Area Forecast Discussion
Issued by NWS Buffalo, NY

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157
FXUS61 KBUF 232150
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
450 PM EST Fri Feb 23 2018

.SYNOPSIS...
Low pressure will move north of the area through tonight with milder
temperatures and rain into early tonight. Above normal temperatures
will then continue through the weekend, with another storm system
bringing another round of rain and windy conditions Saturday night
and Sunday.

&&

.NEAR TERM /THROUGH SATURDAY/...
Temperatures are above freezing across Oswego and Lewis counties and
therefore Winter Weather Advisory has been canceled. Rain will
continue into the evening.

Western and north central New York currently seeing an uptick to
precipitation coverage. This precipitation though will be a rather
quick shot as models showing a window of about 4 hours of stronger
isentropic ascent with a broad corridor of warm air advection ahead
of approaching frontal system.

Aggressive mid level drying in the wake of the approaching frontal
passage will then bring a progressive end to the precipitation from
west to east this evening. Temperatures will be mild ahead of the
frontal boundary within broad southerly flow. The western lake
plains will be the warmest with readings peaking in the 50s. Cooler
air then drains back in tonight under veering low level flow behind
the frontal passage, as high pressure briefly builds across the
northern Great Lakes. This will land lows Saturday morning back in
the lower to mid 30s. Period of warmer air and higher dewpoints over
the ice covered portions of the lakes could yield a period of fog
which may advected across areas just to the east of the lakes
overnight.

Saturday, high pressure slowly retreating off to our north will
provide dry conditions for a good portion of the day. The frontal
boundary which pushes through this evening will lay just south of
the area Saturday and will remain there until deepening low pressure
across the midwest begins to lift it northward after sunset.
Northern edge of expected developing rain shield could sneak into
the western Southern Tier by late in the day. Temperatures should
rebound back into the 40s.

&&

.SHORT TERM /SATURDAY NIGHT THROUGH TUESDAY NIGHT/...
Organizing low pressure over the mid-Mississippi Valley at the start
of this period will rapidly deepen as it tracks to near or over Lake
Superior by Sunday morning...then on to James Bay by early Sunday
evening. As it does so...this low will first push a warm front
across the region later Saturday night and early Sunday...followed
quickly by an occluding cold frontal boundary during Sunday.

This system will bring a period of widespread light to moderate rain
to our region during the Saturday night/Sunday time frame...with the
main driving mechanism for this coming in the form of strong low
level warm air advection and low level convergence on the nose of a
50-60 knot low level jet. Compared to what has been previously
seen...the 12Z iteration of the guidance suite has come in notably
slower overall with respect to the arrival of the precip Saturday
night...so have trended the forecast in that direction with this
afternoon`s update. At this juncture it appears the bulk of the
precip will come between the second half of Saturday night and the
midday to early afternoon hours on Sunday...during which time most
areas appear to be in store for a rough half to three quarters of an
inch of rain. For the most part this should not be enough to cause
much concern for flooding...though with more rain falling to our
south (and across the Allegheny River Basin) on Saturday...there is
still the risk that the Allegheny River could approach flood stage.

While the bulk of our region will again be warm enough to support
just plain rain from this next event...the potential still exists
for a period of mixed precipitation or freezing rain across portions
of the North Country late Saturday night and early Sunday morning...
where surface temperatures could remain below freezing initially in
spite of the overall warm air advection pattern. This could result
in some icing across portions of Jefferson and Lewis counties...
before surface temperatures warm enough to allow a changeover to
all rain by mid to late Sunday morning.

With some of the available guidance packages also suggesting the
potential development of a little weak elevated instability out
ahead of the cold front Sunday morning...it is also not out of the
question that there could be a few weak elevated thunderstorms just
out ahead of/with the cold front as it crosses our region...with the
greatest potential for this lying from about the Genesee Valley
westward. While currently not confident enough in this to place a
mention in the forecast...we`ll have to keep an eye on this
potential over the next day or so.

All the above said...the greatest concern with this system remains
the potential for a period of strong to potentially damaging winds
across portions of our area Saturday night and Sunday. At this
point...the region of the greatest risk remains from the Chautauqua
county shoreline northeastward across the Niagara Frontier to
Monroe county...where a period of strong winds remains possible
following the passage of the cold front on Sunday. While the track
of the low is farther to the northwest than one would typically like
to see and the exact strength of the low level wind field still
remains a bit of a question mark...on the other hand strong dry
slotting in the wake of the front will allow skies to clear rather
nicely during peak heating on Sunday...and this coupled with large-
scale subsidence and modest cold advection will result in the
efficient downward transfer of higher momentum air from aloft to the
surface. With this in mind...it still appears quite possible that
this region will see wind gusts of 50 to 60 mph during the day
Sunday...for which a High Wind Watch therefore remains in effect.

Preceding the cold frontal passage...there will also be the risk of
a downslope wind event later Saturday night into Sunday across
portions of the area. One such region is the Chautauqua Ridge...
where the strength and orientation of the low level wind field is
less than ideal for such an event...but where at least some
marginally advisory-worthy gusts also cannot be totally ruled
out...and where the aforementioned High Wind Watch also begins a bit
earlier to cover this possibility. Of much greater concern for such
an event however are areas east of Lake Ontario...where the low
level wind field is notably stronger (50-60 knots at 925 mb as
opposed to 40-45 knots over the Chautauqua Ridge)...and also better
aligned out of the southeast late Saturday night and Sunday morning.
History has shown that such winds are favorable for both strong
downslope winds to the lee of the Tug Hill...as well as strong
funnelling down the Black River Valley. With this in mind...we have
elected to hoist another High Wind Watch for Jefferson and Lewis
Counties from 09Z-18Z Sunday to cover this potential. In between
these two main regions...some stronger downslope winds are also
likely downwind of the higher terrain in the Finger Lakes Region...
but there winds aloft remain substantially weaker and thus more
likely to support sub-advisory criteria gusts.

With respect to temperatures...this system will also bring our
region a brief but substantial warmup out ahead of the cold front
Saturday night and Sunday...with readings likely to peak in the
mid 40s to lower 50s across the eastern Lake Ontario Region...and in
the mid 50s to lower 60s across the rest of the area. This window
of springlike warmth will be relatively short-lived though...with
modest cold air advection likely to send readings back on a downward
trend following the cold frontal passage.

After all that...Sunday night will then feature a return to much
quieter weather along with diminishing winds as high pressure over
the Ohio Valley ridges northeastward across our region. The newly
established quiet and dry weather will then continue through the
balance of this period...as the aforementioned surface ridge slowly
drifts eastward to the mid-Atlantic coastline and then slightly
offshore through Tuesday night. At the same time...our temperatures
will remain at well above normal levels for this time of year...with
daytime highs only settling back to the mid 40s to lower 50s on
Monday...before climbing back up a category on Tuesday as somewhat
milder air works into the region on the backside of the eastward-
migrating surface ridge.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
Where`s all the cold air??

There is high confidence for above normal temperatures through this
period...as a low amplitude split upper level flow over the country
will include a pronounced sub tropical jet that will extend across
the Lower Great Lakes. This will help to establish a feed of very
mild air out of the GOMEX that will hold our temperatures some 15
deg above seasonal levels for Wednesday and Thursday. As we head
into the weekend...the mercury will gradually settle to near normal.

For those wondering about true cold air...an analysis of H85 temps
shows a pool of sub arctic air (<-20c) near and east of Hudson
Bay...but this airmass will retreat poleward during the coming days.
By the time this forecast period begins (Wedn)...you will have to
travel north of 65N...and more specifically to the Yukon...to find
any arctic air. Given the split flow over the Lower 48...there will
no chance of tapping this marginally cold air.

While confidence is high for above normal temperatures during the
bulk of this forecast period...timing differences between the main
ensemble packages will lead to a fair amount of uncertainty about
the potential for precipitation. As for the day to day details...

A bulging ridge over the eastern third of the country will allow H85
temps to climb above freezing while maintaining enough subsidence to
keep fair weather in place. The warmer air aloft will encourage max
temperatures to reach well into the 50s across the western
counties...with readings generally holding in the 40s across the
North Country.

Timing differences between the models starts to come into play at
this point. A divergent upper level over the region Wednesday night
may be be accompanied by a northward advancing warm front. Greater
consistency would lead to much higher pops...but with the ECMWF
being so slow with a southern stream storm system...will just
maintain our low chc pops for showers for Wednesday night.

A complex storm system based over the Upper Mid West on Thursday
will impact our region with more showers...but will have to be
conservative with pops as radically different scenarios are being
proposed by the main forecast packages. Will maintain 40 pops
regionwide.

As we push through Thursday night and Friday...the potential for
showers will increase while temperatures will decrease. While there
is relatively high confidence in this general scenario...being more
specific with higher pops...p type and temperatures would be
fruitless.

&&

.AVIATION /22Z FRIDAY THROUGH WEDNESDAY/...
Mix of VFR/MVFR cigs this afternoon. Rain will be widespread into
this evening ahead of approaching frontal boundary. Some freezing
rain is expected on the Tug Hill south of KART until around 20z.
There is a possibility of some patchy LLWS this afternoon/evening
with a 40-50 kt low level jet shifting over our region.

Rain will taper off west to east this evening with a frontal
boundary shifting east across the area. All precipitation should be
finished by midnight at the latest. Low MVFR/IFR cigs will linger
through tonight. There is also some fog expected to develop over the
lakes. West winds look to shift some of this fog over KBUF/KIAG/KART
this evening with VSBYS down to IFR.

Finally, since the KJHW observation has not been reporting, we have
continued with a rarely used NIL TAF for KJHW. Safety concerns have
played a major role in this decision, especially given the continued
potential for low/erratic ceilings at this particular site.

Outlook...

Saturday...VFR/MVFR with additional periods of rain developing...
which may be mixed with a little snow or freezing rain across the
North Country at the onset. LLWS also becoming likely.
Sunday...Rain ending with improvement to VFR...though also turning
rather windy.
Monday through Wednesday...Mainly VFR.

&&

.MARINE...
Southerly wind will increase to near 20 knots on the eastern ends of
the lakes as low pressure passes by to our north. The offshore
orientation will keep the highest waves confined to Canadian waters
of Lake Ontario.

In the wake of this system, high pressure will briefly build back
across the Lower Great Lakes on Saturday, before a strong area of
low pressure tracks northeastward across the Great Lakes on Sunday.
This system will bring at least a round of high end advisory-worthy
conditions, with gale force winds possible.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...High Wind Watch from late Saturday night through Sunday
     afternoon for NYZ007-008.
     High Wind Watch from Sunday morning through Sunday afternoon
     for NYZ001>003-010>012-085.
     High Wind Watch from late Saturday night through Sunday
     afternoon for NYZ019.
MARINE...None.

&&

$$

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



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