Area Forecast Discussion
Issued by NWS Milwaukee/Sullivan, WI

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FXUS63 KMKX 181220
AFDMKX

Area Forecast Discussion
National Weather Service Milwaukee/Sullivan WI
720 AM CDT Tue Jul 18 2017

.UPDATE...Areas of fog have quickly thinned and dissipated over
southeast Wisconsin. Latest visible imagery is showing an eastward
moving area of stratus over southwest and west central WI moving
east and will affect western CWA, possibly portions of the east.
This may play a factor in keeping high temps a little cooler than
expected.

&&

.AVIATION(12Z TAFS)...Stratus moving into the west between 1500
and 2500 ft wl likely affect MSN Taf site for a time.

&&

.PREV DISCUSSION... (Issued 337 AM CDT Tue Jul 18 2017)

TODAY AND TONIGHT...Forecast Confidence Is Medium.

Light winds and clear skies allowed areas of fog to develop in
the far southeast overnight. Visibilities have been bouncing most
areas. Areas of fog wl continue through sunrise, however with
clouds increasing from the northwest, will hold of on issuing
Dense Fog Advisory for Racine and Kenosha counties for now.

Also watching area of low clouds pushing SSE from central WI, and
now extends from around Portage to Fond du Lac. This appears to
be diminishing per GOES-16 fog imagery, however may be obscured by
high clouds. The low clouds may hang together through mid-morning
before thinning and dissipating.

Otherwise, a partly to mostly sunny day is expected with
temperatures warming into the 80s. More humid conditions will
return as dewpoints in the mid to upper 60s get pulled into the
area later today and tonight. Weak mid-level short wave moves
across southern WI this afternoon, but low to mid levels remain
dry at that time, so held off on any precipitation with this
feature.

Will use more of a GFS/ECMWF blend regarding precipitation threat
for tonight as stronger NAM is an outlier regarding strength and
speed of short wave trof and surface cold front moving through
southern WI tonight. GFS/ECMWF slower with eastward progression.
However all short term guidance shows enhanced synoptic scale lift
from passing right entrance region of upper jet affecting central
WI this evening.

This will help carry weakening upstream convection from MN and
western WI into parts of southern WI overnight as low level jet
pivots eastward. Also warm air and moisture advection will
contribute to synoptic lift. While mid- level lapse rates not
overly impressive and remain around 6 degrees, MUCapes remain in
the 500-1500 range while bulk shear increasing to 25-30 knots.
Hence can not rule out an isolated severe storm affecting the area
overnight.

WEDNESDAY THROUGH THURSDAY NIGHT...Forecast Confidence Is Medium.

There may be some lingering showers and thunderstorms moving
through the area during the early morning hours on Wednesday.
There is some uncertainty if this activity will linger through the
rest of the day. The GFS keeps the quasi-stationary boundary
generally west to east across the area on Wednesday, with some low
level frontogenesis lingering.

The other models take it to the south of the area, with high
pressure bringing dry conditions by the afternoon hours. For now,
trended downward with PoPs as the day goes on Wednesday.

Onshore winds in the afternoon should bring cooler temperatures
near the lake. There should be partial sunshine in the afternoon
if the precipitation ends. Warm and humid conditions are expected,
with middle 80s well inland and upper 70s lakeside.

Models continue to show potential for an MCS to push east
southeast through central or southern Wisconsin later Wednesday
night into early Thursday morning. Focused warm air advection will
help generate and maintain these storms. The low level jet nose
gradually veers across the area later Wednesday night. The
NAM/Canadian focus the low level jet nose more over southern
Wisconsin, with the GFS/ECMWF focusing more over central
Wisconsin.

There will be ample elevated CAPE and deep layer shear during
this time, so there is concern for strong winds to occur with this
MCS. In addition, precipitable water values and related heavy
rainfall parameters are favorable for heavy rainfall during this
time. Training of storms is possible, with the winds aloft fairly
parallel to the low level jet nose and frontogenesis response.

Expanded likely PoPs east through most of the area later Wednesday
night into Thursday morning to account for the possible MCS. Will
issue another Hydrologic Outlook for this period and into the
weekend. Further headlines for heavy rainfall in this period may
be needed as it draws closer.

The quasi-stationary boundary remains a little south of the
Illinois border by Thursday into Thursday night. The low level jet
weakens somewhat on Thursday, but strengthens again Thursday night
and points into the area. NAM/GFS continue to generate QPF across
the area with continuous warm air advection and some low level
frontogenesis response.

The ECMWF/Canadian are relatively dry during this time. Thus,
some uncertainty here. For now, kept higher end chance/low end
likely PoPs going for later Thursday into Thursday night. If
storms do form, heavy rainfall would be possible, along with gusty
winds and hail, with ample elevated CAPE/shear and favorable heavy
rainfall parameters. Warm and humid conditions will continue as
well.

FRIDAY THROUGH MONDAY...Forecast Confidence Is Medium.

There is another shot at thundestorms, with heavy rainfall
possible, for Friday into Saturday. Low level jet/focused warm air
advection continues to drive QPF formation across the area. The
models are trying to bring an MCS across the region Friday, with
the GFS having another cross the area later Friday night into
early Saturday. Will continue the likely PoPs at times during this
period. Warm and humid conditions will continue during this time.

Models then try to bring cold air advection into the area by
Sunday, finally clearing out the warm, humid and unsettled
conditions. High pressure crossing Lake Superior would then bring
quiet weather into Monday, with more seasonable temperatures.

AVIATION(09Z TAFS)...

Areas of fog will continue into the early morning across the
east. The fog will be patchy dense in the far southeast where
light boundary layer winds and less overnight cloud cover will
occur. Otherwise, watching an area of 3000-3500ft cigs pushing
southward from central WI. The leading edge of the low clouds have
sagged as far south as the Portage and Fond du Lac areas.

GOES-16 showing some decrease in the lower clouds, but may be
just getting obscured by higher clouds. Possible this area of low
clouds holds together and affects several TAF sites early this
morning. Otherwise, expect mainly VFR conditions with a chance for
t-storms most areas tonight.

MARINE...

Areas of fog developed over the inland areas south of KMKE the
last several hours. Light southwest winds likely carrying at least
patchy fog over the nearshore waters into the early morning. The
fog should then thin and dissipate quickly as clouds increase from
the west and better low level mixing after sunrise. Chance for
t-storms tonight as weakening cold front moves through. South to
southwest winds will veer to the west to northwest late tonight.
The winds are expected to remain below Small Craft Advisory
levels.

HYDROLOGY...

Several rounds of thunderstorms are possible across the area into
this weekend, which may produce heavy rainfall. The first will be
later tonight into early Wednesday morning, with storms moving
into the area from the northwest. Locally heavy rainfall is
possible.

The second period will be later Wednesday night into early
Thursday morning, as a possible MCS moves east southeast through
the area. Some storms may linger into Thursday. The third period
will be Friday into Saturday, as more thunderstorms develop and
slide east through the area. More widespread heavy rainfall is
possible during these second and third periods.

All of these periods will have warm and humid conditions, with
abundant moisture leading to favorable heavy rainfall parameters.
This will combine with the potential for training of storms over
the same areas to bring the risk for heavy rainfall.

This would aggravate ongoing flooding across far southeast
Wisconsin, and create flooding issues elsewhere, especially in
urban areas. Once more confidence is gained with timing of these
rounds of storms, expected rainfall amounts will become more clear.
Keep up with the latest forecasts into this weekend.

&&

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

$$

Update...MBK



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