Area Forecast Discussion
Issued by NWS Buffalo, NY

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842
FXUS61 KBUF 292040
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
440 PM EDT Sat Apr 29 2017

.SYNOPSIS...
A warm front will move slowly northward across the region late
tonight and Sunday and produce showers and scattered thunderstorms.
Much warmer air will reach the Southern Tier Sunday, 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 SUNDAY/...
Radar imagery showing a decaying MCS moving east across PA this
afternoon with a large shield of rain staying just south of the
area. To the north of the MCS, a few scattered showers over Lake
Erie will expand northeastward across the Southern Tier from mid
afternoon through early evening, with mesoscale model guidance such
as the HRRR suggesting a few hours of somewhat steadier showers. The
northern edge of the scattered showers may reach as far north as
just south of the NYS Thruway by late afternoon. All of this will
dissipate early this evening as the associated forcing weakens and
falls apart.

Once the early evening showers dissipate, expect a period of dry
weather to last much of the night. Plenty of clouds will remain
across the region with ample mid level warm advection and moisture
over the top of the frontal boundary stalled over PA. The one area
that should see some clearing this evening is the eastern Lake
Ontario region as a bubble of high pressure and drier air builds
into southern Quebec. Lows will drop into the lower 40s in most
locations, with upper 30s for the North Country and upper 40s near
the PA state line.

Late tonight and Sunday morning a developing warm frontal segment
will move northeast across the region. A sharp temperature gradient
in the 900-700mb layer along the boundary will set the stage for a
period of strong isentropic ascent and frontal convergence, aided by
a 40 knot low level jet. Impressive elevated instability develops
along the warm front, with Showalter Index dropping to around -5C.
Warm fronts are notoriously difficult to forecast, especially when
convection is involved. The pattern and strength of forcing seems to
support a solid chance of 2-4 hours of scattered to numerous showers
and thunderstorms along and ahead of the warm front. This would
begin very late tonight across Western NY, then spread northeast
across the area Sunday.

The warm frontal showers and storms will linger through the
afternoon east of Lake Ontario, while across Western NY the showers
will taper off as the warm sector and dry slot become established
behind the departing warm front.

There will be a remarkable temperature gradient across the area on
Sunday. 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 low to mid 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. Best guess at this point is the city
of Buffalo and Northtowns remain cool, while the Southtowns may
break into the lower 70s by mid to late afternoon.

&&

.SHORT TERM /SUNDAY NIGHT THROUGH WEDNESDAY/...
A large stacked low pressure system will impact the Great Lakes
Monday through Wednesday, first bringing very warm air and
thunderstorms to the region Monday followed by below average
temperatures and scattered showers Tuesday into Wednesday.

Sunday night, the surface warm front will finally make some headway
northward across Lake Ontario bringing southerly winds and warmer
air all the way to the lake shore and eventually into the North
Country on Monday. Showers and thunderstorms are likely along and
north of Lake Ontario and into the Saint Lawrence valley as the
front moves northward, with just a chance for showers and isolated
thunder toward the south. Temperatures will be in the 60s across the
Southern Tier with temperatures rising from the 40s into the upper
50s south of Lake Ontario and in the Saint Lawrence Valley.

Much of the area will be dry and warm by Monday morning as we will
be in the warm sector ahead of the approaching cold front. Deep
southerly downslope flow ahead of the front, with 850 mb
temperatures +15 to +18C, will allow for temperatures to rise into
the 70s and 80s. The warmest temperatures, with readings in the low
to mid 80s will be in the Genesee Valley eastward to the North
Country, which will get the most daytime heating ahead of the front,
and likely some breaks in cloud cover for additional solar
insolation. The front will approach western NY mid-day with increase
cloud cover which will keep temperatures limited to the 70s to near
80. With this in mind, the best instability will develop from the
Genesee Valley eastward, and thus as the strongly forced front
approaches widespread showers and some thunder across western NY
should evolve into stronger thunderstorms as it progresses eastward.
Ample deep layer shear of 40 to 50 knots would be sufficient for
severe thunderstorms if enough instability can be realized ahead of
the front. Have kept mention of heavy rain and gusty winds with the
thunderstorms in the forecast, but will have to monitor the
potential for severe storms with damaging winds gusts, which of
course will depend on how well we destabilize ahead of the front.
Will continue to mention in the HWO.

Monday night, the cold front will sweep across the region with
ushering in cooler air and precipitation tapering off from west to
east overnight. On Tuesday the stacked low pressure system will
track from the upper Great Lakes to southern Quebec, while pushing a
secondary front across the region. 850 mb temperatures are expected
to continue falling through the day Tuesday, bottoming out around -
3/-4C by Wednesday. The result will be scattered showers across the
region Tuesday and Wednesday given the cyclonic flow and vorticity
advection combined with steeper lapse rates under the upper-low.
Skies will also remain overcast with the moisture under the low, and
thus temperatures will remain quite cool with highs only in the
upper 40s to low 50s. Additionally, it will feel quite raw with very
windy conditions in the wake of the frontal passage on Tuesday. Cold
air advection with steepening lapse rates will allow for momentum
transfer to the surface, especially funneling up the lakes across
the Niagara Frontier and the Saint Lawrence Valley. Gusts 40+ mph
are expected in those prime locations, with a chance for exceeding
wind advisory criteria. Will continue the mention in the HWO at this
time.

&&

.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 /21Z SATURDAY THROUGH THURSDAY/...
An area of scattered showers will move across the Southern Tier this
afternoon and early evening, with a mix of VFR and MVFR VSBY. Low
level moisture across the higher terrain of the Southern Tier will
bring areas of MVFR and IFR CIGS through this evening. Otherwise the
rest of the area will remain dry with VFR CIGS.

Most of tonight will remain dry, until a warm front moves northeast
across the area with showers and scattered thunderstorms. These will
develop across Western NY very late tonight, then spread northeast
across the area Sunday morning. The showers will produce areas of
MVFR VSBY, with MVFR CIGS becoming more widespread along and ahead
of the warm front. The rain will taper off Sunday afternoon across
Western NY following the passage of the warm front, with CIGS
returning to VFR across lower elevations.

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 stalled across PA will move slowly northward late
tonight and Sunday, finally moving north of Lake Ontario Sunday
night. Northeast winds will increase late tonight and Sunday as the
pressure gradient tightens between strong low pressure moving into
the Midwest states and strong high pressure over northern Ontario
and Quebec. 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 Sunday 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. 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 late tonight and
Sunday 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. With this in
mind, we have issued a Lakeshore Flood Warning for Niagara,
Orleans, and Monroe counties. The northeast winds will quickly
diminish by mid evening Sunday as a warm front moves north
across Lake Ontario.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Lakeshore Flood Warning from 6 AM to 11 PM EDT Sunday for
     NYZ001>003.
MARINE...Small Craft Advisory from 2 AM to 2 PM EDT Sunday for LEZ040.
         Small Craft Advisory from 4 AM Sunday to 2 AM EDT
         Monday for LOZ042-043.

&&

$$

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



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