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000
FXUS63 KGRB 231143
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
643 AM CDT Tue Apr 23 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- There is a chance of thunderstorms this afternoon into early
  this evening, but severe storms are not expected.

- Elevated fire weather conditions are possible Wednesday and
  Thursday due to relative humidities ranging from 15 to 25
  percent. However, lighter winds and temperatures mainly in the
  50s will keep conditions below critical levels.

- Thunderstorms are possible Friday afternoon through Sunday
  evening. It is too early to determine whether they might be
  severe.

&&

.DISCUSSION...
Issued at 355 AM CDT Tue Apr 23 2024

Short Term...Today...Tonight...and Wednesday

Some thunderstorms are expected this afternoon as a strong secondary
cold front approaches. Gusty winds and small hail would be the
primary hazards. Though the Day1 Marginal Risk has shifted back
across southeast WI, no severe storms are expected at this time
for our forecast area.

Leading cold front is working across WI this morning. A few
showers and even some isolated thunder has occurred over north-
central WI. These showers will shift out of far northeast WI
shortly after daybreak. Rest of the morning will be pretty quiet
as shortwave trough axis reaches Lake Superior to southern MN
while secondary cold front drops across northern MN. Ahead of the
front, a thermal ridge will result in soaring temps, especially
over east-central WI where readings by early afternoon will be
well into the 60s if not nearing 70. As the secondary cold front
draws closer, limited instability ahead of the front (low sfc Td
offset somewhat by steep mid-level lapse rates over 8c/km) will
begin to allow scattered showers and some thunderstorms to blossom
mid to late afternoon. These gusty showers and storms will then
sweep southward into the early evening as the front crosses the
region from the north. Even with the mid-level instability,
limited low-level moisture and majority of stronger effective
shear shunted farther south and east should limit potential for
severe storms, though cannot rule out an isolated severe storm
late this afternoon south of highway 29 where heating and insolation
will be maximized.

Front moves through this evening with temperatures quickly falling
off from north to south. Readings by late evening will already
have fallen below freezing over north-central (with maybe a bit of
light snow) and will be heading down to the upper 30s to lower
40s elsewhere. So, after a day with highs near 70 degrees in
parts of the forecast area, readings all areas by daybreak on
Wednesday will be at or below freezing.

Long Term...Wednesday Night Through Monday

Precipitation...Dry conditions are expected to continue Wednesday
night through Thursday night, as dry air from a departing surface
high and a trailing upper-level ridge reside over the region.

Attention turns to Friday through Monday, with a couple rounds of
rainfall and thunderstorms expected to impact the region from two
southern stream systems. Both systems are beginning to take shape
with very similar paths from the central Plains to the Upper
Mississippi Valley northeast across WI. However, for both systems
there are still crucial timing and placement differences that will
ultimately determine rainfall amounts, including where the heaviest
rain will occur, and thunderstorm potential, including severe
potential.

The first system looks to bring precip to the forecast area sometime
Friday afternoon/evening through Saturday evening, with the heaviest
rain occurring Friday evening/night when PWATs around 1.25 inches
are present. There is potential for some elevated thunderstorms
Friday night, but a better chance for surface-based storms looks to
occur on Saturday when the surface low`s cold front sweeps across
the warm sectored forecast area. Although some guidance is
supporting the potential for strong/severe storms, cannot say for
certain there will be some given a slight shift in timing of the
cold frontal passage could change that.

Following the frontal passage Saturday evening, a small window with
no precip appears to occur Saturday night before precip from the
next system arrives sometime Sunday morning and exits sometime on
Monday. The timing of this system is even less certain than the
previous since there is still uncertainty with the first system.
But, a general consensus still has a strong signal for another
round of healthy rainfall amounts and some thunderstorms with this
system.

Temperatures...As low-level winds turn southerly on Thursday,
temperatures will begin to increase to slightly above normal with
highs ranging from the mid 50s to low 60s. Similar temperatures are
anticipated on Friday before the precip arrives. Saturday`s
temperatures will be a challenge to determine this far into the
forecast and given the uncertainty of the cold frontal passage.
However, many models are indicating the forecast area will be in the
warm sector, which could result in highs in the mid 60s to even mid
70s.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 642 AM CDT Tue Apr 23 2024

A strong cold frontal passage late today into this evening will
generate numerous showers and scattered thunderstorms in a
marginally unstable air mass, especially this afternoon into early
evening. Will continue to highlight the best window for thunderstorms
at all of the TAF sites. Ceilings may drop to MVFR behind the
cold front in far northern WI before skies clear out late this
evening into the overnight hours.

West to northwest winds will gradually increase into early this
afternoon, before abruptly shifting to the north and becoming
gusty as the cold front moves through. The strongest winds, with
gusts to near 30 kts, will likely occur in the Fox Valley and
lakeshore areas. Winds will significantly diminish at all
terminals after midnight.

&&

.FIRE WEATHER...
Issued at 355 AM CDT Tue Apr 23 2024

Ahead of a stronger cold front today, elevated fire weather
conditions could develop from mid morning through early afternoon
as temperatures rise well into the 60s, RH values lower to 25-35
percent and southwest-west winds increase to around 15 mph. The
conditions will be brief and limited to mainly east-central WI.

Another period of elevated fire weather conditions is possible
Wednesday and Thursday, as a dry airmass moves into the region.
Afternoon RH values of 20 to 30 percent are possible on
Wednesday, but cooler temps are anticipated in the lower 40s
lakeside to the low 50s inland. On Thursday, RH values will be
slightly higher between 25 to 35 percent, and temps will also be
slightly higher in the mid 50s to low 60s. But, due to the
proximity of the prevailing high pressure system, winds will be
below critical levels on both days.

&&

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

&&

$$

DISCUSSION.....JLA/Kruk
AVIATION.......JLA
FIRE WEATHER...JLA/Kruk