Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 211952
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
352 PM EDT Mon Aug 21 2017

.SYNOPSIS...
The return flow between a surface high off the mid-Atlantic and low
pressure approaching the region will build heat and humidity through
Tuesday. A strong cold front will cross the region late Tuesday, and
may be accompanied by strong to severe thunderstorms. The end of the
week into the weekend will have a touch of fall in the air as a
sprawling Canadian high pressure builds in cooler and drier air.

&&

.NEAR TERM /THROUGH TUESDAY/...
Visible satellite shows sunny skies across most of the region,
with partial cloud cover inland of the lake shadows across the
interior Southern Tier. This should provide excellent viewing
conditions for the solar eclipse in most locations. Temperatures
will top out in the low to mid 80s with upper 80s in the Genesee
Valley due to downsloping.

This evening should be dry, with diurnal cumulus dissipating
rather quickly. However, mid and high level cloud cover will
increase ahead of a deepening low pressure system over the upper
Great Lakes. A strengthening low-level jet will feed warmer,
moisture rich air into the area on the deep southwesterly flow
ahead of the low pressure system. Despite patchy model QPF, suspect
the leading edge of this jet will provide ample lift for a few
scattered thunderstorms to develop late tonight across far western
portions of the cwa. The increasing SSW winds will also keep
overnight temperatures quite warm with most locations not falling
out of the 70s overnight, and also quite muggy as dew points rise
into the upper 60s to the near the 70 degree mark by early Tuesday
morning.

The primary forecast concern continues to be the potential for
severe weather on Tuesday. SPC continues to have the area in a
slight risk for severe thunderstorms with damaging winds the primary
risk.

Model consensus has been fairly consistent with this system, with
12Z guidance trending perhaps a tad faster. The leading edge of a 50
kt 850mb LLJ will continue to lift from SW-NE across the region
Tuesday morning. This will leave robust wind profiles across the
entire region by late Tuesday morning, with more than ample speed
and directional shear to support well organized convective
development. The critical factor appears to be how much
sunshine there is before clouds associated with the shortwave
overspreads the area and limits daytime heating. Forecast
profiles show modest mid- level lapse rates with a long and thin
CAPE very sensitive to small differences in surface temperature
and dew point. If there is significant morning sunshine, then
the risk for severe weather will increase with damaging winds
the primary threat.

Although model guidance is in good agreement on the general pattern,
there is lower confidence in the specifics with model guidance
presenting a wide array of QPF solutions. In general expect
scattered thunderstorms during the morning hours with a limited
contribution from daytime instability but with some large scale
forcing and moisture. Areal coverage will expand during the
afternoon hours with the approach of the shortwave and with
increasing diurnal instability. High resolution guidance tends to
focus this across inland of the lake plains with lake shadowing
diminishing coverage some during the afternoon hours. Forecast
maintains categorical PoPs in these area, though specific timing
varies by a few hours by model.

There is also a potenial for heavy rain with thunderstorms on
Tuesday. PWAT values are forecast to approach 2 inches, with very
heavy rainfall possible. However, the strong flow will result in
fast storm motion which somewhat mitigates this threat, though it
still remains a concern if training were to occur.

Expect high temperatures on Tuesday to be in the 80s, but these may
need to be adjusted a few degrees in either direction depending on
cloud cover.

&&

.SHORT TERM /TUESDAY NIGHT THROUGH WEDNESDAY NIGHT/...
The cold front will finally cross the area during the late
evening/early overnight on Tuesday Night. This will be accompanied
by a few more showers and thunderstorms...ending from west to east.
In the wake of the front skies will clear from northwest to
southeast. As the night wears on however expect some lake
clouds to begin to form by morning.

On Wednesday a broad longwave trough will become established from
the Great Lakes to New England, with a secondary weak cold front
crossing the area during the late afternoon and evening. Any morning
sunshine will fade during the afternoon with the approach of the
secondary cold front and continued increase in lake effect and
upslope clouds. Increasing moisture and convergence over the lakes
may allow for a few lake enhanced showers during the afternoon and
evening. A few lake enhanced showers may continue into Wednesday
night east and southeast of the lakes, however short fetch NW flow
and a fairly dry synoptic scale background will keep this limited.

Temperature-wise the period will be dominated by much cooler and
less humid air.  Highs on Wednesday in the upper 60s to low 70s.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
A long wave trough will remain over the region through the end of
the week. A shortwave will deepen the trough Thursday, bringing
cloudy skies and a chance for a shower. Cool temperatures Thursday
in the 60s, will only improve a degree or two for Friday and
Saturday. Fair weather Friday and Saturday with abundant sunshine
as surface high pressure passes through.

Sunday a weak back door cold front will drop down from Canada,
though limited moisture suggest nothing more than just a few clouds
with this feature. Dry weather is expected to continue into the
start of next week with high pressure again entrenched across the
Northeast. As the upper level trough exits to the east Sunday,
temperatures will be a bit closer to normal in the mid 70s across
the region.

&&

.AVIATION /20Z MONDAY THROUGH SATURDAY/...
widespread VFR conditions will prevail today. Diurnal cumulus
will develop inland from the lake breezes this afternoon across
the interior Southern Tier and Finger Lakes with widely
scattered showers and thunderstorms. However, the TAF sites
should all remain shadowed by the lakes keeping fair weather in
place.

Expect VFR conditions to continue tonight with an increasing
mid-level southwesterly flow late in the night. This may come
close to LLWS criteria, but appears to be just shy of this for
now.

Scattered showers and thunderstorms will develop well ahead of
an approaching cold front Tuesday morning, and expand in areal
coverage in the afternoon. These will result in periods of
MVFR/IFR conditions, and these storms may also contain strong
winds. Also, pinning down the exact timing and location of these
storms will be difficult with high confidence in storm
development but low confidence in the details.

Outlook...
Tuesday night...Showers and thunderstorms in the evening, with
MVFR/IFR conditions.
Wednesday and Thursday...VFR/MVFR with a chance of showers.
Friday and Saturday...VFR.

&&

.MARINE...
Lighter southerly flow today and tonight will direct any waves
mainly toward Canadian waters. Then a strong cold front will cross
the region Tuesday which prompted small craft headlines as
outlined below. A few strong to possibly severe thunderstorms
are also possible Tuesday ahead of the cold front.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Watch from Tuesday afternoon through Wednesday
     evening for NYZ007.
     Lakeshore Flood Watch from Tuesday evening through Wednesday
     evening for NYZ004>006.
MARINE...Small Craft Advisory from 10 AM to 8 PM EDT Tuesday for LEZ020.
         Small Craft Advisory from 10 AM Tuesday to 5 AM EDT
         Wednesday for LEZ040-041.
         Small Craft Advisory from 10 AM to 8 PM EDT Tuesday
         for LOZ030.
         Small Craft Advisory from 2 PM Tuesday to 11 PM EDT
         Wednesday for LOZ043>045.
         Small Craft Advisory from 2 PM Tuesday to 2 PM EDT
         Wednesday for LOZ042.

&&

$$

SYNOPSIS...APFFEL
NEAR TERM...APFFEL
SHORT TERM...LEVAN
LONG TERM...THOMAS
AVIATION...APFFEL
MARINE...APFFEL/CHURCH



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