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655
FXUS61 KBUF 091117
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
617 AM EST Mon Dec 9 2019

.SYNOPSIS...
Low pressure will track across the central Great Lakes today with
above normal temperatures and rain showers, followed by colder air
and lake effect snow showers for mid week. High pressure will move
across the region and bring a return to dry weather for Thursday
night and Friday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Regional radars showing reflectivities increasing along the north
shore of Lake Ontario up through the Saint Lawrence Valley early
this morning. Upstream ground truth showing just light rain with
temperatures in the upper 30s to lower 40s. This activity will
continue to push slowly south through the morning hours and
will tend to weaken as it encounters some drier air in place
across the area. Temperatures have warmed enough above freezing
to end the threat for freezing rain across Lewis County.

An upper level trough will dig across the Great Lakes region through
tonight, while a surface low strengthens as it tracks across the
central Great Lakes today and into southwestern Quebec by tonight. A
south-southwest low level jet of 45-50 knots will provide warm air
and moisture advection and favorable jet dynamics will support a
round of widespread rain. Rainfall amounts heavily influenced by
upsloping, with higher resolution guidance showing localized amounts
exceeding an inch, while the majority of the area will get between a
half inch and an inch of rain. The steadiest rain will be this
afternoon/evening, then will taper off from west to east tonight.

Temperatures will be well above normal during the period with high
temperatures actually being met during this evening in the 40s
across most areas, and even some lower 50s near the Lake Erie
shoreline where downslope warming will be most pronounced. The cold
front will enter western New York late tonight with southwesterly
winds gusting to 35-45 mph. This front may also produce an
additional round of rain showers.

&&

.SHORT TERM /TUESDAY THROUGH THURSDAY/...
A still deepening surface low will move across central and northern
Quebec on Tuesday, with a trailing cold front crossing the eastern
Great Lakes early in the morning. The majority of the large scale
forcing and moisture transport will be associated with the warm
advection regime well ahead of the cold front. The actual cold front
will only produce a few brief showers as it moves east across the
area early in the day. Developing upslope flow will allow showers to
linger a little longer across the higher terrain east of Lakes Erie
and Ontario. Precipitation will change over to wet snow in these
upslope areas, but any accumulation will be minimal. Temperatures
will still be in the 40s before daybreak, but will drop through the
30s during the day with moderately strong cold advection in the wake
of the cold front. Wind gusts will reach around 30 knots in the
morning northeast of the lakes in the wake of the cold front.

A weak secondary cold front will move south into the area late
Tuesday afternoon and evening. This feature may produce an uptick in
snow showers along the south shore of Lake Ontario as it captures a
weak lake convergence band and moves it onshore. This may produce
some light accumulations southeast and east of Lake Ontario.
Following this secondary front, shear and dry air will likely force
Lake Ontario lake effect to end completely for a time overnight
Tuesday night. Off Lake Erie, the close proximity of surface high
pressure will continue to introduce dry air and shear in the
boundary layer, greatly limiting lake effect potential despite
deepening cold air. Expect nothing more than a few snow showers
Tuesday night across the western Southern Tier. Accumulations will
be light Tuesday night, with no more than an inch or two southeast
of the lakes.

Wednesday a mid level trough and associated clipper low will pass
well north of the area, with the clipper cold front crossing the
eastern Great Lakes during the mid to late afternoon. The cold front
itself will do little, with nothing more than a few light snow
showers. It will introduce a more favorable lake effect environment
however. Convergence along the advancing cold front will merge with
pre-existing lake induced convergence over both lakes, resulting in
intensifying bands of lake effect snow. Lake induced equilibrium
levels will briefly rise to 10-12K feet with the passage of the
clipper as moisture improves and a pool of cold air aloft crosses
the eastern Great Lakes. A favorably deep mixed phase layer will be
located within the cloud bearing layer, yielding dendritic growth.

Off Lake Erie...

Expect a band of relatively light lake effect snow showers to move
north out of the Southern Tier and across Buffalo in the morning,
ending up north of Buffalo across the Northtowns and Niagara County
by late morning. This band will begin to move back south and east
across the area by early to mid afternoon as the cold front reaches
eastern Lake Erie. The band will intensify as it crosses the Buffalo
area from a boost of synoptic support from the cold front, and
increasing lake induced instability. The strong band will then move
onshore from Buffalo all the way down to Chautauqua County and push
inland through the Southern Tier through the late afternoon. The
band of snow will continue for a few hours across the western
Southern Tier Wednesday evening before quickly weakening as
inversion heights lower, shear increases, and moisture decreases
with high pressure building over Lake Erie.

The fast pace of the southward band drift will greatly limit
accumulations, even though the band of snow may become quite heavy.
Early estimates suggest 2-4 inches from the Buffalo Metro area into
portions of Genesee County, and 3-5 inches across the higher terrain
of Southern Erie/Wyoming counties into the Chautauqua Ridge. Even
though amounts will be limited, if the brief burst of heavy snow
materializes it would produce very difficult travel for a few hours.
Winds will also gust 30-35 knots around this time, producing blowing
and drifting snow.

Off Lake Ontario...

Expect a similar trend just a few hours later. Tuesday morning
disorganized lake effect snow showers over the lake will push to the
northeast end to near Kingston Ontario, possibly clipping Cape
Vincent in northern Jefferson County. The band of snow will then
intensify during the mid to late afternoon as the cold front
approaches and synoptic support/instability improve. This
intensifying band of snow will then move southeast across Jefferson
County, crossing the Tug Hill region during the evening. The band
will reach maximum intensity during this time frame as the best lake
parameters line up with a period of upslope flow across the Tug
Hill. The band will continue to march steadily southeast overnight,
with a WNW to ESE oriented band of weakening lake snow moving into
northern Cayuga/Wayne counties. A few snow showers may also clip the
south shore farther west overnight from eastern Niagara to Monroe
counties as boundary layer flow veers.

Similar to Lake Erie, the short residence time of the band in any
one location will limit accumulations. Expect 2-4 or 3-5 inches
across the lower elevations of the eastern Lake Ontario region, with
up to 6 inches across the Tug Hill Plateau. Wayne and northern
Cayuga counties may see a few inches overnight. While amounts are
not overly impressive, this band may produce difficult travel for a
few hours with heavy snow and 30-35 knot wind gusts producing
blowing and drifting.

Thursday a few lake effect snow showers may linger in the morning
southeast of Lake Ontario with minor additional accumulations. High
pressure will then build east across the eastern Great Lakes, ending
the lake effect snow and bringing some partial clearing.

&&

.LONG TERM /THURSDAY NIGHT THROUGH SUNDAY/...
High pressure will move east off the east coast Thursday night. Warm
advection in its wake will bring an increase in clouds across the
eastern Great Lakes, but any precipitation will stay well north of
the region. Warm advection continues Friday as a mid/upper level
ridge builds into the northeast states. The warm advection will
produce some cloud cover, but dry weather will continue. After a
chilly start, temperatures will warm into the upper 30s to lower 40s
in most areas.

A complex system will then cross our region this weekend. A southern
stream trough will support low pressure developing over the Gulf
coast Friday. The resulting low will move almost due north, with one
low center moving up the west side of the Appalachians and into the
eastern Great Lakes by Saturday, with a second weak lee side low
moving up the eastern seaboard. This southern stream trough will
phase with a northern stream trough moving towards the western
lakes, allowing an expansive area of deep layer ascent to move north
across our area late Friday night and Saturday. This will produce
another round of rain areawide. There is a small chance the rain may
start as a wintry mix across the North Country if precipitation
begins early enough, otherwise this looks to be an all rain event as
temperatures warm into the 40s on Saturday.

The northern stream portion of this phasing system will then cross
the eastern Great Lakes Saturday night and Sunday, producing
additional chances of rain and wet snow showers as colder air moves
back into the region. Highs Sunday will be in the mid to upper 30s.

&&

.AVIATION /11Z MONDAY THROUGH FRIDAY/...
Ongoing warm air advection will support increasing low level
moisture reaching the western Southern Tier with MVFR cigs
developing by 12z, then over the rest of Western and Central NY
before 18z.

Rain showers, possibly starting as mixed precipitation over
inland cooler valleys of the North Country, will accompany the low
level moisture early this morning. KART will be all light rain.
Rain will then spread across the rest of Western New York this
morning, then spread east by afternoon. Expect the southerly
downslope flow to keep all TAF locations in the MVFR flight
category with the exception of KJHW which does not benefit from
southerly downsloping.

A 40-50 knot LLJ will largely remain through this morning, which
will result in LLWS.

Outlook...

Tonight...Areas of MVFR/IFR in rain.
Tuesday...MVFR with rain showers ending from west to east. Snow
showers possible late.
Wednesday...IFR within lake effect snow east of the lakes.
Thursday...VFR with localized IFR in lake effect snow showers
east of the lakes.
Friday...VFR.

&&

.MARINE...
Winds will stay generally out of the south today at 10 to 15 knots,
but could be a bit higher east of Rochester this morning.

A windy period is expected through much of the week as a strong cold
front crosses the area Tuesday, followed by moderate westerlies
Tuesday night through Thursday night. Marine headlines are
outlined below.

&&

.HYDROLOGY...
A period of warmer temperatures will cause snow melt which will
add to run-off from rain on today through Tuesday. For most
basins, rainfall amounts will be less than an inch and the snow
pack in place is limited, resulting in little risk for
flooding.

There is a bit more snow pack in place in the Black River basin,
with Snow Water Equivalent (SWE) values in the 1-2 inch range.
Model guidance shows a bit more QPF here, with 1-2 inches across
localized favorable upslope regions. Latest RFC forecast has
Boonville reaching action stage by late Tuesday, with Watertown
poised to reach it just beyond the forecast period. This is in
line with latest MMEFS ensembles which show a fairly high
likelihood that these points will reach action stage but very
low probabilities of flooding.

Based on this, have added a mention of this risk to the HWO. The
most likely outcome remains action stage, but the situation
bears watching. The greatest risk is if temperatures are warmer
than expected. If this happens then the additional snow melt
would be a concern, but for now there is not enough confidence
to issue any flood headlines.

&&

.BUF WATCHES/WARNINGS/ADVISORIES...
NY...None.
MARINE...Small Craft Advisory from 10 PM this evening to 4 AM EST
         Thursday for LEZ040-041.
         Small Craft Advisory from 4 AM Tuesday to 1 AM EST
         Wednesday for LOZ043-044.
         Small Craft Advisory from 1 AM to 10 PM EST Tuesday for
         LOZ042.
         Small Craft Advisory until 10 AM EST this morning for
         LOZ045.

&&

$$

SYNOPSIS...Apffel/TMA
NEAR TERM...Apffel/TMA/Zaff
SHORT TERM...Hitchcock
LONG TERM...Hitchcock
AVIATION...Apffel/TMA/Zaff
MARINE...Apffel/TMA
HYDROLOGY...Apffel