Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 220803

Area Forecast Discussion
National Weather Service Buffalo NY
403 AM EDT Tue Aug 22 2017

A strong cold front will cross the region later today and tonight
with several bands of strong to severe thunderstorms crossing the
region. Some storms may contain damaging winds and torrential
rainfall. Rain will end from west to east this evening following the
cold front. Much cooler weather will then build into the region for
Wednesday through next weekend with a few spotty showers possible
Wednesday and Thursday. High pressure will then bring dry weather
Friday through the weekend.


Radar imagery showing a few scattered showers and isolated
thunderstorms from the Niagara Frontier into the Genesee Valley and
western Finger Lakes early this morning. These are forming within a
warm advection regime along and ahead of a weak warm frontal
segment, which will move northeast across the area this morning. The
scattered showers and isolated thunderstorms will reach the eastern
Lake Ontario region by mid morning before exiting the area. Behind
this, much of the morning should be rain free.

A very busy and complex scenario will then unfold this afternoon and
evening across our region. Here are the details...

Synoptic Pattern...

A mid level trough will sharpen and dig across Ontario and Quebec
late today and tonight, with mid level height falls and DPVA
overspreading our region this afternoon and evening, providing
strong large scale ascent. A surface low will deepen to the north of
the lakes, with a strengthening cold front surging southeast across
the Great Lakes late today and tonight. As is often the case, most
of our thunderstorms will develop along a pre-frontal trough as it
interacts with our local lake breeze boundaries, well ahead of the
actual cold front.

Expect storms to initiate along the southern edge of the Lake Erie
lake breeze from NW PA into SW NY around midday. A cluster of
showers and storms may also develop over or near Lake Erie and
spread into the Niagara Frontier around midday in response to the
increasing large scale forcing and prior to the stable lake shadow
becoming established. The lake breeze storms will then rapidly
expand and move northeastward across the rest of the area, reaching
the Genesee Valley during the early afternoon before moving to the
eastern Lake Ontario region by mid to late afternoon.

High resolution model guidance suggests a second round of storms may
develop across NW PA and SW NY during the mid to late afternoon,
spreading through the western Southern Tier and into the western
Finger Lakes. Meanwhile, strong southwest winds will force a stable
lake shadow to develop northeast of Lake Erie, likely bringing a
period of dry weather to the Niagara Frontier following the midday

This evening the actual cold front will sweep east across the region
with a few more scattered showers and thunderstorms, which will end
through the late evening and early overnight as the cold front
clears east of the area. A strong push of subsidence and drying
behind the cold front will bring clearing overnight.

Severe Weather Potential...

Moderate instability will develop inland from the lakes by early
afternoon as surface temperatures climb into the low to mid 80s and
surface dewpoints approach 70. Warm mid level temperatures will keep
MLCAPE tempered somewhat, peaking in the 1000-1200J/kg range. This
will be offset by the strengthening large scale forcing, which will
aid in deep layer ascent and deep convective overturning. The low
and mid level wind field will become quite strong in the warm sector
this afternoon, with 0-6KM effective bulk shear increasing to 40-45
knots. The combination of instability and shear become quite
favorable for organized severe weather by early afternoon.

Parameter phase/space and strong linear synoptic forcing suggest the
initial storms will quickly grow upscale into bowing line segments,
with damaging winds being the primary risk. These bowing segments
will move very rapidly, with forward speed likely to exceed 50 mph
once they become organized. Large hail risk will be fairly low given
the weaker mid level lapse rates, but a few of the initial discrete
cores may produce isolated marginal large hail. Tornado potential is
fairly low but non-zero, with low level flow remaining locally
backed ahead of the line especially across Central and Eastern NY.
This could pose some risk of a brief QLCS type spin-up, especially
where any discrete cells merge with the advancing line of storms.

Heavy Rainfall Potential...

PWAT rises to around 2 inches ahead of the cold front and warm cloud
depth is very deep, suggesting that storms will be efficient
torrential rain producers. Storm motion will be very fast and linear
however, which should prevent any organized risk of flash flooding.

Synoptic Scale Winds...

Outside of the severe weather potential, southwest winds will become
quite strong today, especially in the normally windy spots northeast
of Lake Erie. Following the pre-frontal trough and initial
convective line, winds of 40-45 knots just off the deck will develop
over and northeast of Lake Erie. Lapse rates should steepen
sufficiently to allow some of this to mix down during the afternoon
and early evening, with typical funnelling down Lake Erie aiding in
stronger winds. This will likely produce some gusts of 45-50 mph
across the Niagara Frontier. These winds will rapidly drop off this
evening with the loss of daytime mixing and passage of the cold


Wednesday and Thursday a large upper-level low will meander slowly
eastward across Quebec, while a couple of shortwave embedded in the
cyclonic flow will move across the region.

The first shortwave will dive southward out of Ontario province and
across the region Wednesday into Wednesday night. Marginal moisture
associated with this wave will combine with some added lake moisture
as the attendant surface front crosses the lower Great Lakes. This
will produce scattered showers across the region, which will be
locally enhanced by the northerly upslope flow into the Tug Hill,
and higher terrain from the western Southern Tier to the Finger
Lakes with the frontal passage. Outside of the scattered showers,
broken stratocumulus cloud deck will bring only spotty sunshine to
the region. 850 mb temperatures Wednesday will remain around +9 to
+10C, which will keep daytime highs in the low to mid 70s when
combined with the cloud cover. Showers will dissipate Wednesday
evening in the wake of the trough passage and with the loss of
heating. Overnight lows will bottom out in the upper 40s to mid 50s.

Cooler air will filter into the region Wednesday night and Thursday
in the wake of this frontal passage, with 850 mb temperatures
falling to around +5 to +6C for Thursday. This cooler air will
elicit a lake response, that will be mitigated by fairly dry air
and a stout inversion to start the day. Thus, this lake response
will mainly manifest as low cloudiness, which will help to keep
daytime highs limited to the mid to upper 60s. A sharper trough axis
will then move across Lake Erie Thursday late afternoon/evening.
Expect enough low level convergence with the attendant surface
boundary, along with some added lake moisture, to produce some
scattered showers and even perhaps thunderstorms as the mid-level
lapse rates steepen up. These will most likely be focused south of
Lake Ontario across the Niagara Frontier and Lake Erie late in the
day. As this trough moves over Lake Erie by late afternoon, any line
of showers and/or thunderstorms moving over the warmer lake waters
would be prone to produce waterspouts, and have included the chance
in the forecast. Showers and any thunderstorms will dissipate with
the loss of heating with clearing skies. Overnight lows will reach
the mid 40s to low 50s with the cooler airmass in place.


Friday through the weekend, the aforementioned upper level low will
make its exit eastward while a sprawling Canadian high pressure
system builds slowly eastward across the Great Lakes region. This
will bring a stretch of cooler and drier air more reminiscent of
fall, but with fairly light winds. There will be some day-to-day
warming as the airmass modifies with high temperatures slowly
building from the upper 60s Friday to the mid 70s by Monday. Nights
will be cool and crisp, with overnight lows ranging from the low
40s in inland valleys to the low 50s along the lake plains. The only
fly-in-ointment for the weekend will be a backdoor cool front
passage on Sunday. While the change in airmass will be fairly
minimal, there maybe just enough forcing and moisture with the front
to spark a few stray showers Sunday, mainly in the North Country.


VFR conditions will prevail through the early morning hours as a
warm front brings mid level clouds and a few scattered showers and
isolated thunderstorms, moving from southwest to northeast across
the region.

The showers along the warm front will exit the eastern Lake Ontario
region by mid morning, leaving VFR to prevail through midday with
areas of mid level clouds crossing the area. Expect one or more
lines of strong to severe thunderstorms to develop across Western NY
around midday or early afternoon, then spread very rapidly northeast
across the rest of the area through late afternoon. These storms
will produce a brief period of heavy rain, gusty winds, and local
IFR as they sweep across the area. A few storms may be severe with
surface wind gusts exceeding 50 knots.

In the wake of the line of storms, a few more widely scattered
showers and thunderstorms will develop from late afternoon through
evening, with VFR prevailing most of the time. These will end
overnight from west to east as a cold front crosses the area.

It will become quite windy today even away from the thunderstorms,
especially northeast of Lake Erie where gusts may peak at 35-40
knots during the afternoon and early evening near KBUF and KIAG.

Wednesday and Thursday...VFR/MVFR with a chance of showers.
Friday and Saturday...VFR.


SSW winds will begin to increase this morning, then become southwest
and become strong this afternoon on both lakes Erie and Ontario,
bringing solid Small Craft Advisory conditions. One or more lines of
strong to severe thunderstorms will likely cross the region with
locally higher winds and waves.

A cold front will sweep east across the region this evening with
winds becoming northwest and diminishing on Lake Erie, with winds
and waves likely dropping below Small Craft Advisory criteria
overnight. Winds on Lake Erie will become west and likely increase
again Wednesday with another round of Small Craft Advisory
conditions. Winds will remain westerly and much stronger on Lake
Ontario overnight through Wednesday, with solid Small Craft Advisory
conditions continuing through Wednesday night before winds become
northwest and diminish by Thursday morning.



Strong southwest winds will develop over Lake Ontario late this
afternoon and early evening, then become westerly tonight
through Wednesday. The increase in winds and wave action will
bring and increase in shoreline erosion and lakeshore flooding
to the east half of Lake Ontario when combined with the already
high lake levels. The static lake levels have dropped since late
spring and early summer, so the impacts of this event are not
likely to be as significant as earlier in the season, but still
enough to warrant a Lakeshore Flood Warning.

In addition, the line of strong to severe storms expected to
cross Lake Ontario this afternoon may produce a seiche on the
lake, with a brief rise in water levels at the east end and a
sudden onset of stronger winds and higher wave action.


NY...Lakeshore Flood Warning from 5 PM this afternoon to 11 PM EDT
     Wednesday for NYZ007.
     Beach Hazards Statement from this afternoon through this
     evening for NYZ007.
     Lakeshore Flood Warning from 10 PM this evening to 11 PM EDT
     Wednesday for NYZ004>006.
     Beach Hazards Statement from 1 PM EDT this afternoon through
     this evening for NYZ010-019-085.
     Wind Advisory from 2 PM this afternoon to 8 PM EDT this
     evening for NYZ001-002-010-011.
MARINE...Small Craft Advisory from 10 AM this morning to 10 PM EDT this
         evening for LEZ020.
         Small Craft Advisory from 10 AM this morning to 2 AM EDT
         Wednesday for LEZ040-041.
         Small Craft Advisory from 10 AM this morning to 10 PM
         EDT this evening for LOZ030.
         Small Craft Advisory from 2 PM this afternoon to 6 AM EDT
         Thursday for LOZ042>045.
         Small Craft Advisory from 5 PM this afternoon to 11
        PM EDT this evening for SLZ022-024.



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