Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 240500

Area Forecast Discussion
National Weather Service Green Bay WI
1200 AM CDT Mon Jul 24 2017

Updated aviation portion for 06Z TAF issuance

.SHORT TERM...Tonight and Monday
Issued at 311 PM CDT Sun Jul 23 2017

Timing the end of thunderstorm chances and the potential for fog
are the main concerns in the short term.

An east-west surface trough and upper level short wave combined
with an unstable air mass (CAPES of 1000-1400 J/kg and little
CIN) across east central Wisconsin to form isolated/scattered
showers and thunderstorms that were moving through the area as of
20Z. Surface to 1.5 km lapse rates were in excess of 8.5 C/km but
mid level lapse rates were not very steep, so convection should
weaken and die off with loss of daytime heating.

A tighter surface pressure gradient across much of the forecast
area will shift to the southeast as high pressure moves into
Wisconsin so winds should further diminish. Skies are expected to
clear with the arrival of the high and an upstream mid level
ridge. The clearing skies and diminishing winds bring the
potential for fog to the area later tonight into early Monday
morning. North central and central Wisconsin appear to have the
highest potential for fog issues later tonight. Have not mentioned
dense fog in the forecast, but would not be surprised if it did
develop. The evening shift should be able to get a better handle
on this as a trend starts to develop during the evening.

After any fog dissipates on Monday skies should be sunny and
temperatures will top out a little below normal, with most
locations reaching the 70s.

.LONG TERM...Monday Night Through Sunday
Issued at 311 PM CDT Sun Jul 23 2017

Main focus for the extended period will be the next chance of
storms and a severe weather threat Tuesday night into Wednesday
night. High pressure systems will bookend this mid week system
with quiet weather.

Upper ridge and surface high pressure area drifting over the
region will bring dry quiet weather Monday night into Tuesday.
Clear skies, light winds and cooler temperatures in the mid to
upper 40s across the north should allow some fog to develop,
especially across north central and far northeast WI.

A few models lift some warm air back into north central Wisconsin
late Tuesday afternoon as a short wave drops into the Northern
Plains. Will continue the dry forecast for Tuesday and a good
portion of Tuesday evening, but with a developing upper diffluent
pattern Tuesday night, will spread pops into the area from west
to east, as a likely Northern Plains thunderstorm complex
approaches. Deep layer shear will be on the increase toward
Wednesday with developing westerlies while increasing dewpoints
aiding with producing cape values between 1500 to 2500 j/kg. There
is a period of time where the westerlies hold over the area
through Wednesday evening, potentially slowing the cold front
passage and allow a slower movement of storms, and leading to
locally heavy rainfall. A secondary trough is then progged to turn
the flow aloft more northwest later Wednesday night to push the
front and convection southeast of the area. Nevertheless plenty
of details to work out in the coming days.

High pressure then rebuilds back into the region for the rest of
the week and into at least the first half of next weekend. Long
range progs are trending to another short wave trough with a
frontal system for the later half of next weekend to track over.

.AVIATION...for 06Z TAF Issuance
Issued at 1200 AM CDT Mon Jul 24 2017

Mainly sct to bkn MVFR cigs across the eastern portion of the
forecast area late this evening. Satellite imagery shows that the
western edge of the bkn mvfr cigs stretches from near IMT to west
of OSH and was drifting WSW. Although trends suggest the cloud
deck is continuing to erode, bkn mvfr cigs will likely persist
over much of eastcentral Wisconsin through about 08Z. Clouds will
likely hang on the longest across eastern Wisconsin. Skies have
cleared over much of northcentral Wisconsin with patchy dense fog
forming near the Wisconsin/Michigan border. Fog will likely expand
southward across much of northcentral Wisconsin mainly impacting
the RHI TAF site.

An upstream mid-level ridge and surface high pressure moving into
the area should keep things dry through Monday. Expect any patchy
morning fog and areas of MFVR cigs to give way to VFR conditions
by 15 UTC Monday.



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