Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 231330

Area Forecast Discussion
National Weather Service Buffalo NY
930 AM EDT Fri Jun 23 2017

Showers and thunderstorms associated with a warm front will continue
to impact the North Country this morning. Showers and thunderstorms
will become more widespread by midday as tropical moisture surges
across the area in advance of an approaching cold front. Some of
these storms will feature brief heavy downpours. The cold front will
cross the area late tonight into Saturday, with cooler temperatures
and occasional showers prevailing through the latter half of the
weekend into the first of next week, as broad upper level troughing
digs across the Great Lakes.


Warm frontal rain that impacted the eastern Lake Ontario region
overnight is moving northeast across the Saint Lawrence Valley this
morning, with a few additional showers lingering across the eastern
Lake Ontario region. The general trend in this area will be for
decreasing rain over the next few hours, before additional showers
arrive later today. Farther west, An area of showers and a few
isolated thunderstorms is moving into Western NY, and will continue
to overspread areas west of the Genesee River through late morning
and midday as a subtle mid level shortwave and associated small low
level jet maxima approach the region.

The soupy airmass in place across the region will provide fertile
ground for the formation of additional showers and thunderstorms
going into the afternoon, with the arrival of a pre-frontal trough
this afternoon serving as the impetus for more widespread
development across much of the forecast area. The most widespread
convection by mid to late afternoon should focus from the western
Southern Tier into the Finger Lakes, to the southern Tug Hill region
as a stable lake shadow tries to develop northeast of Lake Erie
later today.

Given PWATs that will be exceeding the 2.0 inch mark, this
convective activity will likely bring localized heavy rain.
Regarding the severe threat, while shear profiles are fairly decent,
with 0-6km shear of around 35kts, instability will be marginal at
best, given abundant cloud cover and showers that will prevail for
much of the day, and skinny CAPE profiles. Given these negatives, do
not expect much of a severe risk this afternoon or evening.

Precipitation will gradually shift east tonight, as the pre-frontal
trough moves through. Regarding temperatures, abundant cloud cover
should keep temperatures limited to the mid to upper 70s across most
locales, though it will certainly be muggy across the area, with
dewpoints approaching the 70 degree mark. A few areas from the
Genesee Valley to Central NY will rise into the lower 80s by late
morning and midday prior to the arrival of showers. Cooler and
somewhat drier air will be slow to filter into the region, with lows
tonight only bottoming out in the low to mid 60s.


A broad 500mb longwave trough is forecast to pivot over the Great
Lakes this weekend in the wake of this afternoons cold front. The
depth of this trough is a bit anomalous for mid-late June showing up
as 2-3 SD below normal. Cool air aloft within this trough (850mb T 6-
8C) will steepen daytime low and mid level lapse rates supporting
gusty afternoon winds and cumulus development inland from lake
shadows. Latest 00z mesoscale guidance continues to indicate that a
lake breeze convergence boundary will set up somewhere between the
NY Thruway and Lake Ontario from Niagara Falls east across Rochester
to about Newark. This will be the focus for a chance of showers and
weak thunderstorms Saturday with a another chance area across the
Saint Lawrence River Valley as a narrow stream of vorticity crosses
the Lower Great Lakes. Cold air advection will support more much
more comfortable temperatures with highs in the low to mid 70s in
most locations along with lowering humidity. Any lake breeze
convection Saturday evening will end shortly after sunset leaving
partial clearing overnight. Expect the drier airmass to allow cool
low temperatures dipping into the mid 50s on the lake plains and
around 50 across the interior Southern Tier and Lewis County.

On Sunday, GFS/EC models show a mid level shortwave trough and
embedded vort max shifting through the base of the Great Lakes
trough and crossing western and central NY. Synoptic scale lift from
this shortwave and low-level forcing from a surface trough will
contribute to chances of showers and thunderstorms. The highest
probability is during the afternoon and evening closer to the
passage of the surface trough but cant rule out a lower end chance
even during the morning hours. Otherwise a diurnal cu field is again
expected along with gusty southwest winds ahead of the trough where
deep mixing should allow 25-30kt winds to mix to the surface.
Temperatures will again run a little below normal with highs
expected to top out around 70. Dewpoints only around 50 will yield
very comfortable humidity levels. In the wake of Sundays surface
trough, 850mb temps become cool enough to bring a potenial for some
summertime lake effect east of the lakes. 850mb temps bottoming out
around +6C with lake temps +21C should support at least lake effect
clouds and even the risk for a chance of showers. Overnight lows
will again dip into the low-mid 50s and even upper 40s in the cooler
interior western Southern Tier and Tug Hill.


The longwave trough will remain over the Great Lakes Monday and
Tuesday before medium range model consensus indicates trough axis
will quickly pivot across western and central NY through Tuesday.
This pattern will continue to support cool temperatures with
comfortable humidly. Daytime highs topping out in the 60s to
perhaps about the 70 degree mark. The cool air aloft within the
trough (again 850mb T around 6C) will support mainly a diurnal
threat for showers and possibly some thunderstorms. Lake effect
clouds/showers can also continue to be possible with steep low level
lapse rates. Taller shower/storm cells in this cool environment may
bring some small hail with a sub-10kft freezing level.

Surface high pressure centered over the southern Appalachians will
ridge north across the region Wednesday allowing for dry weather
with mainly clear to partly cloudy skies and temps still a degree or
two below normal. By Thursday, the high is forecast to shift off the
Mid-Atlantic coast with southerly winds bringing a return of
increasing moisture and warmth across western and central NY. Highs
look to push into the upper 70s to around 80. This may also result
in a few showers and thunderstorms later in the day Thursday.


Rain and a few isolated thunderstorms across the North Country will
diminish through midday as the warm front moves northeast of the
area. Farther west, steadier showers and isolated thunderstorms will
increase across Western NY through midday as a weak mid level
disturbance moves into the area. Some of the heavier showers will
produce areas of MVFR and isolated IFR today, with mainly VFR
between the areas of rain. The one exception will be across the
higher terrain of the Southern Tier, where abundant low level
moisture will produce areas of IFR CIGS today.

Another round of -shra/tsra will move across the forecast area this
evening, ahead of an approaching cold front. Given abundant
moisture, expect MVFR conditions within showers, with brief periods
of IFR. The rain will end from northwest to southeast tonight with
the passage of the cold front.


Saturday through Tuesday...Mainly VFR with a chance of mainly
afternoon showers and isolated thunderstorms.
Wednesday...Mainly VFR with a chance of showers.


South-southwesterly winds will freshen today ahead of an approaching
low pressure system. While this will bring choppy conditions to the
northeastern ends of the lakes this afternoon, waves should remain
below advisory levels. Showers and thunderstorms will periodically
affect the lakes through the next 24 hours however, as a warm front
lifts across the region, with a trough crossing the lakes this
afternoon into tonight ahead of an approaching cold front.

The aforementioned cold front will cross the lakes late tonight into
Saturday. Westerly winds behind the front will be relatively modest,
however, and sub-SCA conditions are expected through Saturday night.
The passage of a prominent upper level disturbance will bring more
substantial winds/waves to the lakes Sunday however, with small
craft advisories likely being needed. The conditions will be short-
lived however, as high pressure begins to nose in across the lower
Lakes Monday.





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