Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 201640
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
1140 AM CDT Sun Aug 20 2017

.UPDATE...

A mesoscale convective vortex (MCV) tied to early morning storms
over the mid Missouri valley was progressing across eastern Iowa
late this morning. A few waning showers were associated with this
feature. The MCV will drift east across northern Illinois today,
possibly triggering a few more showers or a thunderstorm as peak
heating arrives. Some of these showers could brush southern
Wisconsin, so added a slight chance for southern portions of the
area for today. Otherwise, we`ll see high level broken to overcast
sky cover persisting. These clouds could thin a bit towards early
evening as high level moisture decreases.

We should see a quiet start to the night, with some mid and high
clouds blanketing the region. A low-level jet will point into
Iowa, before veering into southwest Wisconsin late tonight. This
will bring focused warm/moist convection to the mid levels, with a
resulting frontogenesis response. The strength of this response
varies among the guidance. Upstream, look for mid-level wave
energy to aid a surface trough as it sinks southeast into central
Wisconsin. Our chances for showers and thunderstorms should
increase from late tonight into early Monday morning, before
waning as the low-level jet veers and weakens by late morning.
Some of the convective allowing models push a mesoscale convective
system (MCS) across southern Wisconsin during the 11-14z Monday
time frame, but will await the full suite of 12z guidance to weigh
support for this solution. Subsidence behind the MCS could keep
us dry through the afternoon before thunderstorm chances increase
towards evening/overnight. It still looks like we`ll have some
clouds to contend with during Eclipse time unfortunately.

&&

.MARINE...

Southerly winds will increase to 10 to 20 knots today, with the
highest winds and waves generally north of Port Washington. Winds
and waves should remain below Small Craft Advisory levels for much
of next week. Showers and thunderstorms are possible tonight into
Monday morning, and again from Monday evening into Tuesday
morning. Some of the thunderstorms could be strong.

&&

.PREV DISCUSSION... (Issued 541 AM CDT Sun Aug 20 2017)

UPDATE...

The HRRR had been persistent all night in bringing some of that
Iowa convection across parts of southern Wisconsin. The latest
run, along with the experimental version, is now keeping it just
south of the border. Will hang onto that chance until more
confidence is gained that it will indeed stay south.

AVIATION(12Z TAFS)...

VFR conditions are expected through most of the TAF period. There
is some patchy fog around the area early this morning that will
burn off by 13-14z this morning. We may see some showers and
storms around the area later tonight into Monday morning that
could briefly drop conditions to MVFR levels. More organized
showers and storms are expected later Monday and Monday night.

PREV DISCUSSION... (Issued 314 AM CDT Sun Aug 20 2017)

TODAY AND TONIGHT...Forecast Confidence Is Medium.

The main question for today is whether some of the convection
across Iowa will sneak into far southwest Wisconsin later this
morning and afternoon. The models are a mixed bag with it. They
seem to be latching onto some sort of MCV, or mid level short wave
and firing off convection with it. We will struggle to have
enough moisture and instability to support that. Given trends at 2
am this morning, it seems reasonable to doubt that scenario. The
better airmass is southwest of Wisconsin, so the progress of the
convective complex to the west should stay just out of the state.
Will carry a very small POP southwest of Madison just in case,
but, in general, the area should be dry and warm today.

There`s a better chance, though not great, that we could see some
showers and storms tonight. Again, the better focus should be
across Iowa where convergence along a frontal boundary, combined
with a more unstable airmass, should result in a complex of storms
that would eventually trend east southeast with time. We will be
within a zone of rather deep warm and moist air advection across
southern Wisconsin tonight with elevated instability and an
increasing low level jet. The models do show some showers and
storms developing within this region across southern Wisconsin.
Worthy of a 30-40 pop for now.

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

Models are generally showing the low level jet and steady warm air
advection continuing to pump warm and humid conditions into the
area Monday into Monday night. There may be some debris clouds
from convection passing through or to the south during the
morning. Frontal boundary remains to the north of the area as
well. There may be somewhat of a lull in any showers and
thunderstorms Monday, with rather weak upward vertical motion
fields. Kept chance PoPs for now. There may be a good amount of
cloud cover midday into middle afternoon Monday, which may limit
the viewing of the solar eclipse.

High PoPs for showers and thundestorms will continue for Monday
night, as the best low level frontogenesis response with the cold
front pushes through the area. This combines with fairly strong
upper divergence from right entrance region of 250 mb jet streak.
There is fairly robust mean layer CAPE, though some models are
overdone on dewpoints. Deep layer bulk shear is around 30 to 35
knots.

This combination brings a risk for strong to severe storms, per
SPC Day 2 slight risk over most of the area. This may be in the
form of an MCS moving through the area Monday night. Some models
suggest this may shift to the south of the area, while others
bring it east through the area. Heavy rainfall would also be a
concern, with precipitable water values around 2 inches and
favorable warm cloud depth. Slow storm propagation may allow for
some training of storms.

Hung onto PoPs mainly Tuesday morning in the the area, until the
front is totally out of the area. The afternoon may end up dry,
with strong cold air advection pushing into the area from the
northwest. It may take some time for dewpoints to drop to more
comfortable levels Tuesday. Closed low at 500 mb northeast of
Lake Superior will bring northwest flow over the area into Tuesday
night. Cooler and drier conditions are expected for Tuesday
night.

WEDNESDAY THROUGH SATURDAY...Forecast Confidence Is High.

This period looks fairly dry and cool over the area, as northwest
flow at 500 mb prevails over the region. There is decent
agreement with high pressure sliding slowly southeastward across
the region Wednesday into Friday, before sliding to the east into
Saturday. Temperatures look to be below seasonal normals, with
highs in the 70s and lows in the 50s. This will be an early taste
of autumn air.

AVIATION(06Z TAFS)...

VFR conditions are expected through the TAF period. The
exceptions would be some early morning fog with reduced
visibilities through about 13-14z this morning. But, that fog
should be mostly out in the countryside and may hit KMSN. There is
a chance of showers and thunderstorms tonight. Any stronger
convection would bring a temporary lowering of conditions. Look
for light winds this morning turning southerly and increasing to
about 10 to 15kts.

MARINE...

Southerly winds will increase to 10 to 20 knots today, especially
up toward Sheboygan. The highest waves will also be up toward
Sheboygan. The southerly winds will relax a bit on Monday and
eventually shift to the northwest on Tuesday in the wake of a
surface trough and cold front moving through. Winds and waves
should remain below Small Craft Advisory levels for much of next
week.

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

Update...SPM
Today/Tonight and Aviation/Marine...Davis
Monday THROUGH Saturday...Hentz



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