Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 191121

Area Forecast Discussion
National Weather Service La Crosse WI
621 AM CDT Thu Apr 19 2018

.SHORT TERM...(Today and tonight)
Issued at 239 AM CDT Thu Apr 19 2018

Main short-term fcst concerns are temperatures today/tonight and
fog potential later tonight (not wintry precip for a change).

06z data analysis had a broad area of low pressure centered over the
mid Atlantic states to Can Maritimes and broad high pressure ridging
thru the plains states. Mid level shortwave that brought snow to
much of the area Wed was quickly exiting with the last of the precip
already confined to northern IL. Clouds were clearing from north to
south across the region, with clouds over the northern 2/3 of the
region already limited to sct-bkn cirrus over MN. Where skies have
cleared temps falling into the upper teens and 20s, with 18F already
at KCMY over the fresh snow cover.

19.00z model runs initialized well. Model in good agreement for the
longwave pattern to be progressive with ridging building east across
the central CONUS, ahead of the next strong troughing digging
into the SW CONUS. Short-term fcst confidence is good this cycle.

For the short-term, after several days of winter weather headlines
and bouts of wintry weather, finally a transition to quieter and
warming weather. After the Wed system continues exit quickly east/SE
overnight, the main features for today/tonight will be ridging
building aloft and sfc high pressure building in from the plains.
lower and higher clouds already clearing from the north as the
system departs and northerly flow behind it spreads across the area.
BL mixing/drying from the north should scour out any patchy fog
early this morning as dew points drop into the teens. Deep, dry 925-
500mb airmass progged over the area today/tonight, with clouds
limited to some cirrus ahead of a weakening shortwave dropping
across MN. Northerly low level flow actually spreads a little low
level warm advection in from the north today. Even so, there is
another batch of fresh snow cover to melt and with high albedo.
Models differ on depth of diurnal mixing today. Even with the snow
cover, mid April sunshines should be able to mix to at least 925mb,
with mixed 925mb temps supporting highs generally in the 40s.

Center of the high is overhead tonight with light winds and what
should be clear/mostly clear skies for some stronger radiational
cooling over the snow cover. With BL moisture loading from the
melting snow today, strong radiational cooling tonight and model
soundings showing a shallow/strong low level inversion later
tonight, added patchy to areas of fog across the entire fcst area
later tonight into Fri morning. One more colder night tonight, with
the blend of guidance lows mainly in the upper teens to mid 20s
looking good.

.LONG TERM...(Friday through Wednesday)
Issued at 239 PM CDT Wed Apr 18 2018

For Friday thru Saturday night: main fcst concern this period is
warming temperatures.

Model runs of 19.00z in good agreement for ridging aloft to continue
building across the Upper Midwest Fri thru Sat night as the
southwest CONUS low/trough tracks to the lower MS valley by 12z Sun.
Fcst confidence is good this period.

A tranquil period of weather is set to continue Fri thru Sat night
as ridging at the sfc and aloft remain over the region. Little low
level warm advection this period with the center of the sfc-850mb
high nearby and light flow. Airmass will continue to modify in place
with late April sunshine. Snow cover/melting snow cover will
continue to temper the warming Fri. Some mid/high clouds from the
southern stream low trough expected to spill northeast across the
region and temper the warming on Sat. For now, blend of guidance
highs in the mid 40s to lower 50s Fri and mostly in the 50s for Sat
look good.

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are warming temps Sun/Mon, mainly rain chances Tue/Tue night,
melting snow and runoff and potential impacts on area rivers.

Medium range model runs of 19.00z showing the ridging aloft to
remain over the region thru Mon then reasonable agreement for a
northern stream shortwave trough to drop into/across the Upper
Midwest Mon night into Wed. Day 4-7 fcst confidence is average to
good this cycle.

Decent confidence for the dry, quiet, warming string of days to
continue Sun/Mon with ridging aloft and the sfc ridge axis remaining
over the region. 925-850mb ridge axis looking to slip far enough
east of the area for at least some weak warm advection in SW low
level flow Sun/Mon. Much of the snow cover should be gone by Sun,
with mixed 925mb temps producing highs in the upper 50s to lower 60s
Sun and mostly in the 60s Mon, near the late April normals!
Shortwave trough to drop across Tue brings increasing moisture and
cold front with it. Timing/detail differences as expected at the day
6/7 time range, but some mostly 30-50% rain chances Tue/Tue night
good for now.


.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 621 AM CDT Thu Apr 19 2018

VFR conditions will prevail today with northerly winds near or
below 10 kts and some scattered high clouds. Winds will become
light tonight under the high pressure ridge axis over the area.
Areas of fog likely will develop, especially given the moist
ground. Brought IFR visibilities into KLSE/KRST after midnight for
now, although LIFR conditions are possible early Friday morning.


.HYDROLOGY...(This Weekend into early next week)
Issued at 239 AM CDT Thu Apr 19 2018

More sunshine and warming temperatures today/Fri will produce
snowmelt across the area. The snowmelt looks to accelerate
Sat/Sun/Mon with even warmer daytime temps and nighttime lows only
dipping to near or staying above freezing. Runoff from the melt will
lead to rises on area streams and rivers. Besides the Trempealeau
River at Dodge possibly exceeding flood stage this weekend, the rest
of the area streams and rivers are expected to just see significant
within-bank rises. Rises on the Mississippi River will be more
into the middle and later part of next week as it will take some
time for the meltwater from central MN and northern WI to flow
south and reach our area. Those with interests or property close
to streams and rivers will want to stay up to date on latest river
forecasts in the coming week.




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