Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 280000

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

.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 00Z TAFS through 00Z Tuesday evening)
Issued at 700 PM CDT Mon Mar 27 2017

Enhanced northerly flow over the past hours has brought in low-
level drier air and lifted ceilings a bit into the MVFR/VFR
category. This flow is suggested to continue through the evening
and should keep ceilings steady as boundary layer begins to cool.
At some point, this cooling should begin to lower ceilings.

However, there has been some suggestion in the model guidance that
this drying could actually clear some areas of sern MN into
north-central WI. Satellite does indicate clear skies over much
of MN and northwest WI in the drier air. This will bring problems
with light winds and already low dewpoint depressions and could
cause rapid onset dense fog and LIFR conditions at KRST. Will
continue to monitor satellite clearing trends and try to update
TAF accordingly as needed. For now, the 00Z Tafs have a similar
flavor as previous forecast, with stratus and IFR fog by morning.

Model forecast guidance suggests the winds will become more
easterly overnight which will help to hold the stratus in place
and prevent the fog from forming. Will monitor for this but right
now, satellite cloud motion indicates the clearing trend may

In either case, VFR should build in rather quickly Tuesday.



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