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Area forecast discussion
National Weather Service Buffalo New York
544 PM EDT sun Apr 30 2017

a warm front will move slowly northward across the region tonight
with clouds and a few scattered showers and thunderstorms. Very warm
air will briefly push into the area Monday before a cold front crosses
the area in the afternoon and evening. This front will produce a line
of showers and thunderstorms with the potential for damaging winds and
heavy rainfall. Much cooler and unsettled weather will then settle into
the region for the rest of the week.


Near term /through Monday/...
as of 540 PM...a broken line of showers and storms has been holding
together pretty well over the past hour as it has pushed eastward
into southern Ontario...Eastern Lake Erie...and Chautauqua County.
For this reason...have upped pops to likely to cover the passage
of this feature across western New York early this evening...with
this activity expected to fade as it pushes further east into the
Genesee Valley and Finger Lakes. No other changes at this time...

Tonight, the surface warm front will continue to move northward and
end up over or north of Lake Ontario by early Monday morning. A well
defined mid level wave will run northeast along the boundary and
promote widespread showers and a few thunderstorms along with heavy
rainfall across southern Ontario. With the northward retreating warm
frontal boundary, the majority of this will stay north of the
international border with just a few scattered showers and isolated
thunderstorms brushing our area overnight, mainly in areas close to
Canada. Warm advection in the wake of the warm front will allow for
steady temperatures this evening, then slowly rising temperatures
overnight aided by developing southerly downslope flow.

On Monday expect the same basic pattern to persist through early
afternoon, with steadier rain across southern Ontario and a few
scattered showers or isolated thunderstorms clipping our region,
with plenty of dry time. There should be enough breaks in the
clouds, and certainly enough strong warm advection to allow for
moderate instability to develop by early to mid afternoon across
western New York. Highs should reach the mid to upper 70s at the western
end of the state, and low 80s from the Genesee Valley to central New York
with a later onset of thicker clouds and showers. The north country
should also push into the 70s, except perhaps right along the Saint
Lawrence River.

The main concern will be severe weather potential from mid afternoon
through evening as a fairly sharp pre-frontal trough crosses the
area. Model guidance has been consistent the past few days in
developing a broken line of strong convection on this boundary.
Latest 12z guidance is an hour or two faster, bringing the boundary
into western New York at an ideal time to take advantage of peak heating
and instability. Strong, mainly unidirectional shear profiles and
sufficient SBCAPE support the idea seen in mesoscale model guidance
of a broken line of strong storms, with bowing segments capable of
producing wind damage. With this in mind, have added mention of
damaging winds from mid afternoon through evening as the line moves
east across the area. Storms may also produce very heavy rainfall as
precipitable water rises to around 1.5 inches, but fast storm motion should
minimize any flood risk.


Short term /Monday night through Thursday/...
Tuesday and Wednesday will be rather chilly, overcast and damp by
recent weather standards, as an anomalously deep trough tracks
through southern Quebec. The broad cyclonic flow with moisture and
steep lapse rates under the upper-level trough will bring scattered
rain showers across the region Tuesday, most prevalent in the
upslope regions east of both lakes Erie and Ontario. Wednesday will
feature some diurnally driven showers again enhanced by orographic
ascent, but will be less widespread than Tuesday given departing
synoptic moisture and some heights starting to rise aloft. 850 mb
temperatures will start the day around +2c on Tuesday, while falling
below 0c Tuesday night, with -3c pool of cold air cross the region
on Wednesday morning. Combine this with ample cloud cover and
scattered rain showers, temperatures will struggle to recover at all
from the morning lows Tuesday morning. Highs will remain in the
upper 40s to low 50s both days. Temperatures Tuesday night will only
fall to the upper 30s to low 40s given the persistent wind and cloud

Speaking of winds, winds will pick up on Tuesday with cold advection
in place with and pressure rises behind the departing surface low.
The steep lapse rates and channel of pv aloft will support momentum
Transfer of the 40+knot 925mb jet to the surface across the Niagara
Frontier to Rochester and along the Saint Lawrence valley. Not all
of the 40+ knot jet will be realized at the surface, but could
promote some wind gusts reaching Wind Advisory criteria up to 45-50
mph. Will continue to mention this in the severe weather potential statement at this point. Winds
will start to decrease Tuesday night and on Wednesday, but still
remain breezy from the west as the surface pressure rises continue
in the wake of the departing low.

On Thursday, a developing low pressure system over the southern Ohio
Valley with a departing northern stream jet streak placing US in the
right entrance region will support developing rain from south to
north across the forecast area. We will remain on the north side of
the warm front keeping temperatures cooler, with highs only in the
lower 50s again.


Long term /Thursday night through Sunday/...
there is very high confidence that this will be a wet and unsettled
period. In fact...our forecast area will most likely be on the
receiving end of another significant rainfall...something that we do
not need after one of the wettest april's in recorded history. The
gefs and European model (ecmwf) based ensembles (including the operational runs) are
in strong agreement that an unusually well organized storm for early
may will track from the Tennessee Valley across our forecast area.
This system will be more typical of a mid winter storm...both in its
intensity...upper level support and overall structure. The only
difference is that temperatures will be some 5 deg c too high for a
major snowstorm. That being said...

The pending storm system will be supported by a highly amplified
pattern...again...more typical of mid winter. A very strong...full
latitude ridge will be anchored over the western states while an
anomalously deep trough (-3 std) with an early may return interval
of once every 10 years will be found over the eastern third of the
country. Despite the impressive climatology behind this pattern...
there will initially be two distinct branches within the deep
trough. The wet storm system that will bring a substantial rainfall
to our region will be found within the southern branch...but as we
work through the weekend...very robust energy within the northern
branch will dive south across the upper Great Lakes to phase the two
into a broad closed low that will largely reside over western

Digging deeper into the details of this storm will
already be raining across all of our forecast area as we open this
period Thursday evening...but the most significant rain will be yet
to come. A strongly divergent upper level flow will be in place
across the lower Great Lakes Thursday night...while even stronger
forcing aloft will be generated from a coupled h25 jet. Meanwhile in
the low levels...increasing baroclinicity in the h925-85 layer will
establish a steep surface for a 40-50kt low level jet to impinge up
on. All of this lift will be accompanied by a wealth of Atlantic
moisture that will extend back across our region from the Delmarva
Peninsula. This will boil down to a steady rain Thursday...which
could prove to be heavy at times...mainly over the western counties.
Have taken the lead and raised pops to 90 regionwide.

On Friday...there is nearly a 50/50 split between the various
ensembles as to whether the storm system will become vertically
stacked in the vcnty of Lake Erie or remain 'dynamic' in nature with
a coupled jet and continued low level baroclinicity for another 12
hours. Either way...widespread rain will be found over all of our
forecast area. The sensible difference for the public will be
whether the rain will taper off to light rain and drizzle over the
far western counties...or remain generally consistent through the
course of the day. Again...will be bold and meteorologically
consistent using 90 pops in such a scenario.

The mature storm system will track across our forecast area Friday
night. By this time...there should be the semblance of a dry slot
working across The Finger lakes and portions of the Eastern Lake
Ontario region. This will help to limit the amount of pcpn for those
areas...but a weakening of the overall dynamics should be more of a
factor. As the system drifts over US...the vast majority of the
strong dynamics will be removed. This will only leave relatively
weak low level am not expecting more than showers. As
this event draws may be more accurate to describe it as
light rain and/or drizzle...but that is a mute point from this

On Saturday...the stacked storm sill be in the process of lifting
away to our northeast across the St Lawrence Valley. Cold advection
in the wake of the system will begin and we will be left with the
likelihood of just showers as spokes of positive vorticity advection circulate across the
region from the northwest.

While a deep cyclonic flow will remain over our region for Sunday...
drying will be well underway in the mid levels. This will encourage
the shower activity to taper off from west to east during the course
of the day...which from this Vantage Point looks to be mainly (but
not completely) rain free.

Rainfall totals for this prolonged (roughly 72 hours) event should
range from one to two inches for basin averages...with locally over
2 inch amounts possible.


Aviation /22z Sunday through Friday/...
scattered to numerous showers and thunderstorms will cross western
New York this evening with spotty/brief MVFR/IFR conditions. Otherwise VFR
will prevail in most areas with varying amounts of clouds. Some MVFR
ceilings may develop across the north country including kart as a warm
front moves northeast across the region.

Scattered showers and isolated thunderstorms will continue tonight
through Monday morning, especially near the Canadian border as the
warm front moves north to Lake Ontario and stalls. Expect a mix of
VFR and MVFR cigs, with MVFR favored in areas northeast of the lakes
as a more moist airmass crosses the Cool Lake waters.

Monday afternoon and evening a cold front will sweep east across the
region with a line of strong showers and thunderstorms. Some of
these storms may contain gusty winds and very heavy rain.

Finally, a low level jet will cross the region tonight and produce
low level wind shear at all the taf sites.


Tuesday and Wednesday...VFR/MVFR with a chance of showers.
Thursday and Friday...MVFR/IFR with rain likely.


moderate to strong northeast winds will continue to produce Small
Craft Advisory conditions along the west half of Lake Ontario
this evening. Overnight a warm front will lift north across the
region and bring a temporary end to the stronger winds and high
wave action.

A strong cold front will cross the lower Great Lakes from west to
east from mid afternoon through mid evening Monday, and produce a
line of thunderstorms with strong winds and higher waves.

A period of moderate to strong westerly winds will then develop
Tuesday through Wednesday on both lakes, with the next round of
Small Craft Advisory conditions.


Tides/coastal flooding...
east to northeast winds will continue in the 20-25 knot Range
Lake Ontario through this evening, 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.

Have issued a Lake Shore Flood Watch for Tuesday afternoon
through Wednesday afternoon for Jefferson and Oswego counties.
Moderate to strong southwest to west winds will develop Tuesday
and Wednesday on Lake Ontario. This may result in Lakeshore
flooding concerns on the east end of Lake Ontario. Some of the
most susceptible areas are the bays of Jefferson County such as
Chaumont Bay. Water levels will also likely rise higher on the
upper Saint Lawrence River in the Thousand Islands region as the
wind forces more water down the river, which can produce
flooding on some of the flood prone islands in the river.


Buf watches/warnings/advisories...
New York...Lakeshore Flood Watch from Tuesday afternoon through Wednesday
afternoon for nyz006-007.
Lakeshore Flood Warning until 11 PM EDT this evening for
Marine...Small Craft Advisory until 2 am EDT Monday for loz042-043.



near term...Hitchcock/jjr

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