Area Forecast Discussion
Issued by NWS La Crosse, WI

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368
FXUS63 KARX 091119
AFDARX

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

.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 12Z TAFS through 12Z Saturday morning)
Issued at 519 AM CST Fri Dec 9 2016

Soundings continue to show that the IFR/MVFR ceilings will
continue into this evening...and then the ceilings will become
VFR. Meanwhile the visibilities will remain VFR through the
time period.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...Boyne



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