Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
257 am CDT Wednesday Sep 2 2015
Forecast discussion for routine morning forecast issuance
Short term...today...tonight...and Thursday
issued at 254 am CDT Wednesday Sep 2 2015
Abundant low level moisture and light winds has led to the
formation of fog early this morning across much of central and
north central Wisconsin. Although areas to the east are not as
foggy...visibilities could still drop during the morning hours.
The fog is expected to dissipate later this morning shortly after
The main forecast concern today and tonight will be a thunderstorm
complex across southwestern Minnesota slowly tracking east
associated with a middle level trough which can clearly be seen on
water vapor imagery. This complex is scheduled to track through
the County Warning Area during the late afternoon and early evening hours per the
hi- res model runs. In addition there are also signs of
development ahead of this main system across northern Wisconsin
later this morning and early this afternoon ahead of a Theta-E
surge which eventually congeals into one large complex right over
northeast Wisconsin. This larger complex is scheduled to slowly
track through the area overnight and into Thursday
morning...dumping copious amounts of rain given the copious precipitable water
values approaching 2 inches...long skinny cape profiles...and very
weak wind field owing to a slow moving system. Given the model
solutions will raise probability of precipitation to likely across the forecast
area...focusing on the late afternoon and early evening when the
complex is mostly likely to track through the area.
Despite decent forecast MUCAPE values of 2000 to 3000
j/kg...severe weather potential is low given the very weak wind
field in place with not much in the way of shear. The main severe
threat looks to be a few pulse storms that could approach severe
limits...with a wet microburst producing some isolated wind damage
being the main threat if severe weather does occur. Hence the
marginal risk for severe weather across northern Wisconsin this
afternoon and evening per the Storm Prediction Center day 1 outlook. The marginal risk
for severe weather aside...flooding appears to be the primary
concern across the area today and tonight given the abundant
moisture and slow moving nature of the storms.
The fog forecast overnight will be tricky to say the least as this
complex could keep areas relatively fog free...or foggier given
the increased low level moisture afforded by the thunderstorms. At
this point will keep patchy fog in the forecast overnight.
Temperatures during this period will be tenuous at best...as highs
will be dependent on when thunderstorms develop or move in from
the west and how much temperatures can soar before this happens.
Current thinking is the area should still get into the 80s given
the very warm airmass in place...however the threat of reaching 90
looks lower today given the clouds rapidly approaching from the
west. Lows tonight could be tricky as well given the
thunderstorm...however lows in the 60s are generally expected per
the last few nights. Temperatures on Thursday should be a few
degrees cooler...with highs in the lower to middle 80s...as this
complex slowly tracks east and the expansive cloud shield also
exits the region.
Long term...Thursday night through Tuesday
issued at 254 am CDT Wednesday Sep 2 2015
Above normal temperatures will continue into early next week...
before a significant cool down is expected by the middle or end
of next week (ecmwf).
Interesting scenario setting up Thursday night which will linger
into Friday morning. The GFS/WRF/Canadian depict some precipitation
developing or continuing Thursday night...in response to weak
850mb warm advection and weak 850mb winds over running weak
boundary draped across south of the area. The European model (ecmwf) did not show
this scenario and the offices to the south followed this model.
Have a hard time going dry with several models producing
precipitation...thus kept 20 to 30 percent chances of showers and
storms. Any precipitation would at least linger into Friday
morning if it develops.
Otherwise warm and humid conditions will prevail through the
weekend. Small chance of showers and storms Saturday...but
better chances for showers and thunderstorms will be during the
Sunday into Monday time frame as a weak cold front moves across
the region. Widespread severe weather is not anticipated at this
time...although locally heavy is possible. Much cooler weather
should return to the area by the middle to end of next week.
Aviation...for 06z taf issuance
issued at 1112 PM CDT Tuesday Sep 1 2015
Atmosphere remains moist with dew points in the upper 60s to low 70s
and winds becoming light once again overnight. Expect another
round of fog over much of the forecast area for the remainder of
tonight...with IFR/MVFR visibilities persisting until 14z. Mostly likely
areas to see patchy dense fog with visibilities less than 1/4 mile will
be over northcentral Wisconsin.
Middle-level shortwave trough over the Central Plains is forecast to move NE
toward Wisconsin overnight. This system is slow moving and is
forecast to bring isolated to scattered showers and thunderstorms to western Wisconsin
starting late tonight. As the shortwave disturbance interacts with
more instability during the day on Wednesday...expect somewhat
more widespread shower and thunderstorm activity accompanied by MVFR
The very moist and moderately unstable atmosphere...in concert
with relatively light winds aloft...would favor slow moving and
more disorganized sub-severe pulse-type storms capable of
producing locally heavy downpours.