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Area forecast discussion
National Weather Service Buffalo New York
514 PM EST Sat Dec 7 2019

Synopsis...
high pressure will build across the eastern Great Lakes tonight
before moving off the southern New England coast on Sunday. This
will provide mainly dry weather for the remainder of the weekend.
Low pressure will track across the central Great Lakes late Monday
with above normal temperatures and showers for the start of the work
week, followed by colder weather and lake effect snow showers for
mid week.

&&

Near term /through Sunday night/...
a ridge of high pressure extends from Ohio into the eastern Great
Lakes at the surface, while the eastern edge of an upper ridge
centered over the upper Great Lakes has now moved over New York state. This has
has helped to squelch any lingering snow showers east of Lake
Ontario, with dry weather now found across the entire forecast area.
Likewise, northwest flow lake effect clouds will continue
to gradually erode southeast of the lakes through the evening.

High pressure will drift to the southern New England coast
tonight. Increasing warm advection in the wake of the departing
high will bring an increase in high clouds overnight. Lows will
occur during the mid to late evening, with 20s across western New York
and teens from the Genesee Valley eastward, and even some
single digits east of Lake Ontario. Temperatures will then rise
overnight as a southerly breeze and warm advection increase.

High pressure will continue to move east and off the southern New
England coast Sunday. Ongoing warm advection will continue to
support thickening mid/high clouds, but any rain showers associated
with the next system will remain well to our northwest through early
evening with dry conditions prevailing in the eastern Great Lakes.
Temperatures will warm to above average, with highs in the mid 40s
across western New York and mid to upper 30s east of Lake Ontario.

A weak surface trough of low pressure across the central Great Lakes
Sunday evening will gradually move eastward towards our region
Sunday night. An increasing southwesterly mid-level flow will
provide a broad area of weak mid level warm air advection. Chances
for showers will increase late Sunday night. The southerly flow will
prevent temperatures from dropping below freezing at most locations,
but there is a chance for below freezing temperatures and a brief
snow or sleet mix across the north country with thermal profiles
showing a mid-level warm layer of around +1c. Otherwise, lows will
range from the lower 40s across the lake plains west of Rochester
with mid to upper 30s at most other locations. Rainfall amounts and
any snow accumulation will be minimal.

&&

Short term /Monday through Wednesday/...
shortwave trough over the upper Midwest tracks east and into
the upper Great Lakes on Monday. Deep southerly flow ahead of
this feature will transport copious amounts of Gulf of Mexico
and Atlantic moisture northward across the region. This combined
with a +40 knot low level jet and positioned within the favorable
entrance region of the upper level jet will provide the lift for
widespread rainfall to occur on Monday. Still looking at
rainfall amounts across area basins of 0.30- 0.75 of an inch,
with localized higher amounts. Otherwise, it will be a mild day
Monday with high temperatures getting into the upper 40s to
lower 50s at most locations.

Low pressure will be just to our northwest by Monday evening and
then will track off into Quebec Monday night. As it heads off into
Quebec, it will send a strong cold front towards the lower lakes. It
appears at this point, that the cold front won't arrive across far
western New York until just before daybreak on Tuesday. Also,
widespread rainfall ahead of the front will gradually diminish
overnight as the main forcing shifts to out east. Temperatures
overnight will be low 40s just before daybreak, then drop quickly
behind the front on Tuesday.

..accumulating lake effect snows possible beginning Tuesday
afternoon...

Much colder air will rapidly filter in as cold air advection ramps
across the lower lakes. This will likely spin up the lake effect
snows off both lakes beginning Tuesday afternoon and continue through
Wednesday night. Now for the details...

Arctic air mass will rapidly spill in behind a strong cold front on
Tuesday. While the air mass will definitely be cold enough to
provide the necessary over lake instability (h850t -12 to -13c),
there are some parameters that won't be favorable for a major event.
First and foremost, residence time of the lake effect bands off Lake
Erie and Ontario will be limited at any one location. Right now it
appears the bands will be moving around quite a bit which will limit
snowfall accumulations. Next, guidance has been fairly persistent
showing a fairly dry air mass behind the front and this system lacks
that deep synoptic moisture to maximize snowfall. Now this isn't to
say there won't be some decent accumulations, especially east of
Lake Ontario where upstream connections will come into play. It also
can be said that this might be some of the coldest air so far that
we've seen this season. A second re-enforcing shot of Arctic air on
Wednesday will send h850t down to -18c/-20c, with an associated
shortwave providing some moisture which will enhance ongoing lake
snows. Otherwise, there still remains quite a bit of uncertainty
with this event, especially in terms of specific band placement and
timing. Stay tuned as things could change.

In terms of temperatures, highs on Tuesday will actually occur just
after midnight. Daytime temperatures will fall into the 30s behind
the front. Lows Tuesday night will be in the upper teens to low 20s
by daybreak on Wednesday. Wednesday, highs will average 10-15f below
normal with temperatures peaking in the low to mid 20s.

&&

Long term /Wednesday night through Saturday/...
lake effect snows will still be ongoing Wednesday night with
additional accumulations possible off both lakes. Meanwhile, surface
high pressure over Tennessee and Ohio will begin building into the
lower lakes as we head into Thursday. Winds will begin the gradual
shift from west-northwesterly Thursday morning, to more
southwesterly by late in the day Thursday. Although this will signal
the beginning of the warm advection process, it will be a slow one.
Temps aloft will still be plenty cold enough to support a lake
response, though will be quite muted when compared to the previous
as shear and continued drier air (lowering cap) come into play as
well. Will most likely see the typical south to north 'sweep' of the
lake bands as winds slowly back from west-northwest to southwest
Thursday into Thursday evening. Any lingering snow showers northeast
of the lakes potentially end Thursday night as surface high pressure
moves overhead and progressively warmer air pushes in aloft. By
Friday morning the surface high will have exited off to our
northeast and positioned just off the coast of New England. There
remains quiet a bit of uncertain regarding the Friday to Saturday
time frame. Per latest guidance, the GFS is quiet a bit faster with
the development of a coastal low along the mid-Atlantic and further
west with the upstream system. European model (ecmwf) is slower and holds off any
potential precipitation until Saturday. Have kept low end chance
pops for now with so much uncertainty.

&&

Aviation /22z Saturday through Thursday/...
all terminals in western and north central New York are now seeing VFR
flight conditions as ceilings have either lifted or scattered out.
High pressure across the area will move to the southern New England
coast with widespread VFR flight conditions found across the region
tonight. Ongoing warm air advection will support increasing mid and
high level clouds during the day Sunday. Also, a modest low level jet of 40-50
knots will result in a period of low level wind shear across western New York Sunday
morning before later in the day when these winds mix to the surface
more effectively.

Outlook...

Sunday night...VFR/MVFR. Showers likely, possibly mixed with
sleet or snow across the north country.
Monday...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.

&&

Marine...
winds have diminished across Lake Ontario with high pressure
building into the eastern Great Lakes. A short period of lighter
winds is expected tonight, but south-southwest winds will quickly increase on
Lake Erie and Lake Ontario by early Sunday morning. Small craft
headlines are in effect for Eastern Lake Ontario for the southerly
flow late tonight through early Monday, and these may need to be
extended to Lake Erie although it's a bit more marginal.

A windy period of weather will then continue through much of the
week as a strong cold front crosses the area Tuesday, followed by
moderate westerlies Tuesday night through Thursday night.

&&

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 amount of 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 quantitative precipitation forecast here, with some models in
the 1-2 range across favorable upslope regions. This supports a
low risk for flooding in the Black River basin, but there are
several limiting factors which is probably why mmefs ensembles
(sref/gefs/naefs) are not showing much of a risk for flooding
with rises to about action stage a much more likely outcome. It
appears that the higher rainfall amounts will be localized to
upslope regions and not represent basin averages. Also,
temperatures are not likely to get warm enough to melt all of
the snow pack in place, and rainfall rates will be modest. The
situation bears watching, but no flood headlines are anticipated
in our forecast area unless some of these limiting factors
change.

&&

Buf watches/warnings/advisories...
New York...none.
Marine...Small Craft Advisory from 4 am Sunday to 10 am EST Monday for
loz045.

&&

$$

Synopsis...apffel/jm
near term...apffel/Hitchcock/jm
short term...AR/tma

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