Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 101551 AAA
AFDMKX

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

.UPDATE...

The 12Z NAM has come in, with the biggest change a southward shift
in highest precip amounts. The NAM axis of highest QPF now sits
mainly across northern Illinois, just clipping the southeast
forecast area. Amounts are still pretty high, maxing out at .90"
liquid in the far southeast. The NAM remains the wettest solution.
Current forecast snow amounts sit right on the edge of either a
warning or an advisory. Will evaluate the bulk of 12Z model data
as it rolls and hopefully have headline decisions out early this
afternoon.

Based on the latest NAM along with mesoscale models, looks like
there`s room to start the highest pops a tad earlier this
afternoon. Will likely make those changes this morning.

&&

.AVIATION/18Z TAFS/...

An area of light snow in the southeast appears to be winding down
per latest radar trends. Could briefly see MVFR cigs if right
under this band.

Attention then turns to the approaching snow event. Based on
latest NAM and mesoscale models, might bump the start of the snow
up an hour or two most places. Steady snowfall will spread across
the forecast area this afternoon into early this evening, not
winding down until 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 18:1 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.

&&

.MARINE...

Gusty offshore winds will decrease today into this evening as
pressure gradient further relaxes as weak high pressure ridge
passes through western Gtlakes. Winds will back to the SSE
overnight and increase ahead of developing low pressure in the
Plains. Expect steep low level lapse rate and tightening pressure
gradient to result in wind gusts likely reaching Small Craft
Advisory levels on Sunday ahead of the approaching low pressure
area. Wave heights will build as well. Will let afternoon forecast
make final decision on whether a SCA will be needed and timing.

Low pressure system will cause snow across southern Lake MI
beginning late in the afternoon and evening.

&&

.PREV DISCUSSION... /Issued 323 AM CST Sat Dec 10 2016/

TODAY AND TONIGHT...Forecast Confidence Is Medium.

A fast more or less zonal flow with a 125 knot jet at 250 mb
across Wisconsin. Upper level divergence increases tonight. A
rather low amplitude but strong shortwave moves across the
northern plains tonight. Moderate 700 mb upward motion begins late
afternoon across portions of south central Wisconsin, with strong
upward motion mainly just to the south and northwest of the
forecast area tonight.

Mid and low levels saturate over south central Wisconsin this
afternoon, and over the southeast this evening. A strong 850 mb
low level jet reaches Missouri this afternoon and into Illinois
tonight.

Accumulating snow will likely begin after 3 pm southwest of
Madison, reaching the Madison/Janesville area by 6 pm, and likely
not into Milwaukee until after 8 PM.

Models similar in bringing moderate snow to southern Wisconsin
tonight. The GFS has slightly higher precipitation values before
midnight, but is similar to the ECMWF and NAM after midnight.
Snow totals by sunrise across the south should be in the 2 to 5
inch range, with the highest southwest of madison.

Decided to keep the Winter Storm Watch, since bulk of the snow
begins tonight, and 24 hour storm totals through Sunday are very
close to warning criteria. Also, there has been a slight trend
north, so storm totals will be mainly on the borderline with
advisory/warning just about anywhere, and will let later model
runs dictate which way to go.

Expect a slow rise in temperatures today and then into tonight.

SUNDAY THROUGH MONDAY NIGHT...Forecast Confidence Is Medium.

Models have modest low pressure system sliding northeast through
the area Sunday into Sunday evening, then shifting further
northeast later Sunday night into Monday. The NAM continues to
have a more northerly low track versus the GFS/ECMWF/Canadian
models.

NAM continues to show quite a bit more QPF for Sunday morning
across the area than the other models, with focused warm air
advection and frontogenesis response. The GFS/ECMWF are showing
the best focused warm air advection and frontogenesis response
perhaps pushing further northward toward central Wisconsin during
this time. Sunday has the best shot at strong upward vertical
motion in the dendrite snow crystal growth region as well.

So, where this band of snow sets up will be critical for snow
amounts. As stated earlier, there is enough uncertainty to keep
the Winter Storm Watch going. There should be snow to liquid
ratios lowering with the warm air advection into the area Sunday.
Another 2 to 4 inches of snow is possible Sunday, highest in the
eastern counties. Again, this may change depending on where best
frontogenesis response sets up.

Models seem to be coming into better consensus with the snow
ending quicker than past model runs Sunday night. The 06Z NAM run
is now like the 00Z GFS/ECMWF in bringing strong cold/dry air
advection into the area Sunday evening, cutting down on QPF in the
process. Trended downward with PoPs somewhat Sunday evening, with
dry conditions expected later Sunday night. Not confident that we
will see any freezing drizzle at the end of this system, given the
quick drying of the air column.

Cold air advection Sunday night into Monday should continue, with
some middle to high clouds lingering. Models then show an Arctic
cold front pushing southeast through the area Monday night. The
NAM is slowest with the passage of this feature. The GFS/ECMWF
show varying amounts of QPF Monday evening, with the NAM/Canadian
models dry. Kept forecast dry for now, given the uncertainty in
the models with column moisture. Northwest winds behind the front
will bring in very cold temperatures Monday night.

TUESDAY THROUGH FRIDAY...Forecast Confidence Is Medium.

ECMWF/GFS are showing the Arctic high pressure system sliding
east southeast across the northern Plains into Illinois Tuesday
into Thursday. This should keep the very cold temperatures over
the region during this period. The coldest night should be
Wednesday night, with the high closest to the area and subsequent
light winds. Highs should remain in the single digits above zero,
with lows dropping into the single digits below zero each night
away from the lake.

Models Thursday night into Friday are trying to show cyclogenesis
with another low moving through the southern Plains into the lower
Mississippi River Valley. The GFS is about a day slower with this
system than the ECMWF. For now, kept small PoPs for Friday in the
western counties, until better handle on this system can be made.

AVIATION/12Z TAFS/...

Flurries mainly far south will be ending. Ceilings will gradually
lower through the day, lowering to MVFR.

Cloud bases/VSBYS will lower quickly to IFR/LIFR late this
afternoon, mainly southwest of Madison, and then into the evening
over the rest of southern Wisconsin, as snow arrives from the
west.

Accumulating snow expected to begin around 00Z Sun at MSN and 03Z
at MKE.

MARINE...

Southerly winds may reach Small Craft Advisory levels for a time
Sunday, as low pressure moves through the region. The persistent
along shore flow may also build waves to the 3 to 5 foot range,
with the highest waves toward open waters.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...WINTER STORM WATCH from 6 PM CST this evening through Sunday
     evening for WIZ046-047-057-058-063>065-069>072.

     WINTER STORM WATCH from 8 PM CST this evening through Sunday
     evening for WIZ051-052-059-060-066.

     WINTER STORM WATCH from 4 PM CST this afternoon through Sunday
     evening for WIZ056-062-067-068.

LM...None.
&&

$$

UPDATE...DDV/MBK
TODAY/TONIGHT AND AVIATION/MARINE...Hentz
Sunday THROUGH Friday...Wood


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