Area Forecast Discussion
Issued by NWS Buffalo, NY

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FXUS61 KBUF 201816
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
116 PM EST Tue Feb 20 2018

.SYNOPSIS...
Low pressure will move into the Upper Great Lakes this afternoon...
then on into Quebec tonight and Wednesday. This will result in periods
of rain along with springlike warmth through early Wednesday morning...
with temperatures peaking in the 60s to lower 70s today. While colder
weather will return following the passage of a cold front on Wednesday...
our temperatures will still remain above normal through the end of the
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Shortly after midday...went ahead and updated the forecast for far
western New York to account for the large area of showers rippling
northeastward across Lake Erie...which is attendant to a mid-level
shortwave also tracking northeastward across southern Ontario. While
this area is weakening somewhat and should continue to do so as it
shifts northeastward in association with its parent shortwave...it
will still likely bring some additional showers to areas along and
north of a rough Jamestown to Rochester line...for which PoPs have
been bumped up into the likely to categorical ranges. South of this
line any showers should be increasingly scattered to isolated with
increasing southeastward extent...while the North Country should
continue to experience periodic showers along and north of the slowly
advancing warm front.

Also...based on the above changes and current obs (which mostly
show winds lagging behind previous expectations)...have knocked
back both winds and gusts a bit further across far western New
York this afternoon...and have also tweaked high temps downward
a little where the showers will be most persistent. This said...
some wind gusts to 30-35 mph or so will still be likely across
portions of far western New York this afternoon...and afternoon
highs are still largely expected to range from the upper 50s/lower
60s across the North Country to the mid 60s to lower 70s across
much of the area south of Lake Ontario.

A large sub-tropical ridge will continue to move northward across
the east coast tonight. This along with the warm front moving into
Ontario/Quebec will keep the region mostly dry overnight. A
southerly flow continues and temperatures will hold steady in the
60s. Due to the cold lakes, fog is likely near the shore lines
through tonight.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The slow progression of the cold front will keep the rainfall
potential going through much of the day Wednesday. The front will
eventually push through the area by late afternoon and bring an end
to the rainfall as dry air and cooler temperatures move in. Expect
high temperatures to be well into the 60s east of Rochester, with
slightly cooler highs likely to the west. Highs will come early in
the day with temperatures falling back into the upper 30s and 40s by
late afternoon.

High pressure will build into the upper Great Lakes behind the front
Wednesday night. There is some uncertainty how far south of the area
the boundary will push, and whether a wave of low pressure which
develops along this boundary will clip southern portions of the area
late Wednesday night into Thursday. Model consensus supports at
least a chance of precipitation across the Southern Tier with
diminishing chances to the north which will be closer to surface
high pressure. Temperatures will get marginally cold enough to
support snow, with rain, snow, or mixed precipitation possible.

High pressure will pass just to the north of the region Thursday
night, supporting mainly dry weather. Lows will be mainly in the
20s. There is a small chance for rain or snow late across the
Western Southern Tier depending on the speed of the next system.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
Active weather pattern to continue during this period with several
chances for precipitation.

Low pressure over the Central Plains will be in the process of
moving into the Mississippi and Ohio River Valley on Friday.
Meanwhile, high pressure over New England will slowly exit to the
Northeast. A warm front associated with this approaching system will
cross the Lower Great lakes early Friday. Initially, it will be cold
enough for a mixture of a little snow mixed with rain before turning
over to all rain as southwesterly flow brings in a much warmer air
mass to the region. The surface low will then track just to the west
of the lower lakes late during the day Friday and then push its cold
front through the region. Latest Guidance packages suggest that the
cold front will stall over Pennsylvania Friday night. Several weak
waves of low pressure will then ride along this boundary bringing a
chance of showers to the region on Saturday.

The last in a series of low pressures will quickly develop over the
Central Plains which will bring the best chance for widespread rain
Sunday. The low pressure system will depart the region late Sunday
with precipitation ending from west to east. Quiet and drier weather
returns Monday with surface high pressure building over the Ohio
Valley. Otherwise, highs will remain above climo (+5F - +10F) with
the daytime highs in the mid and upper 40s likely for most
locations.

&&

.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
A warm front draped across Lake Ontario and the North Country will
slowly push northward to the Upper Saint Lawrence Valley by early
this evening...before shifting completely north of the area during
the first half of the night. Along and north of the front...periodic
showers and lower clouds will lead to MVFR conditions and reductions
to IFR at times through early this evening. Meanwhile to the south
of the boundary flight conditions will mostly be dry and VFR through
tonight...except along and north of a rough KJHW to KROC line this
afternoon where another round of showers will likely produce a short-
lived round of MVFR conditions...with brief/localized IFR visibilities
possible.

Otherwise low level wind shear will also remain an issue much of the
time through tonight...with a persistent 45-50 knot low level jet
lingering around 2kft. The exception to this will be across far western
New York this afternoon...where increased mixing could lead to surface
gusts to 30 knots or so while helping to temporarily alleviate any
wind shear.

On Wednesday a trailing cold front will cross the region from northwest
to southeast...with a rough 3-6 hour period of rain accompanying and
following the frontal passage. Coupled with lowering ceilings as the
boundary layer moistens...this will result in an areawide deterioration
to IFR/MVFR flight conditions during the morning hours. As we move into
the afternoon...high pressure and drier air will begin building into
the region behind the front...resulting in the rain tapering off from
northwest to southeast...and flight conditions rebounding to MVFR and
then VFR in a similar fashion.

Outlook...
Wednesday night...Mainly VFR...with a chance of snow showers/MVFR
across the Southern Tier overnight.
Thursday...MVFR with some rain and snow likely across the Southern
Tier and Finger Lakes...and VFR/MVFR with a chance of rain and snow
elsewhere.
Friday...VFR/MVFR with a period of rain.
Saturday and Sunday...VFR/MVFR with scattered to numerous rain and
snow showers.

&&

.MARINE...
Southerly, mostly offshore winds will continue this afternoon as a low
pressure system tracks by to our northwest. Winds will be mainly in
the 15-25kt range on Lake Erie. A Small Craft Advisory is in effect
for Lake Erie through tonight. This pattern will continue through
Wednesday morning. A cold front will then move through Wednesday
with WNW flow behind the front, with likely SCA conditions
developing on Lake Ontario.

&&

.HYDROLOGY...
There remains a significant risk for flooding in the Black River
Basin. The combination of rain and a prolonged period of warm
temperatures will result in significant run-off in the basin. There
is still a basin average of 4-5 inches (water equivalent) with up to
10 inches across higher terrain. The majority of this will melt over
the next two days.

Although there is high confidence in warm temperatures and
associated snow melt, it is uncertain exactly how much rain will
fall. Model consensus has the axis of heaviest rainfall along the
Saint Lawrence Valley, with less rain expected south of Watertown.
Total rainfall amounts of 1-2 inches are likely in the Black River
Basin through Wednesday. There is a chance there will be more if
this axis shifts slightly south.

The greatest risk for flooding is as the Watertown forecast point
due to the widespread nature of the event and higher rainfall
amounts across the Black River basin north of Lowville. MMEFS
ensembles continue to show a likelihood of at least minor flooding
at Watertown, with significant chances of moderate and possibly even
major flooding. This depends on how much rain falls with the
ensembles capturing some model runs which place this axis across the
Black River basin. The risk for flooding is slightly lower at the
Boonville and McKeever forecast points since less rain will fall in
these portions of the basin. Even so, snowmelt and even some rain
may cause at least minor flooding.

In addition to river flooding, the combination of rain and snow melt
may cause localized flooding in the Watertown area. Ponding of water
may close roadways and cause basement flooding, especially if the
axis of heaviest rainfall shifts even slightly south. Also smaller
creeks and rivers are at risk across Northern Jefferson county where
rainfall amounts will be the greatest. Ice jams also may be an issue
with some ice still in place in and along some waterways in the
basin.

Elsewhere, there is much less snow pack in place due to recent warm
temperatures. Rainfall amounts in excess of an inch may cause some
creeks in the Buffalo area and lower Genesee basin to approach flood
stage, but model consensus keeps the steadier rains to the north of
these basins with rainfall amounts of an inch or less expected in
these basins. High flows can still be expected, with some locations
likely to reach action stage.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...Flood Watch through Wednesday evening for NYZ006>008.
MARINE...Small Craft Advisory until 10 AM EST Wednesday for LEZ020-040-
         041.

&&

$$

SYNOPSIS...HSK/JJR
NEAR TERM...HSK/JJR
SHORT TERM...TMA
LONG TERM...AR
AVIATION...JJR
MARINE...HSK/JJR
HYDROLOGY...APFFEL/HSK


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