Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 020837
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
337 AM CDT Tue Jun 2 2020

.SHORT TERM...(Today and tonight)
Issued at 329 AM CDT Tue Jun 2 2020

Lots to talk about in the short term period, from heat/humidity
concerns today to a high likelihood of severe weather for a portion
of the area by this evening. Broad upper ridging over the central
CONUS continues to slowly build into the Upper Mississippi Valley,
nudging H5 heights to around 585 dam, or above the 99th percentile
for early June. RAOB 02.00Z 850mb temps range from the low 20s
from the Quad Cities to the Twin Cities and points upstream.
Guidance shows a plume of these toasty H8 temps advecting eastward
through this morning, hovering around 22-23C across most of the
forecast area. NAEFS guidance indicates this is also above the
99th percentile, if not outside its climatology for early June.
Temperatures only dropping into the lower 70s in most areas early
this morning will give us a head start on the warming process
today. Deterministic guidance is in fairly good agreement for
highs in the low to mid 90s across much of the area, with bias-
corrected guidance trending a few degrees warmer. Given Monday`s
highs in the mid to upper 90s just upstream over southwest MN
into eastern NE (an area that saw nearly full sun), will lean
towards warmer end of guidance for highs today, ranging from
around 90 to the mid 90s. With strong capping in place, we will
have lots of sun (aside from passing thin cirrus) through at
least early afternoon before convection breaks out upstream, so
that will only contribute to the warming cause.

As for dewpoints, did a quick eyeball comparison of how Monday`s
observations panned out vs. model guidance. Seems the RAP/HRRR and
Canadian had a pretty good handle on them, whereas GFS and NAM were
way too high. Guidance does point to an uptick in dewpoints locally
today, suggesting lower to middle 70s out ahead of a cold front
to our north. Some of this is tied to ongoing weak moisture
transport, with model evapotranspiration processes also likely a
contributing factor. Given the latter factor isn`t yet in full
swing across the region, will lean more conservative with
dewpoints hovering around 70 this afternoon. That will yield max
heat indices of 95 to 100 across the majority of the area, just
flirting with heat advisory criteria in some areas. Given the
marginal and short-lived nature of today`s heat (near thresholds
for just a few hours this afternoon), will hold off on any
advisory.

Main focus of the short term continues to be likelihood of severe
weather developing late this afternoon into this evening. A slow-
moving cold front will be laid out from southwest MN into the Upper
Peninsula, eventually dropping south through the area this evening.
Also some support from a vigorous shortwave crossing North Dakota
and reaching Lake Superior by evening. South of the front, a deep
elevated mixed layer will yield an impressive pool of instability
with RAP MLCAPEs climbing to 2000-4000 J/kg across much of the
forecast area, highest just south of the I-90 corridor. Stout
capping in place this morning courtesy of those toasty 850mb
temperatures will erode by mid to late afternoon as the boundary
layer mixes out. Meager deep layer shear will be the only
limiting factor for long-lived, organized storms later today.
Greatest 0-6km bulk shear will reside north of the front, with
only around 30 knots or so expected along the boundary. CAMs still
show a surface-based multi-cell storms initiating along this
boundary around 5pm as the cap breaks. This cluster of storms will
then likely morph into a broken line of convection along the
boundary given shear vectors oriented parallel to the front. While
threat of supercells will be lower given the weak shear,
conditions should be adequate for at least organized multi-cells
with a rather high threat for large hail (especially with the more
discrete initial cells) and damaging winds. For hail: soundings
show big fat CAPE profiles in the hail growth zone and there may
be some rotating updrafts per HREF updraft helicity signal of
>150, but today`s higher freezing levels and initially very dry
boundary layer will be size-limiting factors. For damaging winds,
deeply mixed inverted-V soundings with some mid-level dryness are
present initially, suggesting possibility of microbursts. DCAPE
values >1000 J/kg and cold pool production along the boundary
would favor potential for strong winds reaching the surface. A
brief isolated tornado or two can`t be ruled out given some low
level hodograph curvature. However, not too impressed by low level
shear/helicity which does not overlap well with projected
reflectivity, and LCL heights will be rather high initially. SPC
Day 1 enhanced risk encompassing majority of the CWA seems well
placed, with the highest risk expected to be centered along and
south of the I-90 corridor. Severe threat should largely diminish
by around midnight with lingering storms exiting our far south
late tonight.

Only other concern will be locally heavy rainfall as a plume of
PWATs approaching 2" and a deep warm cloud layer develops over the
area this evening, parallel to the boundary. If boundary progression
is slower than currently anticipated, this could yield some slower
moving storms repeating over the same areas and thus a threat for
excessive rainfall. Current soil moisture modeling and 6-hr flash
flood guidance suggests there shouldn`t be much problem handling 1
to locally 2 inches of rainfall, but will watch trends closely,
especially in the more complex terrain areas prone to greater
impacts.

.LONG TERM...(Wednesday through Monday)
Issued at 329 AM CDT Tue Jun 2 2020

Perhaps a straggling shower or storm across far southern Grant
County to start the day on Wednesday, otherwise high pressure and a
less humid airmass build into the region. Temperatures won`t be as
hot with highs in the low to mid 80s. The rest of the period looks
to present occasional shower and storm chances, the first arriving
Thursday afternoon as flow aloft becomes more zonal and carries a
weak shortwave through the area. Perhaps a stray shower or storm on
Friday, otherwise looks like a brief period of drier, slightly
cooler weather to start the weekend as high pressure builds in from
the north. An amplifying ridge will slide over the region Sunday
into Monday with temperatures warming back up and occasional ridge-
rider shortwaves providing additional shower/storm chances.

&&

.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1109 PM CDT Mon Jun 1 2020

VFR conditions are expected through early Tuesday afternoon, with
thunderstorms moving into the TAF sites by late afternoon into the
evening. These may bring periods of MVFR or lower visibility
reductions in heavier rain.

South to southwest winds around 5-15 knots and mostly clear skies
are expected to continue into Tuesday morning. There may be some low-
level wind shear at KLSE before diminishing by early morning.
Otherwise, mostly sunny skies are expected Tuesday into early
afternoon. Still watching potential for some cumulus to develop, but
so far confidence remains low and have not included it at this time.
By later afternoon into the evening, thunderstorms are expected to
move into the TAF sites from the west/northwest. Some of these could
be severe. Storms look to push off to the southeast later in the
evening. For now, have kept VCTS in the TAFs until a higher
confidence time period can be pinned down for storms. Winds will
turn from southwesterly during the afternoon to more northerly by
late evening as a front moves through, remaining generally 5-15
knots, except for near thunderstorms where gusts could be much
higher.


&&

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

$$

SHORT TERM...Kurz
LONG TERM...Kurz
AVIATION...Lee


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