Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 270802

Area Forecast Discussion
National Weather Service Buffalo NY
402 AM EDT Tue Jul 27 2021

A cold front will cross the eastern Great Lakes today and produce
showers and scattered thunderstorms. A few storms may be severe with
isolated damaging wind gusts possible. The showers and storms will
taper off this evening, with some fog and drizzle possible overnight
through Wednesday morning. Dry weather will return for Wednesday
afternoon before another cold front brings a chance of showers and
thunderstorms later Wednesday night through Thursday.


Satellite imagery showing mainly clear skies across the region early
this morning, with just a few patches of mid/high clouds. Some
patchy fog will continue across the Southern Tier valleys and also
across the North Country through about 8AM.

A mid level shortwave will advance from the Upper Great Lakes this
morning to the Lower Great Lakes by this evening. The synoptic scale
trough will be modulated by ongoing convection upstream, with
mesoscale convectively induced vorticity maxima moving downstream
and enhancing ascent in some areas. The Lower Great Lakes will
become situated in the right entrance region of a 100+ knot H25 jet
this afternoon, giving an added boost to synoptic scale support. At
the surface, a cold front draped across Southern Ontario will
advance south this afternoon and evening in response to flow
adjustments from the incoming synoptic scale trough and composite
outflow boundaries from organized convection.

In a big picture sense, dry weather will prevail most of this
morning. Showers and thunderstorms will then enter areas near the
Canadian border around midday, spreading southeast across the rest
of the area during the afternoon. There may be several waves of
showers and scattered thunderstorms crossing the area during the
afternoon and evening, with the timing of each tied to convectively
modulated vorticity maxima born from upstream convection.

Organized severe weather potential will be closely tied to corridors
of enhanced mesoscale forcing from any meso-vorticies that emerge
from upstream convection this morning. Radar and satellite trends
coupled with high-res CAMS guidance suggests an area of mesoscale
forcing may reach Western NY around midday, then advance into
Central NY by mid to late afternoon. Increasing deep layer shear,
moderate instability, and increasing synoptic scale forcing suggest
the potential for upscale convective growth into clusters and bowing
segments with isolated damaging winds the main risk. If a well
defined MCV were to emerge from upstream convection, a corridor of
greater severe weather potential may develop with a more
significant damaging wind threat and the possibility of an isolated
tornado. The main severe weather risk looks to be roughly noon to
4PM from Western to Central NY.

Following the first round of convection, a complex scenario of
remnant outflow boundaries, lake breeze boundaries, and a synoptic
scale front will evolve over the Lower Great Lakes from mid
afternoon through early evening. If the airmass recovers following
the first round of convection this may allow for secondary
development of showers and scattered storms. The severe weather risk
will likely be lower (but non-zero) with this second round, with a
few gusty thunderstorms possible from late afternoon through

PWAT values briefly rise to around 1.75" this afternoon supporting
potential for heavy downpours with any storms that develop. Fast
storm motion will likely mitigate the risk of flooding, unless
training can develop along a boundary.

The showers and storms will taper off from northwest to southeast
this evening as the mid level trough moves east of the area. Weak
north/northeast flow will develop overnight, with moisture trapped
beneath a frontal inversion. This will likely set the stage for
plenty of low clouds, patchy drizzle and fog overnight through
Wednesday morning. The clouds, drizzle, and fog will eventually
break for increasing sunshine later Wednesday as a drier airmass
advects into the region.


Overall a mid-level longwave trough will persist across the
Northeast through the remainder of the week and into early next
week. Therefore to initiate the period, a subtle shortwave will
push a surface low and attending frontal features across the
region beginning late Wednesday night through early Friday. As
such expect a warm front to cross the region late Wednesday
night before the arrival and passage of the cold front Thursday.
With arrival of the warm front, moisture rich air will begin to
filter into Western New York as seen by increasing PWAT values
from west to east, aiding in the production of a few showers
across far Western New York.

Then due to the aforementioned shortwave`s entrance into the region
Thursday, instability will begin to increase, along with the wind
regime. Overall, this will aid in the development of showers and
storms Thursday. By Thursday night, a few surface lows will ride
along the boundary, which will keep the chances for showers and a
few possible storms into the night due to newly added convergence
zones along with the previously mentioned moisture.

Cooler temperatures will filter into the region from a now
northwesterly flow regime Friday morning, which may cause
temperatures to drop down around +6C. The cooler temperatures aloft
may enhance a subtle lake shower response Friday. Outside of this,
daytime heating will also aid instability Friday afternoon, which
will again support afternoon showers and a thunderstorm or two.


Subtle mid-level ridging Friday night and Saturday ahead of the next
shortwave arriving later in the period for Sunday. That said, expect
cooler and dry weather through at least Saturday. With the next
arrival of the shortwave from northwestern Ontario, its associated
low will cross southern Ontario where it will also drag its
associated cold front across the region sometime Sunday. Behind the
front Monday, cold air will advect into the region and initiate the
start of a cooler period of weather.

Ahead of the front Saturday and Sunday, expect highs in the low to
mid 80s. Though with the aforementioned cooler air arrival Monday,
expect cooler day with highs in the upper 60s and low 70s.


Mainly clear skies will continue through this morning with VFR
prevailing in most areas. Patchy fog will continue across the river
valleys of the Southern Tier and also the North Country with local
IFR. Any local IFR in fog will dissipate by around 12Z.

A mid level shortwave and cold front will move southeast across the
eastern Great Lakes this afternoon and evening, producing several
rounds of showers and scattered thunderstorms. The chances of
convection will arrive first near the Canadian border by around 16Z
Tuesday, then progress south and east to the PA border and Central
NY by 20Z Tuesday, lasting into the early evening. The specific
timing and placement of showers and storms will be closely tied to
subtle features born from upstream convection, which will be
difficult to time this far in advance. Any heavier showers and
storms will produce brief/local MVFR to IFR conditions, with VFR
still prevailing outside of storms.

Most of the showers and storms will taper off from northwest to
southeast this evening, although a few scattered light showers or
some patchy drizzle may persist overnight. North to northeast flow
behind the cold front and abundant moisture trapped beneath the
inversion will allow areas of MVFR/IFR CIGS to become widespread
tonight, with some patchy fog producing some VSBY restrictions as


Wednesday...Areas of MVFR/IFR early in low clouds and fog, improving
to VFR.
Thursday...VFR prevailing, with a chance of showers and
thunderstorms and associated brief/local CIG/VSBY restrictions.
Friday and Saturday...Mainly VFR.


Winds in general will remain relatively light today and tonight.
Scattered thunderstorms from midday through this evening may produce
locally higher winds and waves. Northeast winds 10-15 knots will
develop late tonight and Wednesday behind a cold front moving south
of the lakes. This will produce choppy conditions Wednesday,
especially along the south shore of Lake Ontario.





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