Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 131629
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1129 AM CDT Wed Mar 13 2024

.KEY MESSAGES...

- One last round of anomalous warmth today with highs in the
  mid to even upper 60s.

- Rain showers move in tonight and persist into Thursday
  evening. There is substantial variability in where the
  heaviest rain will fall and a southward shift in the system
  has been noted in recent model runs.

- Temperatures moderate somewhat for Thursday through Saturday
  morning with a shot of colder air for late Saturday into
  Monday. Possibly some light snow showers on Sunday as well.

&&

.DISCUSSION...
Issued at 400 AM CDT Wed Mar 13 2024

Today: Last Day of Late Spring Warmth

The surface boundary that passed through yesterday afternoon has
shifted into central Iowa and northern Illinois as of early this
morning, yet the post-frontal airmass remains very shallow and
mixes quite readily with even a light wind. Temperatures across
the region early this morning reflect this assessment, with
readings 15-20 degrees warmer on higher elevations versus lower
valley dwellings. This airmass mixes quickly with the onset of
daytime heating and highs today once again top out well into the
60s. With easterly flow and increasing high clouds ahead of our
upcoming weather system, there is some uncertainty with just
how warm temperatures will climb. However, no matter how you
slice the forecast, it looks to be a very pleasant day.

Some of the shorter range guidance over the last 24 hours have
tried to saturate the 800-700-mb layer and generate enough
upright instability to pop a few elevated showers late this
afternoon and evening. Did not make too many changes to the
previous forecast with the 20-40% PoPs, but the theta-e
advection with last few runs of the HRRR and RAP has not been as
strong and these CAMs have either scaled back their coverage of
any precip or gone completely dry. If these trends verify,
precipitation will not arrive until after midnight to 3am for
most locales.


Tonight - Thursday Night: Rain Showers, Uncertainty in Amounts

Longwave troughing easily noted in water vapor imagery early
this morning over the Rockies amplifies over the course of the
day with a shortwave ejecting out of the Four Corners region in
tandem with a 100-kt jet streak this afternoon. Moisture return
is already taking place over TX and OK early this morning and
this surge of higher theta-e air works northward today ahead of
this shortwave. Broad lee troughing is taking place along the
High Plains with a disjointed baroclinic zone ahead of this
troughing that won`t consolidate and organize until later this
afternoon and evening.

A surface low ejects from the OK/TX panhandle region this
afternoon and lifts northeastward into Kansas this evening. The
short and medium range guidance are struggling mightily to
resolve the convective influences along the lifting warm front
ahead of this low for tonight into Thursday. Indeed, recent
trends over the last 36 hours have been for a more progressive
system in which the deeper moisture/convection stay to the south
and shunt eastward. The 13.00/06Z HRRR progs have been latching
onto this trend in developing a large MCS whose outflow
boundary halts the northern progression of the warm sector and
inhibits much in the way of precipitation from reaching the
forecast area. The storm total QPF from the last few runs of the
HRRR bring almost nothing north of I-90, whereas the global
models that are not factoring in this MCS potential and bringing
the warm sector/precipitation farther north.

Needless to day, the forecast precipitation probabilities and
amounts are of very low confidence and have the potential to
come down very quickly with updates today. The faster
progression of the system leaves a weaker and faster departing
deformation zone in its wake that dissipates before the thermal
profiles can support snow. Therefore, have tempered back PoPs
for tonight and lessened any mention of a rain/snow mix.


Friday - Saturday Morning: Quieter Weather, Still Warm

An omega block develops just off the West Coast by Friday
morning and causes the longwave trough responsible for our
Thursday system to bifurcate with the northern stream wave
tracking over the northern CONUS on Friday and the southern
stream energy forming a cutoff low over the desert southwest.
Surface cold air advection will not be all that strong with this
first wave and NBM highs are still clustered tightly in the
lower 50s. The cooler airmass aloft will likely allow more
daytime cumulus/stratocumulus to develop, but that should be the
main sensible weather impact.

A downslope airmass advects into the region Friday night into
Saturday morning ahead of a much stronger shortwave that drives
straight southeast from the Canadian arctic. While highs on
Saturday should still top out in the 50s early in the day, the
surface cold front slams through by midday and expect steady or
falling temperatures through the afternoon and evening hours.


Saturday Night - Tuesday: Turning Colder, But for How Long?

This wedge of polar air finally hauls temperatures back to near
or even slightly below average for Sunday and Monday (yes, it
took an airmass straight out of the Arctic Circle to do this).
A solid 20-30-mb difference between the surface trough and an
upstream high pressure cell over the Canadian prairies will
result in post-frontal northwesterly winds pushing 20-30 mph,
driving wind chill values into the lower teens by Sunday
morning.

The main surface low and forcing remains offset to the
northeast over the Great Lakes and Ontario, keeping much of any
precipitation outside of the forecast area on Saturday. A
backdoor cold front swings through Sunday afternoon and may
steepen the boundary layer lapse rates enough to generate snow
showers during the afternoon hours, with gusty winds possibly
resulting in some reduced visibilities in any of these showers.

Sunday night into Monday morning are shaping up to be the
coldest of the forecast period, featuring lows in the teens and
wind chills in the single digits. The ridge evolution beyond
Monday remains a source of high model variability and will
heavily influence temperatures for next week`s forecast. At this
time, there exists a 10 to 15 degree spread in the NBM 50th
percentile guidance envelope, but trends in the EPS/GEFS inputs
are starting to favor a warm up beginning Tuesday into
Wednesday.

&&

.AVIATION /18Z TAFS THROUGH 18Z THURSDAY/...
Issued at 1126 AM CDT Wed Mar 13 2024

CIGS: gradual increase in high clouds tonight with lowering to a
MVFR/IFR deck after 12z Thu (50-60% shot for IFR). The low cigs
should hold through the afternoon with improvement likely Thu night.

WX/vsby: rain showers start to spread in from the south near/after
12z Thu. The pcpn will battle the dry airmass currently in place
across the local area, serving as a northward deterrent to the rain
chances. A few of the meso models suggest the pcpn might not make
the I-90 corridor - a trend that will be monitored. For now, will
ride -shra for at least the morning hours of Thu. If -shra are
realized, there could be periods of MVFR vsbys before shifting east
by 00z Fri.

WINDS: sub 10 kt easterly winds becoming northeast/northernly toward
12z Thu and increasing. Gusts in the low to mid 20 kts Thu morning,
into the afternoon. Winds will drop off Thu evening with relaxing of
sfc pressure gradient.

&&

.FIRE WEATHER...
Issued at 400 AM CDT Wed Mar 13 2024

One last day of deep mixing and low humidity values is on the
docket for much of the region before cooler temperatures and
increasing dew points bring an end to the fire weather concerns.
Have continued an aggressive approach to dewpoints and humidity
values this afternoon using the HRRR/NBM 10th percentile
output--though did not go fully with the HRRR on account of the
increasing high clouds that may temper mixing to a slight
degree. This brings afternoon RH values down to 20 to 25
percent throughout southeast Minnesota and much of southwest
into central Wisconsin.

Winds today will be from the east at 5-10 mph, with a stray
gust to 15 mph around peak heating/mixing. Given the lighter
winds today, have refrained from any elevated fire condition
wording and have just mentioned these conditions in the HWO and
fire weather planning forecast. A few light showers may impact
areas along and south of I-90 this afternoon, with the bulk of
the wetting rains coming late tonight into Thursday morning.

&&

.CLIMATE...
Issued at 400 AM CDT Wed Mar 13 2024

For today, a cooler airmass aloft and increasing mid to high
clouds raises questions as to exactly where our high
temperatures will peak out. However, we should come within a
few degrees of the record highs at both La Crosse and Rochester
and the probability of reaching these records is around 20 to 30
percent.

                    Today
Location       Forecast / Record
-----------------------------------
La Crosse        68 / 71 (2015)
Rochester        64 / 66 (2012)

&&

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

&&

$$

DISCUSSION...Show
AVIATION...Rieck
FIRE WEATHER...Skow
CLIMATE...Skow


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