Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 310825
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
325 AM CDT Sat Jul 31 2021

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Sunday
Issued at 324 AM CDT Sat Jul 31 2021

Potential for severe thunderstorms this afternoon and early
evening, and air quality due to smoke, will be the main forecast
concerns.

A cold front was dropping south from the Lake Superior region
early this morning, and producing a small cluster of showers and
thunderstorms over western Lake Superior and the western U.P.
of MI. An area of low stratus and fog was moving through central
WI.

A cold front will drop south through the forecast area this
afternoon and evening. The front will combine with the passage of
a short-wave trough in the afternoon to produce a band of showers
and scattered thunderstorms. With SBCAPE of 1000-1500 j/kg, deep
layer shear of 25 to 35 kts and favorable wet bulb zero heights,
conditions should be supportive of isolated strong to severe
thunderstorms. The region remains in a Marginal Risk of severe
thunderstorms, which seems reasonable. Smoke models show smoke
returning to northern WI this morning, then overspreading the
rest of the region during the day. The Wisconsin DNR has already
posted an Air Quality Advisory for the forecast area for today,
and suspect that near-surface smoke will bring at least minor
visibility restrictions during the day. High temperatures will be
in the middle to upper 70s.

The storms will decrease across the region this evening, with
perhaps a few lingering showers over north central WI. Lows should
drop into the 50s again, as a Canadian air mass gets reinforced.

Gusty north winds will continue to pull cooler air into the region
on Sunday, with highs only expected to reach into the upper 60s
and lower 70s.

.LONG TERM...Sunday Night Through Friday
Issued at 324 AM CDT Sat Jul 31 2021

The latest ensemble means continue to show a flattening of the upper
air pattern next week.  As the mean western ridge folds into the
western Great Lakes region and the upper flow weakens, should see
warmer air invade the region.  Storm chances will generally remain
low over the period with no signs of any significant shortwave
impulses passing nearby.

Sunday night through Monday night...Quiet conditions will prevail on
Sunday night with high pressure centered across the northern Plains
but extending into Wisconsin.  May see some cirrus or mid-deck of
clouds arrive late ahead of a weak shortwave diving southeast into
the region.  Some guidance spits out light precip on Monday with the
heat of the day interacting with this shortwave.  Instability is
meager at under 500 j/kg and prefer to leave the forecast dry.  With
light winds and clear skies, remained on the colder side of guidance
for Monday night.

Rest of the forecast...The ecmwf continues to drop a weak cold front
into the region on Tuesday while the gfs keeps it north of the U.P.
border.  Some signs of a mid-level shortwave that gives credence to
the chance of storms.  But given the model differences, prefer not
to stray far from the NBM initialization.  Mid-level height rises
then are projected to occur on Wednesday and Thursday.  Light precip
chances return late Thursday into Friday when another low amplitude
wave approaches the region.  Widespread 80s are anticipated by
Wednesday.
&&

.AVIATION...for 06Z TAF Issuance
Issued at 1044 PM CDT Fri Jul 30 2021

No significant changes in forecast reasoning for the 06Z TAF
issuance. The primary aviation weather concern will be the impact
of a cold front dropping south across the area tomorrow. Opted to
keep the mention of thunder close to the front, and rather limited
in duration. Timing may still need to be adjusted with later
issuances. A secondary issue is the impact of smoke from western
wildfires. Based the forecast for his on HRRR output. Carried a
SCT skycon for the smoke aloft and a 4-6SM vsby restriction late
tonight into early Saturday for smoke/haze at the surface.
&&

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

$$

SHORT TERM.....Kieckbusch
LONG TERM......MPC
AVIATION.......Skowronski


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