Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 280446

Area Forecast Discussion
National Weather Service La Crosse WI
1146 PM CDT Mon Mar 27 2017

Issued at 950 PM CDT Mon Mar 27 2017

GOES-16 Fog product is really awesome to have every 5 minutes.
Clearing of stratus is now roughly along I-90 and overall
continuing south. In last frames of fog imagery, there could be
some cloud forming on northern side of stratus seemingly to slow
its progression from diurnal cooling.

Short term model guidance /HRRR/RAP/ both insist that clearing
line will be progress to about roughly central WI to KMCW
overnight and winds veer to east and halt progression. With
dewpoint depressions of 3-5F in the clearing, fog should set in
over next hours and become dense. A Dense Fog Advisory is being
discussed with the NWS offices in the area.

For now, have increased the fog coverage in the forecast and will
work to get some stronger fog messaging out.


.SHORT TERM...(This evening through Wednesday night)
Issued at 246 PM CDT Mon Mar 27 2017

Early this afternoon, an area of low pressure was over southern
Missouri while a ridge of high pressure was centered over Canada
with the ridge axis nosing south into the Upper Midwest. While not
a strong flow, just enough cyclonic curvature between these two
features to help maintain some weak lift over the region.
Soundings show plenty of moisture trapped below the low level
moisture and in the cyclonic flow, this was maintaining the clouds
over the area. The models have been insistent in showing the ridge
to continue nosing farther south and bringing in some drier air
that allows the clouds to break up. Given the amount of clouds
currently shown by the satellite and that there will not be much
change in the weak flow pattern tonight, the clouds may have a
hard time breaking up until Tuesday morning. If the clouds were to
scatter out, with light winds and plenty of low level moisture,
would tend to think this would be a good set up for fog formation.
For now, will go with the slower clearing trend and keep skies
cloudy through the night.

The ridging aloft and at the surface will not last long over the
Upper Midwest before the flow turns back to the southwest ahead of
the upper level low coming out of the southern Rockies. The
models have been separated into two solutions but it appears there
has been a bit of southward shift by the 27.12Z ECMWF and a
northward shift by the 27.12Z GFS. There are still some
differences between these in regards to a northern stream short
wave trough cross the Upper Midwest Wednesday night into Thursday
as the upper level low is approaching from the southwest. The
ECMWF remains the strongest with this northern stream wave but may
be trending weaker with it which in turns does not draw the
precipitation as far north as some of the previous runs. The GFS
has always been much flatter with this northern stream wave and
continues this trend with the latest run.

As this system starts to approach Wednesday, there will be some
weak to at times moderate pv advection in the 500-300 mb layer
that will spread over the area. The warm air advection ahead of
the system is expected to send a few rounds of 1 to 3 ubar/s of
up glide on the 290K isentropic system over the area, especially
across the south. The low level moisture transport does not look
to be overly strong but does start to come into the southern
sections of the forecast area late Wednesday afternoon into
Wednesday night. Expecting this to be enough forcing to allow
showers to start moving into the area Wednesday afternoon and
continue across mainly the southern sections Wednesday night.

.LONG TERM...(Thursday through Monday)
Issued at 246 PM CDT Mon Mar 27 2017

Looks like the forcing from this system will continue into
Thursday morning before starting to shift off the east starting in
the afternoon but especially for the evening. This should keep the
deformation band over the area into the afternoon with just some
lingering activity expected into Thursday evening. Behind this
system, weak high pressure should build in to end the work week
and start the weekend, but the upper level flow will remain
southwest with another system expected to come out of the southern
Rockies. Differences between the models on how far north this next
system will be along with how strong the ridging ahead of it will
be over the eastern portion of the country. For now, will just
have some low precipitation chances to close out the weekend.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1146 PM CDT Mon Mar 27 2017

MVFR stratus has now cleared to the south of KRST and is right on
the KLSE airfield at 0445Z. With dewpoint depressions in the low-
levels of less than 3F, nearly calm winds will favor fog
formation by morning. The fog at KRST could be in the LIFR

At KLSE, because the clouds are just now clearing, the fog will be
a bit more delayed, but still should onset in the 08-10Z time
frame. IFR is expected at KLSE unless the cloud line does not
clear south of the field.

The fog is still expected to lift during the morning hours but
could be dense for several hours.




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