Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 230842
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
342 AM CDT Sat Mar 23 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

- There is a 50 to 90% chance of at least 6 inches of snow
   accumluations across parts of central, north-central, and far
   northeast Wisconsin on Sunday afternoon through Monday morning.
   Confidence on these percentages is highest across north-
   central WI where there is a 80% chance of at least 8 inches of
   snow. Gusty winds of 30 to 35 mph could also create areas of
   blowing and drifting snow, and make for very difficult driving
   conditions during this time frame. The Monday morning commute
   will likely be severely impacted.

 - Precipitation will likely change over to a wintry mix or rain
   for a time from Monday afternoon through Monday night, before
   changing over back to snow on Tuesday and Tuesday night over
   north-central Wisconsin. Minor snow accumulations are possible.
   A rumble of thunder may also be possible on Monday and Monday
   night.

&&

.DISCUSSION...
Issued at 342 AM CDT Sat Mar 23 2024

Short Term...Today...Tonight...and Sunday

High impact system begins to impact area Sunday afternoon. Until
then, seasonably cool and quiet.

Lingering clouds and flurries with weak trough exiting south of
Manitowoc. Just a few passing stratocu clouds dropping across
central and north-central as even with a NW flow of cold air
crossing Lake Superior, very dry airmass upstream has limited
lake clouds over northern WI. After the lingering clouds exit
east-central WI early, skies will be mostly sunny. A few more
clouds will develop with cooler air aloft and daytime heating.
Mix of sun and clouds covers it. Despite chilly start with
readings in the single digits central to north-central, temps
will rebound with more sunshine into the lower 30s on average.

Lows not as chilly tonight with increasing mid and high clouds
in broad warm air advection pattern downstream of deepening
western CONUS trough. Strong warm air advection and increasing
mid-level FGEN and isentropic ascent eventually overcomes dry
mid-levels to result in snow spreading across the region mainly
in the afternoon. Forecast soundings and HREF probs for 1"/hr
snowfall point to patchy light snow/flurries by early afternoon,
increasing deeper into the afternoon and toward evening with
western tier of counties seeing 50-80 percent probabilties of
exceeding 3" of snow in 6hr ending at 7pm. With wind gusts
increasing to over 25 mph, we`ll start seeing areas of blowing
and drifting snow develop. Winter Storm Watch has been issued
for those areas starting at 21z/4pm. Much more on that in the long
term as that is when the bulk of the heavy snow/blowing snow
occurs.

Long Term...Sunday Night Through Friday

Focus in this part of the forecast continues to be on the heavy snow
and blowing snow potential Sunday night into Monday on the leading
edge of a strong Rocky Mountain cyclone.

Sunday night and Monday...A strong cyclone remains on track to lift
northeast across the central Plains on Sunday night and into the
northern Mississippi Valley on Monday.  Ahead of this cyclone,
strong warm advection will be occurring through this period with
additional forcing via upper divergence in the right rear quad of a
jet streak on Sunday night followed by the left front quad of a
southern stream jet streak on Monday.  While forcing is substantial,
moisture isn`t lacking either as ensemble means point to
precipitable water values increasing to 175% of normal.  The highest
probabilities of precip rates of 1/4" per 6 hours will occur
northwest of the Fox Valley (50-70%) from 7 pm Sunday through 1 pm
Monday.

All this said, there remains significant differences regarding qpf
amounts and thermal profiles.  The general consensus over the past
48-72 hours was to surge the warm air aloft north across the area on
Monday morning, and this hasn`t changed in this forecast cycle.  As
a result, a change over from all snow to rain from south to north
remains the most likely outcome from late Sunday night through early
afternoon on Monday.  Therefore, the greatest risk for heavy snow
remains over north-central WI where the change over will occur last.

The GFS ensemble mean remains the highest with qpf during this time
period, and almost double the EPS and CMCE.  QPF in the ensemble
means has been trending upward over the past 24 hours of runs,
though it`s more drastic in the GFS ensemble mean than other
ensemble systems.   The large range of qpf is also apparent in the
operational models as well.

The practical approach is to remain conservative with qpf amounts
and towards the mean/median and minimize error.  This would put snow
amounts in the 6-10 inches range from central to north-central to
far northeast Wisconsin (north and west of the Fox Valley). In
combination with the snow, winds will gust to 30 or possibly 35 mph.
The highest probability of gusts greater than 30 mph will occur over
central WI which increase impacts even with lower forecast snow
amounts.

In the end, the combo of snow and wind warrants a Winter Storm Watch
from Marathon to northern Marinette county lines.  Timeframe will be
from 4 pm Sunday through 1 pm Monday.

Rest of the forecast...Precipitation changes to a wintry mix (north-
central WI) and rain for the region on Monday afternoon and Monday
night as the cyclone passes over northwest Wisconsin.  As colder air
returns on the backside of the cyclone, a change over back to snow
could lead to minor accumulations over north-central WI on Tuesday
into Tuesday night.

Colder and breezy conditions are then expected on Wednesday and
Thursday before temps warm back up ahead of the next storm system
for Friday.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1025 PM CDT Fri Mar 22 2024

Skies will clear from northwest to southeast overnight, with VFR
conditions and high based stratocumulus clouds Saturday afternoon
and evening. A winter storm will bring snow to the area Sunday. It
will likely change to rain Sunday night east of an Iron Mountain
to Stevens Point line. Significant snow accumulations are likely
where the precipitation remains all snow.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from Sunday afternoon through Monday
afternoon for WIZ005-010>013-018>021-030.
&&

$$

DISCUSSION.....JLA/MPC
AVIATION.......RDM


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