542 
FXUS63 KGRB 300437
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1037 PM CST Fri Dec 29 2023

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- 0.5-2" of snow possible Saturday night into Sunday as a clipper
  system moves through the Great Lakes.

- Highest snow amounts expected across east-central WI, including
  the Fox Valley Sunday morning.

- Short period of gusty north winds (20-30 mph) as snow ends
  Sunday afternoon and evening could create minor blowing and 
  drifting snow on east- west oriented roads.

&&

.DISCUSSION...
Issued at 207 PM CST Fri Dec 29 2023

Short Term...Tonight and Saturday

Main forecast concerns will be on cloud trends late tonight and
whether any precipitation could work into parts of north-central
WI late Saturday afternoon.

The 19Z MSAS surface analysis showed a weak ridge of high pressure
that extended from the Upper MS Valley into western Ontario. A
weak area of low pressure over extreme northwest MN is a clipper
system headed in our direction. Visible satellite imagery
indicated skies had cleared over most of the area with some clouds
hanging over east-central WI.

The surface ridge (and its associated upper ridge) to move into
the western Great Lakes region tonight providing for mostly clear
skies and a light west-northwest wind. However, little change in
the air mass overhead could bring another round of low clouds and
patchy fog to parts of northeast WI after midnight. Confidence is
low as to exactly where/if these clouds actually develop. Models
bring the weak clipper low pressure into western Lake Superior
toward daybreak, but a lack of moisture ahead of this system would
preclude any precipitation. Min temperatures to range from the 
lower 20s north, middle to upper 20s east-central WI.

This clipper will continue to move southeast and enter northeast
WI Saturday afternoon. However, upper support trails back to
northwest WI/northern MN where a mid-level shortwave trough to be
situated. Since both moisture and forcing are lacking through the
day, most if not all of the forecast area should stay dry. We will
see clouds increase in the afternoon and have kept a slight chance
pop over parts of north-central WI late in the day. Max
temperatures on Saturday to range from the lower to middle 30s
north-central WI, mainly upper 30s east-central WI.

Long Term...Saturday Night Through Friday

 Snow...

Changes: 
Only slight changes to snow totals from the previous forecast. Did
increase northerly winds a few miles per hour for Sunday afternoon
and evening as a tighter pressure gradient and increasing winds at
925mb could result in gusts of 20-30 mph, mainly for the Fox
Valley and east-central WI.

Details:  
The main period of focus of the forecast is a clipper system
bringing a likely chance (60-80%) for snow beginning late Saturday
in northwest WI and spreading southeast before ending mid-Sunday
afternoon. The primary forcing with this system is an area of mid-
level convergence that is progged to pass directly over WI. One
factor that my limit snow accumulation is a lack of strong forcing through
the dendritic growth zone. Accumulations of about 0.5-2" are 
expected with the highest totals across the Fox Valley and east- 
central WI. There does look to be a short period Sunday afternoon 
and evening during which gusty (20-30mph) north winds could create
some blowing and drifting snow, mainly impacting east-west 
oriented road.

Uncertainty:
The primary area of uncertainty with this system is whether
northeast winds coming down the bay and lakeshore Sunday morning
could produce lake enhanced snow for a 3-4 hour period before
winds become more northerly by Sunday afternoon. It is likely 
that given the short duration for which lake enhancement could 
occur snow totals won't be greatly increased, however, if the low 
center continues to trend further east snow totals for areas south
of the bay and along the lakeshore may need to be increased.

Rest of the forecast...
Dry conditions are expected to return to the area for much of the
first half of next week with a high pressure system over the
southern CONUS extending into the region, a cold front associated
with a surface low tacking over central Canada could produce 
light snow Wednesday morning, mainly across the Northwoods, but 
impacts look minimal at this time. Another clipper type system may
also bring some precipitation towards the end of next week, but 
given the that this system is a week out left NBM PoPs for now.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1032 PM CST Fri Dec 29 2023

The latest models this evening have backed off on 
the freezing fog potential for portions of central and 
north-central Wisconsin. There will still be fog, but it
not anticipated to be dense. However, with clear skies and 
lights winds and temps already near the dewpoint temperature,
would not be surprised if there was some patchy dense fog 
in a few spots around sunrise or even a few hours after sunrise. 
Look for MVFR or lower CIGS to overspread the entire area
Saturday afternoon and evening with a round of light snow which 
should linger into It has been awhile since the area has seen an 
accumulating snow. 

&&

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

&&

$$

DISCUSSION.....Kallas/GK
AVIATION.......Eckberg