Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 231747
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1147 AM CST Thu Feb 23 2017

.UPDATE...
Issued at 1147 AM CST Thu Feb 23 2017

Upgrades to blizzard warnings will be coming shortly for much of
southeast MN into portions of northeast IA. Concern has greatly
increased due to a number of factors:

1. Strong lift signal provided by coupled jet structure, strong
mid-level frontogenesis and steep (7C/km) 700-500mb lapse rates.
All are likely to produce heavy snow, with 23.09Z SREF progs
hinting at potential for 2 inch per hour rates late tonight into
Friday morning. These alone could result in a 1/4 mile or less
visibility.

2. Increasing northeast wind as surface low tracks from KS to the
Chicago area. Steep low level lapse rates combined with 40-45kt
developing at 900mb by 12Z should mix down and result in 30-35 kt
gusts. These gusts supportive of Blizzard Warning.

3. Deformation band of snow will track across the area Friday
afternoon/evening, bringing another period of heavy snow with
strong winds. Gusts may be just slightly less than what comes from
Friday morning.

Given more flat, open terrain across southeast MN and northeast
IA, these areas are most susceptible to blizzard conditions. Rural
roads could become impassible.

Look for more adjustments to hazards this afternoon. Note that the
12Z model cycle has been trending further southeast, increasing
the likelihood that the Winter Storm Watches will need to become
Warnings.

&&

.SHORT TERM...(Today through Friday night)
Issued at 1118 AM CST Thu Feb 23 2017

A longer short-term this cycle to encompass the impending winter
storm to end the week.

Main fcst concerns this period are the impending major winter storm
tonight into Fri night, precip types and snow amounts with it.

06z data analysis had low pressure moving across northern lower MI.
This low was dragging a cold front SE across the region early this
morning with cooler high pressure was building into the northern
plains and Upper Midwest behind it. Temperatures still well above
normal across the region early this morning, but were on a stronger
downward trend as lower level cold advection was spreading across
the region. Elsewhere, the low to impact the region tonight into Fri
night was taking shape over eastern CO and western KS as strong
shortwave energy was coming thru the central/southern Rockies.

23.00z models look to have initialized well. Models finally starting
to converge on a common solution for the troughing/energy coming
thru the Rockies this morning as emerges into the central plains
today. Better consensus as the trough deepens and closes off a mid
level low tonight/Fri and moves it toward/into IA by 00z Sat. Trend
favors a slightly slower/stronger solution at 00z Sat, then leans
toward a slightly faster solution as the mid level trough would
pass Fri night. The stronger solution Fri/Fri night produces a
southward shift of the lower level features/sfc low. Some lower
level detail differences yet, but better than the past several
cycles. Overall fcst confidence for today thru Fri night is
average to improving this cycle.

For the short term, 23.00z GFS/ECMWF/Gem actually offering a
decent consensus on the strong sfc thru mid level system set to
lift across the Upper midwest tonight thru Fri night. It`s the NAM
that remains an outlier that impacts the fcst confidence, and
potentially precip types and snow amounts across the area later
tonight thru Fri night. For now have continued to side with a
model/ensemble consensus and included some weighting of the NAM
solution. Plenty of moisture (PWs in the 3/4 to 1 inch range in
the inflow airmass) and deep layered forcing/lift to spread across
the area tonight thru Fri night, producing a significant swath of
precip across the area. Consensus QPF totals thru Fri night in
the 1 to 1.5 inch range across much of the fcst area. Stronger,
slight further south solutions of the GFS/ECMWF/Gem would drop
most of this as snow. Southern end of the fcst area would still
see some rain, mainly tonight into Fri as the boundary layer
remains on the warm side or the mid level drying (dry slot) erodes
the ice in the clouds for a time late tonight thru early
Fri afternoon. NAM holds onto more warm air aloft tonight into
Fri, as well as pushes the mid level drying and loss of ice in
the clouds much further north across the fcst area. NAM results in
more of the precip along/south of I-90 as a wintry mix, holding
snow amounts down. Favoring the model consensus of a colder sfc-
700mb solution and shift of the heavy precip axis more southward
into the fcst area, raised snow totals across much of the fcst
area. This consensus now produces 6+ inches over roughly the NW
2/3 of the fcst area, with potential for a swath of wintry mix
centered on a KCCY-KLSE-KISW line. At this time, upgraded the
previous winter storm watch to winter storm warning. Given the
slight slowing at 00z Fri, and bulk of any precip this evening as
-RA until the BL layer fully cools, did push the warning start
time back to 06z tonight. With it being night, along with brisk/
gusty NW winds and some continuing snow, did extend the warning
thru 12z Sat. South of this, where confidence in precip types/snow
amounts still carries some uncertainty, will be issuing a winter
storm watch from roughly a New Hampton IA to Viroqua and Adams-
Friendship WI line northward to the warning, 06z Fri thru 12z Sat.
Late note, the 23.06z NAM has trend a bit south, now more in line
with 00z GFS/EC/Gem, making the southward addition/extension of
the watch look very good. Continued with a TS mention across
roughly the S half of the fcst area late tonight/Fri morning when
models push 100-200 J/KG MuCape to near the S edge of the fcst
area centered on 12z Fri. This with some stronger low level
lifting and plenty of moisture.

Stayed with a consensus of the guidance highs/lows for today thru
Fri night, with highs/lows back to near late Feb normals by Fri/Fri
night.

.LONG TERM...(Saturday through Wednesday)
Issued at 325 AM CST Thu Feb 23 2017

Main fcst concerns for the day 3 thru 7 period are: colder temps for
the weekend then warming into next week, small -RA/-SN chances much
of the Sun-Wed period.

Model runs of 23.00z in decent agreement for the Fri trough to
quickly exit east Sat then another NW flow wave to drop into/across
the region Sat night/Sun. Good agreement for renewed troughing
moving into the western conus Sun thru Mon, forcing a downstream hgt
rise over the north-central Conus for this same period. Agreement
remains good for the troughing to progress into the central Conus
Tue with some portion of the energy moving across the Upper Midwest
Wed. Confidence in the day 3-7 period is average to good this cycle.

Given the impending major winter storm in the short-term, spent
rather little time looking beyond Sat morning. Colder NW flow/Can
high pressure on track to build across the area for Sat/Sun, with
what is looking to be the first near/below normal temps in a
while. Plenty of detail differences, but the NW flow `clipper`
wave could bring a little -SN to the area late Sat night/Sun. This
handled by the small consensus precip chances for now. With the
progression of the pattern and rising hgts over the region, temps
back to near/above normal for Mon-Wed look well trended. Will have
to watch for some of these later period temps being too warm over
what may be a significant snow cover from the storm centered on
Fri. Again with plenty of detail differences in both lower level
features and shortwave detail in what would be troughing
approaching/moving into the region Tue/Wed, consensus of mostly
30-40% -RA/-SN chances Tue- Wed look good for now. Consensus
highs/lows for Sat-Wed look well trended for now, but again will
need to watch many of these over the expected snow cover. May
well see some sub-zero lows Sun night/Mon morning but plenty time
to detail that.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1118 AM CST Thu Feb 23 2017

After an afternoon with VFR visibilities and MVFR ceilings,
expect conditions to deteriorate as precipitation develops and
intensifies over the TAF sites. Northeast winds will also
increase as low pressure bringing the precipitation approaches
from the central Plains. Gusts are expected to increase to around
25 kt at 06Z, with 30-35KT Friday morning. The precipitation,
starting in the form of rain, looks to switch over to snow shortly
after 06Z.

Plan on ceilings to drop to IFR or lower at both sites by 04Z,
with visibilities of IFR by 08Z. Potential exists for LIFR to
VLIFR visibilities at both sites late in the night into Friday
morning, especially at RST where winds will cause extensive
blowing of the falling snow. Conditions look to remain nearly
steady through Friday afternoon as well.

Regarding accumulations, 2 to 4 inches appears likely after
midnight, with another 2 to 4 inches Friday morning. Highest
totals are forecast at RST, but could change.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Storm Watch from late tonight through late Friday night
     for WIZ041>044-053.

     Winter Storm Warning from midnight tonight to 6 AM CST Saturday
     for WIZ017-029-032>034.

MN...Winter Storm Watch from late tonight through late Friday night
     for MNZ096.

     Blizzard Warning from midnight tonight to 6 AM CST Saturday for
     MNZ079-086>088-094-095.

IA...Winter Storm Watch from late tonight through late Friday night
     for IAZ010-011-019.

     Blizzard Warning from midnight tonight to 6 AM CST Saturday for
     IAZ008-009-018.

&&

$$

UPDATE...AJ
SHORT TERM...RRS
LONG TERM....RRS
AVIATION...AJ


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