Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 210802
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
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.

&&

$$

Today/Tonight and Aviation/Marine...Hentz
Sunday through Friday...Marquardt


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