Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 081757 AAC
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1157 AM CST Fri Dec 8 2017

.UPDATE...

After looking through 12Z guidance, opted to slow down the timing
of the steadier accumulating snowfall tonight. Looks like the
northeast could get clipped with some light accumulations in the
early evening, with a band of widespread snow then moving through
from northwest to southeast late evening into the night. Still
expect amounts in the 1 to 2 inch range, with the higher amounts
northeast and lower southwest.

&&

.AVIATION(18Z TAFS)...

Delayed the timing of the steadier accumulating snowfall a couple
hours based on 12Z models coming in a little slower. Still
thinking 1 to 2 inches of dry snow...with the steadier snow
arriving around 03Z northwest and 06Z southeast. A little light
snow or flurries are possible before the accumulating snow
starts. The snow will taper off west to east later tonight into
early Sat morning.

Will see visibilities and ceilings come down with the
accumulating snowfall. Thinking visibilities down to around 2
miles, but will keep an eye on upstream snow as it approaches and
adjust the vis forecast as needed. Ceilings will return to VFR by
late Sat morning. Could see some blowing snow tomorrow with gusty
winds, especially places that see higher amounts toward 2 inches.

&&

.PREV DISCUSSION... (Issued 937 AM CST Fri Dec 8 2017)

UPDATE...

Snow is still expected for this evening and overnight. Amounts
look fairly similar to the going forecast, but the timing may be
trending a little slower. It looks like there may be a fair
amount of dry air to overcome in the lower levels, especially in
the southeast forecast area. While some models keep the timing
similar to the current forecast, a couple short term models are
delaying the start until 11 pm or 12 am in the southeast. After
the majority of 12Z guidance comes in, will take a close look and
adjust the forecast timing as need be.

MARINE...

Still seeing a few higher gusts out there, so will let the Small
Craft Advisory continue until noon. Another advisory will be
needed for later tonight into at least Saturday evening due to
increasing northwest winds behind a departing low pressure system.

Winds may reach advisory levels at times through the first half of
next week under an active northwest flow pattern.

PREV DISCUSSION... (Issued 538 AM CST Fri Dec 8 2017)

UPDATE...

A line of flurries is making its way south this morning. This is
associated with weak low level frontogenesis. The better forcing
will continue to shift to the east. Low clouds expanded across all
of central and northern WI in the wake of this front.

AVIATION(12Z TAFS)...

The light returns on radar are flurries this morning that are
associated with a weak front. The better chance of MVFR ceilings
will be right behind the front. The low clouds are becoming more
widespread early this morning. These will scatter out by late
morning, but mid clouds should linger through much of the day.

Snow will spread into southern WI this evening and temperatures will
be in the mid/upper 20s during it. Timing is trending a little
slower due to dry air in the low levels. Snowfall amounts of 1 to 2
inches are expected, with lowest amounts southwest of Madison and
highest toward lake Michigan (not due to lake effect).
Snow ratios around 17:1 are expected, which is on the average to dry
side. Winds will increase out of the northwest as the snow tapers
off after midnight. This could cause some blowing snow across
runways.

PREV DISCUSSION... (Issued 315 AM CDT FRI DEC 8 2017)

TODAY...Forecast Confidence is High.

An area of 925-850mb frontogenesis is producing light snow/flurries
over central WI early this morning. The better forcing is going to
peel off to the east as the front slides south. Clouds and
reflectivity returns with the front over west central WI are
weakening so only expecting flurries as it tracks through southern
WI. Expect some lower clouds in its wake over east central and
southeast WI this morning. There may be a period of mostly clear
skies by midday.

TONIGHT...Forecast Confidence is High.

Light snow will spread southward into WI by mid evening. This is a
little later than previous forecasts. There is strong upper forcing
for this quick shot of snow, with strong potential vorticity
advection, mid level q-vector convergence, upper jet divergence, and
warm air advection aloft. The surface forcing is on the weaker side
and there is some dry air in the low levels to overcome before it
snows.

Given the better upper dynamics, time-height cross sections of
models show that the stronger omega is above the dendritic growth
zone. While there will still be dendrites, it appears that the snow
might be a little finer rather than fluffy. The snow ratio will
still be on the mid-high side, around 17:1. QPF is lower and thus
forecast snowfall amounts are now lower, in the 1 to 2 inch range.
The higher amounts will still be toward Sheboygan.

Temperatures during the snowfall will be in the mid/upper 20s where
salt will be effective. Once the snow tapers off after midnight,
temps will be on their way down and northwest winds on the way up.
Blowing snow near the ground could cause some slick spots on
untreated roads.

SATURDAY THROUGH SUNDAY - Forecast Confidence...Medium to High.

Will continue a small chance for light snow Saturday morning across
the far east due to lingering mid-level deformation and nearness of
upper trof.   Otherwise, gradual clearing can be expected on
Saturday.  Blustery north winds gusting of 25 to 30 knots will
likely cause some drifting snow in open areas.  However held off on
introducing any blowing snow due to expected lower snow amounts.

Otherwise, dry conditions are expected for much of the weekend.
Several weak passing mid-level waves caught in fast northwest flow
will bring periods of clouds to the area.  Possible some flurries
may get shaken out of the mostly mid-level clouds, but low level
moisture remains limited.  Hence wl hold off on introducing any
flurries.  Despite the clouds, southwest breezes may allow temps to
break the 30 mark on Sunday.

EXTENDED PERIOD...

SUNDAY NIGHT THROUGH THURSDAY - Forecast Confidence...Low to
Medium.

North to northwest steering winds will remain dominant through this
period.  These winds will carry several fasting moving clipper
systems across the upper midwest and western Great Lakes during this
period.  First of these clipper systems is expected to move from
south central Canada into the region late Sunday night and Monday.
Although confidence is high on this system moving through early next
week, the Medium range guidance in disagreement on track of this
system during this period.  ECMWF showing more run to run
discontinuity with a more southern track across MN/IA while GFS and
GEM taking northerly track across northern and central WI.  More
southern ECMWF track would bring more widespread snow to southern WI
Monday morning with the focus shifted farther north with other
Medium range guidance.  WPC using a blend of all guidance in
carrying low across southern WI, which is in agreement with some of
the GFS ensemble members.  Even if surface low remains north of the
area, warm air advection and isentropic omega ahead of the system
worthy of carrying some pops for now.  More northerly track may also
bring a mixed ptype on Monday.

Another fast moving system will threaten the area with more -sn on
Wednesday.  The main focus for this period will be the continued
very cold conditions.  The coldest air of the season may plunge into
the area behind the Monday clipper.  925H temps tumble to -12 to -
15C on Tuesday.  With increasing snow pack, temperatures may fall
below zero Tuesday night before clouds begin to increase.

AVIATION(09Z TAFS)...Radar is showing light returns across central
WI. This weak front will slip south through mid morning and could
bring a few flurries and lower clouds. The better chance of MVFR
ceilings will be right behind the front toward Sheboygan and
possibly Milwaukee. Elsewhere should remain VFR with a clearing
trend besides some high clouds.

Snow will spread into southern WI this evening and temperatures will
be in the mid/upper 20s during it. Timing is trending a little
slower due to dry air in the low levels. Snowfall amounts of 1 to 2
inches are expected, with lowest amounts southwest of Madison and
highest toward lake Michigan (not due to lake effect).
Snow ratios around 17:1 are expected, which is on the average to dry
side. Winds will increase out of the northwest as the snow tapers
off after midnight. This could cause some blowing snow across
runways.

MARINE...

The gusty southwest winds will diminish through late morning as a
weak front drops into the area. The current small craft advisory is
in effect until noon now rather than 4 pm.

Look for snow to spread into southern WI from the north during the
evening hours. This will taper off after midnight as northwest winds
start increasing. Another small craft advisory will be needed for
gusty northwest winds early Saturday morning through Saturday
evening. A few gale force gusts are possible.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory until noon CST today for LMZ643>646.

&&

$$

Update...DDV
Today/Tonight and Aviation/Marine...MBK
Saturday THROUGH Thursday...Cronce


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