Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 221739
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1139 AM CST Thu Feb 22 2018

.SHORT TERM...(Today and Tonight)
Issued at 401 AM CST Thu Feb 22 2018

Main short-term fcst concerns are precip types/amounts mainly
later today tonight.

Data analysis at 06z had a 1041mb high sprawled across MN/WI.
Clouds were increasing/thickening over the central/SE parts of the
region in deep SW flow aloft. Another cold night across the
north/NW parts of the region with sub-zero temps under
clear/mostly clear skies and light winds. Mid level moisture
already streaming NE over the cold dome, with some mid-level Fgen,
aided by some weak shortwave energy lifting thru the flow,
already enhancing a band of reflectivity/-SN northeastward across
IA toward the fcst area. Per WV imagery, shortwave of concern
could be seen rounding the base of the trough near the AZ/Mexico
border.

Model runs of 22.00z initialized well and have ECMWF data tonight.
Solutions, at least at the mid/upper levels offering a tight
consensus as the shortwave lifts northeast toward then into/across
the region today/tonight. Trend is a bit stronger with the wave as
it lifts into the region tonight. Lower level detail differences
remain, with potential impacts to precip types/amounts tonight.
Thus short-term fcst confidence is not quite as good as one
would like, but still above average.

In the short term, forcing/lift responsible for the band of -SN
early this morning wanes after 12z. Much of this morning thru
early afternoon looking rather quiet until the stronger/deeper
lift ahead of the shortwave arrives for later this afternoon/
tonight. Much of the area faces a drier sfc-850mb layer thru about
21z, with saturation of this layer occurring as the deeper lift
and increasing moisture arrives late today. The stronger, deeper
lift with the shortwave crosses the area tonight with models now
producing more total QPF thru 12z Fri. Can-Gem produces up to 1/2
inch of QPF across parts of the fcst area thru 12z Fri, with
others now more in the 1/4 to 1/3 inch range. But, what will the
precip form be? The other problem is, how far north/west is the
roughly 850-800mb warm layer aloft pulled ahead of the lower level
low/trough. Nam furtherest west/north with this warming, across
most if not all of the fcst area at some point tonight. Others
only bring it north over about the SE 1/2 of the fcst area. Nam
appears as an outlier and is a sizable shift from its earlier
runs. Leaned toward a non-Nam consensus on temps aloft as they
work to control precip types. On top of all that, there are
concerns about the presence of ice in the cloud tops tonight, with
some drier aloft/warmer with cloud tops than others. Given the
strength and depth of lift as the wave approaches tonight, and
some near neutral theta-e lapse rates in the mid levels, have to
believe cloud tops will rise/cool enough for ice in the tops much
of tonight. Trended grids toward more ice in the cloud tops,
especially this evening. A continued stronger signal for cloud top
warming/ice loss after midnight and toward Fri morning as mid
level drying above 700mb spreads in. End result is more snowfall
expected across much of the area tonight and less -FZRA. However,
continued the trend of diminishing/ending the precip later tonight
into early Fri morning as a period of -FZDZ.

In the fcst grids thru tonight, now carrying 2 to 4 inches of
snow over the NW half of the fcst area, up to 2" over the SE
half, along with a glaze of ice north and up to 1/10" of ice
south. Will issue a winter weather advisory for the entire fcst
area for the snow/mix, 12z-12z west, 00z-12z tonight central and
00z-15z tonight/Fri morning NE. In all of this, will still have to
watch total precip potential. If GEM and some of the Hi- Res/
CAMs are more correct with QPF totals pushing 1/2" somewhere over
the NW 1/2 of the fcst area, the snow/ice amounts in fcst grids
thru to night may well end up on the low side.

.LONG TERM...(Friday through Wednesday)
Issued at 401 AM CST Thu Feb 22 2018

For Friday thru Saturday night: the main fcst concern this period is
winter storm #2 and the potential for more snow/icing Sat/Sat night.

Model runs of 22.00z offering improving agreement on the southern
portion of the Rockies trough to eject into the southern plains by
early Sat, then wing thru and lift NE across the mid/upper MS valley
later Sat/Sat night. Overall trend favors a compromise of the
earlier runs on the timing/strength of the passing shortwave trough.
Sat night. Fcst confidence in the Fri-Sat night period is average to
good this cycle.

Brief break with passing shortwave ridging and sfc high pressure
Fri/Fri night. Not much for low level cold advection behind the
system tonight, with trend of Fri being a seasonably mild day
looking on track. Attention then quickly turns to the next moist and
dynamic system to lift across the region Sat/Sat night. Fair share
of lower level detail differences remain, including the sfc-850mb
low strength/track, how far west/north warm air aloft is pulled
across the fcst area as the low passes and QPF production.
Tightening consensus for 1/2" or more of QPF across portions of the
fcst area Sat afternoon/night, with GFS remaining the heaviest.
Based on the latest consensus of lower level thermal profiles,
this would mainly fall as snow across the NW 1/2 of the fcst area,
with potential for another wintry mix over the SE 1/3 to 1/2.

It`s starting to look like another round of winter weather headlines
will be needed across the fcst area Sat afternoon/night and possibly
into early Sun morning. This system is 2 1/2 days out and still the
detail differences, so no winter headlines issued for it just yet,
but will increase messaging in other parts of the product suite.

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are continued mild temps thru the period and some precip
chances toward mid-week.

Medium range models of 22.00z in reasonable agreement for the flow
to become quasi-zonal or broad ridging aloft over the central CONUS
Sun-Tue, with troughing re-establishing over the western CONUS.
Models then at odds with evolution of the western troughing by Wed.
Fcst confidence average Sun-Tue then below average Wed.

Honestly, did not spend a lot of time looking at details of this
period. Behind the Sat/Sat night system, appears region to get a
glancing blow of low level cold advection, with more of a W to SW
lower/mid level flow Sun/Tue and 925-850mb temps supporting highs
near normal Sun to above normal by Tue. Depending on which model
(if any) may be more correct by the middle of next week, stronger
shortwave troughing could be approaching the mid/upper MS valley.
Though little confidence, the consensus rain/snow chances for Wed
look OK for now, as do Wed temps remaining above normal.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1139 AM CST Thu Feb 22 2018

A messy wintry mix will spread into the area late this afternoon,
then ending at KRST by 10z and KLSE by 11-12z. The highest
confidence of a wintry mix will be at KLSE as warmer air aloft
gets dragged northward tonight. KRST looks to remain mainly -SN
with about 3 inches of accumulation expected. A light glazing of
ice cannot be ruled out. KLSE is expected to see around 1 inch of
snowfall and some sleet, along with a light glaze of ice as well
from freezing rain. Otherwise, expect VFR conditions early this
afternoon to rapidly deteriorate into IFR/LIFR category later
this afternoon into tonight as this weather system and associated
wintry mix moves in. Although the precipitation is expected to end
in the 10-12z time frame, IFR stratus is expected to linger
through Friday morning.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for WIZ032-033-041-053>055-061.

     Winter Weather Advisory from 6 PM this evening to 9 AM CST
     Friday for WIZ017-029-034-042>044.

MN...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Friday for MNZ086-087-094-095.

     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for MNZ079-088-096.

IA...Winter Weather Advisory from 3 PM this afternoon to 6 AM CST
     Friday for IAZ008-009-018-019.

     Winter Weather Advisory from 6 PM this evening to 6 AM CST
     Friday for IAZ010-011-029-030.

&&

$$

SHORT TERM...RRS/Lawrence
LONG TERM....RRS
AVIATION...DAS


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