Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211141
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
641 AM CDT Mon Sep 21 2020

Updated aviation portion for 12Z TAF issuance

.SHORT TERM...Today...Tonight...and Tuesday
Issued at 344 AM CDT Mon Sep 21 2020

An upper level ridge was shifting east of the region early
morning, and weak short-wave energy was impacting the northwest
part of the forecast area. Weak radar returns extended from
northern IA into north central WI and the western Upper Peninsula,
but little rain was reaching the surface, due to very dry air
below 700 mb. The pressure gradient was fairly strong over our
northwest counties, and a 40-45 knot low-level jet was also
focused there.

The combination of high clouds and a smoke layer from the western
U.S. wildfires will lead to a partly sunny day today. Breezy
south winds will develop, with gusts to 20 to 25 mph in the late
morning and afternoon. The approach of a weak cold front and some
upper level jet energy will combine to produce a small chance of
showers over northern WI this afternoon, but moisture remains very
limited. With steep mid-level lapse rates around 7 C/km, an
isolated thunderstorm or two cannot be ruled out. Highs will be in
the upper 60s to lower 70s, except middle 60s near Lake Michigan.

The weak front and jet energy will sag southeast through the
forecast area tonight, so will have a similar progression with the
small pops for showers/storms. Aside from a few lingering showers
in our southeast counties early in the day, we are looking for
mostly sunny, warm and less windy conditions on Tuesday. It also
appears it will be less hazy, as the smoke conditions aloft abate.
Lows tonight should be in the lower to middle 50s. Highs on
Tuesday should reach the middle to upper 70s, excpet upper 60s
and lower 70s along the Lake Michigan shoreline.

.LONG TERM...Tuesday Night Through Sunday
Issued at 344 AM CDT Mon Sep 21 2020

Main concerns for this forecast period are the various chances for
precip and the continuation of above normal temperatures. However,
temperatures will take a dip to near seasonal temperatures on
Thursday and seasonal temperatures on Sunday due to frontal passages.

Tuesday night through Thursday...Broad upper-level ridging Tuesday
night and dry air from a nearby surface high pressure system to the
south will keep conditions dry into Wednesday. As the upper-level
flow transitions to a mean zonal flow on Wednesday, the ongoing
south/southwest low-level flow is expected to keep above normal
temperatures in the forecast with similar readings to Tuesday in the
middle to upper 70s. While much of Wednesday will be dry, a surface
low pressure system to the west is expected to move into the
southwestern portion of the Upper Mississippi Valley. Ahead of this
system, model guidance indicates f-gen to increase over the
U.P./Wisconsin border Wednesday night. Moisture riding on the
northern portion of the low looks to meet up with the f-gen allowing
chances for showers over far northern Wisconsin Wednesday evening
and overnight. However, model guidance continues to struggle with
the timing and track of the low pressure system Wednesday night and
Thursday. Overall, as the low moves closer to the forecast area,
moisture ahead of the low`s cold front could bring additional shower
chances across northern Wisconsin, possibly into Thursday. There
does appear to be enough instability to create a few thunderstorms
at times. With clouds and possible shower chances lingering, high
temperatures are anticipated to be near normal ranging from the
upper 60s to lower 70s.

Rest of the extended...The low-level south/southwest flow will
quickly return for Friday and Saturday bringing the warm air back
over the forecast area. High temperatures are expected to rise above
normal with values in the middle to upper 70s. Model guidance is in
agreement for dry conditions on Friday, followed by a strong cold
front moving into the region Saturday evening/night with plenty of
moisture and forcing. This front will bring the next chance for
widespread showers and storms to the forecast area late Saturday
into Sunday.
&&

.AVIATION...for 12Z TAF Issuance
Issued at 635 AM CDT Mon Sep 21 2020

VFR conditions will persist through at least this evening. LLWS
over northern WI will taper off this morning, followed by increasing
southerly surface winds with gusts to 20 to 25 kts. A weak frontal
system will bring a few showers to northern WI this afternoon,
and the rest of the area tonight. There is some potential for low
clouds and patchy fog over northern WI late tonight, but overall
confidence is low. LLWS is expected to redevelop this evening,
then diminish late tonight.
&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Beach Hazards Statement through this evening for WIZ022-040-050.
&&

$$

SHORT TERM.....Kieckbusch
LONG TERM......Hykin
AVIATION.......Kieckbusch



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