Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 210847
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
447 AM EDT Sun May 21 2017

.SYNOPSIS...
Scattered showers will be possible this morning, with warming
temperatures and more widespread precipitation spreading from west
to east across the forecast area this afternoon into this evening,
ahead of a cold front that will cross the region late Sunday night
into Monday. Another round of showers will be possible Wednesday
into Thursday, as an upper level trough crosses the region.

&&

.NEAR TERM /THROUGH TONIGHT/...
The synoptic scale pattern across the eastern half of the country is
dominated by two main features of note. The first is a highly
amplified upper level ridge extending from the Carolinas to Hudson
Bay, while to the west, a large low pressure system is spinning over
the Missouri valley. Strongly meridional flow between these two
systems is directing GOMEX moisture across the Ohio valley and into
the Great Lakes, with widespread showers, and a few embedded
thunderstorms noted across eastern Michigan and southern Ontario on
regional radar this morning.

Closer to home, another band of light shower activity extends from
Georgian Bay southeastward to the eastern Finger Lakes. This is
associated with a wing of warm advection aloft and moisture spilling
over the ridge. This shower activity will remain light and scattered
in nature through the night before lifting off to the north as a
warm front pushes through the region. Scattered showers will be
possible again across the Niagara Frontier during the morning hours
as the low-level jet axis begins to press into the area and
isentropic uplift increases. This activity is already being noted
across central Lake Erie early this morning.

The main axis of precipitation will arrive over far Western New York
early this afternoon and will advance across the forecast area
during the afternoon and into the evening hours, following the
progression of the low-level jet axis that will be feeding moisture
into the region. Model-derived instability appears weak, but given
the strength of synoptic lift, a few rumbles of thunder cannot be
ruled out. We may see a break in the precipitation late this evening
before the arrival of a second round later tonight, as the cold
front proper moves into the forecast area. Most areas can expect a
quarter to a half inch of precipitation from this system.

Temperatures should warm into the upper 60s to lower 70s today, as
warm air surges into the region. Areas along the Lake Erie shoreline
will run several degrees warmer, in the mid to upper 70s, due to
downsloping southeasterly winds that may gust as high as 40 mph this
afternoon. Indeed, with the strengthening low-level jet, it will be
quite breezy across much of the forecast area, with gusts of 20 to
35 mph possible across most areas. It will remain mild tonight, as
cooler air behind the front will not arrive in earnest until Monday.
Look for lows in the 50s.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY NIGHT/...
During Sunday night and Monday...modest low pressure over the Upper
Great Lakes will slowly meander its way into central Ontario Province...
while also pushing its trailing cold front across our region. Plentiful
GOMEX-based moisture (PWATs of as much as one and a half to one and two-
thirds of an inch) will be in place for the front to work with...resulting
in a period of fairly widespread showers Sunday night into early Monday...
with a few embedded thunderstorms also possible Sunday night given the
presence of some lingering weak elevated instability. Have continued with
higher-end categorical PoPs for the showers...which should produce another
quarter to half inch of basin-average rainfall.

Following the frontal passage...surface-based ridging and attendant
drier air/subsidence will quickly build across our region during the
course of Monday. This will result in rapidly improving conditions from
west to east...with areas west of the Genesee Valley likely experiencing
a dry afternoon along with partly to mostly sunny skies...and areas
further east also experiencing a return to mainly dry weather as the
afternoon progresses. Regarding temperatures during the first 24 hours
of this period...expect fairly mild lows in the lower to mid 50s Sunday
night...with modest cool air advection then leading to highs mostly in
the mid to upper 60s on Monday.

Monday night and Tuesday...the aforementioned ridging and associated
drier air should allow for mainly dry conditions to continue across
our region...though a few showers still cannot be totally ruled out
across western sections as the next system approaches later Tuesday.
Otherwise temperatures will average out a bit above normal...with lows
in the mid 40s to lower 50s Monday night followed by highs climbing back
into the lower to mid 70s on Tuesday...as modest warm air advection
sets up out ahead of the next system.

Moving on into the last 36 hours of the period...our weather should turn
at least somewhat more unsettled again as additional shortwave energy
drops southeastward from the Upper Mississippi Valley and results in
re-amplification of upper level troughing over the Great Lakes and Ohio
Valley...which in turn will encourage another broad surface low to develop
into the Central/Lower Great Lakes by the end of this period. The above
stated...the medium range guidance also exhibits considerable spread with
respect to both the location and degree of organization of the resulting
surface low...with the new 12Z ECMWF a bit faster/better organized and
consequently wetter sooner for our region...while the 12Z GFS/GEM and
older 00z ECMWF are/were all slower and less organized...which would
allow for considerably drier weather to persist through the end of this
period. Given the resulting forecast uncertainty...have elected to hedge
the forecast toward a somewhat more optimistic/drier scenario for now...
with PoPs only gradually increasing into the mid-high chance range by
Wednesday/Wednesday night. Temperature-wise...readings should remain a
little above average...with highs in the upper 60s/lower 70s Wednesday
sandwiched between lows in the lower to mid 50s both Tuesday and Wednesday
nights.

&&

.LONG TERM /THURSDAY THROUGH SATURDAY/...
While significant weather is not expected during this three day
period...there is fairly high confidence that conditions will be
unsettled for at least the start of it.

Medium range guidance is in general agreement that a large closed
low will be in the vicinity of the Lower Great Lakes and Upper Ohio
Valley for Thursday and Thursday night. Given the variance between
the guidance packages/ensemble members...it would be fruitless to
try to get into the details of where the surface features would line
up. It is safe to say though that showers will be most common during
this initial 24 hours of the long term period. Will use likely pops
for Thursday and high chc pops Thursday night...but can easily see
those numbers climb to `categorical` as the confidence increases in
later forecast packages.

The lumbering closed low will exit across New England Friday and
Friday night. While there will still be the chance for some
showers...the probability will steadily decrease as we push into the
weekend.

On Saturday...a progressive ridge will make its way across the Great
lakes region in the wake of the exiting storm system. This will
favor fair dry weather...so will undercut most of the guidance
package pops which appear to be meteorologically inconsistent with
the large scale pattern.

In regards to temperatures...the mercury will average close to...or
just a bit below normal for Thursday and Friday...as max temps will
be mainly in the mid 60s. Some warming is then expected Saturday
when highs will be upper 60s to lower 70s. Mins will be within a few
degrees of 50 both Thursday and Friday nights.

&&

.AVIATION /09Z SUNDAY THROUGH THURSDAY/...
A band of very light showers extending from Georgian Bay to the
eastern Finger Lakes will continue to lift northeast this morning,
leaving VFR conditions across much of the forecast area through the
morning hours, as the area will be in the warm sector of a low
pressure system centered well to our west, across the upper
Mississippi valley. Showers will move in from the west this
afternoon however, as a pre-frontal trough moves in ahead of a cold
front. Expect showers to move in from 18Z onwards, with MVFR
conditions possible in heavier showers. An isolated thunderstorm or
two will also be possible, however overall instability is marginal
at best, so am not expecting much in the way of widespread
thunderstorm activity.



Outlook...

Sunday night through early Monday morning...MVFR. Showers
likely with a chance of thunderstorms.
Later Monday and Tuesday...Mainly VFR.
Wednesday and Thursday...VFR/MVFR with showers likely.

&&

.MARINE...
Southeasterly flow will strengthen to day ahead of a low pressure
system over the upper Mississippi valley. Warm advection and
offshore flow mean that wave action should be negligible for today
into this evening, with wave action being focused on the Canadian
shores of the lakes, though we may see some choppy waters develop
near the entrance to the St. Lawrence late tonight, just ahead of a
cold front that will cross the region Monday. Winds backing to the
southwest may produce some light to moderate chop across the
northeastern ends of the lakes Monday afternoon into Monday night,
though conditions should remain sub-advisory. Waves will then
diminish Tuesday, with tranquil conditions persisting on the lakes
through the middle of the week as a weak area of high pressure moves
across the region, followed by a broad area of low pressure.

&&

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

&&

$$

SYNOPSIS...WOOD
NEAR TERM...WOOD
SHORT TERM...JJR
LONG TERM...RSH
AVIATION...WOOD
MARINE...APFFEL/WOOD



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