Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 301445
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
1045 AM EDT Sun Apr 30 2017

.SYNOPSIS...
A warm front will move slowly northward into the region today with
showers and a scattered thunderstorms. Much warmer air will reach
the Southern Tier this afternoon, and the entire region Monday. A
cold front will then sweep east across the area late Monday and
Monday night with another round of showers and thunderstorms. Much
cooler and unsettled weather will then set in for Tuesday through
the end of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Radar imagery showing an area of rain and a few embedded
thunderstorms moving rapidly east from Rochester into the eastern
Lake Ontario region. This will bring another 2-3 hours of rain to
the eastern Lake Ontario region, with the back edge rapidly moving
away with rain ending across Western NY. Skies have cleared
temporarily across the western Southern Tier, but much of this will
fill back in by early afternoon as diurnal cumulus develop.

Surface based instability will develop across the Western Southern
Tier inland of the lake breeze during the mid to late afternoon.
This is shown by some mesoscale guidance, though areal coverage
should be sparse.

Temperatures are very chilly this morning on the north side of the
boundary, with upper 30s to lower 40s from the NY Thruway northward.
There will be a remarkable temperature gradient across the area this
afternoon. The Southern Tier should easily break into the warm
sector during the afternoon with highs in the mid 70s. Areas north
of the NY Thruway will likely hold onto northeast winds all day,
aided by flow off Lake Ontario with temperatures in the 50s, and
even 40s along the Lake Ontario shore. The Thruway corridor from
Buffalo to Rochester will be a high bust zone, with just a 10-20
mile difference in the warm front position meaning a temperature
difference of up to 20 degrees. Forecast leans on higher resolution
guidance which has the best chance to resolve the sharp boundary.
This will most likely result in highs in the upper 50s to lower 60s
in the cities of Buffalo and Rochester, however the southern
suburbs/southtowns are likely to warm into the 70s. These
temperatures will not likely be realized until very late afternoon
or early evening.

The surface warm front will gradually move north and through the
rest of the cwa tonight, with an increasing southerly 925mb flow.
Most model guidance develops an area of convection which is forecast
to track across Southern Ontario this evening and weaken as it moves
into the Saint Lawrence Valley later tonight. This will leave most
of Western New York dry tonight, except for areas north of I-90
which may get clipped by this convection.

Temperatures will correspond to the frontal position, with low
temperatures during the evening hours in many locations followed by
warmer temperatures later tonight. Southerly surface winds will
result in further downslope warming across the lake plains. Expect
gusty winds across the lake plains of Chautauqua and Southern Erie
counties where a 40 kt southerly 925mb flow will partially mix with
gusts of 40-45 mph late tonight.

&&

.SHORT TERM /MONDAY THROUGH WEDNESDAY/...
The feature of note for this period is presently on water vapor
imagery passing across the Southern Plains. This feature, a closed
upper level low, will pull moisture northward through the period,
moisture that will fuel thunderstorms Monday, and then cool rain
showers on Tuesday and into Wednesday.

Monday will start dry, warm and humid as our region lies within the
warm sector of a midwest storm system. A line of storms will likely
form across the western Lake Erie waters midday Monday, then
reaching WNY later in the afternoon. Ahead of this line there may be
a little lake breeze convergent showers and thunderstorms over SW
NYS, but the overall synoptic flow is a bit more southerly and may
not allow for much inland penetration of the lake breeze.

These thunderstorms will have the potential to become strong to
severe as a LLJ increases ahead of the upper level low. The 0-6 km
Bulk Shear Wind values of to 65 to 75 knots, and dewpoints rising
into the lower 60s supporting SBCAPE values of a 1000 J/KG or more
along with lift ahead of an approaching cold front should be more
than enough to form a line of strong thunderstorms. The biggest
threat from these storms, if they become severe, will be damaging
wind gusts.

That said, there remains concern that cloud cover, especially over
the Niagara Frontier may limit some of the instability and strength
of the storms, while farther to the east, across the Genesee Valley
and Finger Lakes region more ample sunshine will be possible and
allow for greater instability. This line of showers and
thunderstorms will continue through the evening hours, tracking
across the remainder of the CWA. As they reach the Eastern Lake
Ontario region, likely during the evening or late evening hours, the
daytime instability will be waning, and reducing the strength of the
convection.

Temperatures Monday will rise into the upper 70s and lower 80s, with
mid 80s possible within the southern Genesee Valley. Like many other
warm days this year, the flow is a bit more southerly allowing for
the areas NE of the Great Lakes to also enjoy the warmth with little
flow off the lake waters.

Tuesday moist cyclonic flow will continue across our region as the
upper level low, and surface low pass by to our northwest. Gusty
winds will be likely Tuesday, especially northeast of the lakes, and
across the lake Plains where gusts could reach 40 to 45 mph. A low
level jet of 45 to 50 knots will reside just above the surface, near
2-3k feet. However the clouds through the day will limit the daytime
mixing height, with uncertainty still existing as to how strong
these gusts will become. Will continue to mention the strong wind
potential, as well as the strong to severe thunderstorms in the HWO
for now. Temperatures Tuesday will be much cooler, with highs
ranging through the 50s across the region.

Tuesday night and into Wednesday the upper level low, now an open
wave, will track across eastern Canada, with showers over our region
beginning to end. Also with the upper level and surface low now to
our east, and the loss of the LLJ, winds will diminish Tuesday
night, and not be nearly as windy on Wednesday. Highs Wednesday will
be a degree or so lower than Tuesday.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH SATURDAY/...
As we move into the long term portion of the forecast period...upper
level troughing is again expected to dig across the eastern CONUS
in the Thursday-Friday time frame...though the medium range guidance
packages continue to exhibit significant differences with respect to
both its ultimate amplitude and the amount of time it lingers across
our region.

Digging a bit further into the details...the GFS and ECMWF are both on
the shallower and faster side overall with the aforementioned trough...
a consequence of both closing off a significant chunk of energy into a
closed low over the Deep South by Friday...which then slowly meanders
its way eastward across the Southeastern states next weekend. For our
region...this results in a shallower northern-stream upper trough that
passes across our region Thursday and Friday...with heights then largely
rebounding in its wake next weekend. At the surface...this would result
in a surface low developing over the Tennessee and Ohio Valleys Wednesday
night...then passing by a bit to our south in the Thursday-Friday time
frame...with high pressure and drier air then building into our region
in the wake of this system next weekend. In terms of sensible weather...
this would result in rain chances increasing again for the latter portions
of the work week...with mainly dry conditions then following for next
weekend.

In contrast...the Canadian GEM does not deposit a large chunk of the
energy from the burgeoning trough into a closed low over the southern
states...and instead maintains a much deeper full-latitude trough that
eventually pinches off into a huge closed low over the Great Lakes and
Northeast next weekend. For our region...this would result in a much
wetter and unsettled pattern...as the corresponding surface low would
initially develop to our south Thursday and Friday...before lifting
northward into our region and becoming increasingly vertically stacked
under its parent upper level feature Friday night and Saturday.

Given the above significant differences and continued model-to-model
and cycle-to-cycle fluctuations between these two solutions...for now
will lean close to continuity with increased rain chances indicated
for the Thursday-Friday time frame when all guidance suggests some
semblance of a surface low passing by to our south...followed by a
trend toward drier weather Friday night and Saturday as the guidance
becomes increasingly divergent and forecast confidence decreases.
Temperature-wise...the general presence of troughing and attendant
cooler air aloft for much of this period argues for continued near
to slightly below average readings.

&&

.AVIATION /15Z SUNDAY THROUGH THURSDAY/...
A warm front will move northward into the region today. An area of
steadier showers will move from the Rochester area rapidly eastward
to the eastern Lake Ontario region including KART. There will also
be a few thunderstorms embedded in these showers. Expect mainly VFR
conditions through this morning, outside of localized MVFR/IFR
conditions in heavier showers. The rain will rapidly end from west
to east during the late morning and midday.

This afternoon will be mainly dry, with the exception of the North
Country (near ART) where showers with the warm front will
linger. Instability showers and thunderstorms may develop
across the Western Southern Tier (near JHW) this afternoon.

For tonight, the main area of convection is likely to pass to the
northwest of the region. This will probably clip ART, with mainly
VFR conditions expected for most of the night. Winds aloft will
increase behind the warm front, with LLWS developing late this
evening.

Outlook...
Monday...MVFR to locally IFR with showers and thunderstorms likely.
Tuesday and Wednesday...VFR/MVFR with a chance of showers.
Thursday...VFR.

&&

.MARINE...
A frontal boundary across will move slowly northward into Western
New York today, finally moving north of Lake Ontario tonight. Before
this front moves through, the pressure gradient between high
pressure near Hudson Bay and low pressure developing across the mid-
west will result in a moderate northeasterly flow. This will bring
marginal Small Craft Advisory conditions to Lake Erie southwest of
Dunkirk, and more significant Small Craft Advisory conditions along
the south shore of Lake Ontario to the west of Sodus Bay.

The northeast winds will quickly diminish this evening as the
warm front moves north of the area. The strong low will cross
the western Great Lakes Monday before reaching western Quebec
Tuesday. A cold front associated with this low will cross the Lower
Great Lakes late Monday, with strong thunderstorms possible. Winds
will become west and increase in the wake of this system Tuesday and
Wednesday, with another round of Small Craft Advisory conditions on
both lakes.

&&

.TIDES/COASTAL FLOODING...
East to northeast winds will quickly increase through this
morning on Lake Ontario. Sustained winds will reach 20-25 knots
on the lake, and this will produce significant wave action
along the south shore from roughly the Genesee River west to the
Niagara River. The onshore flow and increased wave action
combined with very high ambient lake levels will result in
increased lakeshore flooding, especially in low-lying flood
prone areas and in bays and inlets. Areas that are particularly
exposed to flooding in northeast winds are the bays and inlets
of northwest Monroe County, including Braddock Bay. A Lakeshore
Flood Warning is in effect for Niagara, Orleans, and Monroe
counties. The northeast winds will quickly diminish by mid
evening as a warm front moves north across Lake Ontario.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning until 11 PM EDT this evening for
     NYZ001>003.
MARINE...Small Craft Advisory until 2 PM EDT this afternoon for LEZ040.
         Small Craft Advisory until 2 AM EDT Monday for LOZ042-
         043.

&&

$$

SYNOPSIS...APFFEL
NEAR TERM...APFFEL/HITCHCOCK
SHORT TERM...THOMAS
LONG TERM...JJR
AVIATION...APFFEL/HITCHCOCK
MARINE...APFFEL
TIDES/COASTAL FLOODING...HITCHCOCK/APFFEL


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