Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 211756 AAB
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1256 PM CDT Sat Oct 21 2017

.UPDATE...

The forecast looks to be on track for this afternoon into Sunday.
Just made some minor edits to the sky cover this afternoon.
Southeast lake breeze should remain near the shoreline this
afternoon, with the gusty south winds inland limiting its
advance.

Wood

&&

.AVIATION(18Z TAFS)...

Gusty south winds are expected this afternoon into early this
evening across the area. Southeast lake breeze should remain near
the shore this afternoon, with the gusty south winds inland
limiting its advance.

Clouds should gradually increase later this afternoon into
tonight from west to east. A period of low level wind shear is
expected between 03Z and 09Z Sunday across western portions of the
area, and 03Z to 13Z to the east.

Cold front will move east across the area Sunday morning,
bringing a period of showers. Mainly MVFR ceilings are expected,
with visibility values around 5 miles or so possible. The showers
should end west to east during the afternoon. South winds will
veer to the west and northwest with the frontal passage.

Wood

&&

.PREV DISCUSSION... (Issued 1029 AM CDT Sat Oct 21 2017)

UPDATE...

Area of showers to the northwest of the area should remain to the
northwest into this afternoon. This is tied to some differential
cyclonic vorticity advection from passing 500 mb shortwave trough
in that area, as well as the low level jet nose.

Clouds will decrease from west to east into the early afternoon
hours. This will allow for ample daytime heating and low level
mixing to allow highs to reach into the middle 70s in most areas.
A few upper 70s are possible. Gusty south winds are expected into
this afternoon, as the pressure gradient tightens and low level
mixing increases.

Mesoscale models suggest that the area should remain dry this
afternoon and early this evening. Low level frontogenesis response
then should move east toward and into the area later this evening
into Sunday morning. Line or broken line of convection associated
with this frontogenesis response is expected to weaken
considerably, as it shifts toward the western counties later this
evening, and into the area overnight into Sunday morning.
Elevated or mean layer CAPE later tonight is rather weak in the
western counties. Thus, not expecting any severe weather.

Dry air on area forecast soundings may limit areal coverage of
showers overnight, before becoming more numerous Sunday morning
with better low level saturation. Showers should move out of the
area gradually Sunday afternoon. Mild temperatures should linger
with south winds tonight into Sunday morning, before the front
moves through. Temperatures should fall after the frontal passage
in the afternoon.

Wood

MARINE...

Small Craft Advisory continues until 21Z Sunday across the
nearshore waters of Lake Michigan. Gusty south winds with a tight
pressure gradient and good low level mixing are expected during
this period. Gusts of 25 to 30 knots are expected, with the
highest gusts tonight. Waves of 4 to 7 feet should develop this
afternoon into tonight, highest toward Sheboygan. These winds and
waves should slowly subside Sunday afternoon, as the pressure
gradient weakens.

Wood

PREV DISCUSSION... (Issued 619 AM CDT Sat Oct 21 2017)

UPDATE...

Isolated ACCAS/sprinkles this morning.

AVIATION(12Z TAFS)...

Low level wind shear across southern WI this morning until surface
winds increase. Low level Wind shear again tonight, especially
southeast. Isolated ACCAS/sprinkles this morning, with showers
and a few thunderstorms associated with a cold front mainly
tonight for south central Wisconsin and towards Sunday morning
for southeast WI. VFR ceilings until early Sunday morning with
MVFR ceilings near a cold front.

PREV DISCUSSION... (Issued 302 AM CDT Sat Oct 21 2017)

SHORT TERM...
Today and Tonight...Forecast Confidence is Medium.

A strong upper trough over the northern and central Rockies will
push into the plains this evening. One weak upper level speed max
is lifting north across southern Wisconsin early this morning,
with the main upper jet pushing into the Upper Mississippi valley
late tonight. Weak upper level divergence this morning, then
increasing again later tonight. 700 mb RH increases today with a
band of saturation moving across late tonight. A 700 mb wind max
of 50 knots and 850 mb wind of 45 knots is mainly over northwest
and west central Wisconsin today, but a second surge lifts across
southern Wisconsin late tonight.

The GFS soundings appear too dry early this morning, with the RAP
soundings showing more mid level moisture, which supports the
ACCAS seen on radar. Isolated areas may see a sprinkle this
morning.

Zero to 1 km CAPE values increase this afternoon and continue
tonight, but this is a modest 300 Joules/kg or so. A little more
far west areas.

An isolated elevated thunderstorm is possible mainly south central
this afternoon, with the main chance of showers and scattered
thunderstorms tonight near the 850 mb trough axis. The far west
areas, west of Madison is in a marginal risk of severe, mainly
for the evening thunderstorms, which would have a wind risk due to
the strong low/mid level winds ahead of the trough, and a
weakening of the cap. The surface cold front should enter the far
west around sunrise Sunday.

LONG TERM...
Sunday...Forecast Confidence is High.

The first push of colder air arrives Sunday behind a surface cold
front that moves through midday. Winds will become northwest and
the gradient will relax after the frontal passage. Precip will end
west to east during the day, maybe lingering into the evening in
the east. During the evening the upper trough swings through and
upper divergence slides east of the region.

Monday through Wednesday...Forecast Confidence is High.

Another front moves through on Monday but the atmosphere remains
pretty dry. There are small pops with steep lapse rates but
overall forcing is weak. A strong mid level trough digs through
the region overnight and into Tuesday. This brings a reinforcing
shot of colder air, with 850 temps dropping below 0. Low temps
look to be in the middle 30s. In addition to the coolest day of
the season so far on Tuesday, there will be a brisk northwest
wind. At the surface, low pressure strengthens over the Ohio
Valley and moves up through the eastern Great Lakes. Some precip
from that system may impact us. On Wednesday another shortwave
moves through bringing another chance for showers.

Thursday and beyond...Forecast Confidence is Medium.

Mid level ridging sets up, bringing slightly milder temperatures, then
models diverge. It looks like another trough will move through,
meaning cooler temperatures, but there are differences in timing.
The GFS and Canadian are showing freezing temperatures at the end
of the week but the ECMWF is milder. Later in the month there is
better agreement on a trough over the Midwest, yielding a cooler
pattern.

AVIATION(09Z TAFS)...

Plan on low level wind shear across southern WI tonight and again
Saturday night. There are strong south-southwest winds just above
the surface due to a tight pressure gradient as low pressure
approaches from the Plains. This will cause gusty southerly winds
during the day Saturday. Isolated ACCAS this morning, with showers
and a few thunderstorms associated with a cold front mainly
Saturday night for south central Wisconsin and Sunday morning for
southeast WI.

MARINE...

Tightening pressure gradient ahead of an approaching strong cold
front will cause gusty south winds today and into Sunday. A Small
Craft Advisory will continue through mid- afternoon Sunday.
Mariners should expect the winds to switch from the south to the
West early Sunday afternoon. Showers will accompany the frontal
passage, along with isolated thunderstorms.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...Small Craft Advisory until 4 PM CDT Sunday for LMZ643>646.

&&

$$

Update...Wood
Today/Tonight and Aviation/Marine...Hentz
Sunday THROUGH Friday...Marquardt



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