Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 110104

Area Forecast Discussion
National Weather Service La Crosse WI
704 PM CST Tue Jan 10 2017

Issued at 701 PM CST Tue Jan 10 2017

Have extended the winter weather advisory based on too many
impacts in the gusting winds in the 30 to 40 mph range. reports of
whiteouts and accidents in southeast minnesota prompted the
extension. but...with the wind gusting in the 30 mph range in
wisconsin and the fresh snow...decided hazardous travel due to
covered roads and isolated whiteouts /kbrf and keau reporting haze
= BLSN/ would warrant the extension there too. MPX matched this
area up with the extension.

we think the winds will diminish over the next hours and the
whiteout pockets will subside. roads will still be messy overnight with
a tantalizing slushy from your favorite store turning to pretty
solid ice with temperatures falling to into the 8-15F range

as soon as things settle down with these hazards...will be
evaluating the morning snow and ensuring the forecast is on


.SHORT TERM...(Tonight through Wednesday night)
Issued at 230 PM CST Tue Jan 10 2017

Main concerns this period are winds this evening, snow chances with
the next wave and round of lift later tonight into Wed evening,
cooling temperatures.

18z data analysis had a roughly 990mb low over SW WI, lifting rather
quickly NE. Liquid precip over all but the far SE/east ends of the
fcst area has changed to snow. Rather impressive display of silver-
dollar sized snowflakes at the ARX office when it changed over
between 1015-11 AM. Deformation band of snow around the north/west
side of the sfc-mid level low/trough was translating rather quickly
east across the area. Vsbys in the heart of the band in the 1/4 to
3/4 mile range. Vsbys already improving to 1 to 2 miles across the
west/SW ends of the fcst area. Brisk NW winds on the back side of
the low spreading across the area, with colder air/falling temps
already spreading across the west 1/3 of the fcst area.

Model runs of 10.12z initialized well. Solutions in good agreement
on the vigorous mid level shortwave/trough axis moving quickly east
of the area late this afternoon/early this evening. Decent agreement
continues Wed/Wed night as a northern stream wave passes across the
north end of the region, while energy coming out of the west coast
troughing arrives into the Upper Midwest Wed night. Short-term fcst
confidence is good this cycle.

For the short term, sfc low-700mb trough moves quickly NE to the
east end of the MI U.P. by 00z, taking the snow with it. Any
lingering -SN near/after 00z down to sct flurries as colder/drier
air and post low/trough subsidence spread quickly across the area.
Distance-speed tool has the west edge of the post-low cloud deck to
near the MS river by 00z. Tightest pressure gradient moves across
the area later this afternoon into early this evening. So far, NW
winds in this tighter gradient area over SW MN into western IA have
mainly been 20-30mph gusting 30-40mph. Considered a wind advisory
for at least the west 1/3 to 1/2 of the fcst area thru early this
evening, but with winds sub-advisory to this point, will hold off
for now. Will continue with the winter weather advisory thru 00z for
the winds, some drifting/blowing snow and the refreeze on untreated
roads/walkways as temps fall into the teens to mid 20s.

Respite from the snow rather brief across the north 1/2 to 2/3 of
the fcst area. Next round of stronger lower/mid level warm
advection/isentropic lift is already into the north end of the fcst
area after midnight, with this forcing/lift maximized around the -
15C level. Concern later tonight is how quickly with the roughly 925-
800mb layer re-saturate and allow snow to reach the ground. Greater
of the 925-800mb saturation progged over the north 1/4 to 1/3 of the
fcst area by later tonight, with a deep dry 925 to nearly 700mb
layer remaining over the south side of the fcst area. Consensus snow
chances back in the 50-70% range across the north of the fcst area
to remaining dry across the south half by 12z Wed quite reasonable.
Stronger/deeper lift spreads across the north 1/3 to 1/2 of the fcst
area Wed. Continued with snow chances 50-70% across the north end of
the fcst area much of Wed. These blend well with neighboring grids,
but may not be high enough. Potential for another 1 to 2 inches of
snow along/north of hwy 29 across WI. Yet another round of
increasing mid/upper level forcing/lift, ahead of the energy
ejecting out of the west coast troughing, spreads across mainly the
central/south parts of the fcst area late Wed night. Continued -SN
chances mainly along/south of I-90 thru Wed night good for now.
Blend of the guidance lows/highs for tonight thru Wed night looks
well trended at this time.

.LONG TERM...(Thursday through Tuesday)
Issued at 230 PM CST Tue Jan 10 2017

For Thursday thru Friday night: main fcst concerns this period are
lingering -SN chances Thu morning, -SN chances again Fri night.

Model runs of 10.12z in good agreement as a cold trough settles over
the northern great lakes Thu, then quickly giving way to more
westerly flow aloft by Fri. Decent agreement for yet another W-NW
flow shortwave to come across the northern plains and approach the
region Fri night. Fcst confidence in the Thu thru Fri night period
is generally good this cycle.

Moisture/lift over the south end of the fcst area from later Wed
night exits rather quickly Thu morning. Rather deep/dry airmass to
then settle across the region for Thu afternoon thru Fri as hgts
rise and arctic high pressure settles across the region. With the
arctic high overhead and what should be clear skies and light winds,
along with 925mb in the -12c to -18c range, Thu night/Friday shaping
up to be a very cold period. Consensus lows mostly 0F to -15F Thu
night and highs Fri mostly 5F to 15F.  With a fast zonal flow and an
approaching shortwave Fri night, the next round of lower level warm
advection/isentropic lift, along with an increase of mid/high level
moisture spreads across the area by later Fri night. The small
consensus -SN chance for much of the area by later Fri night looks
good at this time. With an increase of clouds and a return of south
flow/warm advection, the consensus lows near normal for Fri night
look good as well.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are -SN chance Sat, precip chances Sun night into Tue,
temperatures thru the period.

Medium range model runs of 10.00z/10.12z in reasonable agreement for
a weaker NW flow shortwave to pass across the region Sat. Detail
differences on handling of waves in the flow increases for Sun, even
more so for Mon into Tue. Bigger differences by Mon/Tue concern the
phasing, or not, of northern/southern stream features over the
mid/upper MS valley for early next week. Fcst confidence average for
Sat, then below average by Mon/Tue.

Northwest flow wave passing to pass on Sat looking to spread a round
of weaker mid/upper level lift across the area. With sfc high
pressure over the area, questions concern the amount of moisture
available for snow production. Some small -SN chances on Sat okay
for now. Sunday would be a between systems day with sfc high
pressure ridging over the area, for what should be a dry/seasonable
day. For early next week, less phased model runs offer a drier
solution for Mon/Tue as the southern stream portion moves east thru
the mid MS valley. More phased runs pull the southern stream energy
toward/into the upper MS valley and spread precip chances northward
and across much of the area as early as Sun night and continuing as
late as Tue. Lower level warm advection ahead of whatever form the
trough ends up taking spreads across the region for Mon/Tue. Enough
so for precip type issues with any potential system for early next
week. Given low confidence in the details, will stay with the
model/ensemble consensus for these day 6/7 periods. Somewhat better
confidence that temps for Sun-Tue will be above normal.


.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 700 PM CST Tue Jan 10 2017

Satellite would indicate an area of clearing is heading in before
clouds try to reform and lower by morning. Confidence is not high
on the details of the morning forecast into Wednesday. it appears
a snow band will be around and could affect the taf sites
Wednesday with ifr snow.

winds are gusting in the 30-40 mph range with BLSN now. the winds
will diminish as the evening continues.


WI...Winter Weather Advisory until 9 PM CST this evening for WIZ017-

MN...Winter Weather Advisory until 9 PM CST this evening for MNZ079-



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