081
FXUS63 KMKX 211538 AAA
AFDMKX

Area Forecast Discussion...UPDATED
National Weather Service Milwaukee/Sullivan WI
1038 AM CDT Sat Sep 21 2024

.KEY MESSAGES...

- Thunderstorms developed over northeast Iowa earlier than
  anticipated this morning, so we increased storm chances from
  Madison and west earlier in the day.

- Another day with above normal/summer-like temps in the mid to
  upper 80s today. However cooler/near normal temps are
  expected behind the cold front Sunday through the start of the
  week with daily highs in the upper 60s and lower 70s.

- Increasing shower and thunderstorm chances (50-80%) return
  this evening through Sunday. Cannot rule out a stronger
  thunderstorm or two capable of producing gusty winds this
  evening west of I-39 corridor, but confidence remains on the
  lower end.

- Most of southern WI will see some beneficial rainfall from
  the cold front passage with amounts generally ranging from
  0.1-0.7 inches with locally higher amounts closer to the WI/IL
  border or with any stronger thunderstorms.

&&

.UPDATE...
Issued 1037 AM CDT Sat Sep 21 2024

Convection over the Twin Cities is being supported by 925-850mb
warm air advection on the nose of a decaying low level jet. Mid
level clouds just started becoming bubbly over northeast Iowa
(accas clouds) and thunderstorms developed quickly in the
Decorah area. These elevated storms are associated with 850mb
warm air and moisture advection and are supported by over 1000
j/kg elevated CAPE and 30 kt of bulk shear, along with a weak
ripple in the upper level flow (vorticity advection). This
development is much earlier than anticipated, so I will be
amending our forecast to add thunderstorm chances west of
Madison midday. The eastward extent of these storms is
uncertain, but the low level jet axis is progged to across most
of southern WI this afternoon. The waa is expected to weaken
with its eastward extent, so I am not anticipating convection to
get much farther than Madison. The 06Z GFS might have captured
what is happening pretty well.

Cronce

&&

.SHORT TERM...
Issued 409 AM CDT Sat Sep 21 2024

Today through Sunday night:

As the surface high pressure gradually slides east through the
morning, expect the dry airmass in place continue to limit fog
development through daybreak. Also seeing some scattered light
reflectivities pop up with the low-level WAA upstream along the
Mississippi River Valley early this morning. However, that dry
air will limit any of this activity from precip as it pushes
eastward across the CWA through the morning.

Expect southerly flow to pick up through the day ahead of the
incoming cold front. Will once again see summer-like temps in
the mid to upper 80s this afternoon and given yesterday`s trend,
would not be surprised to see a few locations creep toward 90F.
Accompanying the warmer temps will be gradually increasing
precip chances. While the 00z CAMs, such as the HRRR suggest we
could see showers and storms develop as early as later this
afternoon, the model trend favors the drier air to win out and
limit the earlier development, especially as the forcing lingers
further west at this time as the 06z runs suggests. While the
06z HRRR still seems to bullish developing a line of showers
between 00-06z that is able to tap into the instability with
mean MUCAPE values around 1000 J/kg from the afternoon, still
seeing disagreement between the the models with timing and
forcing. I think the biggest hurdle for this early evening
thunderstorm activity is the timing of upper-level dynamics just
being offset from the surface forcing. Nevertheless, if the
mid- level shortwave trough ejecting out ahead of the main mid-
level trough out across the Plains matches up with the timing
of the surface front better, then we could see a few stronger
thunderstorms develop this evening. If this ends up
materializing, given the model forecast sounding, could see a
few storms capable of producing gusty/downburst type winds given
the the dry air aloft paired with the elevated instability.

Otherwise, the better upper-level forcing (right entrance
region of the upper-level, 500mb dCVA, 700mb frontogenesis)
tends to align better overnight after 09z through Sunday
morning. The overnight timing of this forcing would limit the
stronger thunderstorm potential (diminishing instability and
deep-layer shear <35 kt), but would favor more widespread shower
activity through Sunday morning as the front pushes east.
Models prog the cold front to clear the CWA from west to east
through Sunday afternoon with drier conditions establishing
themselves by the evening.

Overall, pinpoint exact rainfall amounts remains a challenge
given the aforementioned issues of timing and alignment of the
forcing. nevertheless, there will be the potential for most of
the area to see some much needed rainfall. Areas generally north
of the US-18/I- 94 favor the lower amounts ranging from few
hundredths to around a quarter of inch. Meanwhile areas along
and south of US-18/I-94 have a 50-70% chance of seeing up to
around a half an inch or more, while the higher potential to see
an inch or more remains south of the Cheddar Curtain in IL.
However, if any stronger thunderstorms materialize and/or
rainfall trains over the same area then we could see some
localized higher rainfall amounts nearing the one inch mark.

Wagner

&&

.LONG TERM...
Issued 409 AM CDT Sat Sep 21 2024

Monday through Friday:

While Monday looks to start of dry with building surface high
pressure and post cold front conditions, there are hints of the
mid- level trough working its way through the area. This mid-
level disturbance paired with some lingering moisture may spur
some scattered shower activity on the backside of departing
surface trough and northside of another incoming surface low
into Tuesday. Thus low-end shower chances (<40%) linger Monday
and Tuesday.

Behind the weekend cold front expect a cooler, Canadian airmass
to settle across the region for the start of week. Building
high pressure Monday will bring temps more in line with normal
for this time of year with daily highs in the the upper 60s to
lower 70s and nightly lows in 40s-50s.

Otherwise midweek onward, the pattern becomes a bit more
uncertain given the long range model difference and variability.
However, the general pattern as suggested by the WPC Cluster
Analysis does favor upper-level ridge building over the Upper
Midwest through the later half of the week, which would bring a
return of above normal temps. There are hints from the 00z
deterministic models where an undercutting mid-level trough sets
up somewhere south of our neck of the woods. So will monitor
trends to see how the pattern shapes up in the coming days.

Wagner

&&

.AVIATION...
Issued 1037 AM CDT Sat Sep 21 2024

Thunderstorms developed over northeast Iowa earlier than
anticipated this morning. They should not reach Madison before
mid afternoon and I am not sure they will hold together, but
confidence is low. Thunderstorm chances will increase from late
afternoon through the evening hours and continue through Sunday
morning. It will be hard to pin down a 4 hour period when they
are most likely at each TAF site due to the scattered nature of
the storms and lack of organized forcing. Showers and storms are
most likely just ahead of and along a cold front that will be
slowly crossing southern WI Sunday morning through afternoon.
Severe storms are not anticipated although a few stronger gusts
are possible.

Cronce

&&

.MARINE...
Issued 409 AM CDT Sat Sep 21 2024

Expect southerly winds to increase through the afternoon as low
pressure lifts northeastward up through MN into Ontario, which
will in turn drag a cold front across WI and Lake Michigan
overnight tonight into Sunday. Expecting to see showers and
thunderstorms to develop along the cold front and track across
the Lake through Sunday. Will then see southwesterly winds turn
northerly behind this front overnight Sunday into Monday. Given
the strong cold air moving over the warmer lake waters,
expecting to see gusty northerly winds during this timeframe
with gust up to 30 knots and a few gale force gusts cannot be
ruled out, especially for the southern portions of Lake
Michigan. Will see winds weaken a bit for Monday as high
pressure builds in across the Great Lakes region, but the
northerly component will persist through the start of the week.

Wagner

&&

.MKX WATCHES/WARNINGS/ADVISORIES...
WI...None.
LM...None.
&&

$$

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