Area Forecast Discussion
Issued by NWS La Crosse, WI

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041
FXUS63 KARX 280810
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
310 AM CDT Tue Mar 28 2017

.SHORT TERM...(Today through Wednesday)
Issued at 310 AM CDT Tue Mar 28 2017

Early this morning water vapor imagery indicated prominent upper
lows across the Desert Southwest and Ohio Valley regions with
another weaker shortwave approaching northern Minnesota within zonal
flow. At the surface, a very baggy surface pressure gradient has
resulted in light winds over the area. Low stratus have been very
slowly sinking southward from central WI southwest into northern IA.
Areas of locally dense fog have been developing in the clear air
to the northwest of the clearing line across northeast Iowa,
southeast Minnesota, and north-central WI. Will have to monitor
how far the clearing line reaches through early morning, as well
as the coverage of fog.

Drier air will continue to work into the area today as high
pressure centered north across Manitoba/Ontario noses
southeastward into the Upper Midwest. The main question today will
be how quickly the low stratus/fog persists. It may take into the
afternoon for clouds to clear northeast IA/southwest WI, but with
925 mb temps warming to 8-10C and increasingly sunny skies, temps
should warm into the 50s in most areas.

However, by tonight mid and high clouds will be on the increase
ahead of another southern stream upper low coming out of the
Desert Southwest. NAM/GFS soundings indicate low-levels will
initially remain fairly dry into Wednesday. As low-level warm
advection increases, gradual moistening of the low-levels should
occur, with showers increasing, especially west of the
Mississippi during the day.

.LONG TERM...(Wednesday night through Monday)
Issued at 310 AM CDT Tue Mar 28 2017

The upper low will move from the plains towards the Mid-Mississippi
Valley Wednesday night to Thursday night. The overall thinking
remains the same that the highest precip chances will be Wednesday
night into Thursday morning as stronger low to mid-level warm
advection/frontogenesis lifts across the region. One point of
uncertainty remains how far north the precip will reach as surface
high pressure off to the northeast may help impede the northward
progress of the precip. The GFS has continued to trend farther
north with the precip shield closer to the ECMWF, while the
GEM/NAM are slightly farther south. Consensus suggests the highest
chances will be along and south of I-90, with much of the precip
exiting by late Thursday. Some of the model variability likely
stems from differences in how/if the upper low interacts with a
northern stream wave, and the placement of the upper low/strongest
forcing. However, overall, model agreement has been reasonably
good with this system. Precip amounts look to be highest across
southwest WI into parts of northeast IA, where over half an inch
is possible, with decreasing amounts northward. A period of mixed
precip is possible especially late Wednesday night into Thursday
morning depending on boundary layer thermal profiles before temps
warm sufficiently to change any mixed precip to rain. Any wet
snow accumulations look minor at this time.

Behind this system, a split flow pattern will continue through the
remainder of the extended period. Initially weak ridging is
expected Friday and Saturday with temps recovering back to near or
above average. 00Z models bring a few weak upper waves across the
region during the weekend with only low precip chances warranted
right now. Another upper low is expected to eject from the
Rockies early next week. The 28.00Z ECMWF shows much stronger
interaction between this system and a northern stream wave
compared with the GFS which shows little interaction and
suppresses the upper low well south. Needless to say, confidence
is low late in the period.

&&

.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
category.

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.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Baumgardt



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