Forecast Discussion for GRB NWS Office
FXUS63 KGRB 131140

Area Forecast Discussion
National Weather Service Green Bay WI
640 AM CDT Mon Jul 13 2020

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 402 AM CDT Mon Jul 13 2020

High pressure was situated over the forecast area early this
morning. With mostly clear skies and light winds, patchy fog and
low clouds had developed, especially in north central WI.

Any lingering fog will dissipate early in the day, followed by
scattered cumulus clouds during the heating of the day. Highs will
be in the upper 70s and lower 80s, except slightly cooler near
Lake Michigan, where a southeast wind will develop in the afternoon.

Mainly dry conditions are anticipated for this evening, but WAA on
the nose of a 30-35 knot LLJ and increasing moisture and elevated
instability will bring a chance of thunderstorms to the northwest
half of the forecast area overnight. With H8 Lifted Indices of -3
to -5 C, deep layer shear of 30 to 35 knots and WBZ heights of
10-11K feet, a few stronger elevated storms may be capable of
producing hail over north central WI late. Lows should drop into
the lower to middle 60s.

On Tuesday, a slow moving cold front will make its way into north
central and central WI. Precipitation chances will increase to
likely to categorical as this occurs, with only slight chance/chance
pops expected for the Fox Valley/lakeshore areas during the afternoon.
Heavy rainfall will be a good possibility near the frontal boundary,
as PWATs increase to around 2 inches. The threat of severe weather
is still somewhat uncertain, mainly given concerns about the degree
of instability. Deep layer shear of 35 to 50 knots and 0-3km SRH
around 200 will be in place over north central and central WI in
the afternoon, but models offer differing views on how much
instability will be present. Given the expected cloud cover and
ongoing shower activity, suspect the more conservative ECMWF
(SBCAPE around 500-1000 j/kg over central WI) will end up
verifying better. With all of this in mind, the SPC day 2 outlook
of a SLGT risk for our far southwest counties, and MRGL risk over
much of north central/central WI and the western Fox Valley looks
to be on target. Highs should range from the lower to middle 70s
northwest to the upper 70s and lower 80s south and east.

.LONG TERM...Tuesday Night Through Sunday
Issued at 402 AM CDT Mon Jul 13 2020

A subtropical upper high will dominate the southern half of the
CONUS through next weekend, while the main upper jet to run across
southern Canada/northern CONUS. A broad shortwave trough is
expected to stretch from the northern Plains into the Upper MS
Valley through mid-week before weakening and allowing for the mean
flow across the northern CONUS to turn zonal. Precipitation
chances to be tied to a cold front/embedded shortwave trough which
appear to be focused on Tuesday into Wednesday and possibly next
Saturday. Temperatures to be at or slightly above normal into
mid-week, then above normal with more humidity late week.

Showers/thunderstorms appear likely across the forecast area
Tuesday night as a slow-moving cold front moves across WI.
Moisture will not be a problem as dew points climb into the middle
60s with PW values of 1.5-2.0 inches. Lift will also not be an
issue between the cold front and a diffluent flow aloft. Lastly,
mid-level lapse rates will be rather steep (> 6 C/KM). Putting
this together points to a wet night with locally heavy rainfall
surpassing 1 inch in many locations. As for a severe threat,
instability weakens during the evening, while shear values
increase. There should be enough instability early Tuesday evening
(MUCAPES over central WI at 1000-2000 J/KG) to at least bring
stronger storms into parts of northeast WI that will need to be
watched. Min temperatures Tuesday night to range from the middle
50s north-central WI, middle to upper 60s east-central WI.

Precipitation chances for Wednesday to come down to how much push
the cold front can get as surface waves ride along the boundary.
The NAM has the least push and brings another swath of potentially
heavy rain to east-central WI. Meanwhile, the CMC is most
progressive with the front by taking the boundary almost to
central IL by 00Z Thursday. Have continued the trend of keeping
higher pops over east-central WI, but not bring the rain amounts
advertised by the NAM. North-central WI would be dry on Wednesday
with a mix of sun and clouds. Max temperatures to be in the upper
70s to around 80 degrees.

Models all show the precipitation shifting to our east by
Wednesday night as a weak area of high pressure begins to build
into WI. Look for decreasing clouds and a diminishing wind through
the night and this could lead to the development of patchy fog
during the pre-dawn hours. Min temperatures to range from the
lower to middle 50s north-central, to the lower 60s east-central
WI. This high pressure to keep the weather quiet over northeast WI
on Thursday with a bit more clouds north as a shortwave trough
moves across northern sections of the Great Lakes. Max
temperatures to generally reside in the lower to middle 80s.

The high pressure shifts east Thursday night and allows for a
return flow to develop over WI. As stronger WAA pushes into the
region on Friday, this would bring a chance of showers/
thunderstorms into at least central WI during the afternoon hours.
Max temperatures will continue to warm along with an uptick in
humidity. Look for temperatures to reach the middle to upper 80s
and dew points to rise through the 60s.

Chance for showers/storms will continue Friday night into at least
Saturday morning as a warm front lifts northeast across the area.
May need to watch the strength of these elevated storms Friday
night as lapse rates steepen. If we can clear out later Saturday
(cap to strengthen), temperatures could soar in the upper 80s to
lower 90s with a couple of middle 90s not out of the question as
8H temperatures reach the lower 20s celsius.

The chance of showers/thunderstorms return Saturday night into
Sunday as a cold front is forecast to push across WI. Depending on
the middle and upper support, some of these storms could be on the
strong side, but models differ on this support, thus just
something to watch for the end of the weekend. Max temperatures on
Sunday to be just a tad cooler as readings to generally be in the
85-90 degree range.

.AVIATION...for 12Z TAF Issuance
Issued at 634 AM CDT Mon Jul 13 2020

Any lingering low clouds or patchy fog will quickly mix out early
this morning, followed by scattered cumulus development and some
passing high clouds. VFR conditions will prevail until late

A LLJ will bring a more moist and unstable air mass into north
central and central WI later tonight, leading to scattered
thunderstorm development. Some of the storms could produce hail.
Added a tempo group for thunderstorms and associated MVFR
conditions at RHI, and mentioned VCTS at the AUW/CWA TAF sites.
Also added LLWS at RHI/AUW/CWA late tonight.




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

NWS GRB Office Area Forecast Discussion