Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 151725

Area Forecast Discussion
National Weather Service La Crosse WI
1225 PM 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 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1225 PM CDT Tue Aug 15 2017

MVFR strato-cu will continue to lift/mix out early this afternoon
with diurnal warming and a bit of drier air advecting in from the
E/NE. Good VFR expected by mid afternoon, thru the evening into the
early overnight hours as weak high pressure holds over the western
great lakes. The leading moisture return ahead of low pressure
coming across the central/northern plains spreads across the area
centered on about 12z Wed. This over a weak sfc-850mb warm frontal
zone. Mdt signal among models for a band of SHRA/isolated TSRA to
lift NE across the taf sites during the late overnight hours or
early morning hours of Wed. Included a period of SHRA at KRST with
only a VCSH mention at KLSE (further east) for now. Left out VCTS/
TSRA/CB mention for now due to limit instability and lower
confidence in coverage of any TSRA with the precip band.

Much of the moisture return progged to be near/above 805mb, so left
cigs late tonight/early Wed more in the 3k-4k ft range. More clouds
later tonight looking to limit late night/early morning BR/FG
development and kept this to a minimum in the tafs this cycle.




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