Area Forecast Discussion
Issued by NWS Buffalo, NY

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000
FXUS61 KBUF 082126
AFDBUF

Area Forecast Discussion
National Weather Service Buffalo NY
426 PM EST Sun Dec 8 2019

.SYNOPSIS...
Low pressure will track across the central Great Lakes Monday
with above normal temperatures and rain showers for the start of
the work week, followed by colder weather 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 MONDAY NIGHT/...
Early this afternoon there is high pressure off the southern
New England coast and a weak surface trough of low pressure
across the central Great Lakes. This will provide dry weather
through this evening, however mid and high clouds will spread into
the region during this time. There will be broad mid-level warm air
advection with a 40-50 knot low level jet across the area which will
result in gusty southerly winds. Winds will gust to around 30 mph at
most locations, except to around 40 mph across favored downslope
areas through this evening.

A southwesterly flow and approaching low pressure will provide a
broad area of warm air and moisture advection. Chances for showers
will increase after midnight tonight. First across the North
Country, then spreading southeast across western and central New
York. The southerly flow will prevent temperatures from dropping
below freezing at most locations, but there is a chance for mixed
precipitation across the North Country. Thermal profiles show a warm
layer of around +2c with surface temperatures around freezing in the
Lewis county valleys tonight. Precipitation may start as snow,
sleet, or freezing rain before changing to all rain Monday morning.
It`s worth noting that temperatures dropped below zero across
portions of the North Country last night, leaving frozen ground
surfaces. Will issue a brief Winter Weather advisory for Lewis
County tonight. Elsewhere, low temperatures will range from the
lower 40s across the lake plains west of Rochester with mid to
upper 30s at most other locations.

An upper level trough will dig across the Great Lakes region
Monday and Monday night, while a surface low strengthens as it
tracks across the central Great Lakes Monday and into
southwestern Quebec Monday night. A persistent SSW 50 knot LLJ
will provide warm air and moisture advection and favorable jet
dynamics will support periods of rain during the period. QPF is
heavily influenced by upsloping, with high resolution guidance
showing localized amounts of 1-2 inches while the majority of
the area will get between a half inch and an inch of rain. The
steadiest rain will be Monday afternoon, and then will taper off
from west to east Monday night.

Temperatures will be well above normal during the period with
highs in the 40s across most of the area on Monday, with lower
50s near the Lake Erie shoreline where downslope warming will be
most pronounced. The warmest conditions will probably be Monday
evening, during a brief dry period just ahead of the cold
front when temperatures will be in the upper 40s to around 50.
The cold front will enter western New York late Monday night
with southwesterly winds gusting to 40-45 mph. This front may
also produce an additional round of rain showers.

&&

.SHORT TERM /TUESDAY THROUGH WEDNESDAY NIGHT/...
Deepening low pressure system will track off across Quebec Canada
Tuesday and then arrive over the Ungava Bay by early Tuesday
evening. While it tracks northeast, it will send a sharp cold front
from west to east across the Lower Lakes. Cold air advection on the
back side of the front will send a MUCH colder airmass surging into
our area. While temperatures initially will begin in the 40s, look
for them to fall into the 30s over the course of the day with windy
conditions and precipitation winding down behind the front. With
that said and as was previously discussed, there is some suggestion
of a weak frontal wave developing along the front on Tuesday, per
12Z NAM, ECMWF, and now GFS. While this won`t add anything of
significant precipitation wise (possibly some wet snow flakes), it
will potentially delay the exit of the front to our east.

Lake effect snows beginning late Tuesday afternoon and continuing
through Wednesday night...

Right now, it just doesn`t look like there will be a decent lake
effect response initially or even significant snowfall in the long
run. While the air mass will grow colder with time (H850T -12C/-13C
by 00Z Wednesday), other factors will be working against well
organized and intense lake effect snows. First and foremost, there
is a lack of deep synoptic moisture behind the front. It`s a fairly
dry air mass behind the front and then profiles show some decent
shear. Finally, the BIGGEST factor is residence time. Per 12Z
guidance and incorporating all the above factors, expect a weak lake
response east of both lakes beginning late Tuesday afternoon and
then continuing Tuesday night. Lake effect snows will likely focus
across the higher terrain east of Lake Erie and Ontario with light
accumulations occuring overnight.

Wednesday, a shortwave and associated low will race by to our north
through Quebec Canada. Wind flow ahead of this feature will back
winds more to the southwest directing lake snows off Erie to the
north into the Southtowns and Buffalo Metro area. Off lake Ontario,
lake snows will get redirected across central Jefferson county. Lake
snows are still expected to remain fairly weak ahead of this feature
as it travels north. As was previously said, convergence occuring
along the cold front with the clipper may merge with the lake effect
snows which may yield greater intensity for a few hours Wednesday
afternoon. Boundary layer flow will then become westerly by
Wednesday evening in the wake of the clipper, carrying lake effect
snow into the western Southern Tier off Lake Erie, and the Tug Hill
region off Lake Ontario. Flow then becomes northwest later Wednesday
night, with weakening lake effect snow spreading out southeast of
both lakes.

The peak intensity of this lake effect snow event will likely be at
and just after the passage of the clipper, Wednesday afternoon and
evening. The most favorable conditions are relatively brief and the
bands will be in motion, which will limit the potential for heavy
accumulations in any one location. Warning criteria snow is possible
during this time window if the bands become strong enough, but
confidence is low given uncertainties with respect to placement,
intensity, and longevity of the heavy snow.

Wednesday night, in the wake of the cold front and clipper, lake
snows will be focused southeast of both lakes under NW`erly flow.
Lake snows will also gradually weaken overnight as high pressure
over Wisconsin rapidly tracks east behind the departing clipper. A
drier air mass will work in across the Lower Lakes and with a
strengthening subsidence inversion (lowering equilibrium levels)
lake snows will weaken as we head into Thursday.

In terms of temperatures, this will be some of the coldest air so
far this winter season. Highs on Wednesday will likely only be in
the low to mid 20s. Combine that with a brisk wind flow, and it will
feel more like temperatures are in the low teens to single digits.
Wednesday night, winds will subside but it will be cold with
temperatures falling back into the teens south of Lake Ontario, and
single digits across interior locals east of Lake Ontario.

&&

.LONG TERM /THURSDAY THROUGH SUNDAY/...
Any lingering lake effect snow showers southeast of the lakes
Thursday morning will end as high pressure builds over the eastern
Great Lakes. Otherwise expect some partial clearing outside of lake
effect clouds. Highs will be in the mid 20s in most locations. The
surface high will still be close by Thursday night, with associated
light winds allowing lows to drop into the teens in most areas, and
single digits east of Lake Ontario.

The surface high moves off the east coast Friday, with a
continuation of dry weather. Developing southerly flow in the wake
of the departing high will bring the start of a warming trend, with
highs in the mid to upper 30s. The warming trend will continue
through Saturday with deep southerly flow across the eastern US,
supporting highs back into the low 40s in most areas. A complex
pattern will evolve over the east, with a northern stream trough
moving into the western Great Lakes and a southern stream low moving
north out of the Gulf of Mexico and into the Mid Atlantic states.
Moisture spreading north of this southern stream low will bring an
increasing chance of rain showers Saturday.

Rain showers will continue into Saturday night and Sunday as a
complex storm system evolves over the Great Lakes and Northeast. In
terms of temperatures, it appears that temperatures will average a
little above normal Saturday and Sunday.

&&

.AVIATION /22Z SUNDAY THROUGH FRIDAY/...
High pressure off the southern New England coast will provide mainly
VFR flight conditions late this afternoon with southerly winds
gusting to 20-30 knots through this evening.

Ongoing warm air advection will support increasing mid and high
level clouds through this evening. Low level moisture will
increase fairly rapidly, first reaching the western Southern
Tier where MVFR cigs will increase during the afternoon hours.

Rain showers, possibly starting as mixed precipitation, will move
into the North Country late tonight. At KART these will likely be
all rain and on the light side. These will spread across the rest of
Western New York Monday morning, with cigs lowering to MVFR roughly
around when the rain starts. Steadier rains will move in during the
afternoon. There will be lots of low moisture, but 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 aloft late tonight and Monday,
which will result in LLWS at some locations.

Outlook...

Monday night...Areas of MVFR 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...
High pressure off the southern New England coast late this afternoon
will support a moderate southerly flow with 15 to 25 knot winds
across the eastern ends of Lake Erie and Lake Ontario where
headlines are in place.

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. Small craft headlines may
need to be expanded.

&&

.HYDROLOGY...
A period of warmer temperatures will cause snow melt which will
add to run-off from rain on Monday and Tuesday. For most
basins, rainfall amounts 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, 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...Winter Weather Advisory from 11 PM this evening to 7 AM EST
     Monday for NYZ008.
MARINE...Small Craft Advisory until 4 AM EST Monday for LEZ020-040-041.
         Small Craft Advisory until 10 AM EST Monday for
         LOZ045.

&&

$$

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


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