799
FXUS63 KGRB 201143
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
643 AM CDT Sun Apr 20 2025

Updated aviation portion for 12Z TAF issuance

.KEY MESSAGES...

- Widespread precipitation arrives this afternoon/evening and
  continues into Monday with rainfall totals of 0.75-1.20" for
  most locations. Some river rises are expected early this week,
  but widespread flooding not expected.

- The rain will mix with or change to snow across north central
  Wisconsin late tonight into Monday, with a minor accumulation
  expected on grassy surfaces and possibly some slushy roads.

- Cooler temps arrive to start the new week, but a warm up is on
  tap by Wednesday, with periodic rain (storm?) chances through
  the week.

&&

.DISCUSSION...
Issued at 347 AM CDT Sun Apr 20 2025

Today into Monday:

Any patchy fog will lift/burn off shortly after sunrise this
morning. Otherwise, look for dry and quiet conditions through
early afternoon. Then, as strong WAA/isentropic lift, a slug of
moisture (PWATs to ~1"), and a LLJ develops ahead of a low
pressure system and shortwave over the central Plains, a
widespread area of rain will spread into the area later this
afternoon and evening. The steadier rain looks to reach the Hwy 29
corridor between 5-7pm and the Hwy 8 corridor between 7-10pm this
evening. Periods of showers will continue overnight with
additional rain likely on Monday as the low pressure and shortwave
move overhead. Most of the rain will exit from southwest to
northeast by sunset Monday, except for the far north/east where
some precip could linger into the early Monday evening. Elevated
instability (MUCAPE up to ~500 J/kg) arrives this evening into
Monday morning, bringing the chance for some embedded storms.
Probabilities for over a half inch of rain running between 80-100%
(lowest in east central WI) and greater than an inch ~40-60%
(highest in central WI). This supports most locations seeing
between 0.5-1.20", but a few 1.25"+ amounts are possible, mainly
north and west of the Fox Valley. HEFS guidance for Babcock (near
where the heavier rain the last few days and the highest chance
for flooding) showing ~1.5" of rain needed to get near flood
stage. So with under an inch of rain for most, rivers having a
couple days to recover, and river levels pretty low in general, do
expect some river rises and ponding on some roads and/or low
lying areas, but not expecting any major hydro concerns.

As for the wintry part of this system, temp profiles show all rain
with the initial surge of precip, but as the low pressure
approaches and winds turn to the NE/N late tonight into Monday,
colder air will get ingested into north central WI. Soundings
showing nearly isothermal profiles at times overnight, hovering
around the 0 degree mark. This will make for a tricky rain/snow
forecast, plus a small warm layer looks to be present for a time,
bringing in the chance of a little sleet as well. Precip intensity
could allow for a switch over to all snow as well. Will keep a
rain/snow mix starting after midnight and especially toward the
pre-dawn hours, then a rain/snow mix most of Monday, with some
periods of all snow possible. Probability for 1"+ ticking up to
40-60% and 2+" still ~10-30%, so will increase totals across
north central WI, especially Vilas Co., where we will have
0.5-1.5" going. Higher totals are possible if a change over to all
snow occurs earlier or for a longer period. Still think the
majority of the accumulation will be on grassy surfaces, with road
surface temps not forecast to drop below 40, but some slushy
roads are possible where the snow occurs the longest. WSSI-P only
showing some 10-40% chances of minor impacts, likely due to the
warmer/wet ground and strong April sun.

Southeast/east winds will gust up to ~30 mph later today into
tonight, decreasing for a time on Monday when the low moves
overhead, then shift to the west/northwest by Monday afternoon as
the low departs and becoming gusty again. The gusty winds, along
with building waves, will bring hazardous conditions on Lake
Michigan and the bay.

Highs today will climb into the 50s across of the area, with
cooler 40s near the shores of Lake Michigan. The precip and clouds
will help hold up lows tonight, with most spots in the in the low
to mid 30s (north) and low 40s (south). It will be cooler on
Monday as CAA dominates the region, with highs ranging from the
upper 40s to near 40 in north central WI to the upper 40s to
around 50 in east central WI.

Rest of the Week:

A fast/zonal flow over the northern CONUS will promote an active
pattern with multiple chances for rain and a few storms. The next
low pressure and shortwave arrives late Tuesday morning into
Tuesday evening, bringing chances for showers and perhaps a few
storms. This will be a weaker system with less moisture, so
expected more isolated to scattered activity. A stalled frontal
boundary, approaching low and eventual cold frontal passage will
keep chances for showers, and a few storms mainly over the south,
on Thursday and Friday. If current timing holds, high pressure
rolls into the Great Lakes on Saturday, bringing dry conditions.

Temps will rebound back closer to normal on Tuesday with above
normal readings expected on Wednesday, when most of the area will
see highs in the mid 60s to low 70s. Temps look to drop back
closer to normal for the end of the week.

&&

.AVIATION...for 12Z TAF Issuance
Issued at 643 AM CDT Sun Apr 20 2025

Patchy low clouds will continue this morning, bringing
LIFR/IFR/MVFR ceilings to a few spots, especially at MTW and
south/west of CWA and ATW. Otherwise, look for VFR conditions
through early afternoon. Conditions will deteriorate to IFR this
evening as a low pressure system and widespread rain moves in from
the south. Rain and fog will is expected much of tonight into
Monday morning. Some thunder will be possible, especially across
east central WI. Will include some PROB30 groups for GRB/ATW/MTW.

Southeast winds will gust to 20 kts today and up to 30 kts
tonight. When surface winds drop under 20 kts, LLWS is expected
this evening and continue into Monday morning winds around 2000 ft
increase to ~45 kts.

&&

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

&&

$$

DISCUSSION.....Bersch
AVIATION.......Bersch