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Area forecast discussion
National Weather Service Buffalo New York
657 PM EST Wed Nov 20 2019

Synopsis...
mainly dry weather is expected tonight and Thursday as high pressure
builds into the area. A cold front will then cross the region
Thursday night and Friday, producing a round of rain showers and
windy conditions.

&&

Near term /through Thursday/...
a high pressure just off to our west covering the Ohio Valley
wil slide east across the region overnight. There will be a weak
surface flow with low level moisture remaining trapped beneath
an inversion. This said, satellite imagery shows some breaks in
the cloud cover early this evening. Where skies clear,
temperatures will quickly drop and there will be a risk for low
stratus or fog to develop.

In general, low temperatures will be in the lower 30s, except
east of Lake Ontario and the western southern tier valleys where
mid to upper 20s will be commonplace. This said, temperatures
will vary depending on subtle cloud cover differences.

Thursday, developing low pressure will be sent northeastward by the
southern stream jet from the Central Plains into the central Great
Lakes by late in the day. A strengthening southwest flow within the
warm sector of the system will help temperatures climb into the mid
to upper 40s across much of western New York, with even a few lower
50s possible. Better mixing should help to mix out the expected
lower cloud cover early in the day, for at least some brief partial
sunshine, before upstream cloudiness overspreads the area from the
west during the afternoon. Widespread rainfall developing in advance
of the system cold front should start to makes its way into far
western New York by late afternoon. Wind speeds across far western
New York should steadily increase through the afternoon with some
gusts near 35 mph quite possible in favored downslope areas along
the Lake Erie shoreline from Ripley to Dunkirk.

&&

Short term /Thursday night through Saturday/...
the main concern through the short term period will be the potential
for strong, gusty winds behind a cold front, and the potential for
Lakeshore flooding.

A pair of mid level shortwaves will cross the central and western
Great Lakes Thursday night and begin to sharpen and phase with time.
The resulting surface low will track from northern lower Michigan
Thursday night to southern Quebec by Friday morning, deepening with
time. There remains a good deal of spread in model guidance with
respect to the track and timing of the strongest surface low
response, and this has resulted in significant differences in the
handling of winds aloft and surface wind potential. The NAM and its
high-resolution variants continue to be much weaker and farther
north with the surface low, resulting in dramatically lower wind
potential. The GFS, Canadian Gem, and European model (ecmwf) are stronger and farther
south, resulting in stronger winds aloft within the Post-frontal
airmass. Given the emerging consensus from the GFS/Gem/ECMWF, we are
leaning towards the stronger solutions with respect to wind.

A strong cold front will sweep east across the eastern Great Lakes
Thursday night, entering western New York shortly after midnight, then
reaching the Eastern Lake Ontario region before daybreak Friday. If
the verifying pattern is similar to the GFS/Gem/ECMWF consensus,
strong winds aloft will last for several hours following the cold
frontal passage. The combination of cold advection, a moderately
strong pressure fall/rise couplet, and a moderately strong
stratospheric pv intrusion will support strong subsidence and
downward mixing of stronger winds from aloft. This may support at
least advisory level gusts along the Lake Erie shore and extending
northeast across the Niagara Frontier to Rochester, and also along
the east and southeast shores of Lake Ontario.

If the upward trend in model guidance continues, warning criteria
gusts cannot be ruled out in these areas. The lack of support from
the NAM and NAM based high-resolution guidance lends low confidence
in the details and magnitude of damaging wind potential at this
time. Continued mention in the severe weather potential statement for now, with wind headlines
possible once models begin to converge on a common solution. The
strong winds will be brief, quickly diminishing from west to east
Friday as the surface low moves off into eastern Quebec.

As far as precipitation GOES, a brief Wing of warm advection will
allow showers to develop from west to east Thursday evening. This
will be followed by another brief period of showers along the cold
front overnight Thursday night. Finally, there may be a brief window
of lake enhanced/upslope showers late Thursday night through Friday
morning east of Lake Erie, and during the day Friday east of Lake
Ontario. High pressure and associated dry air quickly build into the
eastern Great Lakes later Friday, so the lake response will be
brief. Precipitation may end as a little wet snow on Friday,
especially across higher terrain. Any snow accumulation will be
minimal.

Temperatures will rise into the mid to upper 40s briefly Thursday
night ahead of the front, then fall back through the 40s and into
the 30s during the day Friday in moderately strong cold advection
behind the front.

Friday night our storm system will be exiting across the Gulf of the
Saint Lawrence, with surface high pressure nudging across the
eastern Great Lakes. The loss of a strong pressure gradient will
result in much less windy conditions, with light winds and perhaps
just a flurry of snow southeast of Lake Ontario on marginal lake
effect snow parameters. Warm air advection will end any flurries
early, though moisture remaining in the lower levels may result in
more clouds than not through the night and into Saturday.

Saturday this surface high will pass by just to our south, with fair
weather conditions through the day. There may even be a period of
mostly sunny skies Saturday before clouds thicken from the south
later in the day as another weak low advances through the Ohio
Valley. The 12z GFS brings this surface low much farther northward
than the majority of its ensemble members...and have tailored the
forecast closer to the NAM/Canadian/ECMWF track which do not bring a
southern stream shortwave as far north as the GFS. Will have
greatest chances for either rain or snow Saturday night and Sunday
across the souther tier, and then The Finger lakes region...with
perhaps near the Niagara River and Saint Lawrence River remaining
dry through the weekend.

This surface low will exit to our east Sunday, with limited lake
effect possible east of Lake Ontario Sunday night. The cold air
behind this weekend storm system will drop to around -7/-
8c...marginal enough for perhaps a light lake response east of the
lakes...especially Lake Ontario.

&&

Long term /Saturday night through Wednesday/...
low pressure passing by to our south and east will exit off the East
Coast Sunday night. There may be some limited lake response
southeast of the lakes, but this will end overnight as a ridge of
high pressure builds into the eastern Great Lakes. By Monday the 12z
GFS shows a weak mid level shortwave crossing the eastern Great
Lakes, while the European model (ecmwf) shows a continuation of ridging surface and
aloft. Favored the European model (ecmwf) for this time period, with a mainly dry
forecast for Monday.

The longwave trough will begin to reload Tuesday across the northern
plains and upper Midwest, allowing a surface low to rotate through
the western Great Lakes and northwest Ontario. Model consensus keeps
most, if not all of the associated precipitation to our west during
this time frame with mainly dry conditions over the eastern Great
Lakes. The longwave trough will begin to move east across the Great
Lakes Tuesday night and Wednesday, although model agreement breaks
down significantly on the handling of the synoptic scale details.
The GFS takes one, strong low through the upper Great Lakes, while
the European model (ecmwf) maintains two separate systems, with a deepening frontal
wave moving across the eastern Great Lakes. Both scenarios would
bring increasing chances of rain by later Tuesday night and
Wednesday.

Temperatures will trend warmer through the first part of next week
ahead of the re-loading trough. A return to at or below normal
temperatures is then likely by the latter half of next week behind
the system noted above, although the degree of cooling shows plenty
of variability in medium range model guidance.

&&

Aviation /00z Thursday through Monday/...
clouds have scattered out across portions of the area early this
evening, with satellite imagery showing patchy MVFR stratus
covering roughly have the region. Areas which do remain clear
will be at risk for fog or low stratus to develop. With a weak
flow aloft, cloud cover will be difficult to forecast. The
greatest chance for fog and IFR conditions is at jhw due to its
relative valley location embedded in higher terrain. This said,
periods of IFR fog/cigs cannot be ruled out at any location
tonight.

A weak southerly flow will develop late tonight and become a bit
stronger during the day Thursday. This will help mix out and
push northward any lingering stratus with conditions improving
to mainly VFR during the day Thursday.

Outlook...

Thursday night...VFR deteriorating to IFR. Showers. Low level wind shear
possible during the evening.
Friday...MVFR/VFR with a chance of showers. Breezy.
Saturday and Sunday...mainly VFR.
Monday...VFR.

&&

Marine...
a weak pressure gradient will remain in place across the eastern
Great Lakes through Thursday morning, with winds generally 15 knots
or less and waves under 3 feet.

A strengthening area of low pressure will move from the
western/central Great Lakes Thursday afternoon to southern Quebec by
Friday, with a trailing cold front crossing the eastern Great Lakes
Friday morning. Southerly winds will increase ahead of this system
Thursday afternoon and Thursday night, then become southwest on
Friday following the cold front. There will be a period of several
hours Thursday night into Friday when gale force winds will be
possible on both lakes, especially on the eastern end of Lake
Ontario.

&&

Tides/coastal flooding...
another Lakeshore flood event is possible Thursday night through
Friday on both lakes Erie and Ontario. Deepening low pressure
will pass by to the north of the lakes, with several hours of
strong westerly winds expected behind a strong cold front. There
remains a significant amount of model spread between NAM based
guidance and GFS/Gem guidance with respect to wind speeds, but
overall the trend is stronger.

For Lake Erie, the strongest winds will occur late Thursday
night before quickly diminishing during the day Friday.
Sustained winds will likely reach between 30 and 35 knots for a
few hours on Lake Erie. In periods of normal lake levels these
winds would not be strong enough to produce a warning criteria
seiche on Lake Erie, however all of the Great Lakes are at
abnormally high levels. The higher starting Point of Lake Erie
may allow the lake to exceed 8 feet late Thursday night.

For Lake Ontario, the strongest winds will occur Friday morning
before slowly diminishing later in the afternoon and evening.
The combination of High Lake levels, strong onshore winds, and
high wave action will increase the likelihood of Lakeshore
flooding and shoreline erosion on the east half of the lake on
Friday.

It should be noted that while flooding may occur, we do not
anticipate impacts as significant as the Halloween night
Lakeshore flood event.

&&

Buf watches/warnings/advisories...
New York...Lakeshore Flood Watch from late Thursday night through Friday
evening for nyz006-007.
Lakeshore Flood Watch from Friday morning through Friday
evening for nyz004-005.
Lakeshore Flood Watch from Thursday evening through Friday
morning for nyz010-019-085.
Marine...gale watch from late Thursday night through Friday evening for
loz043>045-063>065.

&&

$$

Synopsis...tma
near term...apffel/tma
short term...Hitchcock/Thomas

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