Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 191144
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
644 AM CDT Wed Jul 19 2017

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Thursday
Issued at 345 AM CDT Wed Jul 19 2017

A cold front was working its way through east central WI, where
most observation sites were reporting light northwest winds early
this morning. Showers had ended, and skies were clearing. Areas
of dense fog had developed beneath a weak surface ridge over north
central WI. Plan to handle this travel hazard with Special
Weather Statements.

High pressure will settle over the region today. After early
morning fog dissipates, abundant sunshine will boost temperatures
into the lower to middle 80s.

The main forecast concern will be the track and impacts of an MCS
that is expected to move into the region tonight. Models are in
two camps, with the GFS/NAM taking the main brunt of the MCS to
our southwest, while the ECMWF/GEM take it right through the
forecast area. The main difference in the models appears to be the
position of the returning warm front and the strength and
location of a low-level jet. Experience would suggest that the MCS
will take the farther south track, along the moisture and
instability gradient, with the convection helping to drive the
front farther south. However, all models show an unseasonably
strong upper level jet (125-135 kts) passing to our north
overnight, with the RRQ tracking through GRB CWA. Have compromised
by increasing pops a little bit to account for the strong upper
dynamics, but focusing the heaviest QPF over our southwest
counties. The severe threat should be marginal, as most of the
instability will be elevated, which should limit the wind
potential. Wet bulb zero heights are also expected to be high,
10500-12500 ft, which should minimize the hail threat. Will have
to see how this plays out though, as deep layer shear is very
strong (50-70 kts). Heavy rainfall will be possible, especially
over our southwest counties, as PWATs climb to around 1.5 inches
again.

Showers and thunderstorms should exit the region Thursday morning,
as high pressure returns. Afternoon sunshine will boost
temperatures back into the lower to middle 80s in most places.

.LONG TERM...Thursday Night Through Tuesday
Issued at 345 AM CDT Wed Jul 19 2017

An upper high parked over the central CONUS and stretched from the
central Rockies to the TN Valley, is forecast to gradually weaken
this weekend and shift farther south. This will allow for a low
amplitude shortwave trough over south-central Canada to dig
southeast with time and develop into a broader upper trough over
the northeast quadrant of the CONUS by early next week. Warm/
humid/unsettled weather will continue over northeast WI until this
trough passes by (Sunday night), before cooler/less humid
conditions take over for Monday and Tuesday. We will have to watch
for the possibility for a couple more rounds of strong to severe
storms, along with heavy rains.

The frontal boundary of interest remains quasi-stationary over
northern IL through Thursday night as a weak area of high pressure
moves from WI into Lower MI. Sky conditions to range from mostly
clear north to partly cloudy south and with winds expected to be
light/variable, we could see patchy fog develop over most
locations (mainly after midnight). Prefer to downplay the pops
south and lower temperatures due to less cloud cover. Look for min
temperatures to dip into the middle to upper 50s north, lower to
middle 60s south.

Precipitation chances to be on the increase once again, especially
by Friday afternoon as the front is progged to lift north a bit
and extend from the WI/IL border to northeast IA. Meanwhile, a
weak mid-level shortwave is expected to move through WI to add
some forcing. Highest pops placed over central WI, while far
northeast WI may not see any precipitation until late in the day.
Even though instability looks somewhat marginal, models do
indicate strong shear with 0-6km values of 50-60 knots. Therefore,
severe thunderstorms cannot be ruled out, along with the
possibility of heavy rainfall with PW values of around 2 inches
across central WI. Max temperatures for Friday to range from the
upper 70s near Lake MI, upper 70s to lower 80s elsewhere.

This convective complex would then move east-southeast across the
rest of the forecast area Friday night, favoring central/east-
central WI which would be closer to the frontal boundary. Northern
WI appears to at dry out a little later Friday night, but will
keep plenty of cloud cover around through the night. Min
temperatures to be in the lower 60s north, middle to upper 60s
south. Models then push a cold front into the western Great Lakes
region on Saturday ahead of the upper trough that is progged to
drop southeast toward northwest MN/SW Ontario by 00z Sunday.
Anticipate additional showers/thunderstorms to accompany the cold
front across WI, but there is too much uncertainty on instability
or shear to determine whether any of these storms could become
severe. Max temperatures on Saturday will be comparable to Friday
with readings in the upper 70s north-central WI/near Lake MI,
lower 80s elsewhere.

A few showers and/or storms could bleed into Saturday night, but
there should be a diminishing trend to the pops as the cold front
pulls away and the upper trough rotates into the Upper MS Valley.
This trough swings into WI on Sunday and with cooler air aloft
arriving, we could see at least a chance of showers/thunderstorms,
especially in the afternoon hours during peak heating.
Temperatures are forecast to trend downward with readings on
Sunday in the lower to middle 70s north-central, upper 70s south.

High pressure is then forecast to build toward the western Great
Lakes Sunday night and reside over the area on Monday. Look for
skies to become partly cloudy Sunday night and remain partly
cloudy on Monday. One noticeable change to the weather on Monday
will be the drier air mass in place with dew points in the
comfortable 50s instead of the sultry upper 60s/lower 70s of the
previous week. Max temperatures Monday to be about 5 degrees below
normal.

This area of high pressure is forecast to gradually slide eastward
by next Tuesday, but still be in control of northeast WI weather.
Dry and pleasant conditions will exist with slightly below normal
temperatures and comfortable humidity levels.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 639 AM CDT Wed Jul 19 2017

Early morning fog over north central WI should dissipate by 13z,
followed by VFR conditions into early evening. A thunderstorm
complex is expected to impact the region from late evening through
the overnight hours, but the track is in question. Have used a
concensus of the various model solutions to frame out the most
likely time period for storms at each TAF site.

&&

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

$$

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


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