Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 191844
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
244 PM EDT Wed Jul 19 2017

.SYNOPSIS...
A weak frontal boundary will generate some isolated showers this
evening...otherwise fair dry weather will persist through Thursday.
A complex of thunderstorms with very heavy rain will then drift from
the Upper Great Lakes to our forecast area Thursday afternoon and
evening. Parts of western New York could pick up in excess of two
inches of rain. After a dry uneventful day on Friday...the weekend
will turn unsettled with showers and thunderstorms becoming rather
widespread...including the threat for more heavy rain.

&&

.NEAR TERM /THROUGH THURSDAY/...
A weak frontal boundary over southern Ontario late this afternoon
will push south across our forecast area during the first half of
tonight. Forcing from this boundary is not particularly strong...
and with an elevated cap around 15k ft...there should not be
widespread precipitation. Will thus continue to use a range of
slight to low chc pops...mainly for sites north of the Southern
Tier. Otherwise...it will be a warmer night than the past few with
min temperatures ranging from the lower 60s in the valleys of the
Southern Tier and in the foothills of Adirondacks to the mid 60s
elsewhere.

High pressure centered over the Ohio Valley late tonight and early
Thursday will nose north across our forecast area. While this will
provide us with fair dry weather for the first half of Thursday...
it will become quite unsettled Thursday afternoon.

A complex of thunderstorms (MCS) will develop over the U.P. of
Michigan by daybreak Thursday. This area of heavy rain and
thunderstorm activity will push east-southeast during the course of
the day...advancing to our western counties by about 19z (3 PM).
While there is little doubt that this complex will move across a
large portion of our forecast area...there is a wide spread of
solutions among the short range/high resolution models as to where
the track of the heaviest rain. This will result in a high
confidence for rain...but with a low confidence for rainfall
amounts. That being said...there will likely be a 30-40 mile wide
strip where rainfall amounts will exceed two inches. This is similar
to what happened last week over parts of western New York...but in
that case...we also had a boundary for the heaviest rains to focus
upon. We will not have that boundary this time around. All of this
being said...confidence is not yet high enough to go with a flash
flood watch...so will continue to mention the potential in the HWO
product while also having `heavy rain` in the base grids.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH SUNDAY/...
The remnants of a potential convective complex are likely to be
across the Southern Tier Thursday evening before exiting to the
south and east of the area later in the night. This will be
followed by a gradual clearing trend from W-E as a weak ridge of
high pressure builds into the area. 12Z model consensus
continues to keep our area dry late Thursday night through
Friday. There will be some afternoon clouds along and inland of
lake breeze boundaries, but still a fair amount of sunshine
across the region. Lows will be in the mid to upper 60s on the
lake plains and lower 60s in the cooler Southern Tier valleys
and Lewis County. Friday should be a rather pleasant day, with
moderate humidity and highs in the upper 70s to lower 80s.

A subtle frontal boundary will sharpen across or just south of the
Great Lakes with 12Z model consensus bringing another convective
wave across Western New York or Pennsylvania on Saturday. This
convective wave is more or less shown by NAM/ECMWF/GGEM
guidance, and if it develops it does have the potential to bring
another round of heavy rain in the 1-2 inch range. This may
result in flooding, with isolated severe storms a lesser but
still plausible risk.

This wave is likely to exit late in the day Saturday, potentially
resulting in a brief dry period. While model guidance differs, there
may still be a weak boundary near or just south of the area with
additional mid-level waves possible in cyclonic flow aloft. As such,
it will be difficult to rule out a shower or thunderstorm Saturday
night and Sunday, even though a good part of the time is likely to
be dry.

Temperatures over the weekend will remain rather seasonably with a
significant cold frontal boundary expected to remain to the north.
Highs depend on the development and track of any convection, areas
which remain dry likely to reach 80 while rainy areas will be quite
a bit cooler.

&&

.LONG TERM /SUNDAY NIGHT THROUGH TUESDAY/...
By early next week the pattern will begin to amplify, with a trough
sharpening over the Great Lakes Monday and New England Tuesday. The
sharpening trough and associated surface cold front will continue to
support shower and thunderstorm chances through Monday, with cooler
and drier air arriving for Tuesday. Expect slightly below normal
temperatures early next week.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
VFR conditions will be found across the region through this evening
although there will be a low chance for showers or thunderstorms
near Lake Ontario.

Any convection that develops later this afternoon and evening will
then give way to clearing skies overnight. While VFR conditions will
be found across the majority of the region...IFR conditions in fog
and low stratus will be possible for portions of the Western
Southern Tier, including sites such as KJHW, KOLE and KELZ.

On Thursday...any IFR conditions early in the morning will give way
to VFR weather through early afternoon. A complex of thunderstorms
will then move from Southern Ontario across western New York between
19 and 00z. While the threat for severe weather will be low...it is
likely that the complex will contain some very heavy rain. This will
result in several hours of MVFR to IFR conditions.

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

&&

.MARINE...
While a cool front will drop south across the Lower Great Lakes this
evening...a weak surface pressure gradient will remain in place.
This will keep light winds and negligible waves in place with
nothing more than an isolated shower or thunderstorm late this
afternoon or early evening.

Winds and waves will remain below small craft advisory levels on
Thursday...but a complex of strong thunderstorms will move across
Lakes Erie and Ontario during the afternoon. This organized area of
convection could lead to some special marine warnings...as these
storms could generate wind gusts over 35 knots. Mariners should be
alert for any warnings that may be issued.

High pressure over the Upper Great Lakes Thursday night 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.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...None.

&&

$$

SYNOPSIS...RSH
NEAR TERM...RSH
SHORT TERM...APFFEL
LONG TERM...HITCHCOCK
AVIATION...RSH
MARINE...RSH



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