Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 231901
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
301 PM EDT Sun Jul 23 2017

.SYNOPSIS...
A frontal boundary will remain draped across Western New York
through Monday, with showers and thunderstorms forming along this
boundary. A very moist airmass in place will allow for heavy
rainfall from these storms to go along with the possibility of gusty
winds and small hail. High pressure will build towards the region
Tuesday and Wednesday with fair weather conditions.

&&

.NEAR TERM /THROUGH MONDAY/...
This afternoon a warm front has pressed deeper into WNY from the
Ohio Valley...with this front just to the south of route 20A. This
boundary has been the focus for storms thus far this afternoon with
regional radars displaying scattered activity across WNY.

The threat for severe storms and heavy rain will continue through
this afternoon and then overnight.

For severe storms, a partly sunny sky through the morning and
afternoon allowed for instability to increase to around 1500 to 2000
J/KG or SBCAPE. A 35 to 40 knot 0-6 km bulk shear profile exists,
though the strongest/most impressive winds aloft are near the 6km
level and not deeper down into the atmosphere. Still this shear,
combined with SBCAPE values of 1500 to 2000 J/KG will support an
environment for storms to grow upon. The CAPE profiles aloft, and
WBZ heights around 9 to 11K feet suggest that hail formation is
possible...and will continue to mention both hail and gusty winds in
the forecast, along with heavy rain owing to PWAT values of 1.50"
and greater. There will be some concern...as there is not much depth
to the 0-6 km shear, and the upper level trough providing mid level
lift remains well to our west. Still with the frontal boundary
remaining stalled over the region a SPC slight risk for severe
storms will remain in to the evening hours...generally for portions
of WNY. In the vicinity and to the north of the frontal
boundary, wind shear profiles of easterly at the surface and
southwesterly aloft will support the risk of an isolated
tornado.

Well to the north of this boundary, and across the Eastern Lake
Ontario region, a less stable and slightly drier airmass will
hold off storms till this evening...at which point the concern
will be transition from severe in the sense of damaging winds
and hail, to more of heavy rain.

Tonight the surface low will only advance from eastern Lake Erie to
the Genesee Valley. Low level winds ahead of this low will veer some
across the Finger Lakes and Eastern Lake Ontario region to a
direction more favorable for isentropic lift. Aloft as an upper
level trough passes through the Central Great Lakes, upper level
deformation will increase across WNY. With an uptick in the LLJ
overnight, and a very moist airmass of PWAT values ranging 1.50 to
2.00 inches, the increased low level lift will bring favorable
conditions for heavy rainfall across the Eastern Lake Ontario
region. Here one to three inches of rainfall will be likely through
the overnight. Will hold off on any flood watches at this time as
Flash Flood Guidance is up around 3 inches in three hours, and the
heavier rainfall tonight will be spaced out through the night. Will
continue to mention this threat within the HWO product.

Tomorrow heavy rainfall will likely still be in the vicinity of the
North Country, with scattered showers and isolated thunder back near
the surface low which will reside near WNY. This slow moving surface
low will track southeastward through the day, gaining some momentum
as the upper level trough nears. This trough, and its associated lift
will again bring showers and thunderstorms in a destabilizing
atmosphere. Tomorrow SBCAPE values will likely not be as great,
generally around 1000 J/KG, and marginal risk for severe
thunderstorms across portions of WNY will be in place. These
potential hazards will continue to be highlighted in the Hazardous
Weather Outlook.

Temperatures will remain seasonable through the period, with
humidity values ranging through the 60s, and at times reaching the
lower 70s.

&&

.SHORT TERM /MONDAY NIGHT THROUGH WEDNESDAY NIGHT/...
Surface low centered over eastern Lake Ontario Monday evening with
associated mid level trough axis is forecast to shift to the east
through the overnight passing into New England by Tuesday. The 12z
GFS indicates one last embedded shortwave crossing Central New York
where likely/chance POPs have been included Monday night otherwise
just slight chance POPs tapering off west to east across the
forecast area as the low departs.

Have lingered a very low chance of showers early Tuesday across the
eastern Lake Ontario Region with any lingering moisture and upslope
flow. There could also be some slow to break cloud cover with
northerly upslope flow off Lake Ontario. Otherwise, expect dry and
quiet weather Tuesday through Wednesday as surface high pressure and
low amplitude mid-level ridging shifts across western and central
New York. Wednesday night should still remain dry for most although
there is a slight chance of showers across the SLV as the next mid-
level trough crosses the upper and central Great Lakes.

Temperatures Tuesday will run a touch below normal with lingering
cool air aloft and northerly flow in the wake of the storm system.
Highs only expected in the low to mid 70s with 850mb temps down to
+10C. Dewpoints dipping into the 50s will also make for comfortable
humidity levels thanks to the drier northerly flow. Wednesday,
southerly flow develops helping to boost temperatures back to late
July levels with highs forecast in the upper 70s to low 80s except
low 70s across the Tug Hill. Expecting a couple of cooler nights
Monday night and Tuesday night with lows in the mid 50s inland to
around 60 at the lakeshores. Then mugginess will return Wednesday
night as the surface high pressure moves off the New England coast,
opening up the lower Great Lakes to warm and moist southerly flow.
Temps may actually rise late in the night with increasing southerly
flow. Expect lows to bottom out only in the mid to upper 60s with 70
possible for lows near the lakeshores.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
The passage of a potent mid-level shortwave across Ontario Canada
will send its associated cold front towards the Lower Great Lakes on
Thursday. Ahead of this front increasing southerly flow will bring
in a much more humid air mass across the region. Additionally, 850H
temperatures will rise to +13C - +16C which will translate into
highs in the low/mid 80s. The combination of increasing temperatures
and humidity will make for a very uncomfortable mid summer day
across the region. As the front nears the lower Great Lakes
widespread showers and thunderstorms will develop late Thursday and
continue into Thursday evening. Showers and thunderstorms will taper
off from northwest to southeast with the passage of the front as
drier air filters into the Lower Great Lakes Thursday night. Friday
into the weekend high pressure builds into the Lower Great Lakes
with dry weather and temperatures right near climo for this time of
year(U70s/L80s).

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
For the 18Z TAFS mainly VFR flight conditions will be in place, with
some patches of MVFR CIGS. There will be three concerns this TAF
package...

The first concern will be thunderstorms forming across WNY. A
stalled boundary will be the focus, and with plenty of daytime
instability scattered to number showers and thunderstorms are
expected to form. Winds above the boundary layer are not as strong,
and this may hold back on the severity of the storms, but from the
So. Tier to the Genesee Valley, including KJHW and KROC, will be the
primary locations for these thunderstorms this afternoon and
evening.

Later this evening an area of much heavier showers and isolated
thunder will push across CNY, including the KART airfield. Heavier
rain from this activity will bring MVFR visibilities, and localized
IFR flight conditions.

The third concern will be the lowering of ceiling heights across the
region. Within a moist airmass that will remain after the showers
and thunderstorms a deck of low stratus in the MVFR flight range and
So. Tier IFR flight range is likely to form.

Outlook... Monday afternoon and Monday night...Showers and scattered
thunderstorms will gradually diminish from west to east...though
widespread IFR/MVFR ceilings will likely persist in lingering lower
stratus/ stratocumulus.

Tuesday...Improvement back to VFR with any leftover scattered
showers ending early.
Wednesday...VFR.
Thursday...MVFR/VFR with showers and thunderstorms likely.
Friday...Mainly VFR.

&&

.MARINE...
An area of low pressure will pass just to the south of Lake Ontario
tonight and tomorrow, with an easterly wind increasing across Lake
Ontario. This easterly wind will bring the greatest wave action on
the western waters, where a small craft advisory will remain in
place. The surface low will slowly track SEward tomorrow, with winds
and waves diminishing on the lakes. Behind this feature an area of
high pressure will push towards the Eastern Great Lakes Tuesday and
Wednesday with non-SCA conditions.

&&

.TIDES/COASTAL FLOODING...
An area of low pressure will approach our region this afternoon
and this evening, with a low-level easterly to east-
northeasterly flow strengthening on Lake Ontario. As winds
increase this afternoon and evening waves are expected to reach
5 feet on the western waters of Lake Ontario. The east-
northeast component to the wind would direct the highest waves
towards the coastline of Orleans and western Monroe
counties...whose shores have a slight easterly component. For
this reason a lakeshore flood warning is in effect starting 5 pm
today through Monday morning. Farther to west, across Niagara
County, waves are expected to remain just below 5 feet along the
shoreline...and thus will discontinue the watch for this
county.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning until 8 AM EDT Monday for NYZ002-003.
     Beach Hazards Statement until 11 PM EDT this evening for
     NYZ001>003.
MARINE...Small Craft Advisory until 8 AM EDT Monday for LOZ042-043.

&&

$$

SYNOPSIS...THOMAS
NEAR TERM...THOMAS
SHORT TERM...SMITH
LONG TERM...AR
AVIATION...THOMAS
MARINE...THOMAS
TIDES/COASTAL FLOODING...THOMAS



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