Scientific Forecaster Discussion
fxus63 kgrb 191934
Area forecast discussion
National Weather Service Green Bay WI
234 PM CDT Thu Sep 19 2019
Forecast discussion for routine afternoon forecast issuance
Short term...tonight and Friday
issued at 232 PM CDT Thu Sep 19 2019
Above average temperatures and humid weather will continue through
the short-term forecast with isolated to scattered showers and
thunderstorms possible. Fog will likely develop late this evening
through the overnight hours and could become locally dense at times.
Tonight into friday: much of northeast Wisconsin will remain between
systems through this time period. High pressure will be centered
over the eastern Seaboard, while the next low pressure system will
be slowly progressing across the plains. Aloft, much of the eastern
U.S. Will be in a general ridging pattern, while a deep trough digs
across the western U.S. Some weak shortwave energy (left over from
the morning convection/mcs) will linger nearby this evening before
shifting east of the area overnight. Late Friday afternoon, yet
another quick moving shortwave and weak surface trough will approach
the area from the southwest. The shortwaves sliding through the
area, will allow instability to remain in the 500-1500 j/kg range.
Of course, the greater values will be diurnally driven, during the
afternoon and early evening hours. This will allow for enough
instability to touch off some isolated to scattered showers and
The main limit to precipitation coverage will be the lack of any
well defined forcing mechanism. In other words, there are not well
defined fronts or troughs to help focus the showers and storms. The
weak flow in the area will also limit severe thunderstorm potential
as shear values are expected to remain less than 20 knots. The main
threat from any storms that form will be heavy rain potential as the
storms will be fairly slow moving. There will also be plenty of
moisture to work with as precipitable water values continue to range
in the 1 to 1.5 inch range through Friday afternoon. Highs will be
in the upper 70s to around 80 with overnight lows mainly in the 60s.
Slowly diminishing cloud cover this afternoon and evening, may allow
fog to form quickly late this evening into the overnight hours. This
is especially concerning with very moist soils, light winds and
dewpoints still in the mid to upper 60s. Could see some locally
dense fog development overnight, depending largely on how much high
cloud cover lingers across the area. Will have to monitor tonight
as a Special Weather Statement for dense fog or possibly even a
short-fuse dense fog advisory may be needed.
Long term...Friday night through Thursday
issued at 232 PM CDT Thu Sep 19 2019
An amplified mean flow continues over the Continental U.S. At the start of
the period with an upper trough over The Rockies and an upper
ridge that stretched from the southeast U.S. Into eastern Ontario.
The upper trough is forecast to lift northeast into eastern Canada
this weekend and flatten the ridge axis. Meanwhile, another upper
trough will enter the western Continental U.S. Early next week, close off
into an upper low over the southwest Continental U.S. And leave a broad/
shallow trough from southwest Canada to the Great Lakes. Unsettled
weather is expected for most of the weekend, followed by brief
drying early in the week, and more rain chances mid-week.
Temperatures to remain above normal through Sunday, then gradually
settle near normal for most of next week.
Waning daytime heating, combined with the passage of a mid-level
shortwave riding northeast on the backside of the upper ridge,
will continue to bring a small chance of showers/isolated thunder-
storms to the area Friday night. Precipitable water values remain from 1.5-1.75"
with dew points in the 60s, thus locally heavy rainfall is
possible under any of the precipitation. Min temperatures to be
generally in the lower to middle 60s. An upper trough is forecast
to move into the northern plains and help push a cold front into
the upper MS valley on Saturday. A continuous influx of moisture
into WI will keep both precipitable water and dew point values high through the
day and as lift/forcing begin to increase, anticipate
precipitation chances to be on the increase as well, especially by
the afternoon hours. Stronger instability and shear to reside to
our west closer to the cold front, thus any severe threat looks
low on Saturday. Temperatures will remain above normal with
readings reaching the lower 70s near lake mi, middle to upper 70s
The cold front is progged to push across most of WI Saturday night
and be the main Focal Point for additional showers and thunder-
storms. There will be some instability through the night (mucapes
of 500-1000 j/kg and steep mid-level lapse rates), as well as
better shear (0-6km of 30-40 knots). However, forcing is somewhat
weak as the upper trough to be headed toward western Ontario.
While a few stronger storms are possible, do not expect to see a
severe weather event with this system. Of more concern would be
more heavy rain, especially over central/east-central WI if added
moisture from tropical system Imelda gets pulled this far north.
Min temperatures to range from the upper 50s north-central WI, to
the middle 60s east-central WI. Chance for showers/storms to
linger into at least Sunday morning across eastern WI as the cold
front to still be in the process of exiting the region. The main
question is how fast will this front depart? The 12z European model (ecmwf) slows
this front down, thus rain chances could linger through the day.
Cooler/less humid conditions are expected with Max temperatures in
the upper 60s north-central, lower to middle 70s elsewhere.
The southern extension of the Ontario upper trough swings through
the Great Lakes Sunday night, so cannot completely rule out a few
showers, mainly during the evening. Otherwise, a ridge of high
pressure is forecast to build across the upper Midwest later
Sunday night and move into the Great Lakes region on Monday. Skies
will begin to clear with mostly sunny skies expected Monday.
Temperatures will continue their slow drop with readings in the
middle to upper 60s north-central/near lake mi, around 70 degrees
This high pressure ridge slides to our east Monday night with a
return flow in place by daybreak. By Tuesday, the mean flow to
have split over the eastern Pacific with the stronger northern
stream to run from the Pacific northwest to the Great Lakes.
Models show a modest cold front sweeping across the northern
plains by Tuesday afternoon and it seems unlikely that any
precipitation would reach this far east. May place a slight chance
pop for central WI, but anticipate that Tuesday would be dry. Max
temperatures to be in the middle 60s to lower 70s.
The cold front is forecast to reach western WI Tuesday night and
push across the rest of WI on Wednesday. The extent of any
showers/thunderstorms may come down to the strength of an
accompanying upper trough which the models disagree on. Chance
pops will suffice for now with Max temperatures on Wednesday again
in the middle 60s to lower 70s. Another area of high pressure is
set to arrive over the region for Wednesday night into Thursday,
thus dry conditions are expected with temperatures down near
Aviation...for 18z taf issuance
issued at 1234 PM CDT Thu Sep 19 2019
VFR conditions are expected at most taf sites through this
afternoon; however, some MVFR conditions may linger briefly into
the early afternoon hours. Tonight, models continue to indicate
IFR/LIFR conditions are possible at each of the taf sites
tonight. The lowest ceilings and visibilities are expected to be
across the central and north-central portions of the area, where
some clearing is possible. Winds will be light, so any clearing
could lead to rapid fog development, potentially dense.
Intermittent chances of showers and thunderstorms can be expected
through Friday. Timing is difficult to pin down at this point and
confidence in them impacting the taf site is low; therefore, have
left this out of the tafs.