000
FXUS63 KGRB 260912
AFDGRB

Area Forecast Discussion
National Weather Service Green Bay WI
412 AM CDT Fri Apr 26 2024

Forecast discussion for routine morning forecast issuance

.KEY MESSAGES...

 - Elevated fire weather conditions are expected today over northern
   Wisconsin as relative humidity values fall to around 25
   percent and southeast winds increase to 15 to 20 mph with gusts
   to 30 mph. Rain arriving this evening will bring some relief
   to the fire concerns.

 - Two systems will result in several chances for showers and
   thunderstorms tonight through Monday. The main hazard with the
   storms tonight into early Saturday will be small hail and gusty
   winds. There is a potential for strong to severe storms with
   large hail and damaging winds Saturday afternoon into Saturday
   evening, though coverage of storms is still uncertain.

 - Storms on Sunday into Sunday night could produce periods of
   moderate to heavy rain, resulting in chances for isolated flash
   flooding.

 - Conditions hazardous to small craft will be possible at times
   tonight through the weekend on the Bay and Lake.

&&

.DISCUSSION...
Issued at 411 AM CDT Fri Apr 26 2024

Short Term...Today...Tonight...and Saturday

One more day of fire weather concerns, then convection potential
is main story tonight and Saturday.

SE flow is steady early this morning on western edge of high
pressure now centered over southern Ontario up the St. Lawrence
Valley. Dry low-levels tied to the high will hold off most of
showers today ahead of vigorous shortwave trough, deep sfc low
over the central Plains. Given the slower arrival of showers,
elevated fire weather concerns will linger for one more day from
northwest of the Fox Valley on into northern Wisconsin.

Showers will not arrive until very late this afternoon, so a
slower trend continues. Currently, showers and storms are on the
edge of sharp H85 moisture gradient. This will mainly lift across
our region tonight as warm front aloft arrives from the south
ahead of negative tilted mid-level low moving into eastern
SD/southwest MN. Expect a period of showers and storms to lift
south to north across WI, with coverage tailing off after midnight
over southern CWA. Generally, elevated CAPEs and effective shear
have diminished (MUCAPE around 300J/kg and shear less than 30
kts), so think this takes us out of much risk of large hail. Even
so, could see smaller hail (pea to nickel sized) in the isolated
strong storms as mid-level lapse rates are aoa 7c/km. Gusty winds
not out of question as well with stronger winds aloft of decent
low-level inversion. All of these expectations should be covered
with the age-old adage in meteorology to never trust a warm front.
Thus, would not be shocked if an isolated severe storm occurs as the
warm front moves through. Temps will rise late as the warm front
approaches which then brings us to Saturday.

Lingering elevated convection should be on the slow exit northward
in the morning over northern and far northeast WI across the Door
Peninsula. These storms too could carry a small hail threat. By
early afternoon, initial mid-level trough will be over western
Lake Superior while next shortwave trough will be well upstream
over CO/KS. That leaves much of WI within weak shortwave ridging
and sure enough, forecast soundings show capping H8-H7 even over
central to east-central WI ahead of approaching cold front and
into what will be a moderately unstable warm sector (temps mid-
upper 70s and dewpoints into the lower 60s). If convection
initiation can occur, MLCAPES at least 1000J/kg and effective
shear at least 40 kts will support organized convection. From
this perspective it appears larger scale forcing and capping may
limit storm coverage but any storms that do form will be strong to
severe, so a conditional setup. SPC Day2 outlook maintains the
slight risk for severe for large hail and damaging winds and we
continue to message the severe potential at this time. Being
farther away from forcing ahead of the cold front and not as much
into the unstable airmass, north-central WI will see less of a
chance of any showers and storms by Saturday afternoon.

Long Term...Saturday Night Through Thursday

The first of two systems will move across the area Saturday night. A
cold front will move across northeast Wisconsin Saturday evening,
bringing the chances of showers and thunderstorms to the area. The
biggest concern tonight is the bufkit soundings are showing a warm
layer between 700-800mb that could inhibit or reduce the cover of
storms as the cold front moves across the area. Except for the NAM,
models are not impressive with QPF totals which would suggest isolated
to scattered coverage of storms if it can break the warm layer. The
NAM was considerably more bullish with activity along the cold front.
If storms do get going, CAPE values of 1,000 to 2,000 J/KG and 0-6km
shear values of 35 to 45 kts will support strong to severe storms.
The best chance for the strong or severe storms will be across
northeast Wisconsin, especially along the Interstate/Highway 41
corridor east to the lake.

The front is expected to sag southward Saturday night, likely being
aided by outflow boundaries from any convection and a more dense
air mass as winds turn northeast off the colder waters of the bay
and Lake Michigan. Latest MET/MAV guidance indicating northeast
winds sustained at 12 to 16 kts at Green Bay by 12z Sunday. With
this in mind, the boundary should be well south of the area Sunday
morning, which may impact the northward extent of the thunderstorm
activity. For the morning hours, only have a slight chance of storms
south of Highway 29. With easterly flow, thick cloud cover and
rain expected through much of the day, lowered high temperatures
by 4 to 7 degrees. The warm front will try to approach our
southern counties by late in the day, which will result in the
chances of storms pushing northward.

The better chances for stronger storms will be Sunday night
as the warm front lifts northward across the area. The cold front
then is expected to move across the area on Monday which will
bring continued chances of showers and storms. If this feature
slows down any, will need to consider the possibility of stronger
storms across eastern Wisconsin due to the heating of the day
and better CAPE values, especially if there is some partial
clearing. Dry conditions are expected Monday night before the next
system brings another chance of showers and storms on Tuesday
afternoon and evening. For Wednesday into Thursday, there are some
model difference on when a system moves across the area. Did not
make any significant changes in this time frame until there is
better resolution in the timing of this system.

&&

.AVIATION...for 06Z TAF Issuance
Issued at 1020 PM CDT Thu Apr 25 2024

Dry conditions will prevail until mid-afternoon on Friday, with
increasing high/mid clouds. Showers are expected to overspread
the region from SW to NE mid-afternoon through early evening,
with numerous showers and scattered thunderstorms impacting the
region Friday night. Conditions should remain VFR until the
steadier showers arrive Friday evening, then deteriorate to MVFR
and localized IFR conditions thereafter.

Southeast winds will increase and become quite gusty by late
Friday morning. LLWS is possible at the western TAF sites toward
daybreak/early Friday, then regionwide Friday night.

&&

.FIRE WEATHER...
Issued at 411 AM CDT Fri Apr 26 2024

Elevated fire weather conditions are expected today as the rain
showers are further delayed across the region. North-central and
northeast Wisconsin will continue to see elevated fire weather
conditions as relative humidity values fall to as low as 25
percent during the afternoon while southeast winds of 15 to 20
mph with gusts to 30 mph occur along with temperatures near 60.
A special weather statement has been issued to highlight the
elevated fire weather conditions. Rain showers finally arriving
during the evening will bring some relief to fire concerns.

&&

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

&&

$$

DISCUSSION.....JLA/Eckberg
AVIATION.......Kieckbusch
FIRE WEATHER...JLA