Area Forecast Discussion
Issued by NWS Buffalo, NY

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147
FXUS61 KBUF 210007
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
807 PM EDT Sat May 20 2017

.SYNOPSIS...
High pressure across eastern Canada will maintain dry weather
for most of the night, then a warm front will into the area
early Sunday. Showers and a few thunderstorms will spread into
the area Sunday into Sunday night ahead of a cold front that
will cross the area Monday. Another disturbance and associated
frontal boundary is expected to bring another round of showers
Wednesday.

&&

.NEAR TERM /THROUGH SUNDAY/...
A ridge of high pressure extends from Quebec into Northern New
England early this evening. Meanwhile a sharpening mid-level
thermal gradient will lift from SW-NE into the region late
tonight. The departing high will maintain mainly dry weather
through this evening outside of possibly a sprinkle from the
remnants of weakening upstream convection. A mid-level boundary
will provide focus for some showers and possibly an isolated
thunderstorm late tonight across Western New York. Mesoscale
guidance develops spotty showers along this axis after midnight.

Otherwise, a strengthening southeasterly flow will generate
downsloping winds off the Chautauqua Ridge later tonight and
into Sunday, with local gusts to 40 mph possible. This will
also make the Lake Erie shoreline the warmest portion of the
forecast area overnight, with lows in the lower to mid 50s, with
40s from the Genesee valley east.

The mid-level boundary will continue to lift northeastward on
Sunday, but the surface boundary with warmer air will lag behind
this. Meanwhile, showers and thunderstorms will develop ahead of
an approaching cold front Sunday afternoon. Most mesoscale
guidance shows there will be a break between these two features,
with a few hours of dry weather possible late morning and early
afternoon across Western New York. After this, model consensus
continues to suggest a solid line of showers will move into
Western New York late Sunday afternoon. Once the area emerges
into the warm sector, despite clouds and rain temperatures
should push into the upper 60s to lower 70s.

&&

.SHORT TERM /SUNDAY NIGHT 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 /23Z SATURDAY THROUGH THURSDAY/...
Expect widespread VFR conditions tonight, with increasing mid-
level clouds well into the VFR category. There is a small chance
for MVFR cigs across the Southern Tier including JHW tonight,
with otherwise high confidence in VFR conditions tonight. Also,
a strengthening low level jet combined with low- level
inversion may result in spotty areas of low level wind shear but
model guidance suggests this will not quite meet LLWS criteria.

A warm front will spark some showers and isolated thunderstorms
late tonight and Sunday morning as the boundary lifts from SW-NE
across the area. Expect mainly VFR conditions with this.
Showers and a few thunderstorms will develop ahead of a cold
front Sunday afternoon, with these moving into Western New York
late in the afternoon. This will bring periods of MVFR/IFR
conditions in heavier showers.

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...
Northeasterly winds around a high centered just southeast of James
Bay will gradually diminish and shift to the southeast overnight.
Based on this, have dropped small craft headlines for all our
waters.

Winds will veer to the southeast Sunday ahead of an approaching cold
front before veering to the west and freshening behind a cold front,
expected to cross the lakes Monday. Conditions may approach advisory
conditions Monday afternoon on the east end of Lake Erie, but
otherwise, conditons are expected to remain sub-advisory level
into the middle of the coming week.

&&

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

&&

$$

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



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