Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140752

Area Forecast Discussion
National Weather Service La Crosse WI
252 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 06Z TAFS through 06Z Monday night)
Issued at 1052 PM CDT Sun Aug 13 2017

A weakening batch of showers and a few thunderstorms continues to
work east out of southern Minnesota and northern Iowa, and should
impact RST and to a lesser extend LSE through mid morning - just a
little faster than earlier expected. Overall, precipitation should
exhibit a weakening trend as it outruns the better instability,
though a brief period of heavier rain is possible for RST along
with a thunderstorm or two. As the rain departs, we will be
watching the possibility that lower MVFR stratus will develop.
This is a trend hinted at strongly by most model guidance, though
confidence isn`t the highest given a lack of clouds at the moment,
though with stronger moisture advection still to take place before
sunrise. Any lower stratus should "mix out" into more of a VFR
cumulus cloud deck through the afternoon hours, as winds shift
from the southeast to southwest and eventually north behind a
passing cold front.

Looking just beyond the current TAF period, there have been pretty
good signals for a widespread MVFR or even IFR stratus deck to
redevelop prior to sunrise Tuesday. Will need to watch trends
closely with this one.




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