889
FXUS63 KGRB 131153
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
653 AM CDT Sat Jul 13 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Warmer and more humid conditions today into Monday. Heat index
  values in the 90s are expected over central, east central and
  parts of far northeast WI Sunday into Monday. Heat indices could
  approach advisory criteria for a time in the afternoon across
  the southern CWA on Sunday. Thunderstorm development and clouds
  could also provide relief from the heat at times.

- Potential for a few rounds of strong to severe storms this
  weekend into early next week. Timeframe with the highest chance
  for the severe storms is late this afternoon into into early
  Sunday morning but confidence remains low on exactly how storm
  evolution will play out.

- Cooler and drier conditions expected mid-late next week.

&&

.DISCUSSION...
Issued at 427 AM CDT Sat Jul 13 2024

Short Term...Today...Tonight...and Sunday

Main forecast concerns will revolve around trying to make sense of
the complex convective setup the next 36 hours and identifying
if/when severe weather is most likely. CAMs showing a variety of
different scenarios that could play out, making for a low
confidence forecast. Building heat and humidity through the
weekend will also be bring some heat concerns.

Convective Trends & Severe/Flooding Potential:

For this afternoon into tonight...to no one`s surprise, models
still on different pages with how things will play out and
confidence remains low on exactly how the storm evolution will
behave through tonight. Models struggling with initializing the 3+
areas of convection over the northern Plains early this morning,
especially the first round MN which is not a good sign. Really
hard to pick a winner at this point as a lot has to play out, but
will lean toward the scenario that has the main area of convection
currently in northeast SD and west central MN tracking east/southeast,
along increasing warm/moist advection at 850mb, the instability
gradient and the better effective shear up to 35kts and possible
MCV development. But some of this complex and/or some northward
expansion of it looks to shift eastward, bringing shower/storm
chances into our area later this morning and early afternoon. It
may outrun the better instability eventually, but modest CAPE of
~1000-1500 J/kg and up to 25 kts of shear will at least bring a
small chance of strong to severe storms. What occurs next is the
biggest question. The HRRR is by far the most aggressive showing
the first round morphing/expanding through/across area developing
widespread convection this evening, then a secondary squall line
racing across the area overnight. This scenario would bring a
higher threat for severe weather (hail and damaging wind) and
flooding with multiple rounds of heavy rain (PWATs of 1.5-1.7").
However, the majority of the CAMs have one complex of storms
bringing a quicker shot of severe weather (mainly damaging wind)
overnight as a shortwave in Canada drops southeast. A few models
actually keep things dry overnight, which could pan out of the
action throughout the day stays mainly to our south, but think the
northern shortwave should produce at least some shower and storm
activity.

For Sunday...once any early morning convection exits to our east,
much of the daytime may actually stay quiet as there will be a
lack of any shortwave energy to help focus convection. Models
hinting a maybe a weak shortwave later in the day, but it
currently should stay to our south. Plus, a capping inversion
will be place, so any daytime pop up activity should remain
isolated. If anything were able to develop, SBCAPE up to 3000 J/kg
could produce a pulse strong or severe storm.

Temps / Fog / Smoke:

Heat and humidity will build through the weekend as southerly flow
ushers in a true summer-time airmass with 925mb temps climbing to
between 21-25C. Highs in the mid 80s to around 90 are expected.
Dewpoints are forecast to be mainly in the 60s today (a few 50s
far north) and in the upper 60s and low 70s on Sunday, with some
mid 70s possible. This will bring heat index readings in the 80s
today and in the 90s on Sunday. Could see a 100 heat index on
Sunday if temps or dewpoints are a little higher than currently
forecast. But cloud debris and precip may hinder fully reaching
our high temps. A little relief will be found near the shores of
Lake Michigan as winds remain onshore for much of the period.

Fog has been held in check during the early morning hours for most
spots as slightly drier air near the surface and winds just off
the ground limited development. Bu with calm winds and dewpoint
depressions at/near zero, some patchy ground fog will be possible
through around sunrise. As dewpoints climb through the weekend,
will need to monitor for fog on Lake Michigan, especially on
Sunday, as water temps are still in the 60s.

Elevated smoke was fairly thick across the area on Friday creating
hazy skies, but did not see any impacts at the surface. RAP/HRRR
continue to show some elevated smoke remaining across the region
through the weekend, but not expecting any impacts at the surface.
The smoke may hinder surface heating slightly, along with possibly
holding down any surface-based convection based on how thick it is.
Will need to watch for a much thicker batch of smoke, likely making
it to the surface and creating visibility reductions and health
concerns, that will spread into Lake Superior Sunday afternoon.

Long Term...Sunday Night Through Friday

The main focus in the extended will be on the first 24 hours,
encompassing Sunday night through Monday. After this period, much
quieter weather finally returns to the region for early through
the middle of next week.

Sunday night through Monday...A period of relative quiet is
possible for the early overnight period as active weather from the
afternoon and evening pushes out of the region. Then, we get set
for our third round of potentially strong to severe storms in as
many days. The hot and humid airmass from the weekend will still
be present, providing the afternoon surface instability before a
cold front sweeps across the region during the day. Bulk shear
during this period will be higher than the previous days, with
values around 40 to 50 knots which would support organized,
relatively fast moving convection. Storm initiation is a little
tricky, as storms in Minnesota along the cold front could continue
pushing east early on Monday, crossing the axis of instability
from southeast Minnesota to southern WI or northern Illinois.
Alternatively, these initial storms could diminish overnight only
for new storms to develop as diurnal instability returns and the
cold front arrives Monday afternoon. In either case, the potential
for an organized and fast moving convective system is certainly
there. If this occurs, damaging wind gusts will once again be the
primary convective threat.

Rest of the forecast...Behind the active weather Monday, northwest
flow returns. Temperatures will drop to below normal, with highs
back in the 70s for the middle of the week. This forecast may
still see occasional showers across the north as weak impulses
cross far northern WI, but nothing as active as today through
Monday is expected to be.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 653 AM CDT Sat Jul 13 2024

Mainly VFR conditions are expected today, with some clouds
increasing through the day. Chances for showers and storms will
increase later this morning and afternoon, especially central and
north central WI. But confidence is not high enough to include at
TEMPO group at this time and will stick with PROB30. Uncertainty
still exists on how things will play out later today and tonight,
as guidance remains quite different, still expect a round of
showers and storms late this evening into early Sunday. This
complex could be well organized and bring strong winds and heavy
rain. Lower flying conditions are possible in any shower/storm
through the TAF period, but more likely overnight.

South winds will gust to around 15 kts today. Locally higher gusts
will be possible in/around any storm. Smoke aloft will create
hazy skies at times as well.

&&

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

&&

$$

DISCUSSION.....Bersch/Uhlmann
AVIATION.......Bersch