054
FXUS63 KARX 011936
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
236 PM CDT Sat Jun 1 2024

.KEY MESSAGES...

- Fog looking likely tonight with questions on how widespread,
  how dense it could be.

- Showers and storms overnight Sunday and Monday morning.
  Daytime hours on Sunday will likely remain dry for most.

- Additional storms likely (50-80% chance) on Tuesday afternoon
  and evening. Monitoring the potential for organized severe
  weather.

&&

.DISCUSSION...
Issued at 236 PM CDT Sat Jun 1 2024

* FOG TONIGHT - how widespread, how thick is uncertain. Dense fog
  advisory not out of the question.

Challenging forecast in terms of fog for tonight and accompanying
impacts on the vsby. Morning, early afternoon rain with clearing
just in time for sundown, coupled with light winds at the sfc and
near sfc, lay the ground work for fog development. Generally,
scenarios like this tend to form the fog earlier in the nighttime
hours (closer to midnight as opposed to daybreak the next day),
potentially lifting earlier as winds start to stir toward daybreak.
Also,  is it widespread or more confined to the river valleys
(soundings look like a typical fall fog setup)?

Latest SREF suggests most of the fog will develop from along the
Mississippi River eastward with 40-60% chance for 1 mile or less
vsbys. HREF paints widespread, but lower impacts fog (mostly 2-4
mile vsby). However, it suffers from differences in the short term
models that make it up. For example the FV3 says "no fog" while the
HRRR and NSSL have suggest sub 1 mile is more likely. Blend those
altogether and you get the HREF. Like the outlay of the HREF and
leaning more into the SREF and HRRR for vsby. Outcome would be
widespread fog with a lot of 1-3SM mile vsbys, but also increasing
chances for 1/4SM dense fog.

Still some uncertainties on how this all plays out and will monitor
trends closely as we move into the evening. Might need a dense fog
advisory for portions (much?) of the area.

* SUNDAY NIGHT/MONDAY: Showers and a Few Storms

As we head into Sunday night, a shortwave trough will progress
through the region bringing convection on the nose of some warm
advection associated with an increasing low-level jet. Consequently,
expecting showers and storms to pass through the region during the
overnight hours Sunday and into Monday morning as shown in many of
the recent CAMs of which have been trending later with the bulk of
precipitation towards Monday morning. Currently, the setup with this
convection is not super impressive with timing during the overnight
and morning hours keeping instability fairly limited (200-500 J/kg
of MLCAPE). As a result, not expecting any organized strong to
severe storms but cannot rule out some gusty winds with the 850mb
jet to around 35-40 kts. Additionally, could still be a fairly
healthy rain event as well with high probabilities (70-90% chance)
for over 0.1" of precipitation and moderate probabilities (30-50%
chance) for 0.5" in the local area.

* TUESDAY/TUESDAY NIGHT: Warmer & Another Round of Storms,
  Monitoring for Severe Potential

The synoptic setup for Tuesday features a 500mb ridge sitting
progressing eastbound ahead of an incoming trough and developing
surface cold front ejecting out of the Northern Rockies and Upper
Midwest. Subsidence under the ridge during the afternoon and
southerly surface flow will allow for temperatures to climb well
into the 80s with the 01.00z grand ensemble (GEFS/EC ens/Canadian
ens) showing an inter-quartile range for maximum temperatures at La
Crosse of 79 to 84 degrees for Tuesday. Consequently, expecting
Tuesday to be the warmest day of the forecast period at this time.

Additionally, southerly flow ahead of the aforementioned surface
cold front will allow for fairly robust moisture advection with the
01.12z NAM/GFS having ample 850mb moisture transport. Therefore,
expecting dewpoints and precipitable waters to increase into the
afternoon with deterministic guidance showing dewpoints in the upper
60s to lower 70s along and south of I-90. Corresponding instability
profiles from the increased moisture would be fairly conducive for
convection with the 01.12z GFS having respectable values for MLCAPE
(1500-2500 J/kg). The larger point of uncertainty in this instance
would be the shear profiles. Currently, deterministic guidance has
some differences in the orientation of the aforementioned 500mb
trough and how this interplays with the surface cold front. As a
result, confidence is low in how bulk shear profiles will
manifest ahead of the front. Additionally, the 01.00z grand
ensemble (GEFS/EC ens/Canadian ens) joint probabilities for over
500 J/kg of CAPE and 30 kts of bulk shear are relatively low
(20-40% chance) for the local area. These probabilities are
heavily CAPE driven. Will have to watch how shear profiles
resolve in short-range guidance though with the CSU machine
learning probabilities painting 15%+ mention in our local area
which combined with the favorable instability profiles certainly
means Tuesday will need to be monitored in upcoming forecast
iterations.

Regardless of severe potential, confidence is increasing for healthy
rainfall later Tuesday with the 01.12z GEFS/EC ensemble having high
probabilities (50-80% chance) for at least 0.5" of precipitation for
Tuesday. Furthermore, locally heavy rainfall may be a concern with
warm cloud depth approaching 4km and preciptable waters increasing
to around 1.75" to 2" in the 01.12z NAM/GFS runs. Storm motions will
be fairly progressive so any flooding concerns appear minimal at
this time. In any case, have opted to keep likely mention for
precipitation chances in accordance with the NBM on Tuesday
evening.

* LATE WEEK: Trending Cooler and Some Shower Chances

With the passage of the previously stated cold front, guidance
overall agrees on temperatures cooling off into the later portions
of the upcoming week with very broad trough developing within the
vicinity of the Great Lakes region. This would allow for northwest
upper-level flow allowing for some shower chances and overall
pleasant temperatures for this time of year with strong consensus in
ensemble guidance for highs to remain in the 60s and 70s across
the local area.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1220 PM CDT Sat Jun 1 2024

CIGS/VSBY: some hints for a short (2 hour) window of MVFR cigs this
afternoon, but most meso models favor keeping 3k+ ft for cigs and
will stick with that. Should see clearing by 00z with mostly SKC/SCT
conditions through the overnight.

Questions arise with fog and stratus potential overnight. Mixed
signals in some of the short term guidance, but with rain and
clearing skies/light winds, setup is favorable for fog formation. Is
it widespread or just river valley is another question - which isn`t
clear. For now, probabilities favor along the Mississippi River
eastward, but will bring fog in for both TAF sites. See the
potential for sub 1SM, moreso at KLSE where the RAP/HRRR soundings
mirror what a fall valley fog event would look like. If thick fog
manifests, it would eventually lift as a low stratus layer,
continuing LIFR/IFR conditions into mid morning Sun. How this
evolves will be closely monitored. Anticipate updates to the
forecast based on trends as we move through the evening hours.

PCPN: temp discontinuity will result in a weak sfc boundary
developing in between the sunny/cloudy regions on the western flank
of the current cloud shield. This will work in a ribbon of
instability to spark isolated shra/ts as we move through the
afternoon. Coverage is very spotty, but this has been a consistent
signal in the meso models over the last 24 hours. Won`t add to the
forecast but will monitor and adjust forecast if a TAF site
becomes more likely to be impacted.

WINDS: light wind field, mostly south to southeast through the
night. Should see an uptick in speeds later Sunday as pressure
gradient starts to tighten from the west.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.

&&

$$

DISCUSSION...Naylor/Rieck
AVIATION...Rieck