Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 272300

Area Forecast Discussion
National Weather Service Buffalo NY
700 PM EDT TUE SEP 27 2016

An upper-level low over the upper Great Lakes will drift south
toward the Ohio Valley tonight and Wednesday. Cooler air across the
region will spark a few scattered lake effect rain showers northeast
of Lake Erie this evening, with otherwise mainly clear and breezy
conditions. After a mainly dry day Wednesday, this slow moving upper-
level low will then bring unsettled weather to our region for the
rest of the week and next weekend.


Water vapor satellite imagery shows a large upper-level low spinning
over the upper Great Lakes with several small scale spokes of
enhanced vorticity with associated deeper moisture and ascent
pivoting from western Lake Erie toward western NY. Upstream radars
show little in the way of rainfall associated with these small scale
vorticity maxima, but they will act to create a somewhat more
favorable environment for lake effect rain showers during the
evening. The enhanced moisture and ascent will briefly raise lake
induced equilibrium levels to over 10k feet between 00Z-03Z Wed over
eastern Lake Erie. The low levels remain quite dry, but may moisten
enough to allow a few scattered showers to develop for a few hours.
As of 23Z, radar imagery shows a well defined convergence zone over
Lake Erie, but nothing more than a few sprinkles. As diurnal
influences dissipate with sunset, a few showers may be able to
develop. Mean boundary layer flow direction from the SSW would favor
Niagara County and Grand Island if these showers were to develop.
Inversion heights lower, and moisture diminishes overnight so if any
lake effect showers develop, they will end by shortly after midnight.

Otherwise, most of forecast area including Rochester, the northern
Finger Lakes and the North Country will see clearing skies this
evening as the diurnal cumulus field dissipates. The clear skies
will last through most of the night with just a modest southwest to
northeast increase in thin/high cirrus late tonight. Expect lows in
the upper 40s to lower 50s across the lake plains, with low to mid
40s in some of the cooler Southern Tier valleys and Lewis County.
The radiational cooling will allow for some limited river valley fog
in the typical areas of the Southern Tier late tonight and early

A dry day is in store for all on Wednesday as the upper-level low
settles over northern Indiana. Initially limited moisture and
forcing staying clear of our area will allow for a dry day, however
as Atlantic sourced air is pulled northwestward by the low, we will
see increasing cloud cover from south to north across the region.
Temperatures will also moderate with the developing easterly flow
from the Atlantic, as 850 mb temperatures climb to near +10C. This
will result in a pleasant day, with high temperatures mainly in the
low 70s. Bottom line, Wednesday will be a great day to get any
outside work done before a long stretch of unsettled weather through
the end of the work week (more on that below).


An upper level low will meander from Indiana down into Kentucky and
back into Indiana during this period as it encounters a stout
blocking ridge over the Western Atlantic. This will result in mild
and showery weather across much of Western and North-Central New
York for the latter half of the week as the area comes under the
influence of a easterly conveyor belt of maritime air advecting in
off the Atlantic around the northeastern flank of the broad low to
our southwest over the upper Ohio valley.

Shower chances will gradually increase as we move from Wednesday
evening into Wednesday night as east-southeast flow begins to become
better aligned across the Mid-Atlantic states. The easterly surface
flow down the axis of Lake Ontario combined with relatively cool
temperatures aloft, around +9C, should yield a decent lake response,
producing considerable rainfall across the Golden Horseshoe in
Ontario province, however lake effect showers should largely spare
the New York side of the border, though showers may begin to affect
areas along the Lake Ontario shoreline as we move into Thursday
morning and flow begins to back to the northeast. Temperatures
overnight will reflect the mild airmass advecting into the region
and increasing cloud cloud cover, with temperatures only falling
into the mid to upper 50s.

The period of most significant rainfall across the forecast area
will likely be on Thursday, when the core of the easterly low level
jet arrives over the area, representing the maximum of the moisture
advection and isentropic lift across the region. Expect widespread
rain showers with periods of moderate rainfall, with general
rainfall around a half inch to an inch, with localized heavier
amounts. Lowest pops and QPF will be found across the North Country,
where the northeasterly flow will limit moisture advection. Higher
amounts will be found in the Niagara Frontier near the Lake Ontario
shoreline, where lake enhancement will come into play.

While the heaviest rains are expected on Thursday, when
southeasterly flow across the area will be maximized, the persistent
proximity of the low and general southeasterly orientation of flow
across the area will keep showers in play through the end of the
week. Regarding temperatures, expect readings to remain within a
couple of degrees of climatology during the day, with highs in the
mid to upper 60s, while overnight readings will remain above
average thanks to the abundant cloud cover, with lows in the 50s.


The upper level closed low centered over the Ohio Valley early
Saturday morning is forecast by 12z medium range models to lift
north and east across the eastern Great Lakes on Sunday. This low
will then get picked up by a sharp trough diving across Quebec on
Monday and shift off the coast of Nova Scotia by Tuesday. The
movement of this closed low will be due to a deep trough forecast to
dig into the west coast. This pattern all breaks down into a rather
showery day across the forecast area on Saturday, especially across
western New York with chances of showers also included Sunday and
Monday as the upper low passed near or across New York. Monday night
into Tuesday guidance shows ridging will build over the forecast
area ahead of the approaching sharp western trough. The ridging
should provide dry weather.

Have bumped POPs across western New York to the likely range on
Saturday as both GFS and EC models show a spoke of vorticity
providing lift while cyclonically shifting across the region where a
east to west feed of Atlantic moisture will be in place to support
the higher probability of rain showers. Further east have featured
high chance pops with the lift from the vort lobe being a little
weaker further from the center of the low. As mentioned above chance
POPs have been included Saturday night through Monday for now but
expect that as confidence builds in timing and location of more vort
maxes increases POPs for at least a few hours on Sunday and Monday
may need to be raised.

In terms of temperatures expect seasonal highs in the mid to upper
60s and lows in the upper 40s to low 50s as the polar jet will be
displaced well to our north keeping any cold airmasses from surging
south across the border.


For most of the area, diurnal cumulus will dissipate this evening
and leave mainly clear skies with VFR prevailing overnight. A band
of lake effect clouds and perhaps a few showers will develop
northeast of Lake Erie near KIAG for a few hours this evening before
dissipating after 04Z. If these showers materialize they may produce
brief/local MVFR near KIAG. Some typical river valley fog will form
across the Southern Tier late tonight and early Wednesday, with
local IFR from about 06Z-12Z. This should not impact KJHW.

VFR will prevail through Wednesday with mid level clouds increasing
from south to north during the afternoon in advance of the next
system. A few showers may develop across the western Southern Tier
later in the day, but CIGS/VSBY will remain VFR through late


Wednesday night through Friday...Areas of MVFR/IFR with periods of
Saturday and Sunday...VFR/MVFR with scattered showers.


A brisk southwesterly flow of cooler air will produce continued
advisory-worthy winds and waves on Lake Erie through this
evening...before slackening through the remainder of the night.
Conditions on Lake Ontario will continue to improve this evening
with diminishing winds and waves.

After a brief respite overnight and Wednesday morning...a brisk
easterly flow will develop across the Lower Lakes region between
later Wednesday and Thursday morning...then will persist through
the end of the work week. This will likely require the issuance of
another round of small craft advisories starting Wednesday evening.
Chance for waterspouts will return Wednesday night on both Lakes
Erie and Ontario.


MARINE...Small Craft Advisory until 2 AM EDT Wednesday for LEZ040-041.



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