711
FXUS63 KARX 301742
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1242 PM CDT Thu May 30 2024

.KEY MESSAGES...

- Seasonal weather today with temperatures slowly warming into
  the upper 70s to low 80s for highs by Sunday and into early
  next week.

- Periodic showers and storms Friday into Saturday. There is a
  potential for a narrow corridor of 1-3 inches of rain Friday
  afternoon and evening, but confidence in exact details is low.

- No severe weather is expected through Saturday, but there
  exists a small severe weather signal for early next week.

&&

.DISCUSSION...
Issued at 300 AM CDT Thu May 30 2024

Today/Tonight: Pleasant and Dry

Given the larger evening T/Td spreads and deeper mixing heights
yesterday, valley fog failed to form in most locales across the
region this morning. With this forecast update, did scale back
any mention of fog to just a few tributary valleys before
sunrise.

Early morning water vapor imagery depicts an omega blocking
ridge axis centered along the MN/Dakota borders with anvil
cirrus from upstream convection spilling over the 500-300-mb
ridge and spreading downstream towards the forecast area. Mid to
high clouds from this complex will gradually be on the increase
through the day, but overall expect ample sunshine (especially
east of the Mississippi River) today with high temperatures
notching a few degrees warmer than yesterday as the lower
tropospheric temperatures slowly increase in the wake of the
departing near-surface high pressure cell.

Friday - Saturday: Rainfall Details

The first big question in the forecast revolves around the
rainfall forecast for Friday into Saturday. A negatively-
tilted shortwave driving the ongoing convection across the
northern High Plains lifts into Canada as it carves out the
western periphery of the aforementioned omega block. The upper
level kinematic support likewise lifts northward of the region
and results in the attendant surface boundary over MN slowing
during the day on Friday and then stalling/lifting back
northward Friday night in response to a second shortwave
approaching from the west.

How far southeast this boundary progresses on Friday is a
source of forecast uncertainty with some convective allowing
models pushing it into southeast MN/central WI whereas other
guidance keeps it closer to the Twin Cities metro area. This
southeastward push will be driven by the strength of the daytime
convection, a difficult aspect of the forecast to nail down
with much lead time given the already marginal environment in
the narrow warm sector ahead of the front.

Increasing isentropic ascent over the boundary in the evening
hours on Friday ahead of this second shortwave should fuel a
renewed rounds of storms with the possibility of a corridor of
training cells and enhanced rainfall rates along a southwest to
northeast axis. HREF member maximum QPF outputs show values
within this narrow corridor of 1 to 3 inches, but vary in the
placement of this 10-20 mile wide corridor by over 150 miles.
Concerns also linger with the quality of the moisture within the
already narrow and mixed warm sector with additional convection
to the south possibly robbing the area of any further moisture
return as we go through the night and into Saturday morning.

Sunday - Tuesday: Additional Waves of Storms

A quasi-zonal flow pattern settles in by the latter part of the
weekend with a pair of amplified shortwaves knocking on our
doorstep for Sunday night/Monday morning and Tuesday night. The
evolution of the waves--most notably the latter wave for
Tuesday--is a source of high model variability and thus instills
lower confidence in the convective details of the forecast.
Nevertheless, the reservoir of higher theta-e air will not be
far away and could easily be tapped by these systems as they
move through. The shear profiles and thus severe weather threat
will be dictated by the orientation and position of the upper
troughs and will be hashed out over the coming days. Bottom line
is that the forecast for the early part of next week looks to be
taking a more active turn.

One aspect of the forecast that is of higher certainty is
temperatures. With the zonal flow pattern in place,
temperatures should hold steady in the upper 70s to low 80s for
highs through midweek, though how the convection unfolds will
drive temperatures to some degree and results in a widening of
the guidance envelop beyond Monday.

&&

.AVIATION /18Z TAFS THROUGH 18Z FRIDAY/...
Issued at 1234 PM CDT Thu May 30 2024

VFR conditions are expected through this TAF period across the
region. Timing of any precipitation impacts west of the
Mississippi River has been pushed back, but did go with a VCSH
at RST mid-morning Friday. Precipitation chances increase around
09Z for RST, but still remain low enough (10 to 20%) to not
include in the TAF at this time. Best precipitation chances (50
to 60%) for RST come around 17Z Friday.

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...JAW