Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 090847
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
245 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 06Z TAFS through 06Z Friday night)
Issued at 1020 PM CST Thu Dec 8 2016

Ceilings have improved to VFR levels for both LSE/RST the past few
hours, with even a few breaks in the cloud cover noted just to the
north. Those breaks may briefly work southward in our direction the
next few hours, but overall, do still anticipate additional stratus
developing through sunrise, with ceilings returning to MVFR levels
for a few hours before improving once again into early afternoon
(all while thicker cirrus rolls overhead). Winds from the northwest
will slowly shift westerly and decrease through the day on Friday.


&&

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

$$

SHORT TERM...Rieck
LONG TERM....Rieck
AVIATION...Lawrence


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