Forecast Discussion for GRB NWS Office
069
FXUS63 KGRB 160910
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
410 AM CDT Tue Jul 16 2019

Forecast discussion for routine morning forecast issuance

.SHORT TERM...Today...Tonight...and Wednesday
Issued at 409 AM CDT Tue Jul 16 2019

The main concerns in the short term portion of the forecast will be
the continued very warm and humid conditions across the area along
with isolated to scattered shower and thunderstorm potential.

Today and tonight: A weak cool front/wind shift, stretching
northeast to southwest, will shift south and become east to west
oriented this afternoon before becoming nearly stationary just south
of the CWA this evening. Most locations will remain precipitation
free today into this evening; however, there may be some enhanced
convergence along/near Lake Michigan (ahead of the front) to touch
off some showers and thunderstorms late this afternoon into early
this evening. Instability ahead of the front is expected to be
around 1000-2000 J/kg with deep layer shear values expected to be
around 25 kts. This would be enough to allow for a few strong
thunderstorms to develop during the late afternoon and early evening
hours. The lack of any strong upper-level support and lack of
enhanced shear, would tend to lend less support to a more organized
strong to severe weather threat. In fact, SPC only has the area
painted in a general thunder threat. PWAT values are progged to be
in the 1 to 2 inch range, so any thunderstorm that does develop
could drop a lot of rain in a short period of time. The storms are
expected to diminish from north to south overnight. Highs today will
be in the upper 80s with overnight lows only dropping into the 60s
to around 70 tonight.

Wednesday: The first half of the day will likely be dry across the
area and continued very warm and humid. By mid to late afternoon, a
shortwave is expected to approach the area from the west. This will
steadily increase the chances for showers and thunderstorms into the
afternoon and evening hours. In addition, the aforementioned
stationary frontal boundary will linger just south of the area. The
combination of these features will set the stage for an active
Wednesday afternoon into the evening hours. Timing and placement of
the features will be the biggest challenge and pops will likely be
increased as confidence increases in each of these. Instability
values are expected to be around 1000-2000 J/kg once again;
however, shear values with some upper-level support will increase to
around 35 to 45 kts. This would indicate an increasing potential for
severe weather by late Wednesday afternoon into Wednesday night.
Large hail and especially damaging winds will be the main threats
from the storms. In addition PWATs are expected to remain fairly
high, in the 1 to 2 inch range, through this time period. Otherwise,
very warm and muggy conditions can be expected with highs in the mid
to upper 80s.

.LONG TERM...Wednesday Night Through Monday
Issued at 409 AM CDT Tue Jul 16 2019

Forecast concerns continue to revolve around thunderstorm chances
and excessive heat potential into early next week.  Models have a
variety of detail differences in regards to the thunderstorm
potential, but appear to point towards two main periods, namely
Wednesday night into early Thursday and also late in the weekend. As
sub-tropical ridging builds north, the hottest period appears to be
in the Friday-Saturday time frame.  Will use a general model blend
to mitigate small scale differences.

Wednesday night through Thursday night...Shortwave energy will be
moving across the northern Plains and the Lake Superior region in
the Wednesday night and Thursday time frame. As a modest 30-40 kt
low level jet cranks up ahead of the shortwave, elevated moisture
transport will increase across the region.  Most unstable cape will
also increase as well, in the 1500-2000 j/kg range in a region of
effective shear upwards of 30 kts.  With that magnitude of
instability and deep layer shear, could have a severe mcs threat on
Wednesday night, though best dynamics should remain north of the
region.  The significant severe parameter suggest central and north-
central WI has the best chance of severe storms as instability moves
in from the west. A weak front will then approach the area on
Thursday afternoon. Evolution of thunderstorm activity on Wednesday
night will help determine the storm potential on later Thursday.
Ample mixed layer cape upwards of 3000 j/kg and deep layer shear
estimated at 40 kts are expected to develop Thursday afternoon.
Forcing will be rather weak and some capping will likely be present,
but with that sort of instability, it wouldnt take much to develop
strong storms. Instability should rapidly diminish on Thursday
night, but remain sufficient for a slight chance of storms to
continue.

Rest of the forecast...Sub-tropical ridging will then build north
into the area on Friday, pushing a lingering front into far northern
WI.  This front will lead to low chances of storms at times on
Friday and Saturday, but lack of large scale forcing should limit
coverage and organization.  The bigger story south of the front will
be the hot and humid weather.  Low level temps support highs in the
low to mid 90s.  Combined with dewpoints in the upper 60s to low
70s, heat indices will approach 100 degrees over central and east-
central WI on Friday afternoon and possibly Saturday afternoon
depending upon storm coverage. Then a cold front is forecast to move
across the region sometime on Sunday. Strength of the system along
with magnitude of instability ahead of the front should lead to a
good chance of storms, and a chance of severe weather. Cooler and
less humid air then is expected to move in behind the front for
early next week.
&&

.AVIATION...for 00Z TAF Issuance
Issued at 625 PM CDT Mon Jul 15 2019

Periods of showers and thunderstorms will be accompanied by
MVFR/IFR conditions tonight, with mostly VFR conditions outside of
the precipitation.

Tuesday may have some MVFR ceilings to start the day, but it
should become VFR by midday. Isolated thunderstorms could occur in
the afternoon, especially near lake breezes.
&&

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

$$

SHORT TERM.....Cooley
LONG TERM......MPC
AVIATION.......RDM

NWS GRB Office Area Forecast Discussion