Area Forecast Discussion
Issued by NWS Green Bay, WI

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739
FXUS63 KGRB 221126
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
626 AM CDT Sat Jul 22 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Sunday
Issued at 251 AM CDT Sat Jul 22 2017

The main forecast focus to be on the severe potential later this
afternoon/early evening across most of the forecast area with
regards to a mid-level shortwave trough approaching, along with a
cold front.

The 07z MSAS surface analysis showed one area of low pressure over
west-central IA attached to a quasi-stationary front that extends
east-southeast into central sections of Illinois and Indiana.
Another area of low pressure was located over central SD along a
cold front that stretched from east of Lake Winnipeg into the
central High Plains. The radar mosaic painted several areas of
convection to our south and west associated with each frontal
boundary.

Models are pretty consistent in moving a shortwave trough from
Lake Winnipeg/northern Plains southeast into the Upper MS Valley
today. If clouds can dissipate enough, potential exists for
stronger instability to develop over north-central/central WI with
MUCAPES in the 2K to 3K J/KG range this afternoon. Couple this
instability with deep shear (0-6km values around 40 knots) and
ample low-level moisture (dew points in the upper 60s), and we
could have a few storms approach severe limits this afternoon into
the evening. Large hail and damaging winds would be the primary
threats, although locally heavy rains cannot be ruled out.
Otherwise, the rest of northeast WI to see a warm and humid day
with max temperatures in the upper 70s to around 80 degrees north/
near Lake MI, mainly lower 80s south.

The shortwave trough, accompanied by the cold front, will reach
the Great Lakes tonight. The severe risk would gradually diminish
during the evening hours as daytime heating wanes and instability
weakens. Expect to see showers/thunderstorms move across the
forecast area this evening and gradually decrease from west to
east once the cold front exits overnight. Look for at least
partial clearing over central/east-central WI later tonight,
however the cyclonic flow in the vicinity of the shortwave trough
would tend to keep more clouds over the north through the night.
Despite the partly to mostly cloudy skies overnight, plan to add
some patchy fog due to light winds and lingering low-level
moisture still in place. Min temperatures to range from the upper
50s to around 60 degrees north-central, to the middle 60s east-
central WI.

Small precipitation chances are in the forecast for Sunday,
especially over eastern WI, as the shortwave trough continues to
gradually move toward the eastern Great Lakes. Enough cool air
aloft, combined with daytime heating, may steepen lapse rates
sufficiently to allow for at least a slight chance of showers/
thunderstorms over northeast WI. Sunday will be a cooler day with
improving humidity levels. Look for max temperatures to be only
around 70 degrees far north, around 80 degrees far south.

.LONG TERM...Sunday Night Through Friday
Issued at 251 AM CDT Sat Jul 22 2017

Long term forecast highlights include dry weather to start and
end the week, but a period of active weather in between with a
least one shot at strong/severe storms. Forecast challenge will be
to pin down timing of best precip/storm chances. Temperatures look
to start off a little below normal early in the week, then above
normal for mid-week, but no significant cool down or heat waves
in sight.

Any lingering showers/storms early Sunday evening will quickly
come to an end with the loss of daytime heating and the mid-upper
trough exiting to the east. Look for clouds to clear out (slowest
over the far northeast as flow over Lake Superior could keep
clouds around into early Monday) and dewpoints to fall through
the night, leading to a cool night over northern WI with the
typical cool spots dropping into the 40s. Lows in the 50s are
expected across the rest of the area.

A dry and comfortable day is expected on Monday as surface high
pressure slides across the Great Lakes and mid-upper level ridging
builds in from the northern Plains. Dry conditions will continue
into Tuesday morning has the high/ridging moves into the eastern
Great Lakes.

Medium range models last night focused the next chance for storms
in the overnight Tuesday / Wednesday timeframe, but latest 2 runs
of the GFS/GEM have sped things up, bringing storm chances in
late Tuesday afternoon into Tuesday night as the next frontal and
upper level systems approach the area. ECMWF still the slowest
solution, and brings a couple rounds of storms (Tuesday night and
again on Wednesday). Since there is still some timing
differences and no favorite, will continue to use a model blend
for POPs, which brings in chances across central and north central
WI late Tuesday afternoon, then for the entire area Tuesday
evening/night. Current timing of the GFS/GEM would have much of
the precip out of the area by noon Wednesday. But will linger POPs
across east central WI, especially in the morning, to account for
any trend back to the slower ECMWF solution. If the front slows
down enough, a round of strong/severe storms would be possible
Wednesday afternoon. Plenty of specifics still to be determined as
timing of the front will play a major role on how unstable we
will get and when it occurs. GFS/ECMWF showing 1000-2000 J/kg
ahead of the front, with 0-6km bulk shear of at least 25-35 kts
(closer to 50 kts if you believe the GFS late Tuesday). So there
will be a severe weather threat, but with the timing issues, and
possible instability issues with any convection leftovers from
Tuesday night, hard to get too specific at this point.

Model consensus is to push the front far enough south of the area
Wednesday night through Friday as high pressure builds across the
northern Plains. This will keep any precip associated with the
front out of the area, so will go with a dry forecast. GEM tries
to sneak the front back toward the area late in the week, but the
GFS/ECMWF keep it well south of the area. Favor the later idea and
will keep a dry forecast going into next weekend.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 625 AM CDT Sat Jul 22 2017

Plenty of low clouds will continue across the area this morning
with cigs in the IFR/MVFR range. Any patchy fog over central WI
will dissipate after 14z. Cigs should rise into the low VFR range
this afternoon, however the approach of a cold front and mid-level
trough will bring an increasing chance for showers and
thunderstorms to northeast WI. Vsbys would get knocked down to the
MVFR category under any of these storms which may become severe.
Once the precipitation begins to diminish later this evening/
overnight, light winds and lingering low-level moisture will
likely lead to another round of fog with more MVFR vsbys expected,
along with scattered pockets of IFR vsbys.
&&

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

$$

SHORT TERM.....Kallas
LONG TERM......Bersch
AVIATION.......Kallas



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