Area Forecast Discussion
Issued by NWS Buffalo, NY

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820
FXUS61 KBUF 241039
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
639 AM EDT Mon Jul 24 2017

.SYNOPSIS...
Low pressure will remain draped across western New York and Lake
Ontario through today...then will slowly drift eastward and weaken
tonight and Tuesday. This system will maintain unsettled weather
across our region today...with locally heavy rainfall possible
across the North Country...and a few stronger storms with gusty
winds again possible across the Genesee Valley and Finger Lakes.
Leftover showers will then wind down from west to east as the system
finally departs our region tonight and Tuesday morning...with high
pressure then providing our region with fair dry weather from Tuesday
afternoon through Wednesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
During this period...a potent mid level shortwave trough will dig
from Lake Huron to southern Quebec and the Saint Lawrence River
Valley. At the surface...an associated surface low will sit and spin
over Lake Ontario today...before slowly weakening and drifting into
central New York tonight. As it does so...the surface low will
slowly push an initial cold front across western New York today...
followed by a trailing secondary cold front late this afternoon
and this evening.

Out ahead of the surface low...a southerly feed of rich moisture
will be in place across central New York and the North Country this
morning...which will be lie under a region of low-level convergence/
lift just to the east of the surface low...and also under the influence
of height falls and a mid-level deformation zone out in advance of the
aforementioned shortwave trough. Consequently...showers and embedded
thunderstorms will be found across this particular region this morning
before shifting east/northeastward this afternoon in conjunction with
the parent deformation zone...with locally heavy rainfall remaining
very possible due to the moisture-rich (PWATs of 1.50-1.75 inches)
airmass that will still be in place. With this area of heavier rain
looking to shift out of this area a bit faster than previously
expected and projected rainfall amounts running a bit lower...will
just continue to cover the potential for locally heavy rainfall
with a mention in the HWO and enhanced forecast wording for now.

Elsewhere across the remainder of the region...convection will
be more scattered (for the most part) through mid morning...before
the combination of increasing diurnally-driven instability and the
advancing cold front drives another increase in shower and storm
coverage late this morning and this afternoon. This will be most
pronounced from the Genesee Valley and Finger Lakes over to about
Oswego County...where the initial cold frontal passage will come
during peak heating...and will likely help produce an area of more
numerous and developed convection.

With the best overall instability expected across this latter region
today...0-6 km shear bulk shear values still running between 30-40
knots...and a unidirectional westerly flow in place aloft...it is not
out of the question that a few stronger storms capable of localized
strong to marginally severe wind gusts could develop...which is in
line with the Marginal Risk area portrayed in the latest SPC Day 1
Convective Outlook. With this in mind...will cover this potential
with a continued mention in the HWO...as well as enhanced wording
in the forecast.

Further west...it is becoming increasingly apparent in the 06Z
guidance that a secondary maxima in convective coverage/intensity
will develop this afternoon across northern portions of the Niagara
Frontier...as a developing Lake Erie lake breeze collides with the
secondary front dropping southeastward from Southern Ontario and
produces a region of enhanced low level convergence. For this reason...
have indicated an enhanced region of likely PoPs to cover this
possibility. Elsewhere...convection should generally be a bit weaker
and more scattered through the rest of the day.

Whatever convection that does form today should quickly shift eastward
and weaken markedly with diminishing heating and the passage of the
secondary cold front late this afternoon and evening...with much
lighter leftover showers then slowly diminishing in coverage from
west to east the rest of tonight as the surface low slowly drifts
east into central New York. Plenty of cloud cover will remain in
place however...as a moist northerly upslope flow develops behind
the departing low and encourages the development of widespread
lower stratus.

With respect to temperatures...expect highs to mostly range in the
mid to upper 70s today...except across the North Country where more
widespread precipitation and cloud cover will keep readings confined
to the upper 60s and lower 70s. Lows tonight will then dip into the
55-60 range as cooler and drier air begins infiltrating our region
behind the departing surface low.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
Chances for showers will linger Tuesday across eastern sections of
the area as the upper trough axis moves over the region. Clouds may
be slow to break up within a northerly upslope flow off Lake
Ontario. Temperatures should run several degrees below normal. Most
highs only expected in the upper 60s to lower 70s with 850 mb
temperatures down to around +10c.

The upper trough axis finally moves off to our east later Tuesday
and fair weather returns as we heads into mid week. A developing
southerly flow will help to boost temperatures back to late July
levels for Wednesday with highs in the upper 70s to lower 80s,
except lower 70s across the Tug Hill.

&&

.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 /12Z MONDAY THROUGH FRIDAY/...
Expect a gradual improvement back to MVFR/low VFR during the morning
hours as IFR/MVFR ceilings over far western New York lift with
renewed daytime mixing...and a swath of heavier rain pulls east of
the North Country. Thereafter...we can expect a general mix of MVFR
to low VFR ceilings to continue through the balance of the day as low
pressure sits and spins over Lake Ontario. Additional scattered showers
and storms will redevelop with daytime heating...and should eventually
become more numerous across the Genesee Valley and Finger Lakes as a
cold front trailing from the low crosses that particular region...which
could also see a few stronger storms capable of producing strong localized
wind gusts. Showers and storms will probably also become a bit more
numerous again this afternoon along a convergence zone setting up from
KIAG eastward to near KROC...as a developing lake breeze boundary
collides with a an approaching secondary cold front approaching from
nearby Southern Ontario. Within any heavier showers/storms...brief
localized reductions to IFR/MVFR visibilities will be possible.

Tonight the surface low will slowly weaken and drift eastward...
with leftover showers slowly diminishing from west to east. More
importantly for aviation interests...a moist northerly upslope flow
will set up across the region as the low departs...which will in
turn lead to the development of widespread IFR to MVFR ceilings...
with the worst conditions found across the higher terrain.

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

&&

.MARINE...
Low pressure will remain nearly stationary over Lake Ontario today...
before weakening and sliding eastward tonight and Tuesday.

As the low sits over Lake Ontario today...it will push a trailing
cold front across western New York. In the wake of this front and
in advance of a secondary such front advancing across Southern
Ontario...a strengthening westerly flow will push winds and waves
to SCA levels across Eastern Lake Erie this afternoon...with higher
waves then lingering into this evening. For this reason...a new SCA
has been issued for the New York waters of Lake Erie as outlined below.

Otherwise...winds and waves across the Lower Lakes Region will
remain below SCA levels until very late tonight and Thursday...when
a strengthening northerly flow will set up across Lake Ontario behind
the departing surface low. This will likely necessitate the eventual
issuance of additional SCAs for the south shore of that lake sometime
later on today.

&&

.TIDES/COASTAL FLOODING...
A period of stronger northerly winds is expected to set up across
Lake Ontario late tonight and Tuesday morning as low pressure departs
off to our east. With it looking increasingly likely that wave
action will increase enough to raise the risk of additional erosion
and flooding along much of the south shore of the lake...a new
Lakeshore Flood Watch has been issued for the Niagara to Wayne
county corridor.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Watch from Tuesday morning through Tuesday
     afternoon for NYZ001>004.
     Beach Hazards Statement from noon EDT today through this
     evening for NYZ019.
MARINE...Small Craft Advisory from noon today to 11 PM EDT this evening
         for LEZ040-041.

&&

$$

SYNOPSIS...JJR
NEAR TERM...JJR
SHORT TERM...TMA
LONG TERM...AR
AVIATION...JJR
MARINE...JJR
TIDES/COASTAL FLOODING...JJR



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