Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Green Bay WI
657 PM CDT Tuesday Jul 28 2015
Updated aviation portion for 00z taf issuance
Short term...tonight and Wednesday
issued at 251 PM CDT Tuesday Jul 28 2015
Showers and storms rapidly dissipated early this afternoon as they
moved east into central Wisconsin. Meanwhile...a cold front was
locations across western Minnesota. Showers and storms are
expected to develop along the front later this afternoon and this
evening...and then move across north-central and northeast
Wisconsin later this evening and overnight.
The severe weather potential looks very low as best 0-6km shear
lags behind the front. However...this is the time of year for
wet microbursts...thus will mention gusty winds and locally
heavy rainfall in the hazardous weather outlook. Formal spotter
activation is not anticipated...but spotters are requested to
report rainfall amounts over an inch or wind gusts over 40 miles per hour.
Kind of an unusual event for late July will take place on
Wednesday. Strong low pressure across southern Canada will move
east towards Hudson Bay. Tight pressure gradient and stronger
winds aloft support wind gusts of 30 to 35 miles per hour possible. Mixing
heights on the BUFKIT soundings indicated between five and eight
thousand feet...which could tap winds as strong as 35 knots. Did
increase winds on Wednesday to account for deep mixing and
stronger winds aloft. Based on temperatures upstream...believe
guidance is too warm for highs on Wednesday.
Long term...Wednesday night through Tuesday
issued at 251 PM CDT Tuesday Jul 28 2015
The timing of a cold front and associated precipitation chances over the
weekend is the main forecast concern.
The region will remain in a fast zonal west-northwest flow pattern
through most of the period...with a series of short wave troughs brushing
northern WI...and frontal passages every couple days.
There is good model agreement on a weak cold front moving through
the area Thursday night into Friday. Have added a small pop over
northern WI late Thursday night...when the front and a short wave trough may
combine to produce isolated/scattered light showers.
Weak high pressure will push across the western Great Lakes on
Friday night and Saturday...bringing dry conditions.
Forecast gets a bit muddled over the weekend and beyond...as models
have trended slower with a cold frontal passage. The European model (ecmwf) has
not only slowed the front down considerably...but is also showing
a weaker feature than yesterday. Will lean toward the GFS...which
has a stronger and more progressive frontal passage late Saturday
night into Sunday. There is also concern about where the front
will reside after it moves through...and whether it will remain
close enough to generate additional precipitation early next week...
especially over our southern and eastern counties. Will continue
to focus highest probability of precipitation on Saturday night/Sunday period for now...
but this may be subject to change with subsequent forecasts.
Aviation...for 00z taf issuance
issued at 656 PM CDT Tuesday Jul 28 2015
Confidence in the early part of the taf valid period is not as
good as one would like. Most of the convection late this afternoon
was confined to Iowa. There was just a thin line of showers along
the approaching cold front...in eastern Minnesota...and
isolated/scattered showers ahead of that in western Wisconsin.
Local mesoscale graphics indicated MUCAPE values in excess of 1000j/kg
there at 23z but that area also had 50-100j/kg of cin. There was
a multitude of cloud heights across Wisconsin at 23z but
observation sites all indicated VFR ceilings. Have some MVFR visibilities
fog prior to frontal passage and some MVFR/IFR visibilities/ceilings with
The 22z hrrr had more areal coverage ahead of the front...so not
so sure about prevailing vcsh that was in the forecast but did not
remove since there is still plenty of instability and dew points
were most 65 to 70f in the area. Also...the hrrr brought convection
from Iowa into east central Wisconsin late tonight...toward
daybreak...so thunder may only impact osh/mtw/atw/grb taf sites.
Once the front passes winds will increase considerably. 925mb
winds are forecast to be in the 30 to 35 knots range at 12z so have
sustained winds around 20 kts with gusts around 30 to 33 kts. VFR
conditions should prevail on Wednesday morning.