Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150800

Area Forecast Discussion
National Weather Service La Crosse WI
300 AM CDT Tue Aug 15 2017

.SHORT TERM...(Today and tonight)
Issued at 300 AM CDT Tue Aug 15 2017

Mid-level shortwave ridging will slide across the area today atop
weak surface high pressure, as yesterday`s frontal boundary drops
south of the area. The main immediate concern will be fog/stratus
through the early morning. A moist boundary layer (temp/dew point
spreads within a few degrees) and light winds is quite favorable
for fog, and patches of locally dense fog have developed
overnight. However, a low stratus field has also been expanding
through the early morning hours, keeping visibilities in check
under the stratus. Will continue to highlight the fog with an SPS,
but will monitor for any expansion of the fog over the next few
hours. The fog/stratus should lift and gradually scatter out
through the day with comfortable afternoon temps in the 70s to
low 80s.

Tonight, the first in a series of upper level shortwave troughs
within a broader trough will approach the region. Deep layer
moisture will be on the increase ahead of the trough overnight with
some showers/storms possible later on overnight, especially west of
the Mississippi, with increasing isentropic upglide/warm advection
ahead of a warm front.

.LONG TERM...(Wednesday through Monday)
Issued at 300 AM CDT Tue Aug 15 2017

Wednesday into Wednesday night remains the main time period to watch
over the next several days. Several embedded shortwaves will rotate
through a broader upper trough which is expected to be across the
Upper Mississippi Valley by early Thursday morning. Steady low-level
moisture transport ahead of the trough will result in anomalous deep
layer moisture, with precipitable water nearing 1.75 inches. The
stronger forcing tied to the upper trough will only gradually
spread east on Wednesday as shortwave ridging across Wisconsin in
the morning moves eastward with the highest rain chances during
the afternoon/evening hours. With the broad but not terribly
strong forcing, there is uncertainty in how much rain and clouds
there will be ahead of the trough during the morning into the
afternoon. This will have ramifications for how far northward the
warm front/instability will reach, and the potential for stronger
storms. The NAM/GFS do try to bring a narrow warm sector up
across the area with the surface low across southern Minnesota by
Wednesday evening. Up to 30 to 40 kts of shear is expected, mainly
concentrated in the 0-3 km layer. As a result, there is certainly
some potential for strong to possibly severe storms, but this
will depend on building sufficient instability. Right now
confidence in the warm frontal position and degree of
destabilization is low.

It continues to look like locally heavy rainfall may be the bigger
threat given the degree of moisture and deep warm cloud depths.
The environment would support high rainfall rates in any stronger
storms Wednesday and Wednesday night, so any areas impacted by
multiple or slow-moving storms could see rainfall add up quickly.
At this point, the flood risk remains low given the recent dry
weather and with the system likely to be progressive.

The front will slide through early Thursday morning putting an
end the heavier shower/storms. After the frontal passage, a
quieter late Thursday/Friday period is expected with surface
ridging over the area beneath northwest flow. Temps should remain
seasonable. The 15.00Z GFS/ECMWF bring an upper trough through
Friday night into Saturday with a chance for showers or storms.
Trends point toward a more zonal flow for the weekend into early
next week, with temps warming up a bit, perhaps a bit above
average. Another upper trough passing by to the north on Monday
could bring a threat for showers/storms.


.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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