Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211750

Area Forecast Discussion
National Weather Service La Crosse WI
1150 AM CST Wed Feb 21 2018

.SHORT TERM...(Today and Tonight)
Issued at 347 AM CST Wed Feb 21 2018

The main short-term fcst concerns include returning -SN chances
across the south late tonight and temperatures.

06z data analysis had the Upper Midwest on the east side of Can/
arctic high pressure settling into the central plains. NW winds/
subsidence/drying on the east side of the high were clearing the
lower clouds out of the region as well as advecting in another cold
airmass. Some cirrus remain over region, mainly east of the jet
axis. Midnight temps falling into the single digits and teens across
the area, with wind chills in the single digits above/below zero.

No issues noted with 21.00z model initializations. Solutions similar
for today/tonight as the region remains under SW flow aloft, ahead
of shortwave energy rotating thru the base of the western CONUS
longwave trough. Short=term fcst confidence is good this cycle.

For the short-term, quiet weather today as shortwave energy trails
away to the northeast, hgts rise a bit and cold high pressure builds
into the region. One concern is the cirrus shield. As the hgts aloft
rise a bit today, the upper jet axis retreats NW, leaving the fcst
area vulnerable for cirrus clouds to spread back to the NW over the
area, especially this afternoon, which would impact high temps. Short-
term forecaster has added more cloud cover for this afternoon. The
blend of guidance highs generally in the 20s, some 5F to 10F below
normal, appear reasonable.

Moisture/lift above 850mb quickly increases over the area from the
SW tonight ahead of some weak shortwave energy rippling out of the
Rockies trough. Question is will there be enough saturation below
850mb late tonight for any -SN to occur. Model soundings/X-sections
showing this will be a challenge before 12z, except over the far SW
side of the fcst area, though several produce some -SN over roughly
the 1/3 of the fcst area by 12z. Confidence not overly high in -SN
chances before 12z Thu, but will continue with some small chances
spreading in after 10z, which blends well with neighboring grids.

.LONG TERM...(Thursday through Tuesday)
Issued at 347 AM CST Wed Feb 21 2018

For Thursday thru Friday night: main fcst concerns this period area
precip chances/types/amounts centered on Thu night.

21.00z model runs, (limited ECMWF data available) in decent
agreement as some stronger short-wave energy ejects into the central
plains Thu then lifts across the region Thu night. This followed by
rising hgts/shortwave ridging aloft as the Rockies troughing
progresses into the central CONUS. Some lower level detail
differences, but fcst confidence in the Thu thru Fri night period is
generally good this cycle.

Lower column continues to try to saturate much of Thu as 925-700mb
warm advection/isentropic lift continue over the area. Greater
saturation in the morning is over the south end of the fcst area
then over the west side in the afternoon. Good signal for ice in the
cloud tops (colder than -15C) Thu. Stronger/deeper lift, aided by
the approaching shortwave, and saturation of the sfc-850mb layer
spreads across the area Thu night. However, Thu night there is a
signal for loss of ice in the cloud tops as dry above 700mb spreads
north into the area. The lower level warm advection eventually
spread a warm nose of above 0C air around 800mb northward into about
the SE 1/2 of the fcst area by late Thu evening into the overnight
hours. Given the depth/strength of the lift Thu evening that should
lift moisture high enough for ice in the clouds, some models appear
to quick with the loss of cloud top ice. Have trend grids Thu night
toward more cloud ice and precip as -SN across the area. Eventually
the drying above 700mb does lift N into the area, with precip by
late Thu night mixing with or changing to -FZDZ before ending. The
warming aloft also introduces other mixed precip types over the SE
1/3 of the fcst area late Thu evening and overnight. For now appears
bulk of the precip over the north 1/2 of the fcst area later Thu/Thu
night will fall as -SN, with mainly 1 to 2 inches, with a wintry mix
across the south 1/2. Will leave any potential winter weather
advisory decisions to the day crew who will have another run of models
to work with. High pressure returns for Fri/Fri night, but not a lot
of low level cold advection to follow the Thu night wave. Blend of
warming guidance highs both Thu/Fri look well trends for now.

For Saturday thru Tuesday (days 4 to 7): the main fcst concerns this
period are the potential wintry system centered on Sat night and the
warmer temps thru the period.

Medium range model runs of 21.00z in general agreement for a large
portion of western trough energy to approach and lift thru the Upper
Midwest Sat/Sat night. Hgts over the region progged to rise Sun/Mon
into Tue as troughing gradually builds back into the western CONUS.
Plenty of lower level detail differences already Sat/Sat night which
impact potential sensible weather outcomes, thus fcst confidence in
the day 4-7 period is average to below this cycle.

Significant mid level trough/energy approaching/lifting thru Sat/Sat
night has potential to produce a strong wintry system in the Upper
Midwest early in the weekend. Strong GFS solution Sat/Sat night
actually trends as an outlier. Signal that some form of system to
lift thru the region Sat/Sat night, but rather low confidence on
exactly what and where, as far as any precip types/amounts, at this
time. Model/ensemble consensus for Sat/Sat night left as is.
Transition to a quasi-zonal flow Sun/Mon then ridging aloft Tue to
provide more of a drier and seasonably mild westerly/SW flow to the
region. Some better confidence in the near/above normal consensus
temps in the Sat-Tue period.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1149 AM CST Wed Feb 21 2018

VFR through the period, then expected to deteriorate Thursday

High pressure will hold across the area this afternoon into
tonight, providing quiet/VFR conditions for the KRST/KLSE TAF
sites. A trough of low pressure will then lift toward the Upper
Mississippi River Thursday morning, pushing in thickening cloud
cover in from the southwest. There could be some light snow
moving in across northeast IA/southeast MN by later in the
morning, but opted to keep out of the TAFs for now. Otherwise,
look for gradual dropping of cloud base heights, below 10kft after
07z and below 5kft after 14z -but remaining VFR.

Conditions are expected to deteriorate Thursday afternoon,
especially west of the Mississippi River, as the likelihood of
light snow increases with the trough moving through.
More detail will be provided with the next TAF issuance this




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