971
FXUS63 KGRB 112331
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
631 PM CDT Thu Jul 11 2024

Updated aviation portion for 00Z TAF issuance

.KEY MESSAGES...

- Small chances (30% or less) for showers and storms this
  afternoon and evening, and again Friday afternoon. 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 NE WI Sunday into Monday.
  Heat indices could approach advisory criteria over the southern
  CWA on Sunday, but current probabilities of heat indices
  exceeding 100 F are only around 10 percent. 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.

&&

.DISCUSSION...
Issued at 232 PM CDT Thu Jul 11 2024

Building heat, humidity and severe thunderstorm potential for the
weekend into early next week, are the main forecast concerns.

Even though there is little in the way of dynamic forcing, MLCAPE
of 500-1000 j/kg and weak MLCIN over the southwest half of the
forecast area should result in at least widely scattered showers
and storms this afternoon and early evening. Not expecting any
severe weather, as deep layer shear and winds aloft are very weak.
Slow movement of any storms could result in very localized 0.25
to 0.50 inch rainfall amounts, just like the past couple days.

Later tonight, partly cloudy skies, light winds and high pressure
drifting just south of the forecast area should allow for at least
patchy fog development to occur. Models highlight the greatest
potential over central WI and the western Fox Valley.

Similar setup on Friday, with weak dynamics and developing
afternoon instability across the region. There is weak mid and
upper level ridging, which is certainly a negative factor, and a
weak cap may also be in place. The degree of instability and the
hint of a weak boundary across N WI support the idea of keeping
slight chance pops in place across much of the forecast area
during the afternoon.

Heat/Humidity: Southerly winds will bring an increasingly warm,
humid and unstable air mass into the region. Heat indices in our
southern CWA will approach 90 on Saturday, mid to upper 90s on
Sunday, and low to mid 90s on Monday, though the heat will likely
get beaten back by a few rounds of thunderstorms. The hottest day
should be on Sunday, as a capped atmosphere may hold off storm
development in C/EC during the day. However, ensemble
probabilities of heat indices >100 F (Advisory criteria) are
only around 10 percent in C/EC WI.

Thunderstorm/Severe Potential for the Weekend/Early Next Week:
Strong instability will be in place across the region through this
period, and models show several weak short-waves moving through
zonal flow, and a fairly strong cold frontal passage Monday
afternoon and evening. On Saturday, a short-wave trough is
expected to move through, but timing varys a bit between the
models. With decent instability (CAPE 1-2 K j/kg), but deep layer
shear of only around 20 kts, convective organization is in
question. Agree with the current Marginal Risk from SPC. On
Sunday, strong instability (2-3 K j/kg) and increasing deep layer
shear of 30-40 kts will be more supportive of severe storms.
However, a potent cap will likely limit development over C/EC, at
least until the evening. Farther north, on the edge of the cap,
short-wave energy may kick off some severe storms. A fairly strong
cold, at least by summer standards, will move through the region
Monday afternoon and evening, and be accompanied by strong
instability, increasing mid/upper level wind fields, and fairly
strong deep layer shear. Could see potential for a more widespread
severe weather event Monday afternoon and evening.

A secondary cold front and associated short-wave trough may keep
some showers in the forecast Tuesday into Tuesday night, but
Canadian high pressure will bring dry weather, cooler temperatures
and low humidity for the mid to late part of the week.

&&

.AVIATION...for 00Z TAF Issuance
Issued at 630 PM CDT Thu Jul 11 2024

Outside of some areas of ground fog that may form late at night,
VFR conditions are expected through Saturday with scattered
Cumulus clouds in the afternoon and early evening.

&&

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

&&

$$

DISCUSSION.....Kieckbusch
AVIATION.......RDM