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000
FXUS63 KARX 161801
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
101 PM CDT Tue Apr 16 2024

.KEY MESSAGES...

- Spring storm brings a strong/severe storm risk mostly south of I-
90 this afternoon/early evening with a hail, tornado threat.

- Rainfall from 1 to 2" expected with the storm.

- Easterly winds will gust upwards of 45 mph today with Wind
Advisories in effect for a large portion of the local area.

- Colder air returns for the latter half of the week with
frost/freeze conditions increasingly likely for the weekend.

&&

.MESOSCALE...
Issued at 1253 PM CDT Tue Apr 16 2024

Environment is ramping up across Iowa for  increased severe
storms over the next hours. The main players are the warm front
moving northward that is near of Interstate 80 marked well by
the 60F dewpoint line. There is currently a 15F dewpoint
increase from Dubuque IA to that front. SB/MLCAPES are growing
south of the warm front as it moves north, now up to about 1500
J/Kg. Otherwise, elevated instability exists off to the
southwest in central IA of about 1000 J/KG, all north of the
warm front with little capping. However values are reduced over
northeast Iowa.

As the afternoon progresses, the warm front will try to shift
into northeast Iowa and southwest WI, possibly to Highway 18.
This provides a surface-based storm threat with tornadoes
possible. Otherwise, north of the front storms should remain
elevated, which would limit the threats to hail and wind. Storm
mode is a challenge, with linear structures suggested in the
CAMS over the area. This would mean a more QLCS tornado threat
vs supercells.

These very initial waves of storms through about 230-3pm should
be north of the warm front and somewhat limited by instability
and thus hold less probability of becoming severe. As the warm
front moves north, the CAPE will increase areawide, fueling
enhanced probabilities for severe storms and vigorous updrafts
/4-7 pm/. With pressure falls of 5mb/3hr over northeast IA, it
would favor almost a more easterly surface low direction...which
may limit the northern extent of the warm front /e.g., low
travels along the warm front instead of the low lifting the warm
front synoptic field north/. So, confidence is lower on how far
north the tornado threat reaches beyond the current tornado
watch along and south of Highway 18.

&&

.DISCUSSION...
Issued at 330 AM CDT Tue Apr 16 2024

* OVERVIEW:

Upper level trough rotates northeast of the desert southwest today,
strengthening as it develops a negative tilt with associated
deepening sfc low. Broad northwest-southeast expanse of rain will
develop chiefly from low level thermodynamics and punchy 50-60kt low
level jet/moisture transport ahead of the storm system`s warm front.
Could see a thinner band of showers northeast of this main band,
associated with mid level Fgen.

As the band pushes northeast across the forecast area today,
further, more enhanced convection looks likely on the westward flank
of this band...moreorless on the "backside" rather than the leading
edge. This activity harbors much of the severe risk for later
today.

These showers/storms exit northeast by late evening with another
lobe of shortwave energy progged to shift across the local forecast
area overnight sparking another band of showers and storms that will
pinwheel in a similar fashion as the previous band. Showers, perhaps
isolated thunder, could linger into Wed afternoon, but mostly north
of I-94.


* SEVERE RISK? Locally northeast IA/southwest WI harboring the
  higher threat, but the severe risk could push north into the I-90
  corridor.

The higher threat tied to where the storm`s warm front lays out, with
current indications favoring the MN/IA border. Narrow triangle of
warm/moist air pools up to the warm front an ahead of the system`s
cold front (north-south across western IA by 00z this evening).
SBCAPES upwards of 1200 J/kg develop in the HREF. Winds shear in
this warm sector supports supercell development while turning/strong
0-1 km shear will lend itself to a tornado threat. Large hail also looks
likely with steeping mid level lapse rates, although this drops off
the farther north you move off the warm front. Wind shear is also a
lot "messier" and not nearly as strong minus the 0-1km element.
Gradient winds still high and still some potential for higher/severe
wind gusts...but this looks more like a large hail/tor threat
locally.

Considering how important the positioning/timing of the warm front
and the initial convective/waa wing of pcpn are...expect refinement
to the forecast through the day...where the severe risk lies, and
just what kind of severe wx is possible, could change.


* RAINFALL: 1 to 2" looking likely

Low level moisture transport with PWs of 1 1/4+" set to feed an
expansive heavily thermodynamically forced swath of pcpn today into
tonight. PW anomalies via the GEFS and EPS are +2 while EFIs are
above 0.70 - suggesting the airmass is abnormally moist for this
time of year. Resulting QPF in the model blend has a 50 to 80%
chance for over 1". The mean ensemble members have shown some
downward trend in the potential for more than 2", although the GEFS
still hangs onto 10 to 30% across southeast MN. Considering some of
the convective nature of the expected rainfall, locally higher
amounts pushing 2+" is certainly a possiblity.


* WIND: strong with easterly gusts upwards 45 mph today. Wind Adv in
  effect for much of the local area.

The low level jet ramps up in earnest today, not only providing a
rich source of moisture to fuel showers/storms, but strong winds.
Latest RAP and HRRR bufkit sounding runs suggesting that 60 kts
could dip as low as 2 to 3 kft across parts of the area. While just
above the near sfc inversion, momentum transfer could bring some of
that wind to the surface. Latest HREF pushes a 70 to 90% for periods
of 45+ mph wind gusts for the entire forecast area while recent HRRR
determinist runs jack up an occasional 50 kt gusts. Latest runs of
the EPS and GEFS aren`t nearly that high, with the bulk of their
members settling into the lower 40s mph for gusts. The strongest
winds would be located ahead of the broad swath of rain/storms
advancing northeast across the area today.


* COLD WEEKEND AHEAD: frost/freeze possible

After 850 mb temps peak around +10 C today, colder air flows in post
the spring storm system starting Wed, with sub zero 850 mb temps
settling in for the weekend. 75% of the GEFS and EPS members top out
highs in the 50s with very low end chances that a few sites could
reach 60. For lows, the model ensemble blend holds 40 to 70% chances
for freezing temps Sat/Sun mornings from I-90 south, with roughly a
70 to 90% shot north of there. Early season planters will likely
need to take precautions to protect tender vegetation. Depending on
how sky coverage/winds shake up, frost advisories may be needed for
some locations.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 100 PM CDT Tue Apr 16 2024

Main taf concerns are periods of showers/storms and the potential
for IFR/MVFR conditions at both RST/LSE taf sites through the taf
period. Low pressure will move into northern Wisconsin through the
taf period and spread showers/storms into the area. With the low
pressure tracking into northern Wisconsin and the showers/storms
moving over the taf sites...ceilings/visibility will lower into
MVFR/IFR through much of the taf period. There could be brief
periods of VFR conditions with any breaks of
showers/storms...however timing of VFR conditions is low at taf
sites. Have kept periods of MVFR/IFR conditions at both taf sites
through taf period. In addition...tight pressure gradient across the
area will produce wind gusts of 30 to 40 knots through this
evening.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Wind Advisory until 10 PM CDT this evening for WIZ017-029-
     032>034-041>044.
MN...Wind Advisory until 7 PM CDT this evening for MNZ079-086-087-
     094-095.
     Wind Advisory until 10 PM CDT this evening for MNZ088-096.
IA...Wind Advisory until 7 PM CDT this evening for IAZ008>010-018-
     019-029.

&&

$$

MESOSCALE...Baumgardt
DISCUSSION...Rieck
AVIATION...DTJ