Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 200734

Area Forecast Discussion
National Weather Service La Crosse WI
234 AM CDT Tue Mar 20 2018

.SHORT TERM...(Today and tonight)
Issued at 234 AM CDT Tue Mar 20 2018

Main short-term fcst concerns include -SN chances west of MS River
today and below normal temperatures today/tonight.

06z data analysis had low pressure over KY and high pressure over
northern Ont. Upper Midwest remained between the two, with a cool
and dry east to NE sfc-850mb flow across much of MN/IA/WI. WV
imagery showed a couple of shortwaves dropping SE across the
northern plains, with mid/high clouds thickening/spreading east
across the region ahead of them. Some -SN falling across the eastern
Dakotas and far western MN ahead of the waves and west of the low
level dry air intrusion.

Model runs of 20.00z initialized well. Models present similar
solutions thru tonight as shortwave energy in NW flow aloft ripples
SE into the region. A couple stronger waves to pass SW of the area
today/tonight with another to pass north of the area later tonight.
Short-term fcst confidence is generally good this cycle.

In the short-term, -SN chances west of the MS River today remain a
battle of lower level dry air feeding into the area on NE flow out
high pressure over Ont and the lift with the passing shortwaves.
Little in the way of lower level thermo-dynamic support over the
area with the sfc-700mb layer dominated by cold advection. X-
sections showing bulk of lift today is above 700mb, tied to a bit of
500-300mb PV advection and 300mb ageostrophic flow/divergence
between jets to the NE and SW of the area. Steeper lapse rates in
the 700-500mb layer as well vs. below 700mb. On the positive side
for the -SN chances, the DGZ lowers to around 700mb as the entire
column cools and puts a squeeze on the moisture. Model X-sections
showing sfc-700mb portion of the column remains rather dry along/
east of a KEAU-KLSE-KOLZ line, with better saturation in this layer
today west of roughly a KRGK-KRST-KCCY line. Will continue with
trend of -SN chances mainly west of this line and mainly thru the
mid morning into the early afternoon hours. Any -SN amounts would be
minor and look to mainly be on grassy areas. Clouds, NE winds and
the low level cold advection to limit warming today. Mixed 925mb
temps today only supporting highs in the 30s, with blend of guidance
highs looking quite reasonable. Lower levels ahead of the shortwave
to pass north of the fcst area tonight remain quite dry, for minimal
precip chances tonight. Lows problematic tonight as plenty of
moisture/clouds above 700mb progged to remain over much of the fcst
area thru 12z Wed and act as a blanket. Leaned toward warmer of
guidance lows tonight with the cloud cover and little if any snow
cover remaining over much of the fcst area.

.LONG TERM...(Wednesday through Monday)
Issued at 234 AM CDT Tue Mar 20 2018

For Wednesday through Thursday night: main fcst concern this period
is temperatures.

20.00z models in good agreement for a weaker shortwave to slide SE
across the region Wed, then followed by rising hgts as mid level
ridging is pushed east into the middle of NOAM. Area to remain east
of the ridge axis under NW flow aloft. Fcst confidence is good for
this period.

Passing shortwave trough axis Wed to take bulk of mid/high clouds/
moisture east/south with it. Wed trending to be a partly cloudy/
mostly sunny day under a broad sfc ridge axis. Model soundings
showing mixing to near 850mb Wed afternoon, with mixed 925mb temps
supporting highs in the upper 30s to mid 40s. Hgts aloft rise Wed
night thru Fri night, with the sfc ridge axis remaining near/over
the fcst area. Low level airmass looks to slowly modify in place
thru Thu night. Cool Wed/Thu nights under mostly clear skies/light
winds favorable for stronger radiational cooling, with mixed 925mb
temps on Thu supporting (near normal) highs mainly in the 40s.

For Friday thru Monday (days 4 to 7): main fcst concerns this period
are precip chances/types now mainly Fri night into Sat night then
next precip chances centered on Sun night.

This remains a difficult fcst period. Medium range model runs of
20.00z in reasonable agreement for a lead shortwave out of the west
coast longwave trough to push into/thru the mid CONUS ridging
Fri/Sat. Modest agreement for hgts to rise/ridging aloft to build
across the region Sun. By Mon, models at odds on the evolution of
shortwave energy coming out of the western CONUS troughing, and the
longwave trough itself. Given lack of detail agreement and plenty of
run-to-run variability fcst confidence for Fri-Mon is below average.

Trend of 20/00z models is to offer a delay, with much of Fri now
trending dry across the fcst area. As such, Fri may turn out to be a
quiet, seasonable day will little more than increasing clouds for
most of the fcst area. Deeper layered moisture/lift now progged to
spread across the fcst area Sat night, but plenty of detail
differences due to different strength, timing, track of the
shortwave energy thru the ridging and where it ends by by 00z Sun.
With lack of confidence in the details, will stay close to the
model/ensemble consensus for Fri night thru Sat night. On top of the
precip chances and timing, there is the problem of precip type.
20.00z GFS/ECMWF prog the lower portions of the column cold enough
for the bulk of any precip across at least the northern 2/3 of the
fcst area later Fri night/Sat morning to fall as accumulating snow.
Time will tell, as plenty of time for model consensus to converge on
a different solution. Ridging/rising hgts aloft would bring high
pressure and a quiet day for Sunday. Plenty of timing issues with
next wave out of the trough by Sun night/Mon, lending to reliance on
the model/ensemble consensus for now.


.AVIATION...(For the 06Z TAFS through 06Z Tuesday night)
Issued at 1035 PM CDT Mon Mar 19 2018

Mid/high cloud cover continues to stream eastward across the area
overnight. Expect generally BKN/OVC VFR ceilings through the
period with northeast to east winds around 5 to 10 kts. Light snow
will try to move into southeast Minnesota on Tuesday, with some
visibility reductions/MVFR ceilings possible with any snow.
However, this precip will likely struggle to advance eastward as
it encounters a very dry low-level airmass, so have kept KRST dry
with VFR conditions at this time.




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