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000
FXUS63 KARX 190704
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
200 AM CDT Fri Apr 19 2024

.KEY MESSAGES...

- Blustery with westerly gusts to 35 mph today. Small snow shower
chances (20-30%) north of I-90

- Cold weekend with frost or freezing conditions expected Sat/Sun
morning

- Next shot for widespread precipitation (rain) comes Monday into
Tuesday

&&

.DISCUSSION...
Issued at 200 AM CDT Fri Apr 19 2024

* BLUSTERY TODAY - small shower potential (mostly snow) north of I-
  90 late afternoon/evening

The surface pressure gradient is tightening ahead of an upper level
shortwave trough (dropping southeast out of Canada) and winds will
be on the increase as a result. Mixing in the lower layers of the
atmosphere will promote gusts upwards of 35 mph (per RAP/HRRR bufkit
soundings).

In addition, cold air advection will increase near sfc lapse rates
today, resulting in instability and additional lift for potential
showers. CAMS models - moreso the RAP and HRRR - produce spotty to
scattered convection for the afternoon into early evening
along/north of I-90. RH fields and soundings show enough saturation
for at least aggravated CU, but the sub cloud layer is rather dry.
Precipitation falling through this layer will have to work against
the drying while also promoting cooling. A couple things from that:
1) anything falling out of the cu could turn out to be just virga.
Never reaching the sfc but locally enhancing wind gusts. 2) pcpn
could reach the surface, and because of the appreciable cooling of
that dry layer, it falls as snow. For now, the lift and saturation
looks enough to warrant at least low end chances (20-30%). Despite
expecting highs to top 40 degrees in all areas, for pcpn to reach
the surface the cooling required would make snow showers more
likely. Won`t amount to much if realized.


* RELATIVELY CHILLY WEEKEND - frost and/or freeze for many/most

Cold air settles in for the weekend with 850 mb temps falling to as
cold as -10 C by 12z Sat morning. A consistent signal from the past
several days in the long range guidance, continuing now into the
short term. Latest HREF holds any chance to warm above 50 across
parts of northeast IA/southwest WI today (and perhaps some valley
locations) with virtually a 0% to exceed 50 Sat for highs. EPS and
GEFS ensemble suites show the same with only a few degree spread in
the warmest 75% of their members - which hold 50 or colder. Same
HREF says "everyone" outside of valley locations will experience
freezing temps Sat morning with something similar Sun morning.

Frost and/or a freeze is likely for most across the forecast area
Sat/Sun mornings. Questions arise on whether a widespread frost will
be realized or mostly "freeze". Clouds and winds can act as
deterrents to frost formation by limiting diurnal cooling of the
near sfc/micro layer. The clouds by insolation and the winds by
mixing. With the influx of cold air, temps still progged to fall
into the upper 20s/lower 30s - so perhaps a moot point. However, sfc
dewpoint spreads expected to range from 5 to 10 degrees, and mixing
will work against moisture (dew) formation on grasses/vegetation
tonight (winds expected around 10 mph or greater). It will still be
cold Sat morning, but not necessarily widespread "white". Sunday
might have a better shot with lighter winds. All in all, even if the
"frost" isn`t widespread, the expected freezing temps could/would
cause damage to sensitive vegetation and early season planters
should take action to protect these plants from the cold.


* NEXT CHANCE TO GET WET COMES MON/TUE

The next precipitation maker looks to come out of the PAC NW as a
shortwave trough shifts east Sat night, dropping across the northern
plains/upper mississippi river valley Monday. Cyclogenesis occu.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Frost Advisory from 1 AM to 8 AM CDT Friday for WIZ032-033-041-
     053>055-061.
MN...Freeze Warning from 1 AM to 8 AM CDT Friday for MNZ086-087-094-
     095.
     Frost Advisory from 1 AM to 8 AM CDT Friday for MNZ079-088-096.
IA...Freeze Warning from 1 AM to 8 AM CDT Friday for IAZ008>010-018-
     019-029.
     Frost Advisory from 1 AM to 8 AM CDT Friday for IAZ011-030.rs
as it moves out of the lee of the Rockies, hanging a sfc cold front
southwestward of its attendant sfc low (which looks to track west-
east across portions of MN/WI). Currently, the EPS and GEFS are
suggesting two main pcpn regions with the available lift - one is
the shortwave itself, with current trends favoring northern parts of
the region. Second, along the cold front as low level moisture
transport/jet helps convection fire. This could be south and east of
the local area. Still looks like most (if not all) of the forecast
area could/would see some rain...but outlay of the higher amount
will be high dependent on the track/positioning of these key
variables..which isn`t clear at this time. The model blend currently
paints a 40-70% for at least 1/4" of rain. Westward handing
trough/secondary cold front will likely continue pcpn chances
through Tue.


* GRADUAL WARMING NEXT WEEK - more rain chances on the horizon for
  the weekend

The long range guidance remains in good agreement with amplifying an
upper level ridge as it shifts from the west coast to across the
plains by Thu night. Milder/warmer air will return to the region as
it pools under the ridge. Trends in the EPS and GEFS suggest more of
a gradual warm up with the upper 75% of their ensemble members
warming back into the 60s as we move into next weekend. 10-20% shot
for 70+. CPC outlooks out 14 days also show an increasing likelihood
for warming - venturing into above normal - for the rest of the
month.

Meanwhile, there has been a steady signal for a shortwave trough to
lift out of the desert southwest and meander across the upper
mississippi river valley Fri-Sat, the last weekend of April. Some
differences in timing/locations of the pertinent features, but ample
support from both models` ensemble members for a good shot to get
wet.


&&

.AVIATION /06Z TAFS THROUGH 06Z SATURDAY/...
Issued at 1105 PM CDT Thu Apr 18 2024

As diurnal heating occurs on Friday, the winds will become gusty (25
to 30 knots) and there will be scattered VFR clouds.
While the clouds will be dissipating on Friday evening, a scattered
mid deck of clouds will move in as a shortwave moves to our north.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Frost Advisory until 8 AM CDT this morning for WIZ032-033-041-
     053>055-061.
MN...Freeze Warning until 8 AM CDT this morning for MNZ086-087-094-
     095.
     Frost Advisory until 8 AM CDT this morning for MNZ079-088-096.
IA...Freeze Warning until 8 AM CDT this morning for IAZ008>010-018-
     019-029.
     Frost Advisory until 8 AM CDT this morning for IAZ011-030.

&&

$$

DISCUSSION...Rieck
AVIATION.....Boyne