Special Marine Warning 77, NWS Buffalo, NY

Area Forecast Discussion
National Weather Service Buffalo NY
1050 AM EDT Fri Aug 4 2017

.SYNOPSIS...
A cold front will cross our region late today and early tonight...
generating some strong to severe thunderstorms in the process.
Notably cooler and less humid air will follow the front for the
weekend when afternoon temperatures will only be in the 60s and 70s.
The change to cooler weather will persist through at least the first
half of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Low pressure is near the SOO of the Upper Lakes this late morning
with a cold front draped southward across MI/western OH and down
towards western TN. Convection forming within a pre-frontal trough
across western Lake Erie, OH and down through KY this morning will
increase in coverage area this afternoon as activity passes across
NY.

For the morning update, while there are scattered showers and
thunderstorms across the region this morning, an area of storms
associated with cooler cloud tops over Lake Erie will lift
northeastward, reaching the Northern Niagara Frontier by noontime.
Heavy rain will be possible with this activity as it lifts farther
across Lake Ontario. Later in the afternoon as SBCAPE values
increase the threat for severe storms will increase.

Partly sunny skies will be found over much of the forecast area as
the cold front will slowly approach from the west. As a result of
the wealth of diurnal heating...MLCAPE values will easily reach at
least 1500 j/kg with higher values likely over the Finger Lakes
region. As the airmass reaches its greatest instability late this
afternoon...the aforementioned 40kt low level jet will cross the far
western counties. This will add enough shear to encourage organized
linear convection that will be blossoming along and just head of the
slow moving cold front. Bulk shear values are forecast to reach
close to 40 kts...with the greatest contribution coming from speed
shear. This will favor damaging wind gusts as the primary severe
weather threat...although the slow passage of the convection will
also present a risk for heavy rain. Additional lift for the
thunderstorms will come from a 100 kt H25 jet that will be found
near the Ontario-Quebec provincial border. This will place our
region under the jet entrance region...a favored area of large scale
ascent.

Deep southerly flow will retard the eastward progression of the
linear convection. While individual cell movement should be in the
vcnty of 30 kts...the line is only expected to propagate to the east
at about half that speed. That will encourage training cells within
an airmass that will have PWAT values of 1.5 to 1.75"...so am
expecting fairly widespread rainfall amounts of a half to one inch
in a short period of time. In terms of timing this convection...a
broken line of thunderstorms will move across the far western
counties after 21z...reaching the Eastern Lake Ontario region after
00z.

In the wake of the slow moving cold front...H85 temperatures will
plummet from the mid teens into the upper single digits. This will
establish enough lake induced instability for some lake effect rain
showers...mainly off lake Erie. The 240-250 flow will initially
direct the lake induced showers across much of the Niagara
Frontier...then as we approach daybreak...veering winds will settle
the rain south of Buffalo into portions of the Southern Tier. Lake
induced CAPE values are forecast to be in the vcnty of 1000 j/kg...
but am not confident that the cap will be high enough to allow for
electrification...so will refrain from adding lightning at this
point.

H85 temps in the mid teens C today will allow max temps to climb
into the mid-upper 80s F across the lake plains and in most of the
valleys...while slightly 'cooler' conditions will be found across
the hilltops. Tonight...there will be a fairly wide range of lows
across the region. Mins will range form the mid 50s across the
Southern Tier to the mid 60s east of Lake Ontario.

&&

.SHORT TERM /SATURDAY THROUGH MONDAY NIGHT/...
In the wake of Friday night's cold frontal passage...much cooler and
drier air will continue to overspread our region during Saturday...
resulting in 850 mb temperatures lowering to around +7C/+8C. At the
surface...this will translate into highs only reaching the upper 60s
to lower 70s in most locations Saturday afternoon...with our highest
terrain likely not making it out of the mid 60s. At the same time
surface dewpoints will plunge through the 50s...while a brisk westerly
low level flow and good diurnal mixing will help to promote fairly
breezy conditions...with wind gusts to 25-35 mph expected in many
locations.

The incoming cooler airmass will also be cool enough to support a
lake response during the day Saturday...though this will be tempered
somewhat by strong August diurnal influences and slow but steady
drying of our airmass. Off Lake Erie...a general west-southwesterly
flow will support at least a broken band of rain showers across much
of Erie County initially...with these then shifting southward into
the Southern Tier and breaking up through the rest of the day as the
low level flow veers more westerly...and as the aforementioned diurnal
influences and drying take hold. Meanwhile off Lake Ontario...a general
west-southwesterly flow should act to focus more scattered showers
across Jefferson/northern Lewis counties during the morning...with
the showers then drifting south into the Tug Hill region during the
afternoon as the low level flow becomes more westerly.

Saturday night and Sunday morning high pressure and even drier air
will ridge into our region from the Ohio Valley...while the low level
flow will veer to west-northwesterly and become increasingly sheared.
This will result in scattered lake effect rain showers across the
Tug Hill shifting south into Oswego County and becoming weaker/more
disorganized Saturday night...before falling apart altogether during
Sunday morning. Otherwise...the remainder of the region will experience
partly cloudy and mainly dry weather through Sunday morning. As we move
into Sunday afternoon...modest warm air advection aloft will develop
out in advance of low pressure pushing its way across the central
Mississippi Valley...and this will drive a general west-to-east increase
in cloud cover across areas south of Lake Ontario...with a few light
showers perhaps reaching the western Southern Tier by late afternoon.
Temperature-wise...Saturday night will be cool and comfortable for
sleeping with lows mostly ranging in the lower to middle 50s...with
highs on Sunday then rebounding into the lower to mid 70s under the
developing warm air advection regime.

Sunday night and Monday the aforementioned wave of low pressure will
ripple its way east-northeastward across the Ohio Valley...in concert
with its parent mid level trough that will be digging across the Great
Lakes Region. While there is still some uncertainty with respect to the
exact track and strength of the low...a multimodel consensus suggests
that this system will spread chances for showers back across most if not
all of the area Sunday night...with rain showers then becoming likely
across the majority of the region on Monday as the low passes across
PA and makes its closest approach to our area. At this point see very
little in the way of instability with this system likely passing by
to our south...and as such will continue to keep the forecast free of
any thunder mention. Otherwise the warmer airmass and increased cloud
cover will result in milder overnight lows in the mid 50s to lower 60s
Sunday night...with highs on Monday then mostly reaching into the lower
70s.

Monday night the surface flow will slide further east-northeastward
along the Long Island and New England coastlines and away from our
region...and as a result shower chances across our area should tend
to diminish. With little in the way of an airmass change expected...
overnight lows should again range mostly between the mid 50s and
lower 60s.

&&

.LONG TERM /TUESDAY THROUGH THURSDAY/...
Behind the departing low...the passage of the main upper level trough
axis and a modest northerly upslope flow will both conspire to keep
clouds and a few scattered light showers around into Tuesday morning...
before high pressure and drier air builds across our region Tuesday
afternoon through Wednesday and brings clearing skies along with a
return to fair dry weather. The high will then slowly depart off to
our east Wednesday night and Thursday...with the next (lower-end)
chance of showers arriving on Thursday with the approach of another
weak shortwave.

With respect to temperatures...highs initially in the lower to mid 70s
on Tuesday will rebound into the the mid to upper 70s for both Wednesday
and Thursday as our airmass modifies and we see increased amounts of
sunshine during the two days of this period. Meanwhile...overnight lows
in the 50s Tuesday night will give way to milder readings in the mid 50s
to lower 60s Wednesday night.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
While VFR conditions will be in place across western and north
central New York today...there will the risk for some
thunderstorms...especially later this afternoon and evening.

A strong cold front will slowly approach from the west during the
course of this afternoon and evening...and this feature will
generate a line of strong to severe convection. A fairly solid of
line of slow moving thunderstorms will then cross western New York
after 21z...and cross the Eastern Lake Ontario region after 00z.
MVFR cigs can be expected with these storms...along with the risk
for wind gusts in excess of 50 kts.

In the wake of the slow moving line of thunderstorms...several hours
of VFR conditions will then give way to MVFR cigs during the
overnight hours. IFR cigs will be possible across portions of the
Srn Tier and Finger Lakes regions after 06z.

Outlook...
Saturday......Mainly VFR with a chance of showers.
Sunday...VFR. Monday...VFR to MVFR with showers and thunderstorms
becoming likely. Tuesday...VFR to MVFR with chance for showers and
thunderstorms.

&&

.MARINE...
A southerly flow will be found across the Lower Great Lakes today in
advance of an approaching cold front. While there will be the chance
for some thunderstorms over western Lake Ontario early this
morning...generally fair weather can be expected over the region
through at least early afternoon.

Winds will then freshen this afternoon...especially on Lake Erie
where southwest winds will reach small craft advisory levels. The
increased winds will come as strong to severe thunderstorms will
advance across the region from the west. The storms will impact the
eastern half of Lake Erie and western Lake Ontario late this
afternoon and early evening...before they move across the eastern
half of Lake Ontario during the first half of tonight. This
convection will almost certainly prompt a slew of special marine
warnings...so mariners should pay close attention to the sky and
have the ability to receive weather warnings. It is probably best
for recreational boaters to alter plans for this period.

Southwest winds will continue to freshen across all of the Great
Lakes region in the wake of the cold front later tonight. This will
lead to high end small craft advisory wind speeds late tonight and
Saturday...with significant wave heights...particularly on Lake Erie
and on the northeast end of Lake Ontario.

Much cooler air will accompany the gusty winds late tonight and
Saturday. In fact...the airmass will be cool enough to support lake
induced convection that will yield some lake effect rain showers and
possibly some waterspouts. Using the very reliable Szilagyi research
(nomogram)...a convective cloud depth in excess of 10k ft with a
significant lake delta t will place most of the Lower Great Lakes in
an area of risk for the small scale features. Again...mariners this
weekend should pay close attention to the sky and listen for
possible special marine warnings for these features.

Winds and waves will subside significantly during the second half of
the weekend as high pressure will pass by to our south.

&&

.TIDES/COASTAL FLOODING...
Have issued a coastal flood warning for the entire lakeshore as
westerly winds increase tonight behind a strong cold front.
With these winds remaining strong Saturday and Saturday night
waves will increase on Lake Ontario to around 6 to 9 feet. These
waves combined with an already high lake level will bring an
increase to coastal flooding across low lying areas of Lake
Ontario. Greatest impacts will likely be across the eastern
section of the lake, where waves will be greatest.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning from 8 AM Saturday to 11 AM EDT Sunday
     for NYZ003>007.
     Lakeshore Flood Warning from 8 AM Saturday to midnight EDT
     Saturday night for NYZ001-002.
MARINE...Small Craft Advisory from 5 PM this afternoon to midnight EDT
         Saturday night for LEZ040-041.
         Small Craft Advisory from 5 AM Saturday to 11 AM EDT
         Sunday for LOZ043>045.
         Small Craft Advisory from 8 AM Saturday to midnight EDT
         Saturday night for LOZ042.

&&

SYNOPSIS...RSH
NEAR TERM...RSH/THOMAS
SHORT TERM...JJR
LONG TERM...JJR
AVIATION...RSH
MARINE...RSH
TIDES/COASTAL FLOODING...THOMAS

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Special Weather Statement
National Weather Service Buffalo NY
1123 AM EDT Fri Aug 4 2017

NYZ001>008-010>014-019>021-085-041730-
Niagara-Orleans-Monroe-Wayne-Northern Cayuga-Oswego-Jefferson-
Lewis-Northern Erie-Genesee-Wyoming-Livingston-Ontario-Chautauqua-
Cattaraugus-Allegany-Southern Erie-
Including the cities of Niagara Falls, Medina, Rochester, Newark,
Fair Haven, Oswego, Watertown, Lowville, Buffalo, Batavia,
Warsaw, Geneseo, Canandaigua, Jamestown, Olean, Wellsville,
Orchard Park, and Springville
1123 AM EDT Fri Aug 4 2017

...SEVERE WEATHER THREAT THIS AFTERNOON...

Thunderstorms are expected to rapidly develop and intensify early
this afternoon as they move from the Ohio Valley into Western and
Central New York. Some of these storms may become severe with
strong winds in excess of 60 mph and hail over 1 inch in diameter.
Be ready to take action if a severe thunderstorm warning is
issued for your location. At a minimum, take shelter in a
substantial building.

There will also be a threat for flash floods if multiple storms
move over the same area. Local areas received heavy rain over the
past few days. If you experienced heavy rain on Wednesday or
Thursday, be aware that additional heavy rain might result in
faster runoff with the possibility of a potential flash flood.
Never drive through flooded roads.

ZAFF

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