Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 260246

Area Forecast Discussion
National Weather Service Buffalo NY
1046 PM EDT SUN SEP 25 2016

Clouds will increase from the west tonight ahead of an approaching
low pressure system. A cold front associated with this system will
bring showers and a chance of thunderstorms from late Monday
afternoon into Monday night. Cooler air behind the front will lead
to the development of lake effect rain showers northeast of the
lakes from Monday night into Wednesday, with unsettled weather
prevailing through much of the coming week.


An upper level ridge axis over Lake Huron late this evening will
move overhead in the face of a sharp upper level trough plowing
across the Upper Midwest. Increasing warm advection and increasing
mid/high clouds ahead of this system will result in a warmer night
across Western New York and the Southern Tier, where lows should
range from the low to mid 40s in the sheltered Southern Tier valleys
to the mid 50s along the lakes. Areas east of Lake Ontario will
still be under the influence of high pressure, light winds, and
clear skies for one more night, so expect frost to be an issue
across inland areas of Jefferson County, as well as Lewis County,
where temperatures are once again expected to dip into the mid to
upper 30s across sheltered area. A Frost Advisory has been posted
for these counties.

Clouds will continue to spread east across the forecast area on
Monday, and with increasing southerly winds and warm advection ahead
of the approaching low pressure system and associated cold front,
temperatures will rebound back into the 70s across most areas, with
the possible exception of the North Country and highest hilltops of
the Southern Tier, which will top out in the upper 60s. Late in the
day we will see rain moving into the far western portions of New
York as the cold front approaches the region. While better
instability along/ahead of the front will be found to our south in
PA/WV, given strong dynamics associated with this low pressure
system, cannot rule out a few embedded thunderstorms within the band
of showers that will move into the forecast area late in the day.


This afternoon a shortwave trough of low pressure is seen on water
vapor imagery advancing across the Northern Plains, with a vort max
sliding down the western slope of the trough, deepening the
shortwave. At the surface an area of low pressure is just north of
Minnesota, and this surface low will continue to deepen through
Monday night, while pushing a cold front towards our region. Ahead
of this cold front we already see the moisture building with both
Gulf of Mexico moisture and moisture from convection over Mexico
streaming northward.

The combination of upper level height falls, low level convergence
and lift along both the cold front and low level jet through a deep
saturated low level will continue a band of showers across the
region Monday evening, though the chances for thunder will be
greatly diminishing as MUCAPE values fall and we loss the daytime

A dry slot will follow the cold front Monday night, leaving much of
the region relatively dry. The partly to mostly cloudy skies will
limit overnight cooling, with most areas dropping back to the lower
to mid 50s, which is several degrees above normal.

Behind the cold front, there will be strong cold air advection, such
that temperatures at 850 hpa drop from around +12c Monday afternoon,
to near +2c by Tuesday morning. Several factors will come together
such that a plume of lake effect rain will develop before daybreak
Tuesday off Lake Erie, and then may develop through the morning hours
of Tuesday east of Lake Ontario. Moisture associated with the upper
level trough will return over Lake Erie Monday night, and Lake
Ontario Tuesday as the upper level closed low slowly rotates
eastward. Meanwhile over the lakes the colder airflow will steepen
low level lapse rates, and lake induced CAPE values will reach 1200
to 1800 J/KG.

For Lake Erie...Factors are coming more into alignment for a healthy
band of lake effect rain to form before daybreak Tuesday off Lake
Erie. Confidence that a cold front will cross the lake Monday
evening, early enough such that the upper level closed low will have
ample time to return ambient moisture over the lake. The strong cold
air advection over the lake Monday night, combined with moist
cyclonic flow aloft, and an aligning southwest wind should begin the
formation of a band of rain shortly after midnight over the western
half of Lake Erie, with rains reaching WNY before dawn. The position
of the upper level low over the SOO suggests that winds will be more
of a 220-230 flow early Tuesday morning such that the placement of
the band of rain should be near Buffalo, and across the Northtowns,
and including Grand Island and Niagara county.

Rainfall with the band will peak just past daybreak Tuesday, before
daytime mixing heights increase and wind shear increases some, such
that a weakening of the band to lighter and more scattered in nature
showers is likely. In fact winds may back enough to lift the lake
effect rain almost entirely into Canada for a short time.

Later Tuesday night wind shear is expected to diminish, with a still
southwest wind over the lake. An increase to the band of rain is
possible, though possibly not extending as far inland as the wind
flow will be weakening through the night.

Off Lake Ontario...instability will be weaker, with lake
induced CAPE values closer to 1000 to 1200 J/KG as the core of the
cold air remains closer to the upper level low over the Central
Great Lakes. This, combined with the shorter fetch over Lake Ontario
on a southwest wind and lower omega values will bring weaker lake
effect bands of rain, Tuesday morning over northern Jefferson County,
and into the SLV. Like Lake Erie, the band of rain showers will
likely weaken later Tuesday morning and through the afternoon hours
before again organizing into somewhat of a better band Tuesday
evening and into the early morning hours Wednesday. Wind shear will
increase through the night Wednesday such that the band of rain
will become disorganized overnight.

On Wednesday the upper level low will drop towards the Ohio Valley.
Surface winds will become easterly Wednesday and with warm air
advection aloft, we will not have to worry about lake effect rain.
There may be a few showers that brush by far western New York as
this upper level low drops southeastward, with areas from about the
Genesee Valley eastward likely to remain dry.

As far as temperatures go, highs Tuesday will remain in the low to
mid 60s under the cool flow aloft. On Wednesday the slight warming
aloft should allow for daytime maxes to be a bit warmer than
Tuesday, with highs Wednesday climbing into the mid 60s to around
70. Overnight lows both Tuesday night and Wednesday night will
remain in the mid 40s to mid 50s (colder in inland valleys/warmer
near the lakes) with much colder temperatures not likely owing to
the wealth of clouds over the region through midweek.


Model guidance supports a considerably more pessimistic forecast for
late this week going into the weekend. 12Z runs of the
GFS/GGEM/ECMWF now all show a cut off low stalling in our vicinity
Thursday through Saturday. There remains some disagreement on the
position of the low, but this pattern would result in mostly cloudy
skies with periodic showers. Model consensus initially has the 500
mb low across the Appalachians on Thursday, then it slowly drifts
northward through Saturday.

The steadiest rain with this system will be north of the upper low,
where an easterly flow will tap into Atlantic moisture. Given
uncertainty in the position of the cut-off low, the forecast will
carry rather broadbrush chance POPs with these slightly higher
across southern portions of the cwa. Despite the extended period
with a chance of showers, there will be significant periods of dry
weather too. The low will weaken and may meander off on Sunday which
may keep Sunday dry though forecast confidence is low.

Temperatures will be seasonable to slightly above normal. Highs will
mainly be in the 60s due to the considerably cloud cover and
showers. Lows should mainly be in the upper 40s to lower 50s.
Periods of clear skies will allow for greater diurnal ranges, but
these cannot be timed with any confidence this far out.


VFR conditions will continue to prevail through the bulk of the
overnight into Monday morning as high pressure overhead moves off to
the east.
Mid-level cigs will move in from the west overnight and into Monday
morning ahead of an approaching low pressure system and associated
cold front. Showers will approach far Western NY around 17-18Z and
shift east across western and central NY through the afternoon and
evening. The showers will likely not reach KART until closer to 00z.
There is a risk of a few embedded thunderstorms with timing of the
front matching up with peak heating. Cigs will also lower to MVFR
when showers arrive.


Monday Night...Areas of MVFR with showers likely.
Tuesday and Tuesday Night...Mainly VFR. Areas of MVFR/IFR in
lake effect rain showers northeast of Lake Erie.
Wednesday through Friday...VFR/MVFR with a chance of showers.


A surface high will push east across New England the St. Lawrence
Valley overnight. This will encourage winds to veer to the southeast.
The southeast winds will freshen on Monday ahead of an approaching
cold front...with the winds coming around to southwest by late
Monday and Monday evening. Winds should freshen enough to warrant a
new round of small craft advisories by late Monday. SCA conditions
are then expected to persist through Tuesday as a stationary storm
system will be parked over the Upper Great Lakes.


NY...Frost Advisory from 2 AM to 9 AM EDT Monday for NYZ007-008.



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