596
FXUS63 KARX 221144
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
544 AM CST Fri Nov 22 2024

.KEY MESSAGES...

- Cloudy & Similar Temperatures As Thursday

- Subsequent Precipitation Chances Sunday Night Through Monday

- Little To No Confidence In Thanksgiving Day (Day 6) Forecast

&&

.DISCUSSION...
Issued at 155 AM CST Fri Nov 22 2024

Today:

As the low that drove recent days sensible weather churns over
the Ohio River Valley early this morning on GOES water vapor, an
upstream ridge is returning flow into the Northern Plains. This
resultant airmass is quite dry and warm as far north as
Bismarck, SD with 25th percentile surface/850mb dewpoints and
75th percentile surface/850mb/700mb temperatures according to
SPC climatology. However, this return flow isn`t expected to
make it as far east as the Upper Mississippi River Valley until
potentially tonight. Instead we remain under the influence of
CAA from the departing cyclone tied to an upstream polar low
over central Canada. Therefore we are unlikely to benefit from
diurnal heating as clouds stick around. High resolution model
soundings suggest cloud bases near 1500 due to CAA and 75th
percentile low level dewpoint temperatures near freezing at
International Falls, MN and Green Bay, WI 22.00Z RAOBs.
Therefore, a persistence forecast might score well for
temperatures today. However, could see some clearing from west
to east tonight as the aforementioned anomalously dry air
combats the moist low level air lingering locally.

Temperatures Through The Weekend:

Return flow does abate coldest temperatures through the weekend
aided by the warm sector of a weak low. Gradual warming through
Sunday is expected with long term ensembles (EPS/GEFS/GEPS)
splitting the forecast area with 100% confidence for the 40
degree isotherm. As these global modes tend to have a cold bias
and can`t account for mesoscale phenomena, have stayed with the
National Blend for high temperatures with highs in the 40s to
50 in select spots for Sunday.

Next Precipitation Chances:

Subsequent precipitation chances reach the Upper Mississippi River
Valley Sunday night from a northern perturbation dropping into
a longwave trough (GEFS/EPS) that extends from the Pacific Coast
to the Great Lakes. Initial precipitation chances will be tied
to a filling surface low zonally advecting a baroclinic
boundary that bifurcates the forecast area. A secondary area of
decreased mid level heights skirts southeast out of the main
trough (LREF 60-90%) later Sunday, driving precipitation chances
primarily across northern Wisconsin through Monday. Clustering
analyses disagree strongly on exact location of the diving area
of depressed isoheights and therefore exact precipitation
chances. Regardless, minimal precipitation and impacts are
expected as highest potential remains to the north as well as to
our south. While mostly liquid precipitation type is expected
at this time, freezing temperatures will likely depend on
diurnal heating concurrence.

Thanksgiving Day & Travel Forecast:

Another trough sags southeast Wednesday through Friday,
upstream of a polar jet streak, placing an area of ageostrophic
ascent over the Upper Mississippi River Valley from the jet
streak`s polar entrance region. All long term global ensembles
(EPS/GEFS/CMC) show the main low advecting east-northeast
through the Ohio River Valley during this time. Even so, one
cluster (30% of members) has widespread precipitation across the
Upper Mississippi River Valley during Thanksgiving from more
than 0.5" PWATs, resulting in 5" of snowfall over a longer
duration. Too far out to nail any details as this is only 30% of
solutions and overall guidance is favoring a more southern
trajectory through the Ohio River Valley rather than a northern
route through the Mid Mississippi River Valley. As one would
expect based on trajectory differences, there is a widespread
difference in surface temperatures during this time of 10-15,
above and below freezing.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 544 AM CST Fri Nov 22 2024

LIFR to low-MVFR stratus will continue across the region throughout
the morning hours with fairly robust low-level moisture in place as
shown in model soundings across the recent HRRR/RAP/NAM. As we
progress into the later morning hours and afternoon, some weak
mixing may aid in increasing cigs marginally but generally
expected to remain low-MVFR to IFR levels across the local area.
Continued low-level saturation and weakening winds towards the
evening hours will aid in keeping LIFR to low-MVFR stratus
around into the overnight, perhaps even some fog development
near I-35 as hinted in guidance. Noting some guidance, namely
the recent GLAMP, wanting to lift cigs to VFR overnight at KLSE
with some dry air intrusion. However NAM/RAP/HRRR soundings and
seasonal climatology would not support this, at least for the
overnight hours but will continue to evaluate in upcoming
forecasts. Otherwise, winds will remain between 8-12 kts
throughout the daytime hours and gradually diminish to around 5
kts after sunset while remaining west/northwesterly.

&&

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

&&

$$

DISCUSSION...JAR
AVIATION...Naylor