Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 122030
Area Forecast Discussion
National Weather Service La Crosse WI
230 PM CST Thu Jan 12 2017
.SHORT TERM...(Tonight thru Friday night)
Issued at 230 PM CST Thu Jan 12 2017
Main fcst concerns this period are cold temps/wind chills tonight
then a small -SN chance Fri afternoon/night.
At 18z, low pressure tracking into central Ont was dragging a cold
front across the Upper Midwest. The front was near a KDLH-KMSP-KSUX
line and marked the leading edge of an arctic high building into the
northern plains. Mid-day temps below zero across ND and NW MN behind
the front. Subsidence behind/SW of the shortwave well north of the
area was clearing out the low/mid clouds over MN and western WI
to northern IA. Temps across the area warming a bit with the
sunshine but still mainly in the teens as stronger lower level
cold advection starts to overspread the area.
Model runs of 12.12z initialized well. Solutions very similar as the
pattern remains quite progressive with hgts rising over the region
tonight and the flow becoming westerly by Fri morning. Hgts to
continue rising Fri, in confluent flow over the region. Model
consensus still brings a weak wave into thru this flow Fri night.
With the tight consensus, short-term fcst confidence remains good
In the short term; clear/mostly clear skies expected thru the
evening with diminishing winds as the arctic high builds into MN.
Center of the high settles over MN/NW WI by 12z Fri, with winds
expected to become light/variable. Lower level cold advection on the
front side of the high drops 925mb temps to -12c to -18c by 12z Fri.
Some increase of mid/high clouds late tonight as western moisture
streams in, but much of the night to favor strong radiational cooling
with light/variable/calm winds by late evening into the early
overnight hours. Continued to favor colder of guidance lows for
tonight and fear some lows in the grids for the low laying areas
along/NE of I-94 may still not be cold enough. Next concern is
potential for a wind chill advisory tonight, mainly for SE MN into
north-central WI. Wind chills progged to dip to around -20 this
evening then -20 to -25 overnight into Fri morning. However winds by
this time looking to be light/variable or calm for many locations.
Given the light winds, plan to not issue a wind chill advisory
(which will blend with neighboring grids) and handle it with SPS
products. Low level warm advection already returns Fri, but an
increase of mid/high clouds ahead of the next shortwave coming thru
the westerly flow will temper the warmup. Given the cold start,
consensus highs mostly 5F to 15F above looking reasonable.
By Fri afternoon/evening the increase of mid/high level moisture and
what lift there is with the shortwave moving thru the confluent flow
spreads across the area. As with the past few of these type of
waves, the struggle will be to saturate the drier 925-700mb layer
before any snowflakes would reach the ground. Appears much of what
lift there is will work to saturate the column while the lower level
warm advection mostly goes into warming. Given the weak nature/fast
movement of the lift, only carried some 20% -SN chances for parts of
the area Fri afternoon/night. Clouds Fri night will help keep lows
near the mid January normals.
.LONG TERM...(Saturday through Thursday)
Issued at 230 PM CST Thu Jan 12 2017
For Saturday thru Sunday night: main fcst concerns this period
include warming temperatures thru the period and returning precip
chances Sun night.
12.12z model runs in good agreement for slowly rising hgts and
westerly flow over the region Sat/Sat night, weak and transient
shortwave ridging to move across Sun, with the flow then
transitioning to SW ahead of a mid level low ejecting into the
central plains. Trend favors a slower/stronger solution with the mid
level low into the plains an higher hgts over the region Sun night.
Fcst confidence for Sat thru Sun night is generally good this cycle.
Sat thru Sun dominated by broad but generally weak warm advection
west to SW lower level flow under the slowly rising hgts. Sfc
gradient flow remains on the weak side thru Sat/Sun as another high
moves across the region, but this one of Pacific/central/northern
plains origin. Some sunshine and the lower level warming looks to
allow highs Sat to climb to within a few degrees of normal, then to
near or a few degrees above normal for Sunday. A cooler night yet
Sat night with light winds and clear/mostly clear skies and another
high centered over the region.
Tightening consensus for deeper moisture and increasing lift ahead of
the sfc-mid level low ejecting into the central plains to spread
north into the area Sun night. Lower level warming continues as
well, with nose of the elevated warm layer above 0C lifting north
into the fcst area as well. There are model detail differences Sun
night, especially with how fast the elevated warm layer lifts north,
which would have impacts on precip type as it first moves north into
the area. Confidence also lowered on the precip type details for
later Sun night. Given the tightening model consensus, model blend
precip chances in the 60-80% range across the south 2/3 of the fcst
area after midnight Sun night do look good. For now stayed with a
model consensus on the warming aloft, with the precip starting as a
period of snow as it spreads north into/across the area. The snow
then transitioning to a snow/sleet/-FZRA mix across the south end of
the fcst area by Mon morning.
For Monday thru Thursday (days 4 to 7): main fcst concerns this
period are the precip types/chances/amounts Mon into Tue night,
above normal temperatures thru the period.
Medium range model runs of 12.00z/12.12z offering improving
agreement Mon as a mid level shortwave/trough/low would be lifting
thru the central plains toward/into the Upper Midwest. Plenty of
timing differences by 00z Tue but overall trend looks to favor
slower/stronger of earlier runs with this feature. Timing
differences increase for Mon night/Tue as this feature would move
E/NE across the region. Loose consensus for some troughing to linger
over the western great lakes Wed, with some rise of hgts/ridging
building over the north-central Conus for Thu. Fcst confidence
average Mon then below average Tue-Thu.
Deep layered moisture along with increasing forcing/lift, and
warming lower level temps, spread across the area Mon into Tue as
the sfc-mid level low/trough would lift northeast across the region.
Again, plenty of timing/detail differences, a developing consensus
for a round of precip to spread N/NE across the area Mon into Tue.
latest consensus would have the precip start out as snow or a wintry
mix, transitioning to mainly -RA or -FZRA for much of the area by Mon
afternoon into Tue. Not enough confidence to deviate from the
model/ensemble consensus on the details for Mon-Tue, which in spite
of the timing differences, is rather consistent among the models in
the warming and depth of warming in the sfc-800mb layer to control
precip types. Given some slowing in the evolution of the system
lifting thru the region, allowing for more warming ahead of it to
spread in, a trend toward more liquid precip Mon afternoon thru Tue
does seem reasonable. Some cooling to arrive behind the system for
later Tue/Tue evening, with lingering precip then to end as -SN.
Models then indicating lower level flow remains westerly Wed/Thu
with any arctic air remaining well north of the US/Can border. NAEFS
ensemble mean has 850mb temps 1 to 2 std deviations above normal over
the region for Wed/Thu. After highs warm into the 30s for Mon/Tue,
appears they will continue in the 30s (some 5F-10F above normal)
Wed/Thu. This along with some sunshine to perhaps help melt any
icing from Mon/Tue.
.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1134 AM CST Thu Jan 12 2017
The clouds are gradually dissipating and expecting this trend to
continue through the afternoon. A cold front will pass through the
region to bring the winds around to the west/northwest. Some
potential for a few gusts at KRST behind the front, but these do not
look to last long enough to warrant including in the forecast at
this time. There was a band of IFR/MVFR clouds along this front
earlier this morning, but these have dissipated on the southern end
and what remains of them will pass by well to the north of both
airports. High pressure will then build in for the overnight with
clear skies and diminishing winds. A weak short wave trough will
start to approach from the west Friday morning. This will bring an
increase in clouds with high VFR ceilings expected. Any light snow
from this wave will hold off until well into the afternoon.