644
FXUS63 KGRB 250519
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
1219 AM CDT Fri Apr 25 2025

Updated aviation portion for 06Z TAF issuance

.KEY MESSAGES...

- Rain increases over north central WI into this evening, then
  spreads southeast across the forecast area overnight into
  Friday morning. There is a 40-80% chance of a half inch of
  rain northwest of the Fox Valley, with lower amounts expected
  southeast.

- Northeast to north winds will gust to 25 to 35 mph Friday into
  Friday night, especially in the Fox Valley and lakeshore areas.

- Warm and potentially stormy weather is expected early next week.

&&

.DISCUSSION...
Issued at 230 PM CDT Thu Apr 24 2025

Thu afternoon through Friday night:

700mb moisture continues to keep scattered to broken clouds through
this afternoon with drier air near the surface to 850mb remaining in
place keeping the region dry overall. However, the low levels will
start to moisten back up later this afternoon as showers begin to
push in from the west that are currently in east-central MN.
Models indicate that drier air over the area may fight the incoming
moisture off for a bit but will eventually be overcome later this
afternoon and certainly into the evening. The forcing will also
increase later this evening with an upper level shortwave sliding in
and some low level WAA. Showers will start pushing in with this
moisture and increased forcing and will overspread the area
overnight into Friday morning. No storms are expected with this as
MUCAPE remains 0 across the area, the only slight chance (10%)
for a rumble of thunder would be in the southwest portions of
the CWA.

Showers will gradually push out over the course of the day Friday as
the shortwave and corresponding mid level moisture pushes out.
Moisture and thus clouds may linger a bit longer but precip should
end by the mid to late afternoon across the CWA. Friday night higher
pressure will be pushing in behind this system with the upper low a
bit more delayed but lack of moisture aloft will likely keep
things largely dry overnight. The most recent models have kept
some precip chances Friday night with the upper low tracking
through. There does look to perhaps be a strip of moisture
associated with it which could yield a brief period of showers
though lower levels may prevent much in the way of showers from
reach the ground which is why we only included 20-35% chances
Friday night. Into Saturday morning large scale ridging will
become the dominant upper level feature, drying things out.

Saturday through Thursday night:

This weekend looks likely to be dominated by large scale ridging and
high pressure with dry conditions expected. The only slight change
to that is the potential for some shower activity Sunday
evening/night as a low pressure system nudges in from the west. The
upper levels keep the ridge in place with perhaps some weak
shortwave activity embedded in the ridge. Clouds seem likely with
this especially will 700mb level moistening up as well but the lower
levels remain fairly dry suggesting as this pushes in dry air could
really fight off any rain potential for quite awhile into the
evening before anything would start to fall. In addition, overnight
storm chances would be possible with this as some instability slides
in with this system but any stronger storms would likely be further
west with the better shear.

Into Monday it is becoming increasingly likely that some shortwave
activity aloft could bring some showers and storms during the day
Monday. This may play a role in what might occur later in the day
and especially into the evening but as the surface low pulls through
WI, models indicate the potential for strong storms across parts of
WI. The better severe potential is likely located further south and
west but the southern portions of the CWA will definitely see risks
for strong to severe storms as well Monday evening and night.
Instability will be at least 1000 J/kg and according to models
appears likely to remain surface based well into the evening. In
addition deep layer shear looks likely to be in the 50-60 kt range
certainly keeping things organized into the late evening hours,
especially with the increasing LLJ into the evening.

The question remains around the timing of storms and how much
instability we will still have and whether it will be surface based
by the time it comes through. Currently this remains a situation
with all hazards being possible. CWASP values around 0.8-0.9
somewhere in WI suggests the potential for strong storms with
tornadoes being a hazard. The decreasing instability in models into
the evening also suggests possibly less of a hail threat but with
the main threat likely being wind depending on storm mode. There
still remains some uncertainties however, especially as it relates
to timing because slight shifts in timing could greatly alter what
our potential ends up looking like.

Into Tuesday the front will have come through, very likely
preventing chances for stronger storms. Showers will be possible on
the backside of the low but overall by the evening higher pressure
will be pushing in behind this system with weak ridging overhead.
Models keep things dry Wednesday with actually fairly good agreement
overall. Thursday looks like there will be potential for activity
with a developing low overhead and a shortwave aloft. Behind that
shortwave into Friday models start to diverge a bit, especially in
the upper levels. While large scale ridging will likely be the
dominant feature the shape and track of the ridge starts to change
and will likely impact precip trends into the weekend.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1218 AM CDT Fri Apr 25 2025

Light rain with VFR cigs prevailed across all TAF sites, except in
east-central WI late this evening where dry conditions prevailed.
The light rain will gradually fill in and spread eastward by early
Friday morning, paired with MVFR/IFR cigs. Conditions will improve
from northwest to southeast Friday afternoon and evening as the
rain exits. Increased NE winds, with occasional gusts to 20 kts,
are anticipated tonight, becoming more northerly by Friday
afternoon with gusts to 20-30 kts. Winds subside by the end of
this TAF period.

KGRB/KATW...VFR conditions will continue at these sites with
increased NE winds and occasional gusts to 20 kts through early
Friday morning. A few light showers or sprinkles may move over
these TAF sites during the few first hours of this TAF issuance,
however, confidence was not high enough to include in the TAF
given the scattered nature seen on radar upstream. Some
uncertainty remains for the onset of the better chances of rain
for Friday morning. Therefore, continued with a PROB30 for rain
between 10z to 12z. However, there does appear to be better model
agreement with the steadier rain falling between 15z to 22z, when
IFR cigs are likely. This time period will also be when the
northerly wind gusts will be highest between 20-30 kts. Following
the ending of the rain after 22z, conditions will gradually
improve into Friday evening with the return of VFR conditions.

&&

.MARINE...
Issued at 1105 AM CDT Thu Apr 24 2025

Tonight we will gradually see a low pressure system lift
northeast through the southern Great Lakes area. This will
increase the pressure gradient, especially Friday afternoon and
evening bringing 20 to 30 kt northerly winds to the nearshore.
Thus a Small Craft Advisory has been issued for these breezy
northerly winds from 1am Friday until 4am Saturday and possibly
lingering a bit longer. Waves are not expected to be a concern
given the northerly wind direction.

&&

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

&&

$$

DISCUSSION.....WFO MKX
AVIATION.......Kruk
MARINE.........WFO MKX

NWS GRB Office Area Forecast Discussion