Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 181123

Area Forecast Discussion
National Weather Service La Crosse WI
623 AM CDT Sun Mar 18 2018

.SHORT TERM...(Today through Tuesday)
Issued at 233 AM CDT Sun Mar 18 2018

Water vapor satellite this morning is showing a large trough over
the western part of the country with ridging downstream over the
Midwest. The 18.00Z models are in good agreement that a short wave
trough coming out of the western trough will come out of the
southern Rockies late today and then remain well south of the area
as it moves across the central Mississippi River Valley. The area
of high pressure at the surface currently centered over Iowa, will
remain through much of today before retreating to the north with
the passage of the system to the south tonight. As the high starts
to retreat, the band of clouds over the Missouri River Valley
should gradually spread east and eventually cover the western and
northern sections of the forecast area.

Once the system coming out of the western trough slides south of
the area tonight, broad troughing will set up over the Upper
Midwest into the eastern part of the country. A short wave trough
in the pattern is expected to slide southeast toward the area
Tuesday and weaken as it comes in. The forcing from this wave
looks to be rather weak with only some weak pv advection in the
500-300 mb layer possible Tuesday afternoon and night. This is
also about the same time as a band of weak frontogenesis looks to
move across in the 850-500 mb layer. The best axis of moisture for
this system to work with will remain west of the Mississippi River
as a dry low level northeast flow will set up over much of
Wisconsin from an area of high pressure over Ontario. With this
limited area of moisture, the precipitation chances from this
system will primarily be limited to areas along and west of the
Mississippi River with the highest chances Tuesday morning.

.LONG TERM...(Tuesday night through Saturday)
Issued at 233 AM CDT Sun Mar 18 2018

Upper level ridging will re-establish itself over the central
part of the country behind the Tuesday system, but the medium
range models are showing differences for how long the ridging
stays in control. Long wave troughing will still be in place along
the West Coast with the models showing pieces of energy getting
ejected from this trough, but differ on the timing. The 18.00Z GFS
is much quicker bringing a short wave trough through the ridge
already Thursday night and is then followed by another for
Saturday. The 17.12Z ECMWF is slower bring the first short wave
trough into the area holding it back until Friday night. With
these differences in timing, will have a chance of precipitation
starting Thursday night and continuing into the weekend but will
keep the chances at 70 percent or lower until better model
consensus is seen.


.AVIATION...(For the 12Z TAFS through 12Z Monday morning)
Issued at 623 AM CDT Sun Mar 18 2018

925-850mb winds progged by models to be SW to W 5-10kts this
morning, yet ARX WSR-88D VAD winds south at 20-30kts in the sfc-8K
ft level. Spring bird migration is on this morning, so use any lower
level 88D VAD winds with caution.

Sfc high to weaken/slip east today, with SW winds around the
perimeter of this high working to spread the MVFR/IFR cloud deck
west and SW of the area into NE IA and SE MN. Models not handling
this low moisture/cloud deck all that well, so confidence not all
that high in the cloud fcst at KRST today into tonight. Better
consensus this moisture/cloud will stay west and north of KLSE as
lower level winds back to a drier easterly flow this evening. VFR
conditions expected at KLSE this taf period, with light/VRB winds
much of today, becoming light NE tonight. Carried a period of VFR
clouds/bkn cigs (diurnal mixing/warming to raise cloud hgts today)
at KRST mainly this evening into the overnight hours.




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