Area Forecast Discussion
Issued by NWS La Crosse, WI

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809
FXUS63 KARX 231058
AFDARX

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

.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 12Z TAFS through 12Z Thursday morning)
Issued at 600 AM CDT Tue May 22 2018

Cigs/vsby: low clouds have filled the Mississippi River valley, and
has spread across KLSE. Some ebb and flow to that with a relatively
light sfc/near sfc layer. In addition, it wasn`t just stratus but
also fog. Upstream at KONA vsbys have dropped to 1/4-1/2SM on and
off. Visual at KLSE shows fog hanging across the river, not yet
pushing onto airfield (at least from that vantage point). Scenario
has played out very closely to a fall valley fog scenario. Fog may
yet push up onto the airfield, but could just as likely hold with
low stratus gradually dissipating/lifting by 14z or so. Will wait
til last second to issue TAFs, monitoring obs to see if vsby
restrictions outside of BCFG mention are needed.

Otherwise, mostly a bkn vfr mid level clouds to contend with
today...blow off from line of showers/storms over southern MN and
northern IA. Expecting sct skies tonight, but don`t believe fog
threat is as high -  more wind, a day removed from most of the rain,
greater t/td spread by 00z this evening.

WX: showers and storms continue to approach KRST and radar trends
suggest it should be raining by TAF time. Looks reasonable. Thunder
threat not as high as the pcpn moves east of the available
instability. Meso models (HRRR, Hopwrf) are inline with putting the
breaks on the pcpn at the Mississippi river, diminishing it by 18z.
Going to keep some VCSH at KLSE for now and monitor.

Winds: staying south/southeast today, with some increase into the
afternoon. Not as light tonight with increasing pressure gradient as
sfc high departs to the east.

&&

.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.
&&

$$


SHORT TERM...JM
LONG TERM...JM
AVIATION...Rieck



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