Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 270933
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
533 AM EDT TUE SEP 27 2016

.SYNOPSIS...
Cooler air following in the wake of a cold front will generate some
lake effect rain showers northeast of the lakes today...with these
then falling apart this evening. After a mainly dry day Wednesday...
a slow moving storm system will then bring unsettled weather to our
region for the rest of the week and next weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
During this period...a large closed upper low will slowly meander
its way south-southeastward across the Upper Great Lakes. While we
will be firmly under of the influence of its accompanying cool pool...
the main region of wraparound moisture associated with the low will
remain well to our west over the Upper and Central Lakes...with just
an attendant spoke of this sliding across far Western New York and
adjoining portions of Lake Erie/Lake Ontario today. For most
locations...today and tonight should thus just be mainly dry and a
little cooler...with daytime highs ranging between the mid 60s and
lower 70s today...and nighttime lows falling into the mid 40s to lower
50s tonight. The two exceptions to the dry conditions will be over
and downwind of Lakes Erie and Ontario...where the cool air aloft
and warm lakes will team up to generate some lake effect rain showers
between today and this evening.

Overall...the best setup still looks to be found over and downwind
of Lake Erie this morning...where a somewhat sheared southwesterly
low level flow and the aforementioned spoke of deeper moisture
should be supportive enough for the ongoing scattershot activity to
eventually develop into a broken band of showers and embedded
thunderstorms. This stated...the amount of moisture available is
not looking to be quite as favorable or as long lasting as it had
previously appeared...and this in conjunction with with the above
mentioned shear (and increasing diurnal influences after sunrise)
will probably help to hinder the development of the band a bit more
than previously anticipated. Taking all of this into consideration
in tandem with the current appearance of the activity...have elected
to trim PoPs for this morning back into the likely range. With respect
to the location of the lake effect...slow but steady backing of the
low level flow should result in the showers drifting from their
current position near or a just little south of Buffalo northward
into the Northtowns and Niagara and Orleans counties this morning.

By the time we reach early this afternoon...the combination of
diminishing background moisture...continued slow but steady backing
of the low level flow...and a further increase in shear will result
in the showers weakening and becoming much more scattered while
lifting almost entirely into Niagara County...before sliding west
of the area and falling apart altogether tonight as our airmass
becomes too dry and the low level flow backs to southerly.

Meanwhile off Lake Ontario...the setup appears considerably less
favorable as the background airmass will be drier...and the low
level flow will likely be a bit more sheared and south-southwesterly
overall...which will also translate into a shorter fetch. This
should result in much more scattered showers primarily affecting
Jefferson County and the uppermost Saint Lawrence Valley today...
with this activity then also shifting further westward and falling
apart altogether tonight as the airmass dries out and the low level
flow turns south-southeasterly.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
A mid latitude blocking pattern will be found over North America
during this period...as a staunch Omega block will be anchored by a
stacked storm system over the Ohio Valley. This will result in a
prolonged period where unsettled weather can be expected across our
region...but at least temperatures should average a few degrees
above normal.

The origins to this block will be found with the closed low...the
same one that helped to drive a very rainy cold front across our
region Monday night. While this large feature will be centered over
the Upper Great Lakes as we open this period...it is forecast by an
over whelming majority of the various ensembles to gradually settle
south across the Ohio Valley by late Wednesday. An anomalously
strong sub tropical ridge will blossom over the Western Atlantic in
response to this scenario...and this will only reinforce the block
as we head into the weekend.

A synoptician could easily argue that the Omega Block will evolve
more into a Rex Block with time...as the equatorward drift to the
stacked low will allow for height rises over Ontario and Quebec
where strong surface high pressure will already be firmly in place.
This basic pattern is not dissimilar to the one that helped to get
the ball rolling back in late Spring to direct us towards drought
conditions. The difference is that is displaced some 500 miles
further north...so this time around we should experience frequent
showers. The unsettled weather will come as a result of a persistent
divergent upper level flow around the northeast side of the closed
low that will help to lift an increasingly moist Atlantic based
flow. The resulting pcpn will be enhanced by shortwaves that will
rotate around the axis of the parent upper low.

Getting to the day to day details...

The large closed upper low will drift from Lake Michigan to Ohio
during the day Wednesday. This will back our boundary layer flow in
the process...so that a feed of Atlantic based moisture will be
established from New Jersey and Pennsylvania back across the
Appalachians towards our forecast area. As the moisture deepens...
an upslope component will help in the low levels to support the
divergent upper level flow to produce the chance for light showers
across the Southern Tier and portions of the Finger Lakes. The
remainder of the forecast area should remain rain free as sfc
ridging extending south along the east side of the Appalachians will
keep drier air and subsidence in place. This should allow for at
least partial sunshine from the Finger Lakes to the Eastern Lake
Ontario Region and Thousand Islands. In fact...glorious weather may
characterize conditions from the Tug northward. Temperatures will be
allowed to climb into the 70s for all areas north of the Srn Tier.

During the course of Wednesday night...the upper low will settle
over the Ohio Valley. This will further back the low level flow over
our region...which in turn will lead to a significant deepening of
the Atlantic moisture. While the source of large scale lift should
still come from the divergent upper level flow and orographic
forcing in the low levels (mainly Srn Tier)...the timing of
shortwave passages will now become crucial in nailing down enhanced
areas of rain. Will not try to explain or sort out the differences
between the guidance packages as to to placement of these
features...as this will be an issue until they can be identified in
WV imagery. Given the gradient with the depth of the moisture
though...will taper the pops from south to north with likely pops
being found across the Southern Tier and portions of the Finger
Lakes Region. Have also introduced low chc pops across the North
Country after midnight to account for the variance in guidance...
but can easily see that area remaining dry for one last period.

On Thursday...the vertically stacked storm system will be parked
over eastern Kentucky while strong high pressure will remain
centered over Quebec. This will keep a feed of rich Atlantic
moisture over our region with the likelihood of at lest shortwave
rotating back across our region. Have bumped pops up a category in
most areas...with likely pops now in place over all of western New
York while low chc pops will still be found across the North
Country. This day could have the potential to be rather `ugly`
across the Srn Tier. Because of the thicker cloud cover and higher
potential for showers...temperatures regionwide should not make it
out of the 60s.

While the axis of deepest Atlantic moisture will still be draped
across our region Thursday night...there should be somewhat of a
lull in pcpn. There is some consensus that there will be a break at
this time between pcpn enhancing shortwaves...so will go with just
chc pops for all areas but the Srn tier. This will also align well
with the adjacent CTP and BGM forecast areas. Will maintain this
basic pop gradient through Friday as the medium range ensembles
suggest a gradual northward push of the closed off low to Wrn Ohio.
This would keep the focus for the most active shower activity over
the Srn Tier...while temperatures will be similar to those from
Thursday.

The three day (Wed-Fri) forecast rainfall ranges from an inch and a
half over the western counties to under a half inch north of the Tug
Hill. There is low to medium confidence in these amounts as they
will be highly dependent on the number of shortwaves that actually
rotate back across the forecast area. Stay tuned.

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
A slow improvement can be expected in our weather during the course
of the weekend and particularly as we push into the new work week.
The mid latitude blocking pattern that will have plagued our region
for several days is forecast by the majority of the medium range
ensemble members to gradually fill and lift out across the Lower
Great Lakes and eventually the St Lawrence Valley during this time
frame. The general northward drift to the previous stalled system
will short circuit the feed of Atlantic moisture that will have
become common...while drier air will gradually work its way into our
region from the Ohio Valley and Mid Atlantic region. This will lead
to a day to day diminishing of the amount...and areal coverage of...
the scattered shower activity.

Meanwhile...temperatures will average just a little above normal.
This will largely come as a result of overnight lows being in the
50s. Afternoon temperatures will generally be in the mid to upper
60s.

Looking further head towards the middle of the week...there is
moderately high confidence that a burgeoning ridge will amplify
across the eastern half of the country in the wake of the departing
mid level closed low. This should allow for fair dry weather to
return in earnest Tuesday and Wednesday with day to day warming.

&&

.AVIATION /10Z TUESDAY THROUGH SATURDAY/...
Mainly dry VFR conditions and scattered cloud cover will prevail
across the bulk of the region today...with the exception of areas
northeast of Lakes Erie and Ontario where some lake effect rain
showers are expected. The most numerous activity will be found off
Lake Erie this morning...where the northward-migrating showers
could produce some MVFR conditions at KBUF/KIAG before weakening
and becoming more scattered this afternoon. Off Lake Ontario...
lighter and much more scattered showers will be found from roughly
KART northward...with these much less likely to produce reductions
to below VFR.

Tonight...VFR conditions will prevail...with any lingering lake
effect rain showers north-northeast of the lakes shifting west
of our region and falling apart as the low level flow turns
more southerly...and our airmass dries out even further.

Outlook...
Wednesday...VFR...with just a chance of showers late near KJHW.
Wednesday night through Saturday...VFR/MVFR with periodic rounds
of scattered to numerous showers.

&&

.MARINE...
A brisk southwesterly flow of cooler air will produce continued
advisory-worthy winds and waves on Lake Erie and at the eastern
end of Lake Ontario today. While the conditions on Lake Ontario
will improve this afternoon as winds diminish...the stronger
winds will persist across Lake Erie into this evening...before
slackening through the remainder of the night.

In addition...although conditions will not exactly be ideal for
waterspouts...a few of these still cannot be ruled out over
Lake Erie and western Lake Ontario today.

After a brief respite overnight and Wednesday morning...a brisk
easterly flow will develop across the Lower Lakes region between
later Wednesday and Thursday morning...then will persist through
the end of the work week. This will likely require the issuance
of another round of small craft advisories sometime either later
on today or tonight.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory until 2 AM EDT Wednesday for LEZ040-041.
         Small Craft Advisory from 10 AM this morning to 5 PM EDT
         this afternoon for LEZ020.
         Small Craft Advisory until 2 PM EDT this afternoon
         for LOZ045.

&&

$$

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



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