Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 262338

Area Forecast Discussion
National Weather Service Buffalo NY
738 PM EDT Wed Apr 26 2017

A narrow ridge of high pressure will shift eastward into New England
overnight while keeping mostly clear to clear skies in place across
the region. A very warm airmass will keep temperatures quite mild
tonight, with high temperatures soaring into the 80s on Thursday. A
line of thunderstorms, with a few potentially damaging wind gusts
and heavy rainfall, will cross the region late Thursday and Thursday
night. Relatively cooler, yet still above normal, temperatures will
linger through the weekend.


Water vapor satellite imagery shows an occluded low pressure system
pinwheeling slowly northward along the northeast coastline,
meanwhile another sharp trough is digging through Texas and a
narrow ridge axis extends northward through the Ohio Valley into
western NY. Under this ridging building in over the forecast area,
visible satellite imagery shows skies becoming clear in most areas
this evening with just a thin band of cirrus entering Western NY,
and a few leftover diurnal cumulus in Lewis County.

As we go into this evening the band of thin cirrus will move across
the region while the remaining diurnal cumulus will diminish with
loss of solar insolation. Overnight skies will become clear to
mainly clear. However, as a pressure gradient develops across the
region between a surface ridge axis shifting to our east and
pressure falls over the upper Great Lakes expect increasing
southeasterly winds. The persistent warm advection and downslope
flow will keep temperatures tonight quite mild, in the upper 50s to
even low 60s on the lake plains.

On Thursday, current PV max rounding the base of the broad trough
over the central plains will eject northeastward crashing the
amplified ridge across the northeast CONUS. The result for the
forecast area will be the poleward transport of a very warm airmass
(850 mb temps +15 to nearly +20C) combined with gusty (25 mph)
downslope winds on Thursday. Temperatures will soar into the 80s for
much of the region, with the warmest readings (mid to upper 80s)
across the Niagara Frontier to Genesee Valley where downslope flow
will be maximized. The majority of the day will be dry and sunny,
with any rain holding off until after 4PM in far Western NY and into
the evening from the Genesee Valley eastward.

At the same time, moisture advection into the region will push dew
points into the upper 50s, if not near 60 in spots. This will result
in strong diurnal destabilization under a warm nose aloft, which
will keep us capped through the day. However, later Thursday
afternoon, as the ejecting PV max nears western NY, height falls
aloft will help erode the capped layer, supporting the development
of thunderstorms along a pre-frontal trough.

Timing of this thunderstorm development trended just slightly slower
by an hour or two in the 18Z guidance, which would bring it into
Western NY at or just after peak daytime heating. Combine this with
the increasing speed shear (0-6km of 40 to 50 knots) associated with
the wave, and any thunderstorms that develop will be capable of
strong to potentially damaging wind gusts. This best potential for a
few severe thunderstorms will in the typical corridor southeast of
Buffalo from about Jamestown to Rochester and the Genesee Valley. As
the storms evolve eastward toward central NY and the North Country
overnight, expect weakening as diurnal instability wanes.

The actual cold front boundary will lag the best thunderstorm
potential by a few hours and will probably feature a few widely
scattered weaker showers, but should be a more uneventful passage.
Cooler air will filter in behind the front overnight, with lows
bottoming out in the upper 40s in western NY to the mid 50s toward
central NY.


After a stormy night Thursday night, western and north-central New
York should get a dry break on Friday as a zone of post-frontal
subsidence, accompanied by mid-level ridging moves across the
region. While there should be sunny breaks through the day, there
will likely be at least passing mid-level cloudiness, as fragments
of upper level energy eject across the region out of the Ohio
valley. While the cold frontal passage will yield markedly cooler
temperatures as compared to Thursday`s heat wave...highs will
actually still be a couple of degrees warmer than
average...generally in the mid 60s...with 50s found to the northeast
of the lakes, thanks to post-frontal southwesterly flow and
afternoon lake-breeze circulations.

The forecast becomes much trickier as we move into Friday night and
Saturday. In terms of the big picture, a Pacific jet diving across
the inter-mountain West will reinforce the longwave trough over the
Rockies and High Plains, while the sub-tropical ridge will remain
displaced to the west, across Florida and the southeastern states.
The convergence between southerly return flow around this anti-
cyclone and the cooler, drier air located within the longwave trough
will set up a baroclinic zone that will extend from the southern
Plains to the lower Great Lakes. While this boundary will certainly
serve as the focus for the development of showers during this as a
series of waves move along it, models continue to disagree both with
regards to its exact placement/orientation, as well as the timing of
the waves moving along the boundary. As a result, some guidance
packages suggest a wet start to the weekend, with periods of showers
Friday night into Saturday, as suggested by the GFS/NAM, while the
ECMWF keeps the showers at bay at least until Saturday night. For
the time being, will stick to broadbrushed chance pops, pending
further model runs.

Regarding temperatures, again, there is quite a bit of uncertainty,
as the temperatures will be affected by the final position of the
stationary front relative to the forecast area. The general trend as
forecasted currently though will be for temperatures to cool
somewhat on Saturday, pending a cloudier/wetter regime, though once
again readings should be, at worst near average for late April.


Our weather for the second half of the weekend into the first half
on next week will be controlled by a seasonably deep stacked surface
and mid-level cyclone. This storm system will develop over the heart
of the country on Sunday then slowly deepening while tracking north
to near the vicinity of Lake Michigan by Monday then further north
tracking across Quebec Tuesday and Wednesday.

On Sunday, a southerly low level jet ahead of this developing system
will shift a warm front from PA north into Western NY. This front
will help trigger at least a broken band of showers and possibly
some thunderstorms on Sunday with the exact movement of this front
being the determining factor in temperatures. The GFS lifts the
front north over Lake Ontario by Sunday evening while the EC leaves
the front just south of the east-west corridor of the NY Thruway. As
is usually the case, this makes for a difficult temperature
forecast. We are most sure that at least the Southern Tier should
break into the 70s as the front lifts north, and well to the north,
the North Country will likely not break out of the 50s. In between
these two areas will be a tight gradient of temps, cooler to the
north and warmer to the south. Sort of a middle ground is to place
the Niagara Frontier and Finger Lakes into the 60s but if the EC
verifies then the southern shore of Lake Ontario counties may not
break above the 50s.

The warm front is likely to lift north of the forecast area Sunday
night leaving very mild overnight temperatures. Have kept POPs low
for a chance of a shower feeling its more likely to be dry behind
the front. Temps overnight will either only slip back slightly or
even rise overnight, especially in areas which had yet to see the
warm front lift through. Lows for most will be in the 50s with some
upper 40s possible in the North Country and possibly not falling
below 60 in the Southern Tier.

On Monday, the surface low to the west is forecast to continue to
deepen below 990mb near the vicinity of Lake Michigan while an
associated cold front shifts east across western and central NY.
This front should trigger another threat of showers and
thunderstorms as decent surface based instability and strong shear
is forecast during the afternoon hours. Will have to keep an eye on
Monday for a severe threat. This will be added to the Hazardous
Weather Outlook. Otherwise, Temps ahead of the front look to warm
into the 70s with upper 70s possible in the Genesee Valley and
Finger Lakes. Humidity levels will be pushing toward the somewhat
uncomfortable levels with dewpoints possibly rising to about 60
which would further support a severe risk.

Cooler weather can then be expected behind the cold front on Tuesday
along with a chance of some showers and gusty winds. Strong cold air
advection behind the front will low 850mb temps below 0c which will
translate into very cool temperatures Tuesday afternoon only topping
out in the 50s which is below normal for this time of year. This
strong cold air advection will also support very gusty winds with a
50+ knot low level jet streaking across the eastern Great Lakes. The
track of the low and current forecast strength of the low level jet
could support possible advisory level wind gusts but if the low
level jet verifies stronger than a late season high wind event may
be possible. This will also be added to the Hazardous Weather

Cool temperatures then look to remain in place Tuesday night and
again Wednesday with continued cold air advection as the mid-level
trough axis shifts east across our region. There will be low to
slight chances of showers as well with winds slackening off. Lows
Tuesday night look to dip into the upper 30s to low 40s then highs
on Wednesday only rising into the low to mid 50s.


A weak ridge of high pressure over the region this evening will
drift east towards western New England overnight. This will provide
mainly clear skies and VFR. Model guidance attempts to develop a
field of low level moisture over central PA and then advect this
northward into the Southern Tier late tonight and early Thursday
morning, but this seems overdone. For now will just indicate at SCT
layer in this area, but if it were to become more robust some IFR
may develop for a few hours across the higher terrain of the
Southern Tier.

Thursday southerly flow will increase across the area as a system
approaches from the central Great Lakes. Expect VFR to prevail
through 21Z areawide with increasing mid/high clouds across Western
NY by mid afternoon. A broken line of convection is then forecast to
enter Western NY between 21Z Thu and 00Z Fri with associated brief
heavy downpours and local MVFR/IFR along with the potential for
gusty winds. This line of storms will then move east across the area
Thursday evening, with scattered showers in its wake gradually
ending from west to east overnight as a cold front crosses the area.
There may be a few hours of MVFR CIGS around the time of the cold
frontal passage as moisture convergence increases over the cold


Friday...Mainly VFR.
Saturday...VFR/MVFR with a chance of showers.
Sunday...VFR/MVFR with a chance of showers and thunderstorms.
Monday...MVFR to locally IFR with showers and thunderstorms likely.


High pressure will move slowly east of the region tonight and into
New England on Thursday, while a low pressure system deepens over
the upper Great Lakes. Light winds this evening will become
southeasterly overnight to southerly on Thursday. Winds and waves
will continue to be well below small craft thresholds into Thursday

A cold front will cross the region Thursday night, with showers and
thunderstorms likely ahead of the front. Winds and waves are largely
expected to remain below SCA threshold on the Eastern Great Lakes,
however locally higher winds and waves are possible with any
thunderstorms that develop late Thursday.


The last days of April will finish with above normal
temperatures, warm enough that it will send our climate sites
towards a top 10 warmest April on record. The degree of
anomalous warmth has been greater towards the west, where
climate site Buffalo may reach the warmest April on record.
Below are the warmest Aprils on record.


Rank.....Value (F)....Year


Current April average temperature through April 25th: 49.0F


Rank.....Value (F)....Year


Current April average temperature through April 25th: 49.9F


Rank.....Value (F)....Year


Current April average temperature through April 25th: 45.9F

Temperature records for Buffalo and Rochester begin 1871, while
Watertown`s history is a bit shorter, with temperature records
beginning in 1949.





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