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Area forecast discussion
National Weather Service Buffalo New York
1018 PM EDT sun Oct 22 2017

Synopsis...
warm and dry weather will continue through Monday afternoon. A cold
front will then slowly cross the area Monday night and produce
periods of rain. A few scattered showers and lake effect rain
showers will then continue through much of the week as temperatures
turn progressively cooler.

&&

Near term /through Monday/...
strong high pressure will remain anchored off the New England coast
overnight. Subsidence and dry air will continue to support fair
weather with additional cirrus level cloudiness crossing the area.
Model point soundings continue to Show Low level moisture increasing
across central PA overnight, and some of this may move northward
into interior sections of the southern tier late tonight and Monday
morning. Otherwise increasing southerly flow and mixing will keep
temperatures very mild overnight, with lows in the upper 50s to
lower 60s on the lake plains and low to mid 50s across the interior
southern tier and Lewis County. To put this into proper perspective...
such readings will actually range near or above normal high
temperatures for this time of year.

On Monday high pressure will drift farther east off the eastern
Seaboard. Subsidence and dry air will continue to hold moisture
ahead of the next system at Bay for the majority of the day, with
mid level clouds increasing from west to east. A strong mid level
trough will dig into the western Great Lakes through the course of
the day. DPVA from the first shortwave will begin to reach western
New York by the end of the day along with deeper moisture. This may allow
a few showers to develop at the far western end of the state by late
afternoon. Temperatures will be well above normal again, with highs
in the low to mid 70s on the lake plains aided by south-southwest downslope
flow, while higher terrain remains in the mid to upper 60s.

&&

Short term /Monday night through Thursday night/...
time to 'pay the piper'. After a prolonged stretch of absolutely
gorgeous weather...we can expect a significant shift in the
overlying weather pattern across the country. This change will not
include a polarity shift in temperature anomalies over the nation
but will also include a much more dynamic and progressive
pattern...one that will involve three distinct cold shots. We will
only deal with the first of these colder intrusions in this short
term period.

A phasing of a split flow will take place as we open this period
late Monday and Monday night...as a robust shortwave within the
northern branch will dive south across the plains. This will carve
out a full latitude longwave trough that will capture a southern
stream storm system already found over the upper Ohio Valley. As the
mature southern stream system ejects to the north Monday night...it
will push a surface occlusion across our forecast area. Strong
frontogentic forcing associated with this boundary will be
accompanied by some lift under the right rear entrance region of a
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. This will produce a 3-5 hour swath of fairly steady rain...
some of which could be heavy at times...especially if there is any
embedded convection. The precipitation will be found within a deep
southerly flow...one that will allow for some training while also
retarding the eastward progression of the main swath of rain. This
will encourage a soaking rainfall across the southern tier...but the
resulting downsloping will 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 while the steady pcpn will be in the process of working
across the north country.

The other potential issue for Monday night will be the risk for some
strong gusty winds. A 50-60 kt low level jet will precede the sfc
occlusion and likely will precede the steadier rain. A portion of
these strong winds could be mixed to the sfc...with the greatest
risk area being found along the Lake Erie shoreline where
downsloping could come into play. Am not particularly impressed with
the strength of the forecast inversion and direction of the flow at
inversion level to do anything more than to continue the mention of
strong gusts in the severe weather potential statement product.

On 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 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 across the north country.
Interestingly enough...these readings will still be above normal
late October values.

As a strong h25 jet digs into an already deep longwave trough over
the lower Mississippi Valley Tuesday night...a narrow shortwave
ridge downstream will push across our forecast area. While this will
keep the bulk of our region precipitation free...continued cold
advection within a south to southwest flow could direct some
nuisance lake effect rain showers over parts of the Niagara
Frontier. Will continue to use low chc pops for that area only.

As the aforementioned jet rounds the base of the longwave trough on
Wednesday...it will become negatively tilted while making its way
across the Ohio Valley and parts of the lower Great Lakes.
Negligible height falls with the trough and a lack of any strong jet
dynamics will limit the potential for showers...although continued
lake effect showers should still be in the place east-northeast of
Lake Erie.

On Thursday...the axis of the negatively tilted trough will push
away from our region via the St Lawrence Valley. While the -2c air
at 850 mb will keep some very Minor Lake effect rain showers in place
east of both lakes...rising heights and increased subsidence will
support fair dry weather elsewhere.

&&

Long term /Friday through Sunday/...
the longwave trough over the center of the country will 'reload'
Friday...as the next vigorous mid level disturbance will dive
southwards across the northern plains. This will re-establish the
longwave trough while the next (second) shot of colder air will pour
southwards into the lower 48.

The leading edge of this next cold shot will follow a cold front
that will extend south form a sub-1000mb low over the upper Great
Lakes. This front will make its way across the lower Great Lakes on
Saturday...then is forecast to stall over our forecast area as it
will become parallel to the mean h925-70 flow. Following a nice day
of weather on Friday...this scenario will favor scattered showers
for both days of the weekend.

Looking further ahead towards the following week...a third shot of
cold air will charge southwards...and this one will have the best
chance of supporting our first flakes of the season across our
forecast area.

&&

Aviation /02z Monday through Friday/...
overnight...VFR conditions will continue to largely predominate under
additional cirrus-level cloudiness. The one possible exception to
this will be across interior sections of the southern tier...where
model guidance continues to show an increase in low level moisture
late tonight and Monday morning. Should this actually come to
fruition...it could result in some IFR/MVFR ceilings developing at
kole/kelz late tonight/early Monday...though these would likely
remain away from the kjhw terminal.

On Monday VFR weather will continue to prevail in most areas with
mid level clouds increasing from west to east during the afternoon
ahead of any approaching cold front...which could also bring a few
scattered light showers to kjhw and vicinity late in the day. Some
MVFR ceilings may linger across the higher terrain of the interior
southern tier, western Finger Lakes, and eventually the Tug Hill.
Some low level wind shear will also become possible at kjhw late in the day as a
45-50 knot southerly low level jet develops into the region out
ahead of the cold front.

Outlook...
Monday night and Tuesday...MVFR/IFR with occasional rain.
Low level wind shear possible.
Wednesday and Thursday...areas of MVFR with a chance of showers.
Friday...VFR.

&&

Marine...
modest southeasterlies late this evening will veer to southerly
and increase to moderately brisk levels overnight and Monday out
in advance of an approaching cold front...though given the offshore
direction and stable temperature profiles...conditions across the
New York waters of both lakes should remain below advisory levels.

Monday night and Tuesday the cold front will cross the region from
west to east...with periods of potentially advisory-worthy conditions
possible at the east ends of both lakes Erie and Ontario.

&&

Buf watches/warnings/advisories...
New York...none.
Marine...none.

&&

$$

Synopsis...Hitchcock

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