609
FXUS63 KARX 041636
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1135 AM CDT Sat May 4 2024

.KEY MESSAGES...

- Another round of rain moves through this morning and early
  afternoon, some thunderstorms possible south of I-90. Fog
  possible tonight along and east of the Mississippi River.

- Dry and pleasant weather for Sunday and Monday with highs
  climbing back into the upper 60s to mid 70s.

- An active stretch of weather from Monday night through at
  least Thursday as multiple rounds of showers move through.
  Severe weather risk for Tuesday is trending south.

&&

.DISCUSSION...
Issued at 330 AM CDT Sat May 4 2024

Today into Tonight: Cooler, Showers Especially this Morning

Early this morning, a surface cold front stretched from Omaha,
NE to Ashland, WI and steadily marches southeastward today ahead
of a parent northern stream upper tropospheric trough quite
evident in water vapor imagery over the Dakotas. An MCV
developed across eastern Nebraska overnight as convection from
the previous day congealed into a larger MCS that stretches down
to central Oklahoma. This complex lifts along the frontal
boundary this morning, and while expected to gradually weaken
through the morning, should bring widespread precipitation with
its passage. Indeed, every EPS/GEFS member has measurable
precipitation and every CAM shows the system holding together
with its passage, with minor timing differences driving down the
PoP values in the blended CONSShort data. Therefore, leaned
more aggressive with PoP coverage this morning with the notion
that everyone should see rain at some point this morning.

The MCV drags up a narrow moist sector of 55-60 F dewpoints
today (currently south of Iowa), but confidence is not terribly
high with how far north this airmass advances. Additional
convection is progged to develop along the outflow-reinforced
front in eastern Iowa this afternoon, which could easily cut off
the moisture transport and leave us with little additional
afternoon storm activity. The timing of this morning`s storms
will drive what transpires in the afternoon, but any threat for
storms would exist over southwest WI before exiting before
sunset. The combination of the morning rain and low clouds
lingering for the afternoon will keep highs confined in the 50s
to low 60s on the cool side of the boundary.

Looking ahead to tonight, low clouds in the wake of this system
clear out from west to east from 00-06Z with surface high
pressure building in its wake. The setup is somewhat similar to
what we saw yesterday morning with the widespread fog, but
rainfall amounts today should be less and the clouds clearing
sooner. This may limit the fog threat to east of the Mississippi
River, and indeed the 00Z HREF has (noisy) 20-40% progs for
visibilities under a mile in this region. Did include the
mention of fog with this forecast update to account for this
signal. Lows in favored sandy soil regions bottom out in the low
to mid 30s tonight as well, which may necessitate targeted
frost headlines.

Sunday and Monday: Dry, Warming back Up

Upper level ridging builds in during the day on Sunday and
lasts well into Monday, keeping the area devoid of any
precipitation. Did increase cloud cover for Sunday over the raw
NBM guidance with the 03Z RAP and 06Z HRRR both showing
convective temperatures being reached areawide by midday given
the cooler airmass aloft and steep boundary layer lapse rates.
This should facilitate the development of a cumulus cloud deck
for the afternoon, though with general subsidence through the
column and a stout 40-50C dewpoint depression dry wedge above
the boundary layer, any clouds will be quite flat in nature.
These types of sky coverage scenarios are very difficult for the
blended guidance to depict and thus increased sky coverage just
enough to get some mention of clouds in the worded forecast.

Temperatures climb back into the upper 60s for highs today,
with increasing southeasterly flow for Monday pushing highs up
into the low to mid 70s. Latest trends in the medium range
guidance have been to shift the threat for precipitation back
into Monday night, so did remove the last vestiges of the 15-20%
multi-run NBM blended PoPs from the southwestern forecast area
for Monday afternoon.

Monday Night through Friday: A Wetter Stretch of Weather

Convection over the Central Plains Monday afternoon grows
upscale into the evening as it moves into Iowa, gradually
eroding the already narrow warm sector that stretches north of
I-80. Overall trends in the medium range QPF data show a
weakening system upon its arrival in our forecast area, likely
after midnight for most locales. The overall severe weather
threat for Tuesday hinges on the properties of the upper level
wave pattern and whether or not recovery of the warm sector can
take place in the wake of the morning MCS. The most probable
scenario is that the early morning storms shunt the warm sector
south as the upper wave pivots through, keeping any severe
threat to the south.

The upper tropospheric longwave trough separates from the flow
and morphs into a cutoff low for Tuesday into Thursday,
lingering over the northern High Plains during this time. This
results in multiple, ill-timed waves of precipitation affecting
the region. Temperatures cool down into the 50s to low 60s by
the end of the week under this pattern, with the global guidance
spread increasing in how long this cutoff low lingers beyond
Thursday into next weekend.

&&

.AVIATION /18Z TAFS THROUGH 18Z SUNDAY/...
Issued at 1135 AM CDT Sat May 4 2024

CIGS: IFR/MVFR will hang around through the afternoon, showing some
improvement with a increasing then scattering/clearing of the clouds
near/shortly after 00z this evening. Mostly SKC conditions then
expected through the night and Sunday, although some some scattered,
flat CU could occur Sun afternoon.

WX/vsby: radar trends and CAMS models suggest the -shra will have
pulled east/northeast of the TAF sites by 18z and will trend the
forecast that way. With the rainfall saturating the boundary layer,
and clouds not expected to clear until close to sundown (or after),
some threat for fog. Latest RAP and HREF suggest at least MVFR fog
in the I-94 northward. NAM soundings at KLSE suggest a very shallow
river valley fog could develop, although wind speeds just off the
sfc would work against that reaching the airport. Tough call here
but will lean toward the HREF for now and keep and fog related vsby
impacts out of the forecast. Will monitor and update if needed.

WINDS: northwest becoming light and vrb tonight with some shift back
to the west/northwest Sun but still light.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION.....Rieck