Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 210919

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

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.


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.


Stacked low pressure will be centered over the northern shore of
Lake Superior Monday morning with am occluded front extending east
into southwest Quebec and a cold front extending south from the
triple point into the Genesee Valley. A sharp mid-level shortwave
trough will pivot across northern lower Michigan into eastern
Ontario province through the day with the forcing from this feature
acting to advance the cold front and moisture axis across central
and into eastern NY while a piece of energy breaks off and is left
over the Upper midwest. The movement of the front should allow for
morning rain showers to taper off from west to east with advancement
of the front. Cold air advection behind the front will lower 850mb
temps down to about +4C which should limit afternoon high
temperatures to only the low to mid 60s or slightly below normal.
The cold advection will also allow gusty winds during the day with
gusts 25-35 possible especially downwind of Lake Erie.

High pressure will drift from the Ohio Valley across western NY and
into New England Monday night through Tuesday. This will bring dry
weather with mainly clear to partly cloudy skies while keeping any
precipitation shielded to our northwest and southeast. Temps will
slip into the upper 40s to around 50 Monday night then warm above
normal into the low to mid 70s on Tuesday with some sunshine and
benefit from a return of warm air advection under southerly flow.
Winds should be light.

The progressive surface high and low amplitude mid-level ridging
will move away from NY into New England Tuesday night. This will
open the door to a return of some unsettled weather as the lingering
mid-level low over the Upper Midwest is forecast to dig a deep
trough across the eastern half of the CONUS. Fairly dry 925-700mb
moisture should keep most areas rain free Tuesday night with only a
slight chance of POPs spreading north into the forecast area
overnight. While the 00z GFS seems to keep us dry through the night,
the slight chance covers what the 00z EC shows as a quicker arrival
of 925-700 moisture available for mid-level divergence east of the
trough to work with. Temperatures Tuesday night will be on the mild
side only slipping back into the low 50s. Mid-range chance POPs are
in place Wednesday then with more likely probabilities for rain
coming Wednesday night as a surface/700mb low is shown by models
lifting across the Ohio Valley or southwest Ontario depending on
your model of choice. Temperature-wise, readings should remain just
above average with highs in the upper 60s/lower 70s Wednesday then
again slipping back into the low to mid 50s Wednesday night.


00z model guidance continues to show a deep mid-level tough axis
shifting over the eastern Great Lakes through Thursday then into New
England for Thursday night. The combination of this shifting mid-
level trough and moisture axis crossing our forecast area will bring
widespread high end likely POPs for Thursday tapering back to chance
range for Thursday night.

The lumbering closed low will exit across New England Friday and
Friday night. While there will still be some decreasing chances for
some showers.

Later Friday night into Saturday, a narrow progressive ridge will
make its way across the eastern Great lakes region favoring fair dry
weather which should hold at least through Saturday morning. Slight
chance POPs are held due to potential faster timing with the arrival
of the next system. 00z GFS/EC shows this next system could bring in
rain as early as Saturday afternoon across western NY where chance
POPs have been included.

In regards to temperatures...the mercury will average close to...or
just a bit below normal for Thursday and 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 either side of 50 both Thursday and Friday nights.


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.


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.


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.





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