Area Forecast Discussion
Issued by NWS Green Bay, WI

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120
FXUS63 KGRB 241626
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1126 AM CDT Mon Jul 24 2017

Updated aviation portion for 18Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 336 AM CDT Mon Jul 24 2017

Low clouds continue to be a concern this morning as a stratus deck
stretching from the lakeshore to central Wisconsin continues to
track southwest through the area. however there are signs that
this deck is eroding from the north as far northeastern Wisconsin
is clearing out from the northeast. This clearing trend is
expected to continue this morning, with assistance later this
morning from mixing, which should help in getting rid of this deck
of low clouds. Until then, these clouds should help mitigate the
fog risk across the region and will adjust the grids to remove
much of the fog, especially from the eastern part of the cwa.

Once this deck of low clouds clears out the area is shaping up to
have a fairly decent day as highs stay in the 70s with lower dew
points in the upper 40s to low to mid 50s, making the humidity
level much lower as high pressure tracks through the western Great
Lakes. The one main concern later today will be gusty northeast
winds, which will cause high waves along Kewaunee and Manitowoc
counties, necessitating a beach hazards statement for these areas
given the risk for dangerous swimming conditions caused by high
waves and strong currents.

A fairly decent night is in store for the region tonight as the
high drifts east and low temperatures fall into the 50s across the
area. However as the region gets on the backside of the high on
Tuesday highs will return to around 80 degrees away from Lake
Michigan as dew points creep back up to around 60 degrees across
central and east-central Wisconsin during the afternoon hours. A
frontal boundary will approach the upper Mississippi Valley
Tuesday afternoon, however rain chances are expected to hold off
until later on Tuesday night.

.LONG TERM...Tuesday Night Through Sunday
Issued at 336 AM CDT Mon Jul 24 2017

The mean flow to consist of a broad upper high over the
central/southern Plains and the main band of westerlies running
along the U.S.-Canadian border. Models agree on taking a prominent
closed upper low across extreme southern Canada during mid-week,
which will help send a cold front through WI Wednesday/Wednesday
night with a good chance of showers/thunderstorms. By next
weekend, the upper high to have retrograded into the southwest
CONUs with an upper trough to have developed over the eastern
CONUS. This pattern shift should bring a stretch of quiet weather
from Thursday through Saturday with temperatures around normal for
late July. Primary forecast focus to be on the Tuesday night into
Wednesday night time frame as a cold front moves through WI and
brings a threat for strong to severe thunderstorms.

The models are in decent agreement in taking the upper low into
west-central Ontario, while the cold front reaches far northwest
WI by 12z Wednesday. The GFS in its typical faster bias, already
sweeps at least a chance of showers/thunderstorms across all of
northeast WI Tuesday night. This may be tad quick as low-level
moisture just getting back into the region. Nonetheless, we will
have to watch these storms as they approach since upstream
conditions may be favorable for severe development. Otherwise,
look for clouds to increase over northeast WI with the highest
pops placed over north-central WI after midnight. A mild night on
tap with min temperatures in the lower to middle 60s.

As the upper low continues to track east toward Hudson Bay on
Wednesday, the cold front to push southeast across WI and be the
main focus for additional showers/thunderstorms. The exact timing
of this front will be key as to whether we will be able to
destabilize enough to allow for stronger storms to form in the
afternoon peak heating of the day. Current model trends show only
modest instability (best MUCAPES from 1000-1500 J/KG), but strong
0-6km shear from 30-40 knots (stronger north). Therefore, cannot
rule out a few storms pushing severe limits and SPC has placed
most of northeast WI in a slight risk Wednesday. Heavy rain is
also a threat with PW values in the 1.5 to 2.0 inch range. Max
temperatures for Wednesday to be in the middle upper 70s north-
central/near Lake MI, upper 70s to lower 80s elsewhere.

The cold front does not exit east-central WI until around
midnight, although the slower ECMWF does not have the front
clearing until almost daybreak. Thus, will need to carry pops
through the evening for the entire forecast area, then focus pops
mainly for east-central WI after midnight. Decent subsidence
behind the front should allow for decreasing clouds later
Wednesday night and with drier air advecting into the region, look
for min temperatures to drop into the middle to upper 50s north-
central, lower to middle 60s east-central WI (where clouds to hold
until sunrise). A weak, but sprawling area of high pressure to
extend from the northern/central Plains northeast into Ontario on
Thursday and is forecast to slowly build into WI on Thursday. This
would provide for a mostly sunny day with temperatures near
normal. This would place max temperatures in the middle to upper
70s north-central/lakeshore, around 80 degrees elsewhere.

This area of high pressure to then dominate the Great Lakes
weather from Thursday night through Saturday with dry/pleasant
weather and temperatures remaining near normal under a north to
northeast wind regime.

By the latter half of the weekend, an amplified mean flow to exist
with upper ridging over the Rockies and upper troughing over the
eastern CONUS. This would bring a northwest flow aloft into WI and
the models indicate a weak mid-level system dropping into the
western Great Lakes region on Sunday. This system would provide
the next chance for showers/thunderstorms over northeast WI.
&&

.AVIATION...for 18Z TAF Issuance
Issued at 1126 AM CDT Mon Jul 24 2017

Cooler drier air on northeast winds will continue to erode
lingering MVFR sct-bkn clouds during the remainder of the
afternoon. The MVFR clouds may be more persistent across parts of
central Wisconsin southwest of CWA, and bands of clouds filtering
southwest from the Bay of Green Bay and along Lake Michigan may
continue to produce sct-bkn MVFR clouds for the GRB, ATW, OSH and
MTW tafs this afternoon. Eventually P6SM SKC VFR conditions to
prevail for the area late today into tonight as well as for much
of Tuesday.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement until 7 PM CDT this evening for WIZ040-
050.
&&

$$

SHORT TERM.....Kurimski
LONG TERM......Kallas
AVIATION.......TDH



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