Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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447
FXUS63 KMKX 110549 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1149 PM CST Sat Dec 10 2016

.UPDATE...

The band of higher radar reflectivity that spans from ND to MI is
moderate snow that is associated with strong 700-600mb
frontogenesis. This forcing is expected to remain fairly stationary
overnight and is a little farther north than our previous forecast.
There is a chance that our northern tier of counties may need to get
upgraded to a warning. There is also a chance that southwest WI will
not get as high of snowfall amounts as we were expecting.

&&

.AVIATION/06Z TAFS/...

Steady snowfall will persist through Sunday morning. The snow should
begin tapering off after 8 am Sunday, especially in south central
WI. Southeast WI could see another round of moderate snow during the
mid afternoon hours, but most of that snow should be to our
southeast. Snow will end later Sunday afternoon into early Sunday
evening.

Persistent visibilities of 1 to 2 miles though the event are likely,
with periods down to 1/2 mile probable as well. Ceilings should be
in the 1000 to 2500 foot range until Sunday afternoon. Expect cigs
to lower to IFR by Sunday evening as low pressure tracks WI.

The snow will be of the drier variety through the event, with ratios
starting around 20:1 and ending around 12:1. With this being a
prolonged event, rates will probably not exceed an inch an hour for
most of the event, though brief heavier bursts are always possible.
Storm totals are expected to be in the 7-10 inch range, with highest
amounts now expected from the Dells to northern Milwaukee County.

&&

.MARINE...

Southerly winds ahead of approaching low pressure will likely gust
to 25 knot at times Sunday. This will build waves of 3 to 5 feet
across the nearshore waters on Sunday. Westerly winds will then
become gusty behind the low and associated cold front Sunday
evening, with higher gusts lingering into Monday morning. The waves
will be lower with the offshore winds Sunday evening into Monday,
except for higher waves toward open waters. Have decided to go with
a Small Craft Advisory from 9 am Sunday to 10 am Monday.

&&

.PREV DISCUSSION... /Issued 812 PM CST Sat Dec 10 2016/

UPDATE...

No changes to headlines planned this evening. New model guidance is
now suggesting that we could see slightly higher amounts than
previously forecast between the Dells and Port Washington. This area
is still covered in the warning.

Snow overspread all of southern WI by early evening. Visibilities
are around 2SM on average, and snow is accumulating at rate of a
quarter to half inch per hour. This is very light, fluffy snow due
to a very deep layer with temperatures within the dendritic growth
zone. Snow ratios are about 20:1 and will continue that way through
most of the night. Snowflakes are not very large this evening since
the overall lift is not really strong (omega values around -5
ubar/sec).

There is a band of higher reflectivity on radar over south central
WI that is oriented NW to SE. That band is associated with 700mb
frontogenesis and warm air advection. This is where we are getting
the half inch or more per hour snowfall rates. It will continue to
slide east through the evening. We should see lighter snow for a
period of time from midnight to 3am.

There is a broad area of 600mb frontogenesis draped across southern
MN and central WI right now and associated with light snow. The
stronger 600mb fgen and also 700mb fgen is expected to reinvigorate
over south central/southeast WI from around 09Z/3am to 12Z/6am
Sunday morning and produce an uptick in snowfall rates.

AVIATION/03Z TAFS/...

See update for latest discussion on snow trends.

MARINE...

Southerly winds ahead of approaching low pressure will likely gust
to 25 knot at times Sunday. This will build waves of 3 to 5 feet
across the nearshore waters on Sunday. Westerly winds will then
become gusty behind the low and associated cold front Sunday
evening, with higher gusts lingering into Monday morning. The waves
will be lower with the offshore winds Sunday evening into Monday,
except for higher waves toward open waters. Small Craft Advisory
in effect from 9 am Sunday to 10 am Monday.

PREV DISCUSSION... /Issued 315 PM CST SAT DEC 10 2016/

TONIGHT AND SUNDAY - Forecast Confidence...High.

Snow will continue to spread southwest to northeast across the
forecast area this afternoon into early evening. Strong warm
advection and frontogenetic forcing ahead of approaching low
pressure will keep persistent snowfall going through the
evening/overnight and into Sunday. The snow will wind down west to
east Sunday afternoon into early evening as the low and associated
cold front move through.

Not a lot of change in total liquid equivalent expected with this
storm. A blend of models came in around .05" higher than the
previous forecast. Model soundings are showing an impressively deep
dendrite growth zone for much of this evening and overnight, which
should help contribute to higher snow to liquid ratios. Given the
higher ratios and somewhat higher QPF amounts, total snow ended up
about 1 to 1.5 inches higher than previous forecast. New totals are
in the 7-10 inch range. Given the boost in numbers...decided to pull
the trigger on a Winter Storm warning across the southern 3 rows of
counties. Went with a Winter Weather Advisory in the northern row
where the lowest amounts are anticipated. Travel will become
hazardous given the snow covered roads and significantly reduced
visibilities at times.

SUNDAY NIGHT THROUGH TUESDAY - Forecast Confidence...Medium.

Will end snow early Sunday evening as drier air sweeps in from the
west and mid-level short wave speeds east of the area.  Possible
that some light freezing drizzle may be mixed with the light snow as
it tapers off and ends at the tail end of the event in the early
evening as the atmosphere dries significantly abv 5k feet, however
not enough confidence to include at this point. Although the
precipitation will be ending, low level cold air advection and a
tightening pressure gradient may result in areas of blowing snow
into early Monday.  The snowfall expected tonight and Sunday will be
drier and more susceptible to being elevated.

The winds will settle down and become less gusty later Monday
morning ending the blowing snow threat.  Short range guidance coming
into better agreement on surge of warm air advection and isentropic
omega on 285 theta surface spreading rapidly across southern WI
Monday evening.  Will be adding high chance pops for the evening
with higher snow ratios of 15 to 20 to 1 possibly producing a quick
inch of dry snow. Then a reinforcing push of cold air on breezy WNW
winds is expected on Tue.

EXTENDED PERIOD...

TUESDAY NIGHT THROUGH SATURDAY - Forecast Confidence...Medium.

Southeastward drifting polar low across southern Canada will ensure
Wisconsin and the Great Lakes remain bitterly cold through Thursday.
Later in the period the polar low shifts farther off into eastern
Canada resulting in zonal flow regime setting up across northern
CONUS.  Temps will moderate slightly but likely remain below
seasonal normal.

Zonal flow regime quickly succumbs to amplifying upstream long-wave
troffing over western CONUS on Friday.  Increasing south winds ahead
of long wave trof will usher deeper moisture northward into the
upper midwest, also strengthening baroclinic zone across the area.
This scenario would likely result in areas of -sn returning to the
area on Friday. Medium range guidance showing some agreement on
short wave trof and resultant sfc low pressure developing somewhere
in the southern or central Plains early in the weekend and tracking
northeast toward the Great Lakes on Saturday...however ensembles
showing considerable variability regarding this evening.  Never the
less, the string of weekend snow events for southern WI looks likely
to continue.

AVIATION/00Z TAFS/...

Steady snowfall will spread across the forecast area this afternoon
into early this evening, not winding down until later Sunday
afternoon into early Sunday evening. Persistent visibilities of 1 to
2 miles though the event are likely, with periods down to 1/2 mile
probable as well. Not out of the question to see visibilities drop
briefly to 1/4 mile under heavier bands.

The snow will be of the drier variety through the event, with ratios
starting around 20:1 and ending around 12:1. With this being a
prolonged event, rates will probably not exceed an inch an hour for
most of the event, though brief heavier bursts are always possible.
Storm totals are expected to be in the 6-8 inch range in the
northern forecast area, to 7-10 inches elsewhere.

MARINE...

Southerly winds ahead of approaching low pressure will likely gust
to 25 knot at times Sunday. This will build waves of 3 to 5 feet
across the nearshore waters on Sunday. Westerly winds will then
become gusty behind the low and associated cold front Sunday
evening, with higher gusts lingering into Monday morning. The waves
will be lower with the offshore winds Sunday evening into Monday,
except for higher waves toward open waters. Have decided to go with
a Small Craft Advisory from 9 am Sunday to 10 am Monday.

MKX WATCHES/WARNINGS/ADVISORIES...

WI...WINTER STORM WARNING until 6 PM CST Sunday for WIZ056-062-063-
     067>070.

     WINTER STORM WARNING from 6 PM this evening to 8 PM CST Sunday
     for WIZ057>060-064>066-071-072.

     WINTER WEATHER ADVISORY from 6 PM this evening to 8 PM CST
     Sunday for WIZ046-047-051-052.

LM...SMALL CRAFT ADVISORY from 9 AM Sunday to 9 AM CST Monday for
LMZ643>646.

$$
TONIGHT AND SUNDAY/AVIATION/MARINE...DDV

MKX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER STORM WARNING until 6 PM CST Sunday for WIZ056-062-063-
     067>070.

     WINTER STORM WARNING until 8 PM CST Sunday for WIZ057>060-
     064>066-071-072.

     WINTER WEATHER ADVISORY until 8 PM CST Sunday for WIZ046-047-051-
     052.

LM...SMALL CRAFT ADVISORY from 8 AM Sunday to 9 AM CST Monday for
     LMZ643>646.

$$

UPDATE...MRC
TONIGHT/Sunday AND AVIATION/MARINE...DDV

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER STORM WARNING until 6 PM CST Sunday for WIZ056-062-063-
     067>070.

     WINTER STORM WARNING until 8 PM CST Sunday for WIZ057>060-
     064>066-071-072.

     WINTER WEATHER ADVISORY until 8 PM CST Sunday for WIZ046-047-051-
     052.

LM...SMALL CRAFT ADVISORY from 8 AM Sunday to 9 AM CST Monday for
     LMZ643>646.

&&

$$

UPDATE...MRC
TONIGHT/Sunday AND AVIATION/MARINE...DDV
Sunday NIGHT THROUGH Saturday...MBK



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