Area Forecast Discussion
Issued by NWS La Crosse, WI

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571
FXUS63 KARX 230822
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
322 AM CDT Wed May 23 2018

.UPDATE...
Issued at 322 AM CDT Wed May 23 2018

Observations and calls to several counties across southwest
through central and north-central WI indicate dense fog becoming
more widespread. Accordingly, have issued a Dense Fog Advisory
until 9 am for these areas.

&&

.SHORT TERM...(Today through Thursday)
Issued at 247 AM CDT Wed May 23 2018

Surface high pressure centered over eastern WI/IL will slowly
push eastward through tonight. Areas of locally dense fog have
developed across central WI and in some river valley locations.
Will have to monitor how widespread the fog becomes over the next
few hours, with improving conditions after sunrise.

Elevated convection has been developing over southwest MN into
northern IA in a zone of enhanced mid-level warm
advection/isentropic ascent. This convection will move into
northeast IA/southeast MN through the early morning hours before
beginning to wane as the low-level jet/moisture transport weakens
through the morning. Locally heavy rainfall could occur under any
of the slow-moving storms. Could not rule out a few lingering
showers or a storm through the day in the warm advection regime,
but with very spotty coverage. Highs today will be well into the
70s to low 80s.

Mid-level ridging remains over the area tonight into Thursday with
the stronger northward push of moisture largely remaining west of
the area ahead of a shortwave trough over the northern Rockies.
Despite the limited forcing, large scale low to mid-level warm
advection along with increasing in moisture could result in some
elevated shower/storm potential tonight into Thursday. Right now
northern WI looks to be most favored, but confidence in
coverage with this sort of warm advection convection isn`t
particularly high. Many of the high res models are indicating
little in the way of precip over the area tonight. Thursday highs
will be well into the 80s in most areas with noticeably higher
humidity.

.LONG TERM...(Thursday night through Tuesday)
Issued at 247 AM CDT Wed May 23 2018

The main concerns through these periods include convective potential
on Friday and warm/hot weather through early next week.

Convection will likely fire well off to the west Thursday afternoon
along a surface trough as an upper shortwave trough lifts across the
northern plains. Some of these showers/storms could approach in a
weakening fashion later Thursday night as stronger low-level
moisture transport and large scale height falls overspread the
region.

Still looks like our best chances for storms will be Friday
afternoon and Friday evening. Forcing doesn`t look particularly
strong, but the combination of the approaching upper trough and a
weak surface boundary should be enough to generate convection.
Continued boundary layer moistening ahead of the upper wave
combined with daytime heating may allow MLCAPEs of 1500-2500 J/kg
to build, but deep layer shear will likely be around 25 kts or
less, limiting a severe threat. A few strong storms are possible
though.

Otherwise, a sustained period of very warm temps will be the story
through the holiday weekend. NAEFS 850 mb temp standard anomalies
peak around +2 over the weekend with 925 mb temps consistently
peaking in the 23-27C range through early next week. Expect highs
in the mid 80s to low 90s during this time while lows will be
mostly in the 60s. Heat indices could reach 95 in spots. This
will be the first sustained very warm spell of the season, so
potential heat related impacts could be enhanced, especially with
many outdoor activities over the holiday weekend.

Otherwise, the stronger westerlies remain well north through early
next week with mid-level ridging looking to build back into the
Upper Midwest. All in all, any shower/storm chances look pretty
low through the weekend into early next week so only have
occasional periods of very low precip chances.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1145 PM CDT Tue May 22 2018

Radar at 0430Z showing thunderstorm activity has broken out across
Iowa as forecast and this should begin to work its way toward
KRST overnight. Still some questions on the instability that will
shift in and how much TSRA will be around versus just SHRA.
Confidence was not high enough to TEMPO the TSRA at this TAF
issuance.

Fog is beginning to develop across central WI and this trend
should continue in WI. Big question is the cirrus spreading in
from the Dakotas and if it will be enough cloud thickness to stop
any cooling for dense fog development in the river valley. Stayed
the course for now with the forecast as there is only a 3F
dewpoint depression at 04Z.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 9 AM CDT this morning for WIZ017-029-
     034-042>044-053>055-061.

MN...None.
IA...None.
&&

$$

UPDATE...JM
SHORT TERM...JM
LONG TERM...JM
AVIATION...Baumgardt



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