Area Forecast Discussion
Issued by NWS Green Bay, WI

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FXUS63 KGRB 211142
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
642 AM CDT Thu Mar 21 2024

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

 - Widespread snowfall is forecast to impact the area from tonight into
   Friday afternoon. There is a 40-80% chance of at least 4
   inches of snow accumulations across the area, with the greatest
   chance over central to east-central Wisconsin. Hazardous
   travel is expected on Friday morning, particularly for the
   morning commute.

 - There is increasing confidence that another strong system could
   provide wintry weather late on Sunday into Monday. There is a
   50 to 80% chance of at least 3 inches of snow accumulations
   across the area, but confidence is low to medium with these
   percentages.

&&

.DISCUSSION...
Issued at 337 AM CDT Thu Mar 21 2024

Short Term...Today...Tonight...and Friday

Early spring snow will result in a slow and potentially difficult
Friday morning commute. Advisory issued for all but the far north
central.

High pressure is bringing dry and chilly start to the day. Mid
clouds in warm air advection pattern downstream of clipper shortwave
that brings the snow to the region tonight are spilling over MN
and WI. Plan on intervals of mid-high clouds and some sun today
with highs as warm as the mid 30s. Temperatures drop back solidly
into the 20s this evening as wave approaches. FGEN well ahead of
the system may result in some snow showers into southwest forecast
area by late evening. Otherwise, main bulk of light to moderate
snow arrives just after midnight, quickly spreading from west to
east. Snowfall rates mainly up to 0.5 inch, but boosted by higher
SLRs could see rates 0.5-1.0" per hour occasionally, especially
over southern portions of the area closer to stronger warm air
advection and isentropic ascent. Bulk of widespread snow will
taper 12-15z on Friday especially west and north of GRB. Exception
will be across Door County, where with H85 temps -11c and SE low-
level flow, there could be some minor lake enhancement.

Overall, recent models trended farther south with primary swath
of QPF over 0.50+" now over especially south quarter of WI more
over ARX and MKX forecast area. This makes sense as that is where
strongest low-level moisture transport and FGEN reside. Some
models are even farther south with this primary axis. That
adjustment in guidance is likely what will keep any portions of
our area from seeing warning amounts. At least as it stands now.
Given the adjustments in QPF and taking into account period of
SLRs pushing toward 18:1 overnight as forecast soundings point
to, ended up with fairly uniform snow amounts of 2-4" for all but
the far north. Highest probabilities of amounts over 5" (30-50
percent) just clip the southern part of our area, south of highway
10. So not looking at a big snow event, but given the lack of
snow (last weaker system was in mid February) and the impact this
will have on the Friday morning commute to work and school, have
issued winter weather advisory for all but the far north-central
from 6z-18z/1am to 1pm. Once the heavier snow pulls out of the
area shortly after daybreak Friday, anticipate that more traveled
and treated roads will likely become just wet given the increasing
sun angle and as temps warm into the mid 30s. Until that point
though, the morning commute at the least will be slow and
potentially could be difficult and it certainly will be a good
idea to give yourself extra time before heading out Friday
morning.

Long Term...Friday Night Through Wednesday

Focus in this part of the forecast remains on the impacts from a
large cyclone forecast to pass across the region Sunday through
Tuesday.

Friday night through Saturday night...Generally quiet weather
returns during this period.  High pressure will be building into the
region from the northwest.  Precip from earlier in the day will have
mostly exited to the east by the start of Friday evening.  But as
winds turn to the north, flow off Lake Superior will lead to clouds
surging south across the area along with light snow showers or
flurries near the U.P. border on Friday night into Saturday morning.

After brief clearing on Saturday afternoon, clouds will return
rather quickly on Saturday night.  Increasing isentropic ascent will
lead to sufficient saturation for light snow to move into north-
central Wisconsin after midnight.

Rest of the forecast...As a strong low pressure system emerges and
travels east across the central Plains, strengthening south flow
aloft will lead to increasing frontogenesis within a broad coupled
jet structure from Sunday afternoon through Sunday night.  As a
result, precip should expand in coverage and increase in intensity
during this time, particularly northwest of the Fox Valley and away
from the relatively drier air emanating out of the departing high.
Critical thickness values would suggest a thermal profile conducive
for wintry precipitation over central, north-central, and far
northeast Wisconsin.  QPF of a quarter to a half inch would suggest
several inches of snow will be possible during this time before
warming temps aloft push critical thickness values north of the U.P.
border.  Some potential for a wintry mix to begin by Monday morning,
including freezing rain. The Sunday afternoon through Monday morning
time period will need to get monitored for impactful winter weather.

As low pressure travels northeast and across the northern
Mississippi Valley and northwest Wisconsin, precip type is forecast
to transition to a wintry mix or all rain on Monday afternoon and
evening before ending on Tuesday with light snow over north-central
Wisconsin.  Additional QPF upwards of a half inch will be possible
during this time frame with only minor accumulations of snow
possible.

Colder air will return behind the cyclone for Tuesday and Wednesday.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 641 AM CDT Thu Mar 21 2024

Light winds with intervals of mid and high clouds across the
terminals through the evening. Clouds then thicken into the early
overnight hours with widespread snow and IFR conditions arriving
at all terminals after midnight. Though visibility will be IFR,
appears ceilings will remain low-end MVFR. Even as the snow
arrives, winds will remain light.

Snow will taper light snow and flurries across the region from
west to east on Friday. Plan on mainly IFR conditions in the
morning with improvement to MVFR in the afternoon.

&&

.GRB WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 1 AM to 1 PM CDT Friday for WIZ010-
011-013-018>022-030-031-035>040-045-048>050-073-074.
&&

$$

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


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