Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 131953
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
253 PM CDT Thu Jul 13 2017

Forecast discussion for routine afternoon forecast issuance

.SHORT TERM...Tonight and Friday
Issued at 253 PM CDT Thu Jul 13 2017

A cool Canadian air mass and lingering low-level moisture has
resulted in cloudy conditions across the region this afternoon.
Convergence along a weak surface trof caused some light showers
to develop in eastern WI. In addition, patchy drizzle has been
observed over far north/northeast WI.

Isolated/scattered light showers along the surface trof should
wane after sunset, but short-wave energy and cool north flow off
the Great Lakes will keep some light showers or drizzle across the
far north/northeast. As cloud ceilings lower across the region
overnight, patchy drizzle may also develop elsewhere in the
forecast area. Due to expected widespread cloud cover, have gone
several degrees warmer than guidance for low temperatures.

Have slowed down the clearing trend on Friday, as a well-defined
thermal trof remains entrenched over the region into the early
afternoon. The best bet for clearing will be across north central
and central WI during the mid to late afternoon. High temperatures
should be mainly in the lower to middle 70s.

.LONG TERM...Friday Night Through Thursday
Issued at 253 PM CDT Thu Jul 13 2017

Models continue to show an amplified mean flow this weekend with
an upper trough from western Canada into the eastern Pacific, an
upper ridge over the Rockies and an eastern NOAM upper trough. The
Canadian part of the western trough moves eastward and essentially
flattens out the northern part of the ridge axis early next week,
placing WI on the northern periphery of a hot dome of air centered
over the central Plains. The main forecast challenges will be
severe potential Saturday/Saturday night as a cold front moves
through WI and potential for heavy rain Tuesday/Wednesday as
another frontal boundary stalls somewhere over the Great Lakes.
After Saturday, temperatures look to be a few degrees either side
of normal.

High pressure is expected to stretch from the Midwest northeast
through the Great Lakes region Friday night. Mostly clear skies
will prevail for most of the night, although an increase in high
clouds are shown by the models into northern WI toward daybreak.
These clouds are in advance of a cold front, still well to our
north and west. Despite the mostly clear skies and light winds,
rising upper heights and weak WAA will prevent temperatures from
falling too far. Look for readings to dip into the lower to middle
50s north, middle 50s to around 60 degrees south.

As the surface high continues to drift south and east on Saturday,
winds will become west-southwest and begin to send moisture back
into WI. Dew points steadily rise into the 60`s, aided by the
growing corn crop. Models indicate max temperatures reaching the
lower 80s north-central WI/lakeshore, to the middle to upper 80s
over parts of central WI. This heat/humidity will send MUCAPES
into the 1500-3000 J/KG range with CIN values <20 J/KG ahead of
the advancing cold front. Models also show enough shear (30-40
kts) to sustain updrafts as thunderstorms develop. Latest timing
brings the cold front roughly from MQT-EAU by 00z Sunday and with
favorable CAPE/SHEAR parameters in place, SPC`s slight risk of
severe storms appears valid.

These potentially strong to severe storms would continue to push
southeast across the rest of northeast WI Saturday evening in
concert with the cold front. Canadian high pressure will drop
south from western Ontario and bring a gradual decrease in clouds
from northwest to southeast through the overnight hours. Even with
drier air slowly working into the area, enough wind will persist
through the night such that min temperatures to only fall into the
middle 50s far north, lower 60s east-central WI. This next area of
high pressure settles over the western Great Lakes on Sunday and
provide for a pleasant end to the weekend. Under mainly sunny
skies with much less humidity, look for max temperatures to range
from the lower 70s near Lake MI, lower to middle 70s north and
middle to upper 70s south.

The surface high remains parked over WI Sunday night with mainly
clear skies and light/variable winds. May need to lower
temperatures a bit from the previous forecast and Sunday night may
end up being the coolest night of the extended forecast. As the
high pressure finally begins to shift east on Monday, a return
flow will eventually develop over WI with the onset of weak WAA
and south winds (away from Lake MI). Under mostly sunny skies, max
temperatures on Monday will be in the middle 70s near Lake MI,
upper 70s to around 80 degrees elsewhere.

Clouds are expected to start slowly increasing over northeast WI
Monday night as the next cold front approaches far northwest WI by
daybreak. Precipitation chances become the main concern headed
into Tuesday as the cold front pushes into WI. The CMC and GFS
bring showers/thunderstorms to much of the forecast area, while
the ECMWF appears capped with little if any precipitation. While a
cap is certainly possible, do not feel comfortable with a dry
forecast considering the amount of instability that would be
available and the cold front in the vicinity acting as a trigger.
Prefer to have at least small pops in the forecast with max
temperatures around 80 degrees lakeside, lower 80s north-central
and middle to upper 80s elsewhere.

The problem for the middle of next week will be where this cold
front stalls out as it encounters a sprawling upper high that is
progged to reside from the central Rockies to the Ohio Valley (and
points south). The ECMWF stalls this front over central WI, while
the GFS sends the boundary into southern WI before stalling.
Meanwhile, the CMC takes the front into northern IL. The location
of this front will be critical not only to precipitation chances,
but also heavy rain potential as weak impulses ride along the
boundary over the same locations. Too much uncertainty yet, but
something that will need to be watched in the coming days.

Depending on how Wednesday plays out, Thursday could still be wet
if the ECMWF is correct, or Thursday could see improving
conditions if the CMC/GFS are correct. Taking the consensus model
approach, have split the difference for now and see if we can at
least salvage Thursday afternoon.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1236 PM CDT Thu Jul 13 2017

IFR/MVFR conditions were occurring across most of the region, with
the most variability over central/east central WI, where some
breaks in the clouds had allowed some heating/mixing to occur.
Isolated showers had formed near a surface trof in eastern WI, and
patchy drizzle was occurring over the far north and northeast.

A slight rise in ceilings is expected this afternoon, and
portions of central/east central WI will likely see brief periods
of VFR conditions. After sunset, ceilings should steadily lower
again, with most locations dropping into the IFR category
overnight. Patchy fog and drizzle may also occur. Improvement will
begin Friday morning, as heating/mixing cause ceilings to rise to
MVFR again. Clearing is expected to occur Friday afternoon and
evening, but that is beyond the scope of this set of TAFs.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM.....Kieckbusch
LONG TERM......Kallas
AVIATION.......Kieckbusch



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