509
FXUS61 KBUF 171037
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
637 AM EDT Sat Aug 17 2024

.SYNOPSIS...
A large storm system over the Upper Great Lakes will slowly drift
eastward across our forecast area through Monday. This will result
in unsettled conditions with occasional showers and a chance of
thunderstorms...although there will also be periods of dry weather.
Some of the showers could be locally heavy...with a stronger storm
or two not also out of the question. Following the passage of this
system...Canadian high pressure will build across our region and
provide us a prolonged period of fair weather and comfortable
conditions that will last through the rest of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Broad and vertically stacked low pressure will slowly drift east
across Lower Michigan today. During this morning...the weak warm
front extending eastward from the low should continue to help
generate fairly numerous showers/possibly some embedded thunder
across the North Country and Saint Lawrence Valley. Meanwhile
further to the west...the combined effects of dry slotting aloft and
weak subsidence behind the warm front should help to yield partly
sunny and mainly dry conditions across much of western New York...
with only a 20-30 percent or so chance of a few pop-up showers or
storms.

This being said...the partial sunshine that will be experienced
across WNY this morning will also lead to increasing amounts of
diurnally-driven instability within our already warm and humid
airmass. As we push through this afternoon and the vertically
stacked low to our west draws a bit closer...shortwave energy
rotating around the low will impinge upon this more unstable airmass
and should lead to the development of more numerous showers and
thunderstorms from about the Genesee Valley and Finger Lakes
westward...for which some likely PoPs remain in place from
continuity. Meanwhile...convective coverage should tend to be more
scattered across the eastern Finger Lakes and North Country this
afternoon as this area will lie in between the departing warm
frontal pcpn and the shortwave energy crossing far western New York.

With PWATs remaining up in the 1.5 to 2 inch range...the potential
will exist for some of this convection to produce locally heavy
rainfall...especially if any slow-moving heavier cores can manage to
train over the same areas. The amount of available instability will
also support the potential for an isolated stronger storm or two
featuring strong gusty winds...with environmental shear values
continuing to appear too low to support an organized severe threat/
greater coverage of stronger storms. Otherwise...it will be a warm
and muggy day with highs in the upper 70s to mid 80s...and surface
dewpoints running between the mid 60s to lower 70s.

As we move into tonight...the large stacked area of low pressure
will meander its way into southern Ontario. With this feature slowly
drawing ever closer and additional shortwave impulses circulating
across far western New York...expect the western half of the area to
continue to see a general likelihood of showers and a chance of
thunder...though this activity may tend to diminish somewhat in
coverage through the night as diurnally-driven instability wanes
with the loss of heating and convective overturning. Further east...
pcpn coverage should remain more scattered across the eastern Finger
Lakes and North Country. Otherwise we can expect another warm and
humid night...with lows again ranging from the mid 60s across the
interior of the Southern Tier and North Country to around 70 near
the lakeshores.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Very unsettled weather will continue Sunday through Monday as an
amplified pattern matures across North America. A strong mid level
ridge will move from the Rockies into the Plains Sunday through
Monday, with a deep longwave trough carving out over the eastern
United States. A mid level low within the trough over Lake Huron
Sunday morning will gradually evolve into a open wave as it slowly
crosses the eastern Great Lakes Sunday night through Monday. At the
surface, a weak low vertically stacked beneath the mid level
circulation will approach the eastern Great Lakes Sunday and Sunday
night before weakening into a trough by Monday.

Sunday through Sunday night, DPVA and height falls will continue to
spread east across the eastern Great Lakes, providing a background
environment of large scale ascent. Numerous convectively augmented
vorticity maxima will propagate through the trough, bringing
periodic enhanced ascent to the region. Modest daytime heating will
allow for weak diurnal instability to develop, with subtle
differential heating and terrain boundaries providing low level
focus for shower and thunderstorm development. In general, expect
the most widespread rain to be across Western NY in the morning, and
then expand eastward across the remainder of the area through the
day. There is some suggestion in guidance that a dry slot and/or
modest lake shadow may lower rain chances later in the afternoon
over and northeast of Lake Erie.

Occasional showers will continue Sunday night as the trough axis
slowly crosses the eastern Great Lakes. Thunder chances should peak
Sunday afternoon and evening with the diurnal cycle, although it`s
possible some thunder may linger through Sunday night as weak
instability persists, especially over and near the warm lakes.

PWAT values will continue to run in the 1.5"-1.75" range Sunday
through Sunday night with surface dewpoints approaching 70F. The
ample low level moisture and persistent forcing will continue to
support a risk of locally heavy rainfall and isolated flash
flooding, mainly in locations where embedded convection repeats
over the same areas.

Monday, the trough will continue its slow march eastward. The
surface front will become increasingly anabatic with time, allowing
deep moisture and rain to linger on the cooler northwest side of the
boundary. A broad area of stretching deformation and frontogenesis
arcing from Quebec south-southwestward across the eastern Great
Lakes will maintain wet weather through much of the day. There is
still some chance of a little embedded thunder, but in general the
thunder chances will diminish from northwest to southeast as
instability wanes with the arrival of a cool airmass.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
The long stretch of unsettled weather will come to an end by
Tuesday, with dry weather from midweek through next weekend.

Tuesday through Wednesday a mid level trough will remain over the
eastern US, however the surface front and deep moisture will be
located along the eastern seaboard by Tuesday, then off the coast by
Wednesday. Meanwhile, high pressure will build into the upper Great
Lakes Tuesday and then expand into the eastern Great Lakes by
Wednesday, with associated subsidence and drying supporting the
return to dry weather. Thursday through Friday expansive high
pressure surface and aloft will dominate the entirety of the Great
Lakes, Ohio Valley, and New England with dry weather prevailing.

Temperatures will start off quite cool midweek. Highs Tuesday will
range from the upper 60s for lower elevations to the lower 60s for
higher terrain. High temperatures will then add on a few degrees
each day for the rest of the week, getting back into the upper 70s
by Friday for lower elevations. Nights will be quite cool, with lows
in the 50s close to the lakes and 40s in cooler inland locations.

&&

.AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
Broad low pressure will slowly drift east across Lower Michigan
today. During the morning hours...the weak warm front extending
eastward from the low should continue to help generate fairly
numerous showers/possibly some embedded thunder across the North
Country...with ceilings there lowering to the lower end of the VFR
range...and perhaps to MVFR across the higher terrain. Meanwhile
further to the west...the combined effects of dry slotting aloft and
weak subsidence behind the warm front should help to yield mainly
dry and VFR conditions across much of western New York...with just a
low chance of a few pop-up showers and storms. Any IFR/MVFR ceilings
across the Southern Tier will likely mix out by late morning/early
afternoon.

As we push through this afternoon...the combination of building
diurnal instability and another impulse rotating around the larger-
scale low should help to trigger the development of another round of
more numerous showers and thunderstorms from roughly a KROC-KDSV
axis westward...with more scattered convection expected further east
across the remainder of the area. Flight conditions should be mainly
VFR...except for brief/localized reductions within any heavier
showers and storms...and some lingering lower ceilings across the
higher terrain of the North Country.

As we move into tonight...the area of low pressure will meander its
way into southern Ontario. With this feature slowly drawing ever
closer and additional impulses circulating across far western New
York...expect the western half of the area to continue to see a
general likelihood of showers and a chance of thunder...though this
activity may tend to diminish somewhat in coverage through the night
as instability wanes. Further east...convective coverage should
remain more scattered across the eastern Finger Lakes and North
Country. With respect to flight conditions...expect these to
generally be VFR...again save for brief/localized reductions within
any heavier showers and storms...some lingering lower ceilings
across the higher terrain of the North Country...and redeveloping
lower ceilings across the western Southern Tier overnight.

Outlook...

Sunday...Occasional showers and scattered thunderstorms with
associated reductions to IFR/MVFR...and MVFR/VFR otherwise.
Monday...MVFR/IFR ceilings with showers likely and a chance of
thunderstorms.
Tuesday...Improvement to VFR.
Wednesday...VFR.

&&

.MARINE...
Over the weekend expect a general southerly to southwesterly
flow on Lake Erie and a southerly to southeasterly flow on Lake
Ontario... with winds generally remaining at or below 15 knots.

The above said...there will be a couple periods of slightly
stronger (15-20 knot) SSE flow on eastern Lake Ontario...with
the first of these coming early this morning...and the second
then following later tonight and early Sunday. Fortunately...the
offshore component of the flow will direct the bulk of the
greater wave action across Canadian waters and should preclude
the need for any Small Craft Advisories during these time
periods.

Of greater concern to marine interests this weekend will be the
potential for scattered thunderstorms...with locally higher winds
and waves possible in and near any such storms.

Following the passage of a cold front...northerly flow will develop
later Sunday night...then will strengthen across the Lower Great
Lakes Monday. This will at least bring some choppy conditions to
Lake Erie...with somewhat stronger winds across Lake Ontario
potentially leading to some advisory-level conditions along the
south shore of that lake Monday and Monday night. Conditions should
then gradually improve on Tuesday as Canadian high pressure begins
building into the region...with a slackening pressure gradient
allowing for winds and waves to subside.

&&

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

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...Hitchcock
LONG TERM...Hitchcock
AVIATION...JJR
MARINE...JJR