Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 141104

Area Forecast Discussion
National Weather Service La Crosse WI
605 AM CDT Mon Aug 14 2017

.SHORT TERM...(Today through Tuesday)
Issued at 252 AM CDT Mon Aug 14 2017

A relatively slow moving upper trough will swing across the area
today into tonight. An organized area of mainly showers that has
been working across southern MN and northern IA has continued to
weaken as it progresses eastward into a more stable environment.
However, this area of showers will continue to move eastward into
the morning hours with forcing from the upper wave and some 850 mb
moisture transport nosing into the area. Ahead of the cold front
MLCAPEs of around 500-750 J/kg may build this afternoon,
especially over northeast Iowa into southwest Wisconsin. Isolated
to scattered diurnal showers/storms are possible then through the
afternoon into the early evening, but any storms will remain non-
severe given the limited instability. Weak high pressure beneath
transient mid-level ridging should result in a mainly dry weather
later tonight and Tuesday. However, there is concern that low
stratus or fog may develop tonight. NAM soundings are quite
supportive of this, with the SREF also indicating high fog/stratus
probabilities as well. If this occurs, a good chunk of Tuesday could
end on the cloudy side with weak mixing north of the surface
boundary to the south of the area.

.LONG TERM...(Tuesday night through Sunday)
Issued at 252 AM CDT Mon Aug 14 2017

Another upper level trough will begin ejecting across the central
plains by Tuesday night. Strong moisture transport will occur ahead
of the associated frontal cyclone with precipitable water values to
1.75+ inches over the area. The 14.00Z ECMWF has trended slower and
stronger with upper wave, more similar to the GFS. Broad forcing
associated with upper wave and low-level warm advection and
isentropic upglide will lead to increasing chances for showers and
storms Wednesday and Wednesday night, with shower chances possibly
lingering into Thursday if the system continues to trend towards the
stronger/slower side. Deep layer shear profiles are expected to be
on the weak side, with maybe 30 kts of 0-6 km shear. Widespread
clouds may also hinder destabilization and the potential for
stronger storms. However, locally heavy rainfall still appears to be
the main threat, with warm cloud depths extending to above 4 km and
precipitable water standard anomalies of +2. Still a little early to
have a high degree of confidence on the details of the system, but
it certainly looks to be the best chance for widespread
showers/storms over the coming week, with some potential for heavier

Behind this system, a general west to northwest flow pattern looks
to set up into late week. Overall, temps should remain fairly
seasonable heading into next weekend. Poor agreement and continuity
with timing and degree of amplification of embedded upper waves
exists among models late this week, so confidence in rain chances is
low at this time.


.AVIATION...(For the 12Z TAFS through 12Z Tuesday morning)
Issued at 605 AM CDT Mon Aug 14 2017

Cigs: challenging forecast in relation to sky cover/heights. Area of
low stratus expanding across MN early this morning, behind the
eastward moving area of showers. Meso models project the stratus to
continue to advance east, covering KRST and likely KLSE. Models then
point to mixing toward late morning/afternoon that would bring cigs
back to VFR...with some potential for scattering out in the evening.
Low clouds progged to redevelop back across the TAF sites later
tonight - moreso for KLSE. A lot of "ifs" and "coulds" and
anticipate updates to the forecast as trends become more clear. For
now, will play mvfr cigs for this morning, with vfr then into this
evening and back into mvfr for the overnight (IFR possible).

WX/vsby: most of the shra look to be east and/or south of KLSE and
KRST. May carry a vcsh for a couple hours at KLSE. Some
redevelopment is expected this afternoon, but favored to the east.
Will leave it dry for now.

Vsby concerns then crop up for tonight. If it clears for a period,
fog is likely - and probably sub 1sm. If not, low stratus would be
favored, with mvfr vsbys more likely. Going to side with a stratus
layer for now.

Winds: south winds will go west this afternoon, north tonight with
the passage of a front/sfc trough.




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