NWS Area Forecast Discussion

FXUS61 KBUF 221958

Area Forecast Discussion
National Weather Service Buffalo NY
358 PM EDT Wed Sep 22 2021

A slow moving cold front will bring periods of rain to the region
through Thursday, with the steadiest and heaviest rain tonight and
Thursday as the boundary slowly crosses our area from west to east.
Following the frontal passage, cooler and drier weather will return
for most areas on Friday, except northeast of Lake Erie where some
additional lake effect rain showers will be possible. Another cold
front will then bring the next chance of showers on Saturday.


Upper low over Ohio valley in process of cutting off with sfc low
over eastern Ohio. Sharp inverted trough extends to the northeast of
the low across Lake Erie, western Lake Ontario and into southern
Ontario. East of the trough, warm air advection and persistent low-
level jet has supported widespread, albeit, light showers much of
the day over much of the forecast area. A lull in showers has
developed over WNY and farther into eastern OH/western PA, but this
is also where a line of thunderstorms are beginning to develop in
low CAPE, high shear regime ahead of cold front. Possible that
stronger storm may make it into Chautauqua county early this
evening, though majority of severe convection should stay south and
west of our area.

Later this evening remnants of this convection along with low-level
convergence ahead of approaching cold front will result in
widespread band of showers with some heavy rain over western NY as
far east as the Genesee valley. Only other issue tonight will be
gusty southeast winds as strong as 40-45 mph along Lake Erie shore
in Chautauqua to southern Erie and also over hills of southern Erie
and Wyoming county. These stronger winds will cut out quickly after
midnight as the low-level jet shifts east, but could see some gusty
winds develop late tonight into Thursday morning for northern facing
slopes of Tug Hill and western Adirondacks.

Late tonight into Thursday morning the swath of moderate to heavy
rain will continue to edge eastward while parent upper low and sfc
low spin over central Great Lakes. Strongest moisture transport co-
inciding with sharpest differential divergence btwn low-level jet
and diffluence of upper level jet looks to focus heaviest rain over
cntrl and eastern PA somewhat lessening as it lifts northward across
the Finger Lakes by midday Thursday. Given the strong dynamic lift
with the system still could see some embedded thunder. This band of
rain, no more than 50-75 miles wide by this time, is agreed upon to
slide east across the North Country and eventually into far eastern
NY late Thursday afternoon into Thursday evening. Heaviest rain will
occur south and east of our area it appears on nose of strongest
moisture transport, PWATs toward 2 inches and where models show at
least weak instability. As the rain marches across the North
Country, drying trend takes hold over WNY to Genesee Valley in the
afternoon. Soundings show clouds stick around, but if rain occurs it
will be light and scattered.

With respect to rainfall amounts from here on out the bulk of the
heaviest rain will fall late this evening through late tonight over
far western NY, on Thursday morning Genesee Valley to Finger Lakes
and Thursday afternoon into Thursday evening over North Country.
Totals from tonight through Thursday evening will generally be 1.25
to 2 inches, with amounts over 2 inches toward Cattaraugus and
Allegany counties. Modeled amounts and even looking at 75-95th
percentile amounts from NBM indicate heaviest rain from this event
remains west of our area and also south and east of our area. So
there are no plans to issue any hydrologic watches given both the
prolonged duration of the rainfall and the progressive-enough nature
of the band of moderate to heavier rain along the cold front
tonight into Thursday.

One more night of warmer temps, then temps will remain steady in the
upper 50s to mid 50s over WNY on Thursday. Temps from Genesee valley
to Finger Lakes to North Country could reach mid maybe upper 70s,
before falling off late behind the cold front.

Only showers on Thursday night will be closer to Lake Erie late as
lake effect still on track to develop. Temps will be in the 40s
inland to the lower to middle 50s on the lake plains.


A closed upper level low will be advancing northward towards James
Bay to start this period, while a seasonably cool airmass lingers
over the western Great lakes region. A few scattered showers
are possible Friday before the upper level low exits.
Temperatures of +5 to +6C at 850 hPa over a warm lake water
temperature of around 20-21C will create instability, steep
lapse rates resulting additionally in bands of lake effect rain
to the northeast of Lake Erie Friday, and to a lesser amount
northeast of Lake Ontario Friday afternoon and night. The
ambient moisture source will be quickly passing through our
region, so do not expect the lake effect rain to persist too
long, but still a quarter to half an inch of rain is possible in
heavier bands.

Friday Night, outside rains to the northeast of Lake Ontario, it
will be mainly dry as our region will lie between two moisture
plumes. It will be cool, with overnight lows in the lower 40s to
lower 50s. River valley fog is likely to form across the SW NYS.

Saturday the next upper level low will approach our region. The 00Z
Canadian/ECMWF are much deeper and farther southward displaced with
this low than the 06Z GFS. As an upper level shortwave and weak
surface cold front passes across our region, rain showers will
become likely by Saturday evening. Precipitation will be more
widespread if the deeper solutions of the Canadian and European
models verify. Again behind this upper level low, a cooler return
flow will generate lake effect rains to the northeast of the Lakes
Saturday Night.


Cooler WSW to Werly flow behind the cold front for the second half
of the weekend into the start of next week. Cyclonic flow aloft
combined with 850Ts in the neighborhood of +5C should at least
produce some upslope showers along with some lake effect/enhanced
showers E and ENE of the lakes during this timeframe. Continued with
tailoring of NBM PoPS Sunday and Monday to better reflect the lake
enhancement. Although toned down Monday a bit at this point as the
exact feature placement will determine to what degree any lake
enhancement continues into the start of the new work week. Any
lingering activity would be most favored off Lake Ontario.

General mid/upper level troughing pattern is then favored to
continue over the lower Great Lakes and Northeast through the middle
of next week. ECMWF/Canadian/GFS (big 3) also all advertise the
possibility for an Alberta Clipper like system to slide SE in the
NWerly flow and possibly impact our area toward mid week depending
on the exact storm track. Otherwise, with troughing likely remaining
entrenched over the region, expect a seasonably cool and moist early
fall airmass to remain in place with cyclonic flow aloft and 850Ts
cool enough to generate bouts of lake effect and/or lake enhanced
showers downwind of the Lakes, in addition to any synoptic features
embedded within the flow aloft.

Temperatures through the period will be near to a little below
normal, generally ranging through the 60s, with some upper 50s over
the Tug Hill and western Dacks.


Despite showers lifting through the region ahead of sfc low over
eastern Ohio Valley, CIGS mainly remain VFR with lower IFR/MVFR
exception over higher terrain of Southern Tier. Do expect a gradual
lowering to at least MVFR at western NY TAF sites rest of the
afternoon. VSBY mainly VFR, but have seen brief lowering to MVFR
VSBY during heavier showers.

Tonight the aforementioned surface low will lift northward into the
Central Great Lakes, while slowly pushing a cold front and narrow
band of moderate to heavy rain into western New York. This will
result in flight conditions across western New York deteriorating
into the MVFR and LIFR/IFR ranges across the lower elevations and
higher terrain respectively, while further east general VFR
conditions and scattered lighter showers will be found across the
far eastern Finger Lakes and North Country. Some LLWS will also
again be possible, mainly at KROC where low-level jet is strongest
and there is limited chance of gusts.


Thursday...MVFR/IFR in a band of moderate to heavy rain crossing the
area from west to east...improving to VFR in its wake during the
Friday...Mainly VFR...except for scattered showers early the North
Country...and scattered to numerous lake effect showers/possible MVFR
across the Niagara Frontier.
Saturday and Sunday...MVFR/VFR with a chance of showers.


High pressure draped along/offshore of the Atlantic coastline and a
stalling cold front across the Central Great Lakes will keep a
moderately brisk south to southeasterly flow intact across Lake Erie
and Lake Ontario through tonight. Even so, the southeast flow will
keep the highest waves confined to areas well offshore, with conditions
thus remaining below SCA criteria.

Looking further ahead, conditions should remain below SCA criteria
through Thursday, with a developing moderate southwesterly flow
bringing a period of SCA conditions to Lake Erie Thursday night into






NWS BUF Office Area Forecast Discussion