Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 170507

Area Forecast Discussion
National Weather Service La Crosse WI
1100 PM CST Sat Dec 16 2017

.SHORT TERM...(Tonight thru Sunday night)
Issued at 227 PM CST Sat Dec 16 2017

Main short-term fcst concerns this period are fog potential south of
I-90 late tonight, temperatures thru the period.

18z data analysis had a weak low near KMCW/KCCY with a warm front
east to near KMKE. North/east of the low/front cloud cover and
easterly low level flow keeping temps nearly steady so far today.
Near to south and west of the low and front skies were more sunny to
partly cloudy, with mid-day temps mostly in the 40s.

16.12z model runs initialized well. Solutions quite similar as the
the mid level low near the TX big-bend is quickly ejected into MO by
12z Sun then eastward thru the OH valley by 00z Mon, remaining SE of
the area. Otherwise agreement is good on shortwave ridging to pass
over the Upper Midwest tonight then for a weakening shortwave in the
northern CONUS westerly flow to move across the region Sun/Sun night.
Short term fcst confidence is good this cycle.

In the short term, model soundings showing a rather shallow and
strong nocturnal inversion across much of the area tonight.
Soundings also showing plenty of moisture to be trapped under this
inversion tonight. With a little snow melt today, along with light
winds, and the shallow inversion, have added a fog mention in the 06-
15z grids. This especially across the south 1/3 to 1/2 of the fcst
area where there is a stronger signal for mostly clear to partly
cloudy skies tonight. Quiet, dry, seasonably mild weather continues
Sun/Sun night even as the weakening westerly flow shortwave trough
moves across the region. System lifting out of the TX big-bend area
and into the OH valley tonight/Sun appears to grab any moisture
trying to lift north ahead of the trough moving across the region,
with the 850-700mb layer remaining rather dry. Sun night trends
mostly cloudy for now but if trend is toward more mostly clear/
partly cloudy, may need to add some patchy fog for later Sun night
into Mon morning as well. Blend of guidance lows/highs for tonight
thru Sun night some 5F to 10F above normal look good.

.LONG TERM...(Monday through Saturday)
Issued at 227 PM CST Sat Dec 16 2017

For Monday thru Tuesday night: main fcst concerns this period are
above normal temperatures thru the period and potential for
strong/gusty winds on Tue.

16.12z models in good agreement for strong quasi-zonal flow across
the northern CONUS this period. A northern stream wave to slide
across the northern great lakes Mon night/Tue with some lowering of
hgts, with weak/flat shortwave ridging to quickly build back in
behind this trough Tue night. Fcst confidence is generally good this

One thing the fast quasi-zonal flow across the northern CONUS does
this period is keep the true arctic air bottled up north of the
US/Can border. Mon thru Tue to remain seasonably mild with highs
both days mainly in the 30s (few lower 40s south) and lows Mon
night mostly in the 20s. (Normals: highs mostly 25-30F, lows
10-15F) Wave dropping across the great lakes Mon night/Tue is
moisture starved across the area. It does push a cold front thru
the region Mon night with some 925-850mb cold advection to spread
across the area later Mon night/Tue. With this cold advection,
model soundings showing mixing to near 850mb for Tue, with as much
as 35-40kt of W- NW wind in the top of the mixed layer Tue
afternoon. With some sunshine, this deep mixing still allows for
highs mostly in the 32F to 42F range on Tue, even with the cold
advection. Did raise wind speeds/gusts on Tue a bit above the
model consensus but this may still not be enough if the column
truly mixes to near 850mb. Gradient relaxes rather quickly Tue
evening, with lighter winds and the cooler airmass allowing for
Tue night lows closer to the mid Dec normals.

For Wednesday thru Saturday (days 4-7): main fcst concerns this
period are snow chances Wed afternoon thru Fri morning then colder
temps Fri/Sat.

Medium range model runs of 16.00z/16.12z in reasonable agreement for
weak/flat ridging to progress across the region Wed as strong
troughing digs into the western CONUS. Still a consensus that the
trough will split Wed night with the southern portion closing off a
mid level low over UT/AZ. 16.12z models are more brief with this,
are more progressive and keep the north/south portions of the trough
more together, which would bring more of the trough energy into the
mid/upper MS valley Thu/Thu night. Differences then increase thru
Fri into Sat on strength/timing of a northern stream shortwave to
drop into the trough/region for the weekend. Fcst confidence average
to good Wed, then average to below by Fri/Sat.

Lead warm advection `wing` ahead of the lee 850-700mb low/trough to
develop in the central/northern plains progged to quickly spread a
round of lift and increasing moisture into the region Wed. Some -SN
chances across mainly the north end of the fcst area by Wed
afternoon reasonable at this point. Trend of 16.12z models of
keeping more of the north-south portions of the trough together
produces a stronger deformation band of forcing/lift, and more
precip across the area Wed night thru Thu night. Still between
model and run-to-run variability, but it`s getting smaller. The
consensus 50-80% precip chances to spread SE across the fcst area
Wed night thru Thu night reasonable at this time. Column looks to
be or cool enough for bulk of the Wed-Thu night precip to be
snow with some accumulations looking likely. Model opinions then
differ on the strength/extent of the Can/arctic airmass to advect
into the region behind the departing trough. Either way, appears
to be a strong trend for temps Fri into the holiday weekend to be
below normal. Question is just how much below normal. Consensus
highs in the teens to mid 20s and lows in the 0 to +15F range Fri
thru Sat night look to be a good place to start for now.


.AVIATION...(For the 06Z TAFS through 06Z Sunday night)
Issued at 1100 PM CST Sat Dec 16 2017

Cigs: mvfr cigs are/have returned from the east and north late this
evening, with backbuilding low stratus on an easterly flow. Models
hold mvfr-ifr cigs across the TAF sites well into Monday.

WX/vsby: some concern for fog development outside of the low cloud
shield tonight. At this moment, best chances will be south of
KRST/KLSE. Will keep period of mvfr br for KRST and monitor for

Winds: generally light and variable - more easterly tonight, then
northerly Sunday afternoon.




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