Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 201050

Area Forecast Discussion
National Weather Service Buffalo NY
650 AM EDT Thu Jul 20 2017

High pressure will move east of the area today. A complex of
thunderstorms will move into the area this afternoon and evening.
Some of the thunderstorms could produce heavy rain and gusty winds.
Dry weather will prevail on Friday before a frontal boundary sets up
in the vicinity of the lower Great Lakes, potentially bringing wet
and unsettled weather at times this weekend. Temperatures will
remain near average through the end of the week and this weekend.


High pressure currently across the region will slip off to the east
today. Quiet weather is expected through at least mid day, as mid
and high cloud debris overspread the forecast area from the western
and central Great Lakes.

Quite a bit of uncertainty exists with how the afternoon will play
out. Currently dual thunderstorm complexes upstream. The southern
complex pushing through northern Indiana is waning and basically
irrelevant to the forecast for western New York. Expectation is for
the mature complex across Wisconsin to cross lower Michigan,
eventually reaching the vicinity of western New York this afternoon
probably in the 17z-22z time frame. Certainly questionable about how
far east shower and thunderstorm activity will make it here locally.
Latest models suggest a greater potential for southwestern New York,
generally south of Niagara Falls and Rochester, which will probably
be the northern edge of instability axis on which thunderstorm
complex will track. In addition, depending on the track of the
complex, there may be an outflow boundary that also may promote

As far as thunderstorm hazards, precipitable water values will be on
the rise and the risk for heavy rainfall and local flash flooding
will exist. In addition, high instability combined with decent shear
suggesting that the strongest storms will be capable of producing
damaging wind gusts. Will continue to monitor the severe threat.

Lingering convection this evening will exit to the south and east of
the area later tonight. This will be followed by a gradual clearing
trend as a weak ridge of high pressure builds into the area.


On Friday...lingering weak troughing draped across Lake Ontario at
the start of the day will slide eastward and give way to an area of
weak high pressure settling southeastward from the Upper Great Lakes.
While an isolated leftover shower cannot be totally ruled out across
the North Country as the weak trough departs...the day should largely
be dry and feature a decent amount of sunshine...along with seasonable
temperatures in the upper 70s to mid 80s and moderate humidity levels.

Friday night the aforementioned surface ridge will gradually shift
eastward to central and eastern New York...while the next wave of
low pressure develops into the upper Mississippi Valley...with its
attendant warm frontal boundary extending eastward into Southern
Ontario and Lake Erie. The combination of warm advective forcing
and shortwave energy rippling eastward along this boundary should
result in one or more waves of showers and thunderstorms developing
and riding eastward along the front as the night progresses...with
some members of the 00Z/20 guidance suite suggesting that some of
the leading shower activity could reach far southwestern New York by
daybreak Saturday. Have therefore maintained some lower-end chance
PoPs across the latter area to account for this potential...with
the remainder of the region otherwise more than likely to remain
dry through the night. As for temps...we can expect overnight lows
to be near or slightly above normal...with readings generally
bottoming out in the 60-65 degree range.

On Saturday the main surface low will press eastward to about Lake
Michigan...with its attendant warm front snaking its way eastward
along or near the NY/PA border. Moisture and forcing pooling along
and north of the front should generally support an increase in
precipitation potential from about Lake Ontario southward as the
day progresses...though given the dearth of instability forecast by
the 00Z/20 guidance would expect this to primarily be in the form of
showers...and as such have elected to significantly cut back on both
any thunder and heavy rain potential for Saturday. Otherwise high
temperatures should range from the mid and upper 70s across the
Southern Tier (where there will be more cloud cover and a greater
potential for showers) to the upper 70s and lower 80s elsewhere.

Saturday night and Sunday the surface wave will continue to slowly
make its way eastward along the quasi-stationary frontal boundary...
eventually reaching central NY/PA by Sunday evening. This wave
should provide a further increase in both lift and moisture as it
approaches...with some increase in instability also potentially
coming into play over time. For much of our area...all of this
should result in convection becoming more likely over time...with
overall thunder potential also climbing later Saturday night and
Sunday. In conjunction with precipitable water values pushing to
near two inches and favorable shear profiles...all of this should
result in this portion of the weekend featuring the greatest potential
for heavy rainfall...along with at least a conditional risk for some
stronger to severe storms...with this highly dependent on the amount
of instability that actually develops. With this in mind...will
continue to highlight the possibility of both heavy rain and some
gusty winds/hail in the HWO for Saturday night and Sunday. As for
temperatures...lows should range through the 60s Saturday night...
with daytime highs on Sunday largely running within a few degrees
of 80.


As we move into the longer term portion of the forecast...general
chances for showers and thunderstorms should linger through both
Sunday night and Monday as upper level troughing digs across our
region...and as the axis of the surface low settles southeastward
to the Mid-Atlantic coastline. Any leftover showers should then come
to an end between Monday night and early Tuesday as high pressure
and cooler/drier air begins building into our area...with fair and
dry weather then following for the rest of the period as the high
first settles directly overhead...then slowly drifts eastward to
the Atlantic coastline.

Regarding temperatures...the digging upper trough and arrival of
cooler and drier air will translate into temperatures and humidity
levels falling off through Tuesday...which should feature daytime
highs generally be in the mid 70s and comfortable dewpoints in the
mid to upper 50s. Both will then begin to rebound some on Wednesday
as the axis of the ridge slides to our east and a southwesterly
return flow develops on its backside...resulting in temps climbing
back to around 80 degrees and dewpoints recovering to around 60
or so.


Widespread VFR conditions can be expected to persist through the
morning hours. A 10-15kft deck will spread across the forecast area
from the west, as cloud debris from an upstream MCS over the western
and central Great Lakes makes its way across the region.

Aside from mid/high clouds, area terminals will be VFR during the
morning hours. Model guidance continues to suggest that a complex of
showers and thunderstorms will cross western NY during the
afternoon/early evening timeframe, generally from 17Z onwards. This
convection would likely bring a period of MVFR/IFR conditions, as it
is expected to produce periods of heavy rain and potentially strong
to severe thunderstorms.

Tonight...Mainly VFR...with a chance of showers and
thunderstorms south of Lake Ontario before 03z.
Saturday and Sunday...VFR/MVFR with some showers and thunderstorms.
Monday...Mainly VFR but with a chance of showers.


A weak surface pressure gradient will remain in place through today,
which may allow some local afternoon lake breeze development. Will
definitely need to keep an eye on thunderstorm evolution this
afternoon across Lakes Erie and Ontario, with any storms that do
develop capable of producing very gusty winds and enhanced wave
action, potentially leading to some special marine warnings.
Mariners should be alert for any warnings that may be issued.

High pressure over the upper Great Lakes tonight will then expand
across the lower Great Lakes for Friday and Friday night. This will
result in fine conditons for recreational boating to end the work





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