop to around 4c over Lake Erie by the end of the
day...so this should be just short of establishing any real response
off Lake Erie. The cold advection will lead to a cooler day though...
as Max temps will range from the upper 50s over parts of the southern
tier to the upper 60s east of the Genesee Valley. Interestingly enough...
these readings will still be above normal late October values.

&&

Short term /Tuesday night through Thursday/...
it's finally going to feel more like October than late Summer as
much cooler air will settle in across the lower Great Lakes during
this period.

A nearly stationary upper level trough over Michigan will be in
the process of slowly progressing east into the lower Great Lakes
Tuesday night. A pronounce dry slot ahead of main center of
circulation and trough axis will slowly work its way east across
western and north central New York Tuesday night. This will
provide virtually dry weather for a vast majority of the area
through the overnight hours. Temperatures wise, behind the frontal
boundary expect lows to drop back in the 40s across the vast
majority of the forecast area.

Wednesday, as the axis of the upper level trough continues to near
western New York cooler air aloft will continue to filter into the
region(850t dropping to -2c to -3c). This will help to generate
some lake effect response with rain showers developing off Lake
Erie. Initially, SW cyclonic flow of 220-230 will focus most lake
effect rain showers across Grand Island and northern portions of the
Niagara Frontier. These lake effect rain showers will then drop across
the Buffalo Metro area late in the day Wednesday and the south towns
shortly after.

Wednesday night, as the axis of the upper level trough swings
through west-southwest flow will focus the lake effect rain showers
south of Buffalo across western southern tier.

Off Lake Ontario, although brief a similar scenario will develop
with lake effect rain showers increasing later Wednesday night.
These lake effect rain showers will slowly move south across
Jefferson and Lewis County and affect a portion of Oswego County on
Thursday before diminishing.

Thursday, the upper level trough will exit the region and surface
high pressure will build into the lower Great Lakes. Any lingering
shower activity will diminish by the afternoon with clearing skies
but remaining on the cool side. Temperature wise, highs will top out
in the 50s for most locations which will be near to slightly below
normal for this time of year.

&&

Long term /Thursday night through Monday/...
Thursday night a narrow surface ridge will build across the eastern
Great Lakes with dry conditions. This high will drift to the East
Coast on Friday, but still remain near enough to provide subsidence
and support another dry day. Southerly return flow will increase
between the departing high and another strong trough moving into the
upper Midwest. This will allow temperatures to briefly return to
above normal, with highs back into the mid 60s.

Model guidance begins to diverge after that, with the 12z GFS
notably faster in bringing the next system into our region than the
12z European model (ecmwf). The faster GFS brings showers into the bulk of the region
Saturday as a cold front slowly crosses the area, while the European model (ecmwf)
would keep Saturday dry with the cold front not arriving until later
Saturday night. Given the uncertainty in timing, have just slowly
increased probability of precipitation through the chance range from west to east Saturday
and Saturday night.

Unsettled weather will likely persist Sunday through Monday as the
cold front moves slowly east of the area, and then a significant
wave of low pressure develops along the front and moves northward
along the eastern Seaboard. An associated upper level low will close
off and remain over the eastern Great Lakes, supporting a chance of
rain showers each day. Temperatures will turn significantly cooler
by Sunday and Monday, but it does not appear cold enough to allow
for any wet snow to mix in even across higher terrain.

&&

Aviation /03z Tuesday through Saturday/...
rapidly deepening low pressure will track by to the west of our
region tonight...stalling over the confluence of the upper Great
Lakes on Tuesday. This strong storm system will sweep its associated
cold front across our region in the process...and this will lead to
a significant deterioration in our weather.

Initial mainly VFR cigs as of 0230z will give way to several hours of
showers from west to east overnight...along with cigs/vsbys that will
lower to MVFR levels in many areas. This will especially be the case
across the higher terrain of the southern tier and The Finger lakes...
where some IFR cigs cannot be totally ruled out.

Following the passage of the cold front...the showers will then
sharply taper off from west to east late tonight and Tuesday
morning...with any lower cigs gradually improving to VFR levels.

Along with the showers and deteriorating cigs...winds of 60-70 knots
at 4-6k ft will present the risk for some low level wind shear out
ahead of the cold front tonight. Since a portion of these winds is
expected to mix to the surface...have largely left the mention of
wind shear out of the tafs.

Outlook...
Tuesday night...mainly VFR.
Wednesday...mainly VFR...except at kbuf/kiag where some showers will
be likely...and reductions to MVFR will be possible.
Thursday...mainly VFR with a chance of showers east of Lake Ontario.
Friday...VFR.
Saturday...VFR to MVFR with scattered showers.

&&

Marine...
a storm system over lower Michigan this evening will make its way to
the Soo late tonight and Tuesday while rapidly deepening to around
980mb. This will result in significant freshening of the southeast
winds which will gradually veer to the south-southwest later tonight
and Tuesday. While the highest waves will be in Canadian waters...
conditions will still warrant small craft advisories for most of the
nearshore zones.

Moderately strong southwesterlies will then be in place Tuesday
night through Wednesday...so the small craft advisories will remain
in place for an extended period on Lake Erie. In fact...winds may
actually reach their highest overall levels during Tuesday night...
when sustained winds to around 30 knots and some lower-end gale force
gusts will become possible.

&&

Buf watches/warnings/advisories...
New York...Wind Advisory until 8 am EDT Tuesday for nyz006>008.
Wind Advisory until 2 am EDT Tuesday for nyz012-019>021-085.
Wind Advisory until 5 am EDT Tuesday for nyz013-014.
Marine...Small Craft Advisory until 2 am EDT Thursday for lez040-041.
Small Craft Advisory until 5 am EDT Tuesday for lez020.
Small Craft Advisory until 11 am EDT Tuesday for
loz045.

&&

$$

Synopsis...rsh
near term...jjr/rsh

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