233
FXUS63 KGRB 121751
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1251 PM CDT Fri Jul 12 2024

Updated aviation portion for 18Z TAF issuance

.KEY MESSAGES...

- Small chances (30% or less) for showers and storms this
  afternoon and evening. Severe weather is not expected, but
  storms could produce locally heavy downpours.

- Warmer and more humid conditions arrive this weekend through
  early next week. 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 could also provide relief from the heat at times.

- Increasing moisture and instability, combined with a couple
  short-waves and a cold frontal passage on Monday, will bring
  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 Saturday night into early Sunday
  morning.

&&

.DISCUSSION...
Issued at 418 AM CDT Fri Jul 12 2024

Short Term...Today...Tonight...and Saturday

Main forecast concerns will be if/where to include small
shower/storm chances today and this evening, storm chances and
severe potential on Saturday, fog potential and the increasing
temps and humidity.

Shower/Storm Chances & Severe Wx Potential:

MUCAPE of up to 800 J/kg still present across parts of central
and east central WI early this morning, so could see a stray
shower pop up, but no clear trigger or area to really focus on.
Will add some small PoPs over the east where a couple showers have
popped up, but will keep things dry elsewhere. As surface
instability increases through the morning, SBCAPEs will build to
1500 J/kg. Not a clear cut trigger through the day, but a weak
shortwave and a lingering weak boundary or two, including the lake
breeze, may be enough to touch off isolated showers and storms. A
warm layer just above 700mb, weak ridging trying to build in and
some mid- level dry air will be negating factors. Severe weather
is not expected, but brief heavy downpours are possible as flow
remain weak (slow moving storms) and PWATs remain around an inch.
Soundings also showing an inverted-V again, but still not much
wind to mix into. Any wind would have to come for a storm that can
pulse up and generate some brief wind of its own. Most/all of the
shower/storm activity will end in evening as instability wanes.

For Saturday...many CAMs are now advertising a complex of storms
approaching from the west early in the day, prior to 12z. But the
evolution of this convection remains uncertain, as track/timing/
instability/strength differences remain. As evidence, huge
differences from the 00z and 06z with two completely difference
scenarios portrayed. Leaning towards the scenario that brings just
isolated activity during the day on Saturday with an initial weak
shortwave, either from a weakening complex and/or some pop up
activity, as we will be fighting capping and dry air. Then we will
have to wait until after dark for a larger complex to arrive from
the northwest.

Fog & Smoke Trends:

Fog has been held in check for most of the area during the
overnight hours as winds just off the surface are a little
stronger than previous nights, some clouds have hung around and
moisture is a little less over the north. But expect some patchy
fog to continue to develop, mainly where skies are clearer across
central and north central WI, through sunrise, then any fog would
dissipate shortly thereafter. Fog is possible on northern Lake
Michigan as well, with some dense fog just to our north early this
morning (nothing seen yet on the Washington Island cams). Thinking
the light south winds will push any fog bank north, but will
monitor trends. Fog potential for tonight looks very low, but
can`t rule out a little patchy ground fog early Saturday morning,
especially if/where any heavier rain falls today.

Quite a bit of elevated smoke was observed yesterday, thin in our
area but thicker to our west. Smoke guidance has this area
remaining across the region, with the thickest smoke remaining to
our west/north. HRRR shows some thicker smoke pushing across Lake
Superior and the U.P., which may sneak into the north. Other than
hindering surface heating slightly and providing for a hazy skies
at times, not expecting any other impacts from the smoke.

Building Heat & Humidity:

Above normal temps are expected as southerly flow will continue
to allow temps and humidity levels to increase today into
Saturday. Highs will mainly be in the 80s, with a couple spots
possibly touching 90 in the south on Saturday if the clouds hold
off long enough as 925mb temps climb to the 22-24C. Heat indices
in the 80s are expected today, with the southern CWA likely around
90 on Saturday. Temps near Lake Michigan will remain slightly
cooler.

Long Term...Saturday Night Through Thursday

The attention of the extended will be on the upcoming weekend to
early week period which may see several rounds of active weather.
Behind this, much quieter conditions move in for the middle of
next week.

Storms this weekend...

Saturday evening, diurnal destabilization may have already led to
the development of a storms to our west where the best instability
gradient will be. Although bulk shear remains modest, around 20
knots, if these storms start to become outflow dominant they may
yet make their way eastwards into portions of central WI. A better
bet will be a fast moving shortwave that will cross the area in
the early to mid overnight hours, crossing north-central to
northeastern WI and the Upper Peninsula. The timing on this
shortwave still varies between runs, from late evening to around
midnight, but most mid-range models either initiate or sustain
storms with its passage. The southern extent of any precipitation
with this shortwave remains unclear at this time as well. The
most likely severe threat will be wind with any storms that form,
but a brief hail threat is also possible.

Sunday will be largely influenced by any overnight convection from
Saturday, which may leave a lingering outflow boundary in the
region. Any outflow boundary will be something to watch for
convection across the area on Sunday. Outside of this, capping
does appear more modest than previous model runs suggested, which
could mean an increased chance of convection during the day,
especially for north-central WI where the warm layer is the least
robust. Finally, another weaker shortwave cross the region late in
the day on Sunday, which could serve to sustain some overnight
precipitation.

Monday will be another day to watch for severe weather. The warm
moist airmass will still be in the region ahead of an approaching
cold front. As things stand, this may prove to be the most
organized of the three days, with afternoon and/or evening storms
developing in the region.

Temperatures...Outside of the storm chances, the warm and moist
airmass will also produce a period of very hot weather for Sunday
and Monday. Sunday will be the hotter day, with highs in the lower
90s and dewpoints potentially pushing into the middle 70s or
higher. This would produce heat indices into the middle to upper
90s. After the cold front on Monday, cooler air will bring
temperatures back towards the 70s for next week, which should also
provide a period of quieter weather.

&&

.AVIATION...for 18Z TAF Issuance
Issued at 1251 PM CDT Fri Jul 12 2024

VFR conditions are expected to prevail through the end of the TAF
period. A daytime cumulus field developed late this morning/early
afternoon and is expected to persist through the evening. An
isolated shower or storm is possible this afternoon during peak
heating, although mid-level dry air and capping will likely limit
the coverage of any storm activity. Canadian wildfire smoke may
be present aloft at times, just enough to create occasional hazy
conditions. Winds will remain light before gradually picking up
toward the end of the TAF period. Some ground fog will be possible
tomorrow morning, especially in areas if/where any rain falls
today.

&&

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

&&

$$

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