Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 091714
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1114 AM CST Fri Dec 9 2016

.UPDATE...
Issued at 1055 AM CST Fri Dec 9 2016

Early look at the 09.12Z models show they are remaining consistent
with the weekend snowfall raising the confidence in the forecast.
Still expecting a prolonged period of light to at times moderate
snow starting Saturday afternoon through Sunday afternoon or
evening. Accumulations over this time period should range from
around 4 inches in the Interstate 94 corridor up to 7 inches
roughly along and south of a Rochester to Viroqua line. Will go
ahead an issue an winter weather advisory to cover this event. It
is still possible that a warning may be needed if amounts look
like they will exceed 8 inches, but right now, expecting the
impacts to be relatively low as high snowfall rates not expected
and the winds look light enough that blowing snow not much of a
concern.

&&

.SHORT TERM...(Today through Sunday)
Issued at 245 AM CST Fri Dec 9 2016

Forecast concerns squarely on impactful snowfall for the weekend:
several inches of accumulation with winter headlines likely.

1) Sat/Sat evening: upper level shortwave trough slated to move west-
east across the Northern Plains...slipping across the local area Sat
night. Not overly dynamic...but packing a pretty good thermodynamic
punch. Strong 925-850 mb warm air advection from southern MN
southward...with isentropic upglide 280-295 K sfcs. Good west-east
frontogenetic response - centered mostly along the MN/IA border
southward.

GFS/EC/Canadian favor more of a southern track with the resulting
snow bands, generally south of I-90. The NAM though is staying with
its northern solution, and a lot more QPF (thus snow). Going to
stick with the southern track for chances for now.

2) Overnight Sat/Sun Night: a second, stronger upper level shortwave
follows quickly on the heals of the Sat ripple...and looks to arrive
overnight Sat/early Sun morning.  The best thermodynamics has
shifted east/southeast, but still good frontogenetic response -
perhaps a bit more south. Likely some help from the upper level jet
too.

3) Amounts and Winter Weather Headlines:

For amounts, GFS bufkit soundings points to a dendritic growth zone
as deep as 300 mb at times, with ratios 15/20:1 for Sat/Sat evening.
This shrinks up overnight Sat/Sun morning - mostly 50-100 mb, and
elevated. Ratios still at least 15:1.

As its shaping up now, it looks like a prolonged period of light
snow, with high ratios. Current time/best forcing puts the higher
accumulations between 18z Sat-18z Sun.

Snow amounts from 4 to 7 currently forecast for the Sat/Sun time
period, highest south of I-90. Could be higher if qpf/snow ratios
increase...even just a little bit.

While winds don`t look strong/blustery - might be enough to move the
fluff around in the open/unsheltered areas of of southeast MN and
northeast IA. Usually winds closer to 20 kts start to cause
impactful blowing/drifting issues - but it doesn/t look that windy,
yet.

Given the length of time the snow will be falling, a Winter Weather
Advisory would probably be the way to go, even with amounts
exceeding 6 inches. Won`t have to issue quite yet...let the dayside
work through the next model runs and coordinate with surrounding
offices. Now, if qpf/ratios increase - 24 hours amounts could
approach warning criteria.

Bottom-line: several inches of snow will fall Sat-Sun. There will be
impacts to travel, especially Sat night through Sunday.

.LONG TERM...(Monday through Thursday)
Issued at 245 AM CST Fri Dec 9 2016

Its all about the cold.

Another shot of cold air progged to drop down south/southeast out of
northern Canada for the middle part of next week. 850 mb temps in
the EC/GFS go from around -8 C at 00z Monday to -24 C by 00z Thu.
NAEFS 850 mb temp anomalies are -1 to -2. Highs in single digits
above and lows in the single digits below zero look likely for the
Tue-Thu period. Depending the wind speeds, some wind chill
advisories are possible (likely).

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1055 AM CST Fri Dec 9 2016

Looks like VFR conditions will dominate ahead of the incoming
system for the weekend. The cyclonic flow has weakened enough to
allow the clouds to scatter out as a very weak ridge of high
pressure moves in. Once this ridge drifts across tonight, the high
level moisture will start to move in ahead of the storm system for
VFR ceilings overnight through Saturday morning. Conditions will
deteriorate Saturday afternoon as the snow moves in with IFR
conditions expected.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 2 PM Saturday to 6 PM CST Sunday
     for WIZ054-061.

     Winter Weather Advisory from 4 PM Saturday to 9 PM CST Sunday
     for WIZ032>034-041>044-053-055.

MN...Winter Weather Advisory from 2 PM Saturday to 6 PM CST Sunday
     for MNZ079-086>088-094>096.

IA...Winter Weather Advisory from 2 PM Saturday to 6 PM CST Sunday
     for IAZ008>011-018-019-029-030.

&&

$$

UPDATE...04
SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...04



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