Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 190217

Area Forecast Discussion
National Weather Service Buffalo NY
1017 PM EDT Tue Apr 18 2017

An area of high pressure will shift to the east tonight, allowing
for clouds to increase across the Eastern Great Lakes region ahead
of the next system. Rain showers will overspread the area tomorrow
as a weak frontal boundary moves across the region. The rain chances
will last through the end of the week as an area of low pressure
moves across the Great Lakes, with a few thunderstorms possible
Thursday afternoon and evening as well.


IR satellite imagery showing an expansive area of cirrus
overspreading the area late this evening as a surface high moves
east off the New England coast.

This area of high pressure will settle towards the Canadian
Maritimes tonight, and this will allow for mid/upper level moisture
to increase across the region. This will allow clouds to thicken
overnight and eventually lower into a mid level deck by Wednesday
morning. Latest 18Z model guidance continues to suggest the showers
associated with the next system will hold off until after daybreak

A weak warm front will move northeast across the region Wednesday,
with the best isentropic upglide found along the boundary. In
addition a LLJ of 35 to 40 knots will bring increase low level
convergence, that in the presence of deeper moisture will increase
chance for rain showers across WNY by mid to late morning. Then as
the warm front and lift push east and northward, chances for showers
will increase across eastern zones. The best coverage of rainfall
should come during the afternoon as moisture and instability
maximize across the area. Overall rainfall will range from a tenth
of an inch or so near the state line of NYS/PA upwards to a quarter
to third of an inch farther to the north.

Wednesday night a cold front trailing from the storm system passing
by to our north will reach our region. As this front slowly sags
southward across our region it will continue to support chances for
rain showers. The frontal boundary will slow, and then stall just
south of the state line Wednesday night. Behind the front moisture
will remain plentiful, that with the addition of slightly cooler
airmass, and northern winds, patches of fog will become possible in
most places. Fog may become more widespread along the south shore of
Lake Ontario with moist air crossing the cold lake, and also across
higher terrain where stratus will intersect the hills.

Air temperatures will cool quickly this evening under clear skies,
and light winds, but should begin to rise shortly after midnight as
a southerly flow develops, and clouds increase. The shift in
temperatures tonight will come later across the Eastern Lake Ontario
region. Expect highs on Wednesday in the lower 60s on the lake
plains of Western NY with an added boost of southerly downslope
flow, with mid 50s elsewhere. The cold air behind the cold front
Wednesday night will not be too impressive, with temperatures
nudging downward into the mid 40s to lower 50s.


Thursday into Thursday night will feature widespread showers and
some scattered thunderstorms, as well as a wide range in
temperatures from much above normal across the southern Tier to
seasonably cool in the North Country.

A strong jet stream nearing the California coast late this afternoon
will ultimately lead to cyclogenesis in the central Great Plains by
Wednesday. The low pressure system, will deepen and track into the
central Great Lakes by Thursday. Ahead of this robust low pressure
system, the frontal boundary that will have pushed south of the
region after Wednesday`s disturbance, will move back northward and
stall across the area during the day Thursday. The frontal boundary
will provide plenty of focus for widespread showers and scattered
thunderstorms, but also result some rather interesting temperature
gradients across the region.


The western Southern Tier will most confidently break
into the warm sector, with temperatures markedly jumping into the
upper 60s and low 70s. Would not even rule out some mid 70s there,
should any breaks occur in the overcast. The North Country will
remain on the cool side of the boundary in the northeast winds with
temperatures likely not escaping the mid 40s during the day. Quite
simply there is large potential for busting the high temperature
forecast across the Niagara Frontier and along the south shore of
Lake Ontario, depending on how far north the boundary can progress
against the stubborn northeasterly flow across Lake Ontario from a
strong Canadian high pressure system near the Hudson Bay. The
official forecast brings the boundary about to the I-90 thruway
corridor during the afternoon, keeping locations on the Lake Ontario
side cool with a northeast wind, and warming locations along the
thruway southward into the 60s with a southeasterly wind. A subtle
shift north or south of this boundary could result in temperatures
10 to 15 degrees off from the current forecast.


Showers will likely develop along and near the frontal boundary
during Thursday morning, but coverage will remain somewhat scattered
early in the day. By Thursday afternoon, height falls ahead of the
upper-level low will result in steepening mid-level lapse rates and
increase instability. Also, a plume of deep moisture advection ahead
of the wave will track through the Ohio Valley, with deep moisture
convergence into the frontal boundary by late Thursday afternoon and
early Thursday evening. Expect widespread showers and scattered
thunderstorms to develop along the boundary with the greatest heavy
rain potential along and just north of the surface front (which will
be near the Lake Ontario shoreline and across the North Country).
Just ahead of the plume moisture advection and best synoptic
forcing, there could also be scattered thunderstorm development in
the western Southern Tier along the Lake Erie lake breeze boundary.
Then more numerous showers and thunderstorms are possible across
western NY as the best moisture advection and pre-frontal trough
cross the area. While instability will be somewhat marginal due to
likely lingering cloud cover during the day Thursday, there is
plenty of shear and forcing to warrant the potential for some
isolated damaging wind gusts if any stronger thunderstorms can
develop in the marginal instability. This threat would mainly be
limited to areas south and east of Buffalo across the Southern Tier
and southern Genesee Valley, given the current forecast location of
the surface boundary. A southward shift in the boundary would negate
this severe potential in our area, while a northward shift could add
Rochester and the northern Finger Lakes into the mix.

By Thursday night, the best synoptic forcing will track to our east,
with fairly mild temperatures still in place across the forecast
area. Interestingly, the North Country, including Watertown and the
Saint Lawrence valley, may well see their temperatures jump from the
40s to the upper 50s in the middle of the night as the warm front
finally progresses through. However, as the low tracks east,
temperatures will already begin falling back into the lower 50s
there by morning. Precipitation chances will taper off from west to
east overnight.

Friday and Friday night will be tracking to our northeast, with
drier air filtering into the region. Expect some lingering cloud
cover and perhaps an isolated shower or two. However, much of Friday
and Friday night will remain dry. Temperatures will slightly below
seasonal on Friday, with highs near 50, and lows in the upper 30s to
around 40.


Cooler air aloft behind Friday`s cold frontal passage will likely
keep a decent amount of diurnally driven stratocu lingering around
through the day, with the potential for a few showers over the
higher terrain. Clouds along with a northerly flow of air will
make for a chilly day Saturday, with highs ranging from the
mid 40s to lower 50s. Saturday night looks to be dry with
seasonable temperatures as our weather will be influenced by a
relatively dry northerly flow from high pressure centered over
southeast Canada.

More forecast uncertainty for the Sunday into Monday period, as
latest models showing more spread between a progressive northern
stream upper trough and a developing cut-off low across the
southeast CONUS. The ECMWF is suggesting a stronger northern stream
trough, which would act to suppress the cut-off low south of the
region and keeps the lower Great lakes dry. This is in contrast to
the GFS which is suggesting some chances for rain, as the cut-off
slowly tracks eastward across the southern Appalachians. Although
with much less QPF than it advertised yesterday. Based on these
model trends, will trend this portion of the forecast toward
somewhat diminished rainfall chances. Temperatures will greatly
be influence on the amount of sunshine versus cloud cover. With
a better chance for more cloud cover this time of year, will
lean towards below normal temperatures with mostly cloudy skies
much of the time.


Increasing mid/upper level moisture ahead of the next system will
bring thickening cirrus level cloudiness from west to east late this
evening, lowering into a mid level deck by Wednesday morning. A weak
warm front will move northeast across the area Wednesday, followed
quickly by a weak cold front Wednesday evening. Both of these
features, combined with an upper level trough, will bring increasing
chances of rain showers during the day Wednesday. Expect VSBY to
remain VFR most of the time, with brief periods of MVFR in the
heavier showers. CIGS will gradually lower to MVFR in the afternoon
as low level moisture increases, first across higher terrain and
last on the lake plains.

A few showers will linger into Wednesday night, especially across
the western Southern Tier as the front stalls nearby across PA. CIGS
will deteriorate further to IFR in most locations overnight, and
br/fog will drop VSBY down to MVFR/IFR as well. Fog will be most
prevalent across the higher terrain where low stratus will intersect
the hills, and also along the south shore of Lake Ontario due to
moist air crossing the cold lake waters.


Thursday...IFR with fog/stratus in the morning improving to MVFR.
Showers with a chance of thunderstorms.
Friday...VFR/MVFR with a chance of showers.
Sunday...Mainly VFR, chance of a shower.


An area of high pressure will shift to the Canadian Maritimes
tonight, with its influence still maintaining light winds and waves
in most areas. Northeast winds have increased to 15 to 20 knots
locally at the west end of Lake Ontario, with choppy wave conditions
there into this evening.

Southerly winds will increase on Wednesday ahead of the next system,
and may approach Small Craft Advisory levels on Lakes Erie and
Ontario. That said, the southerly winds will direct most of the wave
action into Canadian waters. A front will sag south across the lakes
Wednesday night, with winds becoming northeast. This may bring near
Small Craft conditions to the south shore of Lake Ontario. Finally,
low pressure will pass to the north of the lakes Thursday night and
Friday with winds becoming west and increasing, possibly to Small
Craft Advisory levels again.





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