Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 222026
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
325 PM CDT SUN OCT 22 2017

.TONIGHT AND MONDAY - Forecast Confidence...Medium.

Showers have increased in areal coverage this afternoon, with some
strong mid level frontogenesis and differential cyclonic vorticity
advection from approaching 500 mb shortwave trough.
Mesoscale models not picking up well on this trend. Will adjust to
high PoPs across the area to account for this activity.

These showers should slowly taper off from west to east into this
evening, as the 500 mb shortwave trough moves through. Dry air
moving into the area, with cold air advection behind the front, will
contribute to help end the showers. These showers may hang on later
tonight in far southeast Wisconsin, as some 850 mb to 700 mb
frontogenesis lingers in that area. Some uncertainty here, so left
small PoPs in this area for now later tonight.

Skies should gradually clear out over most of the area later this
evening and overnight. The far southeast may have clouds and some
showers linger there. Lows should drop into the lower to middle 40s,
with the cooler airmass in place.

A secondary cold front will push east through the area Monday. A
digging 500 mb trough will slide toward the area from the northwest
as well. Various vorticity maxima rotating around the trough should
help bring more clouds and possibly some showers during the
afternoon. Highs in the upper 50s to around 60 are expected.

.MONDAY NIGHT THROUGH WEDNESDAY - Forecast Confidence...Medium.

Unsettled, cool weather can be expected for much of this period.
Amplifying long wave trof over the western Great Lakes will absorb
remnants of srn MS Valley cutoff low Monday night as it pinwheels
northward into the Great Lakes.  End result will be reinensification
of cutoff low pressure over the central Great Lakes. Deeper moisture
will still be in place in the vicinity of the upper low. Layer q-
vector convergence in the 5 to 10 unit range associated with mid-
level ripples rotating around the parent low during this period.
Scattered to numerous -shra should affect much of the CWA as upper
low strengthens over the central Great Lakes, with higher liklihood
in the east.

Upper low gets nudged northward by incoming upstream short wave in
fast northwest steering flow.  This has the look of a "clipper" type
system which moves very quickly southeast from southern Canada
across the upper midwest/western Great Lakes.   All short term
guidance carries this upper wave across central and southern WI
between 09z and 18z Wednesday.  GFS is the most aggressive with low
level warm air advection ahead of this feature while NAM is showing
very little advection and less atmospheric moisture.  ECMWF leaning
toward stronger GFS with a brief period of stronger layer q-vector
convergence moving across southern WI centered at 12Z/Wed but with
less column moisture. Also brief burst of layer frontogenetical
forcing accompanying upper level synoptic forcing.  Cold air will be
firmly entrenched in the area by late Tuesday night with temps
likely to be in the 30s.  For now wl have chance pops for a period
of light rain and light snow affecting southern WI late Tuesday
night into Wednesday morning.  With warm pavement temps, not
expecting any accumulation on roads, but may briefly lower
visibilities and have impact on Wednesday morning commute.  Due to
uncertainty at this time, wl hold off on mentioning in HWO.

Also temps expected to fall into the mid 30s Tuesday night before
clouds increase late resulting in a possibility of frost
development.  However clouds and boundary layer mixing may very well
prevent significant frost from developing.

.EXTENDED PERIOD...

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

Quiet conditions expected to start off between periods of long wave
troffing across the upper midwest and Great Lakes. Brief period of
short wave ridging will result in lighter boundary layer winds, with
surface winds likely 5 knots or lower.  Cold low level temps in
place and overnight lows are expected to dip into the mid 30s many
locations.  However uncertainty remains regarding frost formation
due to likely increasing mid-high clouds expected over the area due
to upstream warm air advection. For now, wl mention possibility in
Hazardous Weather Outlook.

Medium range guidance in better agreement on timing and placement of
another round of amplifying long wave troffing over the plains in
the late Thursday/Thursday night time period. Extended guidance
diverges on timing of eastward progression of upper trof and surface
low pressure.  00Z GFS/GEM carry upper low into the western Great
Lakes and WI by Thursday night, and then upper low remains over the
region through the weekend.  00Z ECMWF slower with eastward
progression, and keeps southern WI warmer through Friday, with
colder air surging in for the weekend.  Limited initial column
moisture with this sytem will retard shower chances, but will
increase during the weekend as the upper low spins over the Great
Lakes. The coldest air of the season will settle in for the weekend
increasing the liklihood of frost and freezing temperatures.

GFS 500H standardized anomolies show large negative anomoly of over
200 meters over the Great Lakes at 00z/30 so unseasonably cold air
likely to persist into early next week.  By 00z/02, the large
negative anomoly has shifted northeast into southern Canada so
indication are for temperatures returning to seasonal levels later
in the week.

&&

.AVIATION(21Z TAFS)...Showers have increased in areal coverage this
afternoon, and should become more scattered into the evening hours.
MVFR ceilings and visibilities are expected at times with the
showers. The showers should diminish from west to east into early
this evening. They may linger later tonight in far southeast
Wisconsin, near the Kenosha area. Some uncertainty here with these
showers lingering.

Clouds should clear out from west to east across the area later this
evening and overnight, though may linger in far southeast Wisconsin.
Light winds are expected. Drier and colder air moving into the area
should prevent fog formation later tonight.

Another cold front is expected to move east through the area on
Monday, Light southwest winds should veer west later in the day
behind the front. Some showers may develop during the afternoon, as
clouds spread over the area. Expecting VFR conditions at this time
for Monday.

&&

.MARINE...

Small Craft Advisory has been extended until 7 PM CDT this evening
across the nearshore waters of Lake Michigan. Gusty northwest winds
behind the departing cold front should linger for another few hours,
especially south of North Point Lighthouse. The pressure gradient
will relax into this evening. This should allow these winds to
weaken. Subsequent waves of 3 to 6 feet should also gradually
subside by early evening.

Gusty northwest winds are expected to develop across the nearshore
waters Monday night and linger into Wednesday morning. This is in
response to a tight pressure gradient developing over the region.
Gusts of 25 to 30 knots are possible during this time, with 35 knot
gales not out of the question Tuesday afternoon and evening.
Waves of 3 to 5 feet are possible toward the open waters of Lake
Michigan, higher over the open waters. A Small Craft Advisory will
likely be needed for Monday night into Wednesday morning.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...

WI...None.
LM...Small Craft Advisory LMZ643>646 until 7 pm this evening.

$$

TONIGHT AND MONDAY/AVIATION/MARINE...Wood
MONDAY NIGHT THROUGH SUNDAY...MBK


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