Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150430

Area Forecast Discussion
National Weather Service La Crosse WI
1130 PM CDT Mon Aug 14 2017

Issued at 1032 PM CDT Mon Aug 14 2017

You know what they say about "the best laid plans". Such is the
case with stratus sometimes, and the forecast has done a 180
regarding how much stratus is expected to develop, given a rapid
loss of upstream stratus earlier this evening. As of a few hours
ago, have steered the forecast heavily toward the idea of more
dense fog development than stratus overnight, given the combo of
clearing skies, light winds, and already very low temp-dew point
depressions. In fact, already seeing the usual suspect sites dip
below 1 mile for visibility, and per trends and the setup, believe
we are headed toward a Dense Fog Advisory type of event later

The one thing to watch remains a smaller patch of lower stratus
developing southward out of the I-94 corridor of central
Wisconsin. Based on RAP soundings and trajectories, looking like
that axis of low clouds may expand for areas east of the
Mississippi River, leaving plenty of real estate farther west to
fog in. However, just where said clouds end up will make a huge
difference regarding fog potential. Simply put, where stratus
exists, fog won`t, and vice versa. Should be a good night to "test
out" GOES-16 fog imagery.


.SHORT TERM...(This evening through Wednesday night)
Issued at 213 PM CDT Mon Aug 14 2017

Local and regional radars still showing a few showers over
portions of central Wisconsin in the cyclonic flow of the
departing short wave trough over the Upper Great Lakes. These
showers could possibly hold onto into the early evening over the
far eastern portions of the area, otherwise the rest of tonight
and into Tuesday look to be dry as high pressure over Minnesota
and western Ontario briefly builds in over the area and as upper
level ridging moves across the region.

The ridging aloft and at the surface will get pushed east of the
area starting Tuesday night. Water vapor satellite imagery
currently shows the next system was over Oregon and northern
California and will quickly move east toward the region. There
looks to be several short wave troughs embedded in the mean trough
with this system. The first of these short wave troughs will move
across the region Wednesday with another for Wednesday night. As
these waves move across the area they look to produce periods of
weak to moderate pv advection in the 500-300 mb layer. The low
level moisture transport looks respectable with these waves, but
never looks to be overly strong. The leading and convergent edge
of this moisture transport looks to start spreading in very late
Tuesday night or early into Wednesday morning and then starts to
diminish Wednesday evening as the first short wave troughs moves
past the region. The warm air advection ahead of the short wave
troughs looks to produce a period of 1 to 3 ubar/s (at times a
little higher) of up glide on the 305K isentropic surface.

The surface front currently across northern Iowa and southern
Wisconsin will continued to get pushed south as the surface high
builds in tonight. This front will start to return back to the
area Wednesday and how far north is gets will determine how much
instability and the coverage of storms that will occur. It
currently appears that the general consensus from the 14.12Z
models is that the front may get back into the southern portions
of the forecast area Wednesday afternoon. Recent runs of the GFS
brought in the most amount of CAPE, but the 14.12Z run now only
shows 500-1000 J/Kg of ML CAPE south of about Interstate 90. The
deep shear still looks to be lacking with maybe around 30 knots of
shear in the 0-3 km layer. This could be enough for a strong storm
or two but would not anticipate much severe weather. A larger
concern is with the possiblity of some heavy rain. The models
remain consistent in showing the warm cloud depths in the 3.5 to 4
km range with precipitable waters approaching 2 inches. This
should allow for some locally heavy rain but with the recent dry
stretch, the area should be able to take some rain and not
concerned about any flooding issues at this time.

.LONG TERM...(Thursday through Monday)
Issued at 213 PM CDT Mon Aug 14 2017

The last of the short wave trough with this system will move
across the area Thursday. By this time, the best low level support
will be well past the area, being tied to the lead short wave
troughs, so only expecting some lingering showers and storms to be
around. Northwest flow aloft looks to set up behind this system
which should allow another short wave trough to move across the
Upper Midwest for late in the week into the start of the weekend.
Differences between the GFS and 14.12Z ECMWF on how strong this
wave may be and how much rainfall that could occur. Either way,
enough to include a small rain chance for Friday and Saturday.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1130 PM CDT Mon Aug 14 2017

Skies continue to remain clear in many areas just before midnight,
with moisture conditions and light winds setting the stage for
fairly rapid fog development through sunrise. Already seeing some
areas dip below 1 mile at times, and suspect RST will rather
quickly dip to LIFR and eventually VLIFR levels with some dense
fog, mainly after 08Z. LSE remains a little more questionable, as
there could be some lower stratus working overhead at times,
hindering the drop in visibility to some degree. Still, given very
low temperature-dew point spreads at the moment and relatively
light winds, do believe that LSE has a pretty good shot at dipping
to 1/2SM or even lower for a time before and just after sunrise.

Any fog should lift through mid morning, perhaps manifesting
itself for a time as an IFR or MVFR stratus deck before VFR
conditions return by late morning with just some thickening cirrus
and diurnal cumulus. Winds will remain under 10 knots the next 24
hours, shifting from the north currently to southeast this
afternoon and evening.




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