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Area forecast discussion
National Weather Service Green Bay WI
358 am CST Thu Jan 17 2019

Forecast discussion for routine morning forecast issuance

Synopsis...
issued at 353 am CST Thu Jan 17 2019

Not as cold with a little light snow today, then below normal
temperatures for the weekend.

The main westerlies flowing across the Pacific are split due to
complex blocking over the northeast Pacific and northwest North
America. The branch of the flow suppressed south of The Block
enters North America along the U.S. West Coast then continues
east across the Continental U.S. As a separate southern stream. The northern
branch of the flow loops around The Block, then frames a large,
deep upper trough over eastern Canada. The large scale pattern
will be stable through the weekend, then full-latitude ridging is
expected to begin building off the West Coast next week. That will
eventually lead to a consolidation of the downstream flow, and a
gradual westward shift and extension southward of the eastern
North American upper trough.

The forecast area will be primarily along the southern edge of the
northern stream throughout the forecast period. That will result
in a temperature rollercoaster as the air mass across the area
oscillates between surges of fresh polar air from Canada and more
moderated air masses from the plains. Temperatures will briefly
return to above normal levels today, drop below normal for the
weekend, rebound close to normal for a time early next week, then
drop off again. There will be at least a few opportunities for
precipitation--including some lake-effect. Amounts will be light
through the weekend, with more uncertainty in the precipitation
potential thereafter.

Though specifics remain unclear, the evolution of the pattern on
the medium range models suggests a major cold outbreak is
increasingly likely beyond the end of the forecast period. The one
factor that could mitigate that somewhat is the current lack of
deep snowcover, though there will be opportunities to build up
a snow cover prior to the Arctic outbreak as the retrogression/
sharpening/deepening of the eastern North American upper trough
will result in the pattern more supportive of snow across the
area.
&&

Short term...today...tonight...and Friday
issued at 353 am CST Thu Jan 17 2019

The main change to the forecast this morning was to increase snow
chances for today. Most of the guidance now generates light
precipitation across much of the area during the day. That fits
well with the Leaf-like structure advancing east from Minnesota on
early morning satellite imagery. Surface observations from that
area generally had 3-6sm vsbys, though a few obs were as low as 1
1/2sm. It's not a real organized snow band, so there wasn't much
opportunity to add much in the way of specific timing. Basically
went with a gradual increase in pops until mid-day, then a slow
decrease during the afternoon. It probably won't snow continuously
either. The situation is something we'd have probably have
described as "occasional light snow and flurries" back in the days
of manually typed forecasts. Model forecast soundings suggest
warm advection and a gradual "shallowing" of the moisture during
the afternoon could lead to some fzdz across the southern portion
of the forecast area as the precipitation ends. Even if it
occurs, the impact would be less significant than with the past
few fzdz events because it will be falling on top of the fresh snow.

Clouds should decrease overnight, except in the north where lake-
effect snow showers are possible. Middle and high clouds will
stream back across the area Friday as the next southern stream
system begins to head east from the plains.

Long term...Friday night through Wednesday
issued at 353 am CST Thu Jan 17 2019

An amplified, progressive mean flow will continue across the Continental U.S.
Through early next week, before the pattern begins to become
blocky with a dominant upper ridge over the eastern Pacific and a
deep upper trough over the central Continental U.S.. two strong Pacific
systems will move across the Continental U.S.. the first system Friday night
into Saturday still appears to only provide a glancing blow to
central/east-central WI. The second system Monday night into
Tuesday is more interesting, but models do not agree on several
key elements. The main story for northeast WI will be the Arctic
air intrusion this weekend with possible wind chill headlines.

The southern stream winter storm is forecast to move from the
Southern Plains toward the mid-MS valley Friday night, while a
weak northern stream shortwave trough drops southeast into the
western Great Lakes toward daybreak. Models are still adjusting as
to how far north to carry light snow into WI with the GFS the
farthest north by reaching up to Hwy 29. Most other models favor
up to Hwy 10. Another factor will be the northeast winds that
could add some lake enhancement across east-central WI. Highest
pops placed across Kewaunee/Manitowoc counties where an inch or
two of accumulation will be possible with less amounts (less than
an inch) farther west. Some light snow showers may also occur over
Vilas County Friday night, but unfavorable trajectories would
limit accumulation potential. The rest of northeast WI to stay dry
and cold with min temperatures from 5 to 10 above zero north-
central WI, 10 to 15 above near Lake Michigan.

Snow chances will continue across eastern WI (synoptic snow) and
Vilas County (lake effect snow) on Saturday, although additional
accumulations over both locations will be minimal. The winter
storm to move up the Ohio River valley with a persistent north-
northeast wind across WI. Much colder air is expected to get
pulled southward into WI with 800 mb temperatures dropping into the
-16 to -21c range by 00z Sunday. Look for Max temperatures to
reach 10 to 15 above zero north, middle to upper teens south.

Arctic air to overspread the rest of the region Saturday night
with high pressure located over southwest Ontario and the winter
storm hitting the East Coast. Winds are forecast to back almost
due north, which would keep any Lake Michigan lake effect snow offshore,
but increase the chances for lake effect into north-central WI
from Lake Superior. Inversion heights still look low on forecast
soundings, thus accumulations for Vilas County to be light. Min
temperatures Saturday night to range from 5 to 10 below zero
north-central, to around 5 above zero near Lake Michigan. North winds of
5 to 10 mph inland, 10 to 15 mph Lakeside, will generate wind
chills of 10 to 20 below zero with values approaching Wind Chill
Advisory criteria over north-central WI.

The area of high pressure will move across northern sections of
the Great Lakes on Sunday and provide for a mostly sunny to partly
cloudy day across the forecast area. While lake effect snow
chances to linger over Vilas County, drier air should mitigate
much in the way of further accumulations. Max temperatures Sunday
to only reach 5 to 10 above zero north, 10 to 15 degrees south.

As the surface high slips to our east Sunday night, a return flow
sets up across WI with stronger warm air advection over the upper MS valley/
Midwest. Do anticipate some high or mid clouds to overspread parts
of WI after midnight which could impact min temperatures. This warm air advection
then spreads across WI on Monday and once the atmosphere
saturates, this could bring US our next chance of light snow,
especially during the afternoon. Max temperatures to warm a bit
with readings in the middle teens north-central, upper teens to
lower 20s south.

Models remain split into 2 camps with regards to the next Pacific
system to impact the central Continental U.S. Monday night through Tuesday.
The GFS and CMC are progressive in sending a weak shortwave
trough/associated surface low into the Great Lakes with the
potential for a moderate snow accumulation. Meanwhile, the European model (ecmwf)
digs the shortwave trough through The Rockies and keeps the track
of the surface low farther south, thus less snow for northeast WI.
There is enough uncertainty in timing, track and strength at this
point to keep confidence on the low side. Have followed the model
consensus solution that focuses highest pops Monday night into
Tuesday morning. Just too early to make any call on snow
accumulations. Temperatures during this time will moderate closer
to normal.
&&

Aviation...for 12z taf issuance
issued at 353 am CST Thu Jan 17 2019

Low clouds with MVFR ceilings have surged north across the area
much more quickly than forecast by any of the aviation guidance
products. Now that they're in place, don't see them going anywhere
until after the surface trough/weak front crosses the area
tonight.
&&

Grb watches/warnings/advisories...
none.
&&

$$

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