Forecast Discussion for GRB NWS Office
FXUS63 KGRB 211121

Area Forecast Discussion
National Weather Service Green Bay WI
621 AM CDT Thu Sep 21 2023

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Friday
Issued at 410 AM CDT Thu Sep 21 2023

The main forecast focus will be on fog development at night, well
above normal temperatures, and precipitation chances.

Abundant low level moisture trapped under a steep low level
temperature inversion has once again led to some dense fog across
portions of northeast Wisconsin early this morning. Currently the
worst conditions appear to be across isolated locations; however,
the fog is expected to be a bit more ubiquitous as the morning
wears on. The fog does not appear to be widespread or persistent
enough for a Dense Fog Advisory at this time but will continue to
monitor through the morning. Any fog will dissipate a few hours
after sunrise as the inversion mixes out.

Another warm day is on tap for northeast Wisconsin with 850 mb
temperatures of 12 to 14 Celsius. Highs today are expected to rise
into the upper 70s to lower 80s away from Lake Michigan with lower
to middle 70s near the lake. Although some models continue to
develop some light precipitation across the region today, the
general overall trend has been dry. MUCAPEs rise to 500 to 1000
J/kg during the afternoon; however, CAPE profiles are very long
and skinny with abundant dry air in the sounding with PWATs of
only around 1 inch. Therefore, will keep the forecast dry given
the lack of forcing to create any showers during the day.

Fog looks to once again be an issue tonight with abundant low
level moisture still in place along with light winds and mostly
clear skies. The models indicate the worst conditions would likely
be across north-central and northeast Wisconsin. Lows tonight are
expected to range from the lower to middle 50s across the north
with middle to upper 50s across central and east-central

Surface winds turn southeasterly on Friday, which should push
temperatures down a few degrees but still well above normal for
this time of year. Highs on Friday are expected to generally be in
the middle to upper 70s away from Lake Michigan, with lower to
middle 70s near the lake. The main story on Friday will be a
chance for showers and thunderstorms across central and north-
central Wisconsin as a mid level shortwave lifts north through
western Wisconsin. Mid level moisture will be a bit better;
however, MUCAPEs will only be a few hundred J/kg along with long
and skinny CAPE profiles and bulk shear around 10 knots.
Therefore, if any storms do develop they are not expected to be
severe or long lived.

.LONG TERM...Friday Night Through Wednesday
Issued at 410 AM CDT Thu Sep 21 2023

The extended forecast will focus on the an approaching upper low
that will move across the Upper Plains this weekend. Overall, the
forecast continues to look drier than previously forecast.

The main features for the beginning of the extended will be an
area of broad high pressure over far northeastern CONUS and
accompanying ridging aloft while an upper low approaches from the
west. Consensus remains in line with previous forecasts on the
track of the upper low, bringing it from the Rockies into the
Dakotas Friday night to Saturday night, before stalling out to
our west. The low then remains mainly west of the region before
gradually merging with the main flow again by the end of the
weekend. This means that the heavy precipitation associated with
the occluding low will stay well west of the area, focused mainly
across Minnesota. For our area, this leaves only weak moisture
advection into the mid- levels Friday night into Saturday but will
lack any focused sources of lift. Thus, while this could still
produce a stray sprinkle or two, felt reasonably confident 70%+ in
lowering pops for the weekend as most will see far more dry
conditions than any rainfall; this is especially true for areas
across eastern Wisconsin. Additionally, removed thunder for any
showers that form as instability and sources of lift will be
fairly lacking. Suffice to say, any sprinkles that remain in the
forecast are expected to be fairly light. Finally, confidence is
growing that if the drier trend holds through the next forecast
cycle, could foresee lowering pops further and creating a much
drier forecast for the weekend.

The changes in the weekend forecast will also impact early next
week. While the exact dissolution of the low is still unclear,
none of the options bring a focused round of precipitation to the
area. Thus, pops continue to fall in the Monday to Tuesday
timeframe as well. Behind this, upper ridging and high pressure
are expected for the middle of next week.

Temperature-wise, expect highs to start around 5 degrees above
normal Saturday before coming to near normal by early to the
middle of next week.

.AVIATION...for 12Z TAF Issuance
Issued at 621 AM CDT Thu Sep 21 2023

Low clouds and fog will continue to plague the TAF sites this
morning; mainly north of a line from KGRB to KAUW. LIFR/VLIFR
conditions will prevail for a few more hours this morning until
the fog mixes out after sunrise. After the fog dissipated VFR
conditions are then expected the rest of today into this evening.
Models indicate additional fog is expected to form later this
evening into tonight as surface moisture gets trapped beneath an
inversion. KRHI looks like it would be impacted the most with
LIFR/VLIFR conditions with some fog possible at KGRB and KMTW but
to a lesser extent.



SHORT TERM.....Kurimski
LONG TERM......Uhlmann

NWS GRB Office Area Forecast Discussion