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120
FXUS63 KARX 180908
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
408 AM CDT Sat May 18 2024

.KEY MESSAGES...

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- Still warm today with spotty morning showers/storms for parts
  of SE MN/Wrn WI, then showers and thunderstorm increasing this
  afternoon/early evening (20-60% high chances WC WI). An
  strong to severe storm cannot be ruled out.

- Unsettled weather pattern continues later Sunday into early
  next week. Multiple rounds of showers and thunderstorms are
  expected. Some potential for severe storms, however at this
  time, we are keeping an eye on Tuesday due to early
  indications that ingredients for severe thunderstorms are
  coming together for parts of eastern IA/southern WI/northern
  IL, especially the DBQ/DVN are. In addition, heavy rain will
  be possible.  Please keep up to the date with the forecast
  into early next week.
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&&

.DISCUSSION...
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Issued at 345 AM CDT Sat May 18 2024

Overview:

Water vapor satellite imagery, 500mb heights, and lightning show a
closed low over Manitoba and another over Alberta with an additional
trough over the lower Mississippi Valley.  Lightning was noted
around midnight from the U.P. into MN with stronger clusters of
storms from northwest Minnesota into the Dakotas.  The leading area
of showers and storms were associated with mid level warm advection
and moisture transport, while the latter were ahead of the cold
front.  Of note on the SPC mesoanalysis was a broad area of DCAPE in
excess of 1500J/kg ahead of the front.  This was conducive to
damaging wind gusts, thus with the storms some reports of 75 to
85mph winds occurred over the western High Plains earlier in the
evening reports of 60mph+ across the Dakotas. Deep layer shear in
excess of 45kts was also noted with each of these areas of storms.

Locally, it was a warm May day Friday with temperatures about
10 degrees above normal with highs in the 70s to lower 80s.
Skies were variably cloudy with gusts 20 to 30 mph+ during the
day. The southwest to northeast track of the showers and storms
continue, decreasing in intensity as they move east toward the
forecast area. As of 08Z...showers were clipping parts of the
northwest portion of the forecast area.

Showers and Thunderstorms with the cold front today/this evening:

The trough over the Northern Rockies migrates northeast into
Manitoba and Ontario through the day today dragging a cold front
across the forecast area this afternoon and tonight.  Surface
dewpoints start out on the somewhat dry side in the 40s and 50s,
however as moisture transport continues to increase and pooling
occurs, dewpoints increase.  The models HRRR/RAP/NAM all seem to be
well initialized related to the dewpoints and through Saturday
the HRRR/NAM forecast an increase into the 50s and lower 60s,
however the RAP pools lower 70s dewpoints with the front. We
have had recent rains and grasses are growing, however given the
lack of crops/ evapotranspiration, this seems overdone. A
strong low level jet this morning has assisted with the
showers/t-storms ongoing and this jet continues, but weakens
through the morning hours as it shifts eastward into the
forecast area through the morning. Also noted was a diffluent
aloft pattern that could support backbuilding of the
showers/storms. Will continue to mention some morning
showers/storms with decreasing coverage later this morning.

MLCAPE increases this afternoon 1000-2000 J/kg per the HRRR/NAM
with isolated to scattered convection increasing along the
front for most of the hi-res models. DprogDT for the HRRR/RAP
showed an increase in the coverage, while the 18.00Z NAM was
still dry. It was noted that the 18.06Z NAM did increase precip
from 18-00Z today. Synoptic forcing is greater farther north,
so confidence is lower with this higher coverage due to heating
the weaker forcing farther south alone. Forecast soundings show
elevated instability and limited shear around 30kts with some
warmer temps aloft which would work again the storms. The HRRR
forecast soundings are a little perkier with stronger shear 35
to 40kts. Our pops have increased compared to yesterday with the
CAMs showing more of a consistent signal and pooling of
moisture along the front 21-00Z. Cannot rule out a severe storm
with the cold frontal passage during peak heating and expected
steep lapse rates. The HREF has mean wind gusts 20 to 30 mph,
however the ens max has the potential for 40 to 60+ with the
60+mph gusts 23-00Z near Marathon/Wood Co. or Grant Co. This is
also when the HREF probabilities of 40dBZ values increase 50 to
75%. The precipitation should generally be east of the forecast
area by 02-03Z. The diffluent pattern lingers over Grant Co. so
will watch for this.

Highs today should top out in the 80s again and with the increasing
dewpoints.  South to southwest winds strengthen ahead of the cold
front. Surface high pressure builds in for the rest of the night
with drier dewpoints in the 30s and 40s pushing in from north to
south.

Showers and thunderstorms increase later Sunday and continue into
Monday:

A deepening trough over the western U.S. sends some energy eastward
across the Plains with a warm front lifting northward across Iowa
Sunday.  With this shortwave, moisture transport increases across
the Plains with the increasing low level jet.  Broad mid level warm
air advection increases across MN/IA Sunday afternoon with shower
and thunderstorm chances increasing west of the Mississippi River.
However, there are some differences in how far east they will make
it during the afternoon.  Showers and thunderstorms become more
widespread Sunday night with the low level jet increasing 40 to
50kts across the forecast area with continued moisture transport.
EC/Canadian/GEFs ensemble probabilities 30-60% for 0.50" or more for
the 24hr period ending 7pm Monday. Related to the severe weather
potential Sunday night, hodographs show some low level turning
of the winds and a straightline hodograph aloft, but weaker mid
level winds with MUCAPE of 1000-2000J/kg. The CIPS analogs show
some 5 to 10% probabilities for hail and wind, so cannot
completely rule it out. Within the southwest flow aloft pattern,
another shortwave quickly follows for Monday with periods of
showers and thunderstorms continuing.

Severe weather potential Tuesday:

A stronger trough arrives Tuesday with deepening surface low
pressure over the Upper Mississippi River Vally Tuesday afternoon.
We have limited data out this far, however there is currently a
consensus that destabilization will occur ahead of a cold front with
a belt of stronger winds rotating through Iowa into southern WI and
northern IL. Current Colorado State University machine learning
probabilities show the ingredients for severe weather coming
together across eastern Iowa, southern WI and northern IL. The
greatest probabilities toward DBQ/DVN.  We will continue to monitor
the severe weather potential and the potential for heavy rain
with this system as well.  Stay tuned for forecast updates.
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&&

.AVIATION /06Z TAFS THROUGH 06Z SUNDAY/...
Issued at 1000 PM CDT Fri May 17 2024

Main taf concerns are shower/storm potential at both taf sites
Saturday afternoon. VFR conditions are expected to prevail tonight
into Saturday morning. Then a surface cold front tracks across the
area Saturday afternoon and evening. With daytime heating and
moisture convergence along the front...showers/scattered storms
will develop along the front early Saturday afternoon. At this
time development of scattered convection along front may
develop over the RST taf site and could impact the LSE taf site
between 20z Saturday to 00z Sunday time frame. For now kept
vicinity showers at both taf sites after 19z Saturday...based on
timing of front/development of convection. Southerly wind
direction will shift to the northwest late Saturday afternoon to
early Sunday afternoon.

&&

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

&&

$$

DISCUSSION...Zapotocny
AVIATION...DTJ