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643
FXUS63 KGRB 201813
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
113 PM CDT Thu Jul 20 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 403 AM CDT Thu Jul 20 2017

Thunderstorms have stayed mainly to our southwest overnight,
where strong moisture convergence was focused on the nose of a
30-40 kt LLJ. The northern edge of the precipitation shield has
impacted parts of central and east central WI, but was shifting
south, and should be out of the area by 12z.

High pressure will build into the region today with clearing
progressing from northwest to southeast this morning, and
abundant sunshine anticipated for the afternoon. Used a blend of
the better performing guidance sets for temperatures, resulting in
highs in the middle to upper 80s, except lower 80s near the
lakeshore.

Clear skies and light winds are anticipated tonight, setting the
stage for patchy fog development overnight. Low temperatures
should range from the lower to middle 50s in the cool spots of
northern WI, to the lower 60s farther southeast.

On Friday, return flow will develop as the surface high shifts
east. An 850 mb warm front will move into our southwest counties
during the afternoon, causing showers and scattered thunderstorms
to develop. Instability will be fairly weak, so severe weather
appears unlikely at this time.

.LONG TERM...Friday Night Through Wednesday
Issued at 403 AM CDT Thu Jul 20 2017

Mean flow to undergo a modest amplification from near zonal over
the northern tier of states through Saturday, to a minor upper
ridge over the Rockies/minor upper trough over the northeast
CONUS. Unsettled weather pattern to persist over northeast WI
through Sunday between the old quasi-stationary front and the
passage of a shortwave trough/cold front. Quiet, cooler, less
humid conditions expected for next Monday/Tuesday before the next
cold front arrives on Wednesday with more shower/thunderstorm
chances.

Showers and thunderstorms will continue into Friday night as the
warm front to reside across southern WI. It appears to be one of
those situations where the stronger instability will be located
south, while the stronger shear will be located north. The 0-6km
bulk shear of 40-50 kts over northeast WI is a concern and may
lead to another round of strong to severe thunderstorms. SPC has
already placed the entire forecast area under a slight risk for
Friday night into Saturday. Have kept the highest pops over
central/east-central WI (closer to the front) and will need to
watch rainfall rates with copious amounts of moisture available
(dew points well into the 60s/PW values pushing 2.0 inches). Min
temperatures to only fall into the lower 60s north, middle to
upper 60s south.

Precipitation chances to persist into Saturday as a weak surface
low is forecast to move along the warm front and a trailing cold
front pushes across WI. A few of these storms could also be on the
strong side if the atmosphere can recover from the earlier
convection. Saturday will be another humid day with dew points in
the middle 60s to around 70 degrees. Temperatures should be a bit
cooler due to cloud cover/precipitation chances with readings in
the middle to upper 70s north-central WI/near Lake MI, lower 80s
south.

Attention then turns to the shortwave trough with the models in
decent agreement in bringing this feature into the western Great
Lakes region later Saturday night. Precipitation chances are
relatively low between the departed surface low/cold front and
approaching trough, but do not see enough instability for the need
to carry any thunder during the overnight hours. Min temperatures
to range from around 60 degrees north-central, to the middle 60s
east-central WI. This shortwave trough swings through WI on Sunday
and with cooler air aloft, cannot rule out a chance for showers/
thunderstorms across the entire forecast area. Any precipitation
would be scattered around with dry hours in between, thus not a
washout by any means. Cooler/less humid conditions will begin to
overspread the region on Sunday with dew points down into the
upper 50s to lower 60s and max temperatures around 70 degrees
north-central, middle to upper 70s central/far northeast WI and
upper 70s east-central WI.

An area of high pressure is forecast to build toward the western
Great Lakes Sunday night and essentially reside overhead on
Monday. Look for any lingering showers in the east to end by early
Sunday evening, followed by a gradual decrease in clouds during
the overnight hours. This will lead to a mostly sunny day on
Monday with slightly below normal temperatures and less humidity.
Max temperatures to be in the lower 70s north/lakeshore, middle
70s south. This high pressure to remain anchored over the Great
Lakes Monday night, leading to mostly clear skies, light winds and
somewhat cool conditions. As the surface high begins to shift east
on Tuesday, a return flow sets up over WI as winds shift to the
south. This should start to tap warmer air and bring max
temperatures into the middle 70s north/near Lake MI, upper 70s to
around 80 degrees south.

The next chance for showers/thunderstorms arrives late Tuesday
night into Wednesday as a cold front and shortwave trough track
east into WI. A potential problem is that the new 00z ECMWF has
slowed the approach of this cold front and actually does not bring
the front through northeast WI until Thursday night! We have
plenty of time to adjust the front`s timing in the coming days.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 113 PM CDT Thu Jul 20 2017

Good flying weather through the evening hours, with
scattered clouds between 3000 and 5000 feet and some mid clouds
above. Some ground fog is possible after 08z tonight, especially
where it has rained recently, and near lakes. A few places could
have IFR conditions for a few hours before sunrise. Good flying
weather for awhile Friday, but showers and thunderstorms are
possible later in the day.
&&

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

$$

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