Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 110458

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CST Fri Nov 10 2017

.SHORT TERM...(Today through Saturday Night)
Issued at 227 PM CST Fri Nov 10 2017

At 2 PM, water vapor imagery showed a short wave trough moving east
through the Dakotas.  Despite clouds, temperatures have warmed into
the 20s across the Upper Mississippi River Valley.

The 10.00z models are in good agreement that the short wave trough,
currently over the Dakotas, will move quickly east across the region
tonight.  There will be moderate 280K isentropic lift ahead of this
trough.  This lowers condensation pressure deficits to less than 10
mb north of Interstate 94 tonight. The soundings suggest that snow
will develop after 8 PM and then continue through 4 AM.  As the
short wave pulls away from the region, there will be a loss of ice
aloft.  This may result in a brief period of freezing drizzle as
this precipitation ends.  Snow amounts for this area will likely be
in the 1 to 2 inch range.

On Saturday, another short wave trough will be approaching the area
from the Northern Plains. The 280 through 290K isentropic surfaces
show that the area will be saturated, but there is only very weak
moisture transport into the region.  As a result, there is concern
that the models are generating far too much QPF.  While temperatures
climb above freezing, surface dew points will be sub freezing, so
there is concern that there could be light freezing rain or freezing
drizzle.  The greatest threat looks to be in north-central and
central Wisconsin where the dry air will be the slowest at moving
out of the region.  Since confidence is low, we will just mention a
possibility of some light icing and not go with a winter weather
advisory at this time.

Secondary mid-level shortwave trough rotates through the area
Saturday night but models show this wave dampening/weakening with
time for decreasing precipitation chances by late evening/overnight.
Warmed overnight lows a few degrees from previous forecast mainly
south of I-90 due to warmer air being in place during the day
Saturday and lack of cold air advection/push. As a result, mainly a
rain or rain/snow outcome is expected south of I-90 while there will
be a small chance of some freezing rain along and north of I-90
where colder surfaces are expected.

.LONG TERM...(Sunday through Friday)
Issued at 227 PM CST Fri Nov 10 2017

Bulk of models take precipitation chances southeast out of the area
by Sunday morning, but the GFS which is an outlier brings some
deformation precipitation across northeast IA/far southwest WI. As
such, kept a small-end chance of rain or rain/snow mix down that
way. For the rest of the day, clouds are expected to clear from west
to east with the passage of the mid-level trough and increasing
subsidence from ridge building in from the Plains. Plan on
temperatures still on the chillier than normal side though with
highs only in the middle 30s to near 40.

Drier and slightly warmer temperatures on tap for Sunday night into
Monday as a fairly strong ridge of high pressure moves across the
region. Highs Monday are expected to be in the middle 30s to middle
40s. which is just a couple degrees below normal.

Rain chances will be seen for Tuesday in warm air advection and
moisture transport nose in to the area ahead of low pressure
crossing the Plains. Highs Tuesday look like they will actually
surpass normal with readings in the 40s to lower 50s.

Models in good agreement in pushing a cold front through the area
late Tuesday night/Wednesday morning with what looks to be a decent
slug of rain as precipitable water values increase into the 3/4 to 1
inch range.

GFS/ECMWF show a strong storm system lifting out of the Rockies into
the region for Thursday night into Friday. Too far out at this point
to assign any degree of certainty, but will continue to monitor for
precipitation type and impact on the area.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1100 PM CST Fri Nov 10 2017

Cigs: cigs will continue to lower, but not as clear cut when MVFR
will move in. Latest sfc obs/satellite trends show an expansive area
of 2.5-4 kft clouds off to the west, moving east. However, latest
models runs suggest the lower cigs could transit just north of the
TAF sites overnight. Better agreement with MVFR pushing in later Sat
afternoon, holding through the evening. Going to trend the forecast
this way.

WX/vsby: area of -sn moving across east-central MN into northwest WI
at late evening. The pcpn is generally holding north of I-90, and
outside of a few flurries, don`t anticipate an impact at KRST/KLSE.
That said, a few observation points have noted a brief period of
freezing drizzle behind the snow. Something to watch, but not enough
confidence to include.

Second shortwave will spin across southern parts of the region on
Sat. Model consensus favors keeping the bulk of pcpn chances
associated with this feature south of I-90, even moreso now in the
later runs compared to earlier in the evening. Warming ahead of this
system would result in full/partial melting of anything that falls,
bringing rain/sleet/freezing pcpn questions (depending on sfc
temps). With trends saying "south" now, confidence doesn`t support
continuing any mention of pcpn at this time. Of course, will monitor
and update if necessary.

Winds: mostly south/southeast through the period, lightening up Sat
evening as the pressure gradient slackens ahead of an area of high
pressure over the northern plains.




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