Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 162256

Area Forecast Discussion
National Weather Service Green Bay WI
556 PM CDT Sun May 16 2021

Updated aviation portion for 00Z TAF issuance

Issued at 408 PM CDT Sun May 16 2021

Warm and dry Monday, then scattered showers at times for the rest
of the week. Dense fog is likely over the lake at times, and it
could occasionally bleed into lakeshore areas as well.

The broad low-amplitude westerly flow across North America will
undergo a gradual but significant amplification. The evolving
pattern will be characterized by a seasonably deep trough near the
West Coast and strong ridging in the East. Continued
amplification combined with the development of a closed upper low
over the western Atlantic will result in a little retrogression of
the mean features and the evolution of the flow into a blocking
regime next weekend.

The forecast area will initial be located within deep-layered
southerly upper flow along the western flank of the evolving
eastern ridge. That will result in warm and increasingly moist
conditions across the area. Temperatures will be above normal
Monday, probably edge back a little for a couple days and then
warm to 6-12F degrees above normal for the rest of the period.
Total precipitation for the period is still tough to estimate,
but a reasonable guess is for amounts somewhat close to normal.

.SHORT TERM...Tonight and Monday
Issued at 408 PM CDT Sun May 16 2021

Besides the concern of fog moving on and offshore along Lake
Michigan, quiet conditions are expected across the forecast area
during this forecast period.

A surface ridge will continue to build over the area tonight,
followed by an upper-level ridge building over the area on Monday.
These ridges of high pressure will bring dry air to the forecast
area keeping skies mostly clear overnight and mostly sunny on
Monday. South to southeast winds on the back side of the high
pressure will pull warmer air into the forecast area. Low
temperatures to range from the middle 40s to lower 50s. Much warmer
temperatures are expected for Monday with highs ranging from the
middle 70s to lower 80s away from the lake, and middle 60s to lower
70s along the lakeshore.

Dense fog along the Lake Michigan shoreline moved into the area this
afternoon. Model guidance continues to show the fog staying put
through early Monday morning with the potential to move inland by a
few miles, especially along Door County. The fog should dissipate,
at the latest, by mid Monday morning, but could dissipate before

.LONG TERM...Monday Night Through Sunday
Issued at 408 PM CDT Sun May 16 2021

The main change to the forecast since yesterday is that a
shortwave rotating around the western flank of the building
eastern CONUS longwave ridge is now progged to be stronger and
more distinct. It is also likely to pass directly across the
forecast area, and reaches the area more quickly. As a result,
increased the rain chance Tuesday afternoon and night. There
should be a lull in the rain chance as the shortwave departs, but
timing of the specific features generating forcing for ascent
breaks down quickly by that point in the forecast. Carried some
PoPs for the rest of the week as the forecast area remains in
deep, moist, southerly flow. But the details of when/where/how
much rain are highly uncertain.

The eastern ridge will close off into an upper anticyclone during
the latter part of the period, and the center of the anticyclone
will edge back toward the Ohio Valley as the pattern becomes
increasingly blocky. That may eventually push the focus of the
precipitation back to the N and W of the area late in the period.

.AVIATION...for 00Z TAF Issuance
Issued at 554 PM CDT Sun May 16 2021

Diurnal cumulus clouds with bases between 4-8k ft will dissipate
early in the TAF period with the loss of daytime heating.
Otherwise mainly high clouds will linger across the region. High
pressure will build into the area tonight into Monday, leading to
good flying conditions. Some patchy fog may develop across the
north-central and central WI TAF sites sometime after midnight
tonight, and then dissipate soon after sunrise. Marine fog may
also drift inland along the lakeshore TAF sites throughout the
night, especially across KMTW.



SHORT TERM.....Hykin
LONG TERM......Skowronski
AVIATION.......Kurimski is the U.S. government's official web portal to all federal, state and local government web resources and services.