782
FXUS63 KGRB 162327
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
627 PM CDT Thu May 16 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Thunderstorms are possible across the forecast area through
  early this evening. A few storms may become strong across
  central and east-central Wisconsin. Small hail and strong winds
  are the main hazards.

- Patchy to areas of fog are expected overnight with clearing
  skies and light winds. The fog may reduce visibilities at times,
  making travel hazardous.

- Isolated thunderstorms are possible Saturday, but most of the
  area will be dry during the day. Lightning and isolated wind
  gusts will be the main concern with any storms.

- Next chances for widespread active weather will be in the region
  early next week. It`s too early to determine severe potential.

&&

.DISCUSSION...
Issued at 237 PM CDT Thu May 16 2024

Short Term...Tonight and Friday

An area of showers and thunderstorms is currently exiting the
lakeshore counties with light showers continuing across north-
central Wisconsin. Hi-res models develop another area of showers
and thunderstorms with the occluded front as it sweeps through the
area this afternoon into the early evening in an area of MUCAPEs
currently building from southwest Wisconsin north to western
Wisconsin. Although MUCAPEs are forecast to jump to 1000 to 2000
J/kg across central and east-central Wisconsin ahead of these
showers and thunderstorms, this seems a bit generous given the
extensive cloud cover, cool temperatures, and low dew points
across the region. A more realistic instability forecast would be
in the realm of 500 to 1000 J/kg or about half of the current
model guidance. In addition, bulk shear late this afternoon into
early this evening is expected to be modest at 20 to 30 knots.
Most of the mesoscale models generally keep any thunderstorms in
check as they track across the area late this afternoon into this
evening; which makes sense given the relatively limited dynamics
associated with the front itself and limited available shear. Any
strong storms that do develop could contain small hail and gusty
winds, but this should be the exception rather than the rule.

Once the occluded front and showers/thunderstorms move off to the
east, clearing skies and light winds should rule during the
overnight hours. This will provide for ideal radiating conditions,
when combined with the recent rainfall, should lead to patchy to
areas of fog overnight. Although the models are targeting
different areas for visibility reductions overnight, they
generally agree that there will be fog somewhere across the area.
Therefore, will introduce fog across the area overnight. Lows
tonight are expected to range from the lower 40s across the north,
to around 50 across east-central Wisconsin.

A weak ridge of high pressure will track through the area on
Friday, which should keep the area dry for much of the day. The
only exception could be far north-central Wisconsin as a warm
front develops across the northern Great Lakes on the backside of
the departing high. Otherwise, Friday will be around 10 degrees
above normal as highs range from the middle 70s across north-
central, to 75 to 80 along and west of the Fox Valley, with 60s
near the lakeshore.

Long Term...Friday Night Through Thursday

Precipitation trends remain the focus of the extended. However,
with the Saturday forecast trending drier, attention now turns to
the potential rain for early next week, which could be another
more active pattern.

Saturday...Unstable air near the surface coupled with the passage
of weak cold air advection aloft will be the main two factors that
could produce some scattered precipitation on Saturday.
Instability values see CAPE get to around 600-900 J/kg in the
afternoon as high temperatures head towards the upper 70s and
lower 80s. However, to get precipitation the dewpoints will also
need to be on the increase during this period, but this will be at
odds with the increase mixing with some of the drier air aloft.
All in all, the forecast is trending to just isolated
precipitation associated with some pulsy storms but severe storms
are not currently expected. Dry conditions then follow behind
this for Sunday.

Early next week...The mean pattern will see broad ridging over
eastern CONUS while an upper low sets up over southern Alberta and
Saskatchewan. For us, this means general southwesterly flow and a
couple of rounds of active weather as a series of shortwaves
crosses into the area. As of this forecast issuance, models have
to come in to better agreement in the passage of two systems, one
during the day Monday and the next Tuesday evening. That said, this
type of pattern can be somewhat volatile in terms of timing
systems this far out, so if you have plans early next week be
prepared for changes in the details.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 624 PM CDT Thu May 16 2024

A few isolated thunderstorms are ongoing this evening across the Fox
Valley and east-central WI. These storms may create MVFR vsbys at
GRB, ATW, and MTW as they move through those areas this evening.
Small hail and gusty winds are also possible with these storms.  The
threat for thunderstorms wanes after 02Z as the occluded front
forcing the storms moves off to the east.

As skies clear out tonight and winds become calm conditions will
become favorable for fog formation across the region, especially
given the recent rainfall. Included LIFR/IFR low clouds and vsbys
at all the TAF overnight. Fog should start to burn off quickly as
the sunrises. Expect all the TAF sites to return to VFR conditions
by 14/15Z.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION.....Kurimski/Uhlmann
AVIATION.......GK