Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 122341

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
541 PM CST Tue Dec 12 2017

...Early Evening/00Z Aviation Update...


Have increased snow potential for late this evening into the
overnight hours across the northern half or so of the forecast
area, where an area of warm advection snow is expected to push
through from west to east. Amounts/impacts should be quite
minimal, with just flurries or very light snow showers expected.

The main band of snow is still expected to affect the region
starting tomorrow morning around daybreak. It still appears that
the heaviest snow will be most likely to fall along and north of a
line from Fond du Lac to Port Washington, with decreasing
confidence in amounts further to the southwest. New 00z guidance
will be coming in over the next few hours, and will hopefully
continue to tighten up expectations for the area of heaviest snow.



The next 24 hours or so will be a tricky forecast for aviation
purposes. Initially this evening, a band of light snow is expected
to push in from the west, likely around or after midnight for most
sites. This activity should be quite light, with just flurries or
light snow showers expected. Vis may drop for a brief time to
MVFR, but impacts should be limited.

Around daybreak, a more organized band of snow is expected to
develop and push southeast into the region. All four terminals
(but especially KUES, KMKE, and KENW) will likely be on the
southwestern edge of the heavier snow, making amounts and impacts,
particularly visibility, quite challenging to forecast. While the
TAFs indicate a prolonged period of 1-2 SM visibility tomorrow
morning and early afternoon, breaks in the snow will be possible,
though are impossible to forecast this far in advance. Where the
heaviest band of snow does set up, several hours of moderate snow
can be expected.

In addition to the main band of snow, winds will come off the lake
tomorrow afternoon and evening, producing some lake effect snow,
which would affect all but KMSN. An additional period of IFR to
LIFR can be expected with any lake effect bands that form.


.PREV DISCUSSION... (Issued 244 PM CST Tue Dec 12 2017)


TONIGHT - Confidence...Medium.

Mid-level clouds from strong upstream warm air advection already
spreading into south central WI this afternoon.  Trend will
continue.  Strongest warm air advection spreads across southern WI
from late evening through the overnight.  Enough low to mid level
moisture connection expected to possibly bring some light snow or
flurries to southern WI during this period.  Bumped up and expanded
pops across most of the area for later tonight.  Potential some
northern areas may receive one half inch to one inch by late
tonight.  Hence Winter Weather Advisory goes into effect for several
north central counties late tonight.

WEDNESDAY into WEDNESDAY EVENING - Confidence...Medium to High.

Short term guidance in good agreement on carrying low level
circulation and surface low pressure across southern WI/northern IL
on Wednesday.  GFS and NAM showing fairly good run to run
consistency with this feature.  Strong layer Q-vector convergence of
up to 50 units passes rapidly across southern WI late tonight and
Wednesday morning.  One worrisome snafu is potential for losing ice
crystals for a time in the southwest CWA as drier air wraps around
low pressure circulation, hence slightly lower confidence.  Did
include small chance for -zl/-zr in the southwest. Low level drying
remains more limited in the eastern CWA during the day. Cross
sections show impressive lift across northeast CWA in the dendritic
growth zone close to an area of sloping frontogenetical forcing.  In
addition, an inverted trof extending northwest from the low level
circulation may increase the convergence along the lakeshore areas
from Milwaukee to Sheboygan in the late afternoon into the early
evening.  Delta-T around 14-15C in the early evening with lake
induced Cape over 250J.  Part of the low level conditionally
unstable layer reaches the snow growth region as well.  With snow
liquid ratios around 15 to 1, expecting synoptic snowfall in the 3
to 6 inch range over northeast CWA.  However snowfall may be locally
heavier due to the elevated instability and lake enhancement.  Added
7" local amounts in the northeast.  Posted Winter Wx Advisory for
all of the northeast, with eastern areas continuing into the

Thursday and Friday...Forecast confidence is medium.

We`ll remain under a cold northwest flow regime through the end of
the work week. Thursday looks cloudy and we could see a few
flurries, but not really worth mentioning. On Friday, there`s a
better looking short wave dropping south through the area and this
could bring a chance for some light snow. At this point, it looks
rather dry, so not too worried about any accumulations.

Saturday and Sunday...Forecast confidence is low.

There continues to be model disagreement with the complicated
pattern this weekend. The upper pattern begins to flatten a bit,
or become less northwest oriented. Low pressure is expected to
develop west of the area on Saturday, but where it goes is rather
uncertain. The GFS has a low moving over Lake Superior while the
ECMWF has a low moving over central/southern Wisconsin. Right
now, most of the precip looks north of the forecast area. We
should warm up into the lower and mid 30s this weekend with the
somewhat milder flow returning. We`ll have to keep an eye on that
low pressure track. These are the kind of system that can drift
south, resulting in some snow for southern Wisconsin. If it stays
north, we`re warmer and drier.

Monday and Tuesday...Forecast confidence is medium.

There`s nothing of significance to latch onto early next week. The
upper flow does look for zonal, and thus more mild than what we`ll
see this week. For now, the forecast is dry, but it`s an active
flow, so we`ll have to watch out for some stronger pieces of
energy that may produce some light, pesky precip.

AVIATION(18Z TAFS)...Stratus continues to become more
cellular across eastern CWA last several hours. This trend should
continue through the late afternoon. Upstream strong warm air
advection over the northern Plains causing a rapid increase in mid
level clouds which have already advanced southeast into southern WI.
This trend will continue and possibly ceilings lowering to MVFR
later tonight.  Some light snow or flurries may accompany this warm
air advection surge overnight.  Accumulating snow associated with
passing low pressure will spread across parts of southern WI
Wednesday morning and afternoon with lower ceilings and vsbys.  May
be some patchy freezing drizzle in parts of south central WI.

MARINE...Potential for wind gusts reaching 25 knots will
continue into the evening.  Will continue Small Craft Advisory for
now, but if wind speeds continue to fall, may be able to cancel
early. Low pressure passing across southern WI/northern IL area on
Wednesday may clip far southern Lake Michigan in the afternoon.
Tightening pressure gradient associated with the low will cause
gusty southeast winds turning to the ENE on Wednesday, eventually
backing to the NNW Wednesday night.  These gusty winds will likely
reach Small Craft Advisory levels for a time.  Lake temperature
remains in the lower 40s, which is preventing freezing spray


WI...Winter Weather Advisory from 6 AM to 9 PM CST Wednesday for

     Winter Weather Advisory from 4 AM to 6 PM CST Wednesday for

LM...Small Craft Advisory until 9 PM CST this evening for LMZ643>646.



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