971
FXUS63 KGRB 122314
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
614 PM CDT Fri Jul 12 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Isolated showers/storms will be possible through this evening.
  While severe weather is not expected, any storms could produce
  locally heavy downpours.

- A few rounds of strong to severe storms are possible from
  Saturday through Monday. The mostly likely times are Saturday
  night, and again Monday afternoon/evening. However, there is
  still high uncertainty.

- Warmer and more humid conditions arrive Saturday through Monday.
  Heat index values in the 90s are expected over central, east
  central and parts of far northeast WI Sunday into Monday.

&&

.DISCUSSION...
Issued at 401 PM CDT Fri Jul 12 2024

Short Term...Tonight and Saturday

Main forecast concerns for the short-term will revolve around
severe weather potential and building heat and humidity. Best
timing for anything severe looks to be late Saturday evening and
into the overnight, although CAMs are currently showing a variety
of different scenarios that could play out.

Showers/storm potential this afternoon/evening... Mid-level
capping and dry air intrusion from high pressure situated over
Michigan`s lower peninsula have kept shower activity at bay
through the early afternoon. However, decent moisture and
instability (~2,000 J/kg CAPE) have sustained a mid-level cu field
across the entirety of the CWA. Some pulse showers/storms may be
possible into this evening, although dynamics don`t look too
impressive given the absence of any surface features or triggers
to focus convection. This being said, anything that does go up may
have the potential to produce brief heavy downpours given PWATs of
1.25" and weak storm motion.

Fog/smoke... Some patchy ground fog will be possible tomorrow
morning, especially in areas if/where any rain falls today.
Some elevated Canadian wildfire smoke was also observed across
the upper Midwest, enough to create hazy conditions. While areas
of thicker smoke will remain off to our northeast, even a thin
layer of smoke may be enough of a hindrance to prevent us from
reaching convective temperatures later this afternoon and evening.

Saturday severe weather potential... Most likely solution currently
shows a complex of storms coming down from the northwest late
Saturday evening into early Sunday morning, mainly before 12Z.
Main source of discrepancy between CAM guidance centers around
shower/storm activity earlier in the day as weak shortwave energy
treks across the Great Lakes. The HRRR solution favors convection
developing off to our west along the axis of greatest instability
Saturday afternoon before dissipating as it encounters drier air
over central Wisconsin. Other short-range models show more
isolated/pop-up activity during peak heating. There are some
questions as to whether or not this will hinder the
east/southeast progression of the MCS later in the evening, with
the most recent suite of CAM guidance favoring a slightly later
arrival than was shown this morning. Current thinking is that any
organized storms would be hard to come by earlier in the day due
to the influence of dry air from departing high pressure and low-
level capping. Inverted-V soundings indicate locally damaging
winds as the primary severe threat for Saturday evening, while
mid-level lapse rates and wet bulb zero heights favor hail as a
secondary threat.

Otherwise, persistent southerly flow looks to make this weekend
rather unpleasant as temperatures climb into the low 90s and a
surge of low-level moisture brings dewpoints up into the low 70s.
The combination of heat and humidity will result in heat indices
potentially reaching the mid 90s tomorrow, and flirting with heat
advisory criteria on Sunday. A persistent southeast lake breeze
will keep temperatures along the lakeshore slightly cooler.

Long Term...Saturday Night Through Friday

The focus is on the first 48 hours or so of the long-term
forecast with the potential for multiple rounds of convection.
Heat and humidity will also be a concern during this time.

Saturday night...Already starting out with high uncertainty at
00Z/Sun. Some indication thunderstorms or a convective system
could be moving across the area at this time, or it could have
dissipated already (as discussed above). Regardless, attention
will quickly turn to a compact shortwave coming through Saturday
night. This is more likely to generate a thunderstorm complex
across northern Minnesota Saturday evening, which tracks into
Wisconsin overnight (timing window anywhere from 9PM to 6PM across
the forecast area). Placement is uncertain, but generally favors
a southeast movement across most of the forecast area. Damaging
winds would be the main threat, with smaller potential for large
hail and localized heavy rain/flooding.

Sunday...A morning break from convection, however the heat and
humidity build during this time. Dew points rise into the lower
70s for most of central and east-central WI (in the mid-60s across
north- central WI). High temps forecast in the mid 80s to low 90s
will yield heat index values as high as 95 to 100 across central
and east-central WI. However, temps could be impacted slightly by
any lingering clouds or smoke aloft, along with any isolated
thunderstorms. While most models favor a cap holding off any
thunderstorms, and there is limited upper level dynamics, will
want to watch for any meso scale interactions which could lead to
some isolated thunderstorm potential. A lot of CAPE will be in
place, so anything which does develop could become severe.

Monday...Once again looking at significant moisture with dew
points in the low 70s, and afternoon highs in the low 80s, and
increasing instability. There are more dynamics at play, leading
to somewhat higher confidence for thunderstorms. A surface cold
front, along with mid-level shortwave, and RRQ of upper jet will
all serve as larger scale lifting mechanisms. Timing and details
are less certain given the prior activity.

Tuesday through Friday...Quieter and more seasonable temperatures
settle into the region.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 606 PM CDT Fri Jul 12 2024

VFR conditions are expected tonight and Saturday. A diurnal
cumulus field will persist into early this evening before
dissipating. Satellite imagery indicates some Canadian wildfire
smoke is aloft, which may cause some hazy conditions at times.
Surface winds are expected to be light through the TAF period.

&&

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

&&

$$

DISCUSSION.....Goodin/KLJ
AVIATION.......Kurimski