Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 291750
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
150 PM EDT Mon May 29 2017

.SYNOPSIS...
Low pressure will slide by our northwest today, with occasional
showers tapering off from west to east following the passage of its
trailing cold front. After this system exits our area, a large upper
level trough will then meander its way across the region through
much of the upcoming week, bringing periodic opportunities for
additional showers and storms along with near to slightly below
average temperatures.

&&

.NEAR TERM /THROUGH TONIGHT/...
Surface cold front currently pushing through the Genesee Valley into
the Finger Lakes. Widespread showers out ahead of the front have
ended across the west. These will taper off across the remainder
of the area almost immediately behind the frontal passage as
much drier air and strong subsidence overspread the region in
its wake. Cloud cover will linger a bit longer behind the front,
but a good deal of sunshine is expected from about the Finger
Lakes westward this afternoon.

While our airmass will turn marginally "cooler" in the wake of the
frontal passage...850 mb temps will still range from +10C to +12C,
and this in concert with the renewed sunshine and good mixing
will help promote widespread highs in the mid to upper 70s from
about the Finger Lakes westward, though areas immediately
northeast of Lake Erie will see cooler highs (in the upper
60s/lower 70s) thanks to a freshening southwesterly breeze off
the cooler lake waters. Further east clouds and precipitation
will linger longer, resulting in highs ranging from the mid 60s
to lower 70s east of Lake Ontario.

Moving on into tonight, drier air and subsidence should result in
mainly dry conditions this evening, before the persistent low over
Ontario Province pushes a secondary cold front into our region
overnight. Coupled with another round of height falls aloft, this
will drive another west-east increase in cloud cover and bring
renewed chances for some scattered showers and thunderstorms.
Otherwise, expect another mild night thanks to a persistent
southerly low level flow in advance of the front, which will result
in overnight lows mostly ranging between 55 and 60 degrees.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY NIGHT/...
While the bulk of this forecast period will be dry...there will be
the likelihood for some afternoon showers and thunderstorms both
Tuesday and Wednesday. The overlying synoptic feature responsible
for the unsettled afternoons will be a large closed off upper level
storm over eastern Canada that will occasionally circulate impulses
of energy and their associated weak surface frontal boundaries
across our region. These subtle features within a moist and
conditionally unstable airmass will be the focus for convection...
with weak lift from the deep cyclonic flow and added diurnal
instability adding to their likelihood. On the flip side of this
scenario will be a stabilizing lake shadow that will temper...if not
completely squelch...the convective coverage within a corridor from
Lake Erie to the Thousand Island region. As for the details...

A 125kt H25 jet streaking by immediately to our southeast will be
the main feature to watch on Tuesday. It will generate a ribbon of
channelled vorticity that will support enhanced convection across
parts of the Southern Tier...Finger Lakes and Eastern Lake Ontario
regions...right where one of the aforementioned surface boundaries
will be slowly pushing east. This boundary will be synoptic in
nature...yet as the day matures...it should intensify as it will be
reinforced by the eastern edge of the Lake Erie `shadow`. While
likely pops were already in place for the affected area...they were
concentrated in the afternoon. Will speed up the timing...as
convection will already be underway as day breaks. Do not expect
enough instability at this earlier time for strong/severe
convection...but that may not be case as this activity pushes
further east into central and eastern New York. The good news is
that the afternoon for these areas may now be dry. Meanwhile...a
strengthening lake breeze off Lake Erie will produce a pronounced/
text book lake shadow from the Niagara Frontier and Rochester Metro
area to the Thousand Islands. Winds within this corridor could gust
as high as 35 mph during the midday and afternoon. H85 temps in the
vcnty of 8-10 C will allow afternoon mercury readings to climb into
the lower 70s in most areas. It will be much cooler in downtown
Buffalo and in the Thousand Islands region.

With the loss of diurnal heating and the lack of a focusing
trigger...leftover showers early Tuesday evening will give way to
fair conditions regionwide. Temperatures Tuesday night will settle
back through the 50s.

Wednesday will feature another day where the eastern lake breeze
boundary off Lake Erie will generate some showers and thunderstorms
over portions of the Southern Tier and Finger Lakes region. Unlike
the day before when the activity was supported/enhanced by
channelled vorticity...the showers and thunderstorms on Wednesday
should be more typical of lake breeze driven convection. That is..
it should wait for diurnal instability to build up through the
morning before becoming initiated during the midday and afternoon.
The downwind shadow off Lake Erie will likely not be as pronounced
as the day before...and while it should still help to `protect`
areas from the Niagara Frontier to the Thousand Islands from
widespread convection...height falls from the passing mid level
trough could be enough to allow some showers or storms to move
through. In regards to temperatures...it will be cooler as H85 temps
near 5c will keep the region in the 60s for the afternoon.

Any leftover showers Wednesday evening will quickly die off as
notable drier air in the mid levels will accompany high pressure
that will nose across the Lower Great Lakes from the Ohio Valley.
The vast majority of Wednesday night will feature dry and relatively
cool weather. Temperatures will drop into the lower 50s across the
lake plains...with 40s anticipated across the Southern Tier and
portions of the North Country.

While a weakly cyclonic flow will still be in place over our region
on Thursday...high pressure centered over the Mid Atlantic states
will help to encourage fair dry weather. It will still be on the
cool side of normal with highs generally ranging from the mid 60s
across the North Country to within a couple degrees of 70 elsewhere.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
A high latitude omega block featuring a staunch ridge over the
Canadian prairies and a closed low over eastern Canada will remain
in place for the bulk of this period. This will keep a deep cyclonic
flow over the Great Lakes region with daytime temperatures over our
forecast area averaging at least 5 deg below normal. The silver
lining to the cool weather is that after we experience some showers
and thunderstorms on Friday...mainly dry weather is anticipated for
the weekend.

On Friday...there is fairly good model consensus that a cold front
will push southeast across our region. Have thus raised pops to high
chc regionwide for scattered showers and thunderstorms. While H85
temps ahead of this front will allow max temps on Friday to climb to
near 70...we can anticipate day to day cooling as we progress
through the weekend.

As the stacked low over eastern Canada pushes east to the maritimes
during the course of the weekend...an expansive surface high will
build south across the Great Lakes from Hudson Bay. This will offer
fair cool weather for our region...although steep low level lapse
rates each afternoon within the fall like airmass could support
isolated showers. The highest chance for such widely separated
showers will be found across the North Country. Max temps both days
of the weekend will be in the 60s.

&&

.AVIATION /18Z MONDAY THROUGH SATURDAY/...
Back edge of showers now moving into northern New York. Lingering
MVFR cigs at KART will improve to VFR by 21z. A line of weak
convection will impact areas from KFZY-KSYR-KRME through 21z
with brief heavy rain showers, but will remain south of KART.

Tonight, expect mainly dry VFR conditions during the evening, before
a secondary cold front pushes into the region overnight and brings a
renewed opportunity for some additional scattered showers and
thunderstorms. While mainly VFR flight conditions should continue to
predominate overnight, some brief/localized reductions to MVFR
cannot be totally ruled out within any showers or storms.

Outlook...

Tuesday through Saturday...Mainly VFR...with localized MVFR possible
in scattered to numerous showers and thunderstorms at times.

&&

.MARINE...
A cold front will pass across the lower Great lakes today as low
pressure moves across southern Ontario. Behind the front, winds will
veer to southwesterly and freshen to around 15 knots, resulting in a
light to moderate chop on the northeastern ends of the lakes. Winds
and waves are expected to remain below advisory criteria.

A secondary cold front will then cross the region later tonight and
Tuesday morning, with another round of scattered showers and storms
marking its passage. In the wake of this second boundary, southwest
winds will again freshen across the Lower Lakes region on Tuesday,
particularly over Lake Erie where winds and/or waves could reach
lower-end advisory levels.

A broad upper level trough will linger across the region through the
middle and latter portions of the week., resulting in a continued
general southwesterly to westerly flow, with this potentially
pushing advisory levels on Lake Erie on Wednesday.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...None.

&&

$$

SYNOPSIS...JJR/TMA
NEAR TERM...JJR/TMA
SHORT TERM...RSH
LONG TERM...RSH
AVIATION...TMA
MARINE...JJR/TMA


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