Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 291814

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

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.


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.


A closed upper level low will meander towards the western corn
belt states by Monday morning. Ahead of the upper level low deep
southerly flow will transport moisture northward from the Gulf
of Mexico.

At surface an area of low pressure forming near SW Texas will slowly
track northward across the Plains, deepening some by a trailing PV
anomaly. This surface low will nudge an elevated warm front
northward, across the Lower Lakes Sunday and to Lake Ontario and
east of Lake Ontario Sunday night. Instability trailing this warm
front will bring chances for showers and thunderstorms into
Sunday night. Greatest chances for storms will come along and
north of the warm front. PWATs rising to an inch, and towards an
inch and a third north of the warm front will fuel these
storms. Though instability will be greater south of the warm
front, there will still feature a decent amount of lift north of
the warm front, and it is this lift that will likely form much
of the rain showers and thunderstorms through Sunday night.
Greatest rainfall totals Sunday and Sunday night will likely lie
along Lake Ontario and points eastward. However instability
south of the warm front will still have chances of forming
convection, and will continue with chances PoPs, with any
thundertorm bringing locally heavy downburst of rain.

Monday will likely start the day dry, warm and humid as our region
lies within the warm sector of this 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. These storms will
have the potential to become strong to severe as a LLJ increases
ahead of the upper level low. 0-6 km Bulk Shear Wind values rise 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 strong
thunderstorms. There will be a little 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. These storms will continue through the
evening hours, tracking across 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. Highs Monday
will likely not be as high as this past Thursday, as 850 hPa
temperatures will be a few degrees cooler, and there will likely be
a bit more cloud potential.

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 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.


Cool and unsettled weather look to be the norm for most days over
the next few weeks as a mean longwave trough remains in place over
the Great Lakes and Northeast, with frequent shortwaves and
associated surface lows bringing periods of showers and fresh
deliveries of cool air.

Tuesday night and Wednesday the deep upper level trough over the
Great Lakes will move east across Quebec and weaken, but will still
maintain enough influence across our region to keep below normal
temperatures in place right through the end of next week. 850mb
temps run from -2C to -4C Wednesday depending on model of choice,
and only improve to around +1C by Friday.

The weather will remain unsettled as well. Tuesday night and
Wednesday the eastward moving trough will combine with cool air
aloft and remaining wrap around moisture to support scattered
showers, especially across the higher terrain with an added boost
from upslope flow. It should briefly dry out Wednesday night as a
bubble of high pressure builds over the eastern Great Lakes and New

By Thursday and Friday of next week another trough is forecast to
dig into the eastern US, and potentially phase and close off into a
mid level closed low. An associated low will form along or inland of
the east coast and move slowly northward. If this materializes, it
will spread clouds and more showers into the region for the end of
the week.


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.

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


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.



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.


NY...Lakeshore Flood Warning from 6 AM to 11 PM EDT Sunday for
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.



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