Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 120918
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
418 AM CDT Tue Mar 12 2024

.KEY MESSAGES...

- Another day of record to near record high temperatures today
  with warm weather continuing through Wednesday.

- Widespread rain moves in late Wednesday night and persists
  through Thursday night. Thunderstorms are possible along and
  south of I-90. Some snow may mix in Thursday night.

- Above average temperatures persist through Friday, with cooler
  air for the weekend to near slightly below average. A few
  rounds of light rain or snow move through during the weekend.

&&

.DISCUSSION...
Issued at 400 AM CDT Tue Mar 12 2024

Today through Wed: Continued Warmth

A sharp meridional trough in water vapor imagery is progressing
east over the northern High Plains and keeps on chugging
through the region today. A 1003-mb surface low pressure cell is
lifting northeast through the Twin Cities and up into western
Lake Superior this afternoon with a trailing boundary slowly
sagging to the southeast over the course of the day ahead of the
upper tropospheric trough. Winds behind this boundary shift to
the west and eventually the northwest over the forecast area by
this evening.

Despite the +10C 850-700-mb thermal ridge shunting to
the east of the forecast area ahead of this wave, strong warm
air advection throughout the night ahead of the boundary has
kept temperatures 20+ degrees warmer than this time last night,
giving us effectively a head start when daytime warming
commences. Therefore, have kept the high temperature forecast
very similar to yesterday despite temperatures at the top of
the boundary layer being 2-4 C cooler than yesterday. Very dry
air near the surface will also be very conducive to heating and
achieving super-adiabatic lapse rates. There will be more high
clouds lingering around compared to yesterday, but these should
thin somewhat this afternoon and overall impacts to temperatures
should not be overly high.

A somewhat surprising trend in the short to medium range
guidance over the last 48 hours has been an increase in
temperatures for Wednesday north of our old "cold front" (which
stalls just south of the area and tightens as a warm front on
Wednesday). This is likely due to the lack of cold air
advection on the poleward side of the front and the continued
influence of a very dry lower troposphere which will aid in
deeper mixing and steeper boundary layer lapse rates.
Temperatures on Wednesday are now progged to reach the mid-60s
despite increasing mid to high clouds and an easterly fetch.


Wed Night - Thu Night: Widespread Rain, Snow Mixes in Thu Night

There still remains some variability in the medium range
solutions as to the timing of the shortwave that ejects from a
deepening longwave trough over the Four Corners region. However,
the various ensemble camps are both compromising and the
differences are much less than last night.

The leading warm air advection wing of rain lifts northeastward
though the region late Wednesday night into Thursday morning
with the greatest probabilities for higher rainfall amounts
(50-60% chance of seeing over 0.50" of rain) residing over
northeast Iowa and southwest Wisconsin closer to the near-
surface baroclinic zone and within the northern periphery of the
MUCAPE corridor on Thursday. There exists about a 50-70 mile
spread in the ensemble guidance as to how far north the warm
front lifts on Thursday, but only 20% of the Grand Ensemble
members bring any SBCAPE into the southern forecast area with
the surface front most likely to stay just to the south.

Depending on lower level moisture profiles, MUCAPE values over
the southern forecast area may reach upwards of 300-700 K/kg
owing to steeper mid-level lapse rates through the hail growth
zone. The forecast hodographs show some potential for updraft
organization, but overall the impacts look relegated to small
hail as reflected in the Marginal Risk that covers the southern
forecast area.

For Thursday night, have continued the mention of a rain/snow
mix along a decaying frontogenesis band that situates itself
along or somewhat north of I-90. The last few runs of the GFS
have been quite pronounced with the frontogenesis band and may
even be producing some snow during the afternoon on Thursday.
That being said, the deterministic run remains a bit of an
outlier and the snow probabilities from the EPS have steadily
decreased over the last 24 hours with drier air undercutting the
frontogenesis band and forcing coming in weaker in the
deterministic EC. These mesoscale features can be difficult for
the global models to resolve correctly and it will be
interesting to see what this fgen band looks like in the short-
range guidance over the next day or so.


Fri - Mon: Cooler, Occasional Rain/Snow Over the Weekend

The pattern takes a decisive shift at the end of the week and
into next week as an Omega Block builds and lingers over the
Pacific Northwest. This locks in northwesterly flow all through
the weekend (sans a brief period of warm air advection Saturday
morning) with several shots of cold air advection, most notably
Saturday night into Sunday. A broad system clips the region
over the weekend, bringing precipitation both with the surface
cold front Saturday and then along a secondary back-door front
on Sunday that pivots around the low racing eastward over
Ontario. This latter wave as the best potential to produce light
snow with surface temperatures struggling to reach into the
mid-30s.

Longwave troughing deepens over the area for the end of the
weekend and into next week, keeping cold air in place through at
least Monday and possibly longer, depending on how soon the
upstream ridge breaks down. The NBM temperature forecast has a
large degree of spread for Monday onward indicative of this
higher uncertainty forecast.

&&

.AVIATION /06Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 1037 PM CDT Mon Mar 11 2024

A period of low-level wind shear will occur overnight tonight as
winds off the surface increase. Otherwise, VFR conditions will
persist with some periodic high clouds. A front will swing
through Tuesday morning with winds becoming NW.

&&

.FIRE WEATHER...
Issued at 400 AM CDT Tue Mar 12 2024

In coordination with state fire weather personnel and
neighboring offices, have issued a statement for elevated fire
weather conditions this afternoon for southeast Minnesota and
western/southwestern Wisconsin. There is a strong signal that
the dewpoints will mix out quickly behind the frontal passage
with the moisture profiles at the top of the boundary layer
similar to yesterday but expanded farther eastward. Leaned
heavily on the NBM 10th percentile and HRRR dewpoints for the
next 24-36 hours of the forecast, which tanks RH values into the
mid teens to lower 20 percent range for nearly the entire
forecast area.

The big limiting factor is wind speeds. Winds veer to the west
and northwest this afternoon areawide in the wake of a weak
frontal passage. The post-frontal pressure gradient will be
strongest east of the Mississippi River this afternoon with
forecast profiles showing the potential for sustained winds of
10-15 mph, gusting at times to 20 mph once mixing heights
increase. To the west of the river, while RH values will be the
lowest, a surface ridge axis is building southeastward into the
region that tempers winds by about 5-10 mph compared to
locations to the east. Winds will be even lighter in northeast
Iowa and with GFDI values not even close to elevated levels,
have refrained from mentioning fire weather concerns.

Fine fuels areawide have steadily dried out over the last few
days with the FFMC values rising from the mid-80s to the low
90s since Sunday. All put together, fires will start easily in
these conditions and may spread if not properly attended to.
Please exercise caution if burning today.

The airmass does not appreciably change for Wednesday and
there remains some risk for humidity values to fall to around
20-25 percent during the afternoon along and north of I-90. The
pressure gradient does start to increase ahead of our next
system Wednesday afternoon, with winds possibly gusting up to 15
to 20 mph again. Wetting rains move in Wednesday night and
bring an end to this round of warm and dry weather.

&&

.CLIMATE...
Issued at 400 AM CDT Tue Mar 12 2024

While temperatures remain quite warm early this morning, the
current forecast has readings falling 5 to 7 degrees below
record high minimums for this date by midnight tonight;
therefore, we are not anticipating any record warm lows to be
set.

For today, questions linger about how much mixing will occur,
which will have an impact on temperatures. The probability that
a record high is set at Rochester is around 30-40%, but less
than 10% at La Crosse. The probabilities slip even further for
Wednesday owing to increasing cloud cover.

                    Today                Wednesday
Location       Forecast / Record      Forecast / Record
-----------------------------------------------------------
La Crosse        72 / 74 (1990)         66 / 71 (2015)
Rochester        67 / 67 (1990)         63 / 66 (2012)

&&

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

&&

$$

DISCUSSION...Skow
AVIATION...JM
FIRE WEATHER...Skow
CLIMATE...Skow


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