Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 201806

Area Forecast Discussion
National Weather Service Buffalo NY
206 PM EDT Thu Jul 20 2017

A complex of thunderstorms will move across western and central New
York this afternoon. 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.


Complex of strong to severe thunderstorms is quickly shifting across
the Genesee Valley into the Finger Lakes near 2pm. These storms are
being forced ahead of a weak surface cold front and mid-level
shortwave trough/vort max. The complex is moving into a region of
building CAPE over 1000 j/kg per SPC Mesoanalysis. 0-6km shear is
also very impressive at 50-60kts supporting the organized
convection. Damaging winds from organized embedded bows will
continue to be the main threat but an isolated tornado or two could
also be possible with strong shear interacting with intersecting
surface outflows. A Severe Thunderstorm Watch remains for Allegany,
Livingston, Ontario, Wayne, and Northern Cayuga Counties
within our forecast area through 8pm.

Expect the leading organized cells of this complex to exit the
forecast area by mid afternoon. Behind the leading cells, some weak
showers and isolated thunderstorms over western NY will clear from
Northwest to Southeast. The far western Southern Tier could also see
some storms potentially triggered by lake breeze or storm outflow
from the convection moving across the Genesee Valley/Finger Lakes.
Not expecting strong storms for the North Country do to the lack of
instability. Temps will push into the upper 70s to low 80s this
afternoon with dewpoints in the upper 60s making for uncomfortable
humidity levels.

Convection will be shifted into south-central NY by this evening
leaving dry weather with some sunshine breaking through along with
lingering humidity this evening. Skies will continue to break
through the overnight but low temps only dipping into the mid to low
60s will make for another mild night.

Friday...An area of weak central Canadian-sourced high pressure will
settle southeast from the Upper Great Lakes. While an isolated
leftover shower cannot be totally ruled out across the western
Adirondacks the day should generally be dry and feature a decent
amount of sunshine. Temperatures will be seasonable in the upper 70s
to mid 80s with a little more comfortable humidity levels as
dewpoints drop back into the lower 60s with northwest flow.


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.


A cluster of strong to severe storms is moving across the Genesee
Valley into the Finger Lakes at 18z. Clearing is occuring to the
Northwest with KIAG/KBUF only seeing a stray shower. Some lingering
MVFR in SHRA at KROC will lift to VFR by 20z. All sites will then
run VFR as high pressure builds in from the northwest tonight with
partial clearing.

Saturday and Sunday...VFR/MVFR with some showers and thunderstorms.
Monday...Mainly VFR but with a chance of showers.


Strong showers and storms have shifted south/east of the lakes this
afternoon. There may still be a stray shower or storm through this
evening but overall the threat for severe storms has ended over the
waters. 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 week.





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