Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180535

Area Forecast Discussion
National Weather Service La Crosse WI
1135 PM CST Fri Nov 17 2017

.SHORT TERM...(Tonight thru Saturday night)
Issued at 205 PM CST Fri Nov 17 2017

The main short-term fcst concerns are precip types/chances tonight
into Sat morning, colder temps Sat night.

18z data analysis had one low in NE Manitoba, with a cold front thru
western MN to another low in the central plains. Abundant moisture
being pulled northward ahead of the cold front, with lower level
saturation so far generally limited to the sfc-700mb layer. Precip
has increased thru the morning with the increase of moisture/lift,
but has been mainly -DZ or very light -RA. Temps have been slow to
warm with the increase of moisture/light precip, but most in the mid
to upper 30s as of 18z.

No issues noted with 17.12z model initializations. Solutions quite
similar as the central/northern Rockies troughing moves across the
northern plains toward MN tonight and across the Upper Midwest on
Sat. Even decent model agreement on a shortwave to drop down the
back side of the trough and across MN/WI Sat night. However, there
are some lower level thermo-dynamic forcing/moisture differences
tonight into Sat morning. Except for the 06z-18z precip chances/
types, short-term fcst confidence is generally good this cycle.

For the short term, broad isentropic lift/ascent continues across
the area into this evening, however higher PW airmass is quickly
shunted east of the area thru the evening. Higher mainly -RA chances
translate east of the area thru the evening as the sfc-850mb
trough/front passes. Behind the front, decent model signal for a mdt
band of 700-500mb FN convergence to move across mainly the NW half
of the fcst area, along with some continued isentropic lift in the
295-305K layer and increasing 500-300mb PV advection ahead of the
trough axis. Models differing on the vertical motion response to
this, the amount/depth of saturation and QPF production. Some,
including hi-res/CAMs, from next to nothing for QPF across the NW
half of the fcst area after midnight, to a robust band of 1/4 to 1/2
inch seen in the NAM. For now will use a model/ensemble consensus
after midnight with precip chances mainly in the 20-50% range,
pending which group of models shows its hand and trend more correct
toward wetter or drier later tonight. Models also differing on how
quickly the lower column cools behind the sfc-700mb front/trough
axis later tonight. Again will stay with a model/ensemble consensus
of a gradual mix with/change to -SN across the NW half of the fcst
area later tonight.

Model differences continue Sat morning, as far as the NW extent of
some 850-500mb FN convergence to pass SE or clip the SE end of the
fcst area. NAM/ECMWF/Can=GEM and about 1/2 the CAM models now keep
the bulk of this lift and lower level saturation to support precip
SE of the fcst area Sat morning. Leaned on the consensus/blend
precip chances for Sat morning. Column from 925mb and above
generally cold enough for any precip as -SN Sat morning. However
models slow to cool the BL behind the front late tonight/Sat morning
under an OVC cloud blanket. BL temps across the SE half of the fcst
area looking to be in the mid/upper 30s 12-15z Sat morning, and will
leave precip types there as a -RA/-SN mix. Should heavier precip/
precip rates per the GFS occur, a quicker changeover to snow and
some minor accumulations would be possible, mainly over the far SE
end of the fcst area. Weather quiets/cools down rather quickly Sat
afternoon (except for brisk NW winds) as a drier/colder Can high
pressure settles into the northern plains/Upper Midwest. Sat night
still trending as a colder night with lows in the teens to lower 20s
under mostly clear skies and lighter winds as the sfc ridge axis
builds in.

.LONG TERM...(Sunday through Friday)
Issued at 205 PM CST Fri Nov 17 2017

For Sunday thru Monday night: main fcst concerns this period are the
warmer temps on Monday.

Models remain in good agreement for NW flow aloft over the region
Sun, quickly transitioning to more westerly for Sun night into Mon
night as Pac shortwave energy comes thru/flattens the western NOAM
ridging. Sun-thru Mon night fcst confidence is on the good side for
a quiet and warming period.

Low level warm advection returns by mid-day Sun, and continues into
Mon night as low pressure tracks east across southern Can. Lower
levels dry for Sun thru Mon night as drier air from the central/
northern plains is advected east. The only clouds much of this
period looking to be some cirrus clouds from time to time as some
500-300mb moisture spills thru the NW then westerly upper level
flow. Sfc pressure gradient gradually increases for Sun thru Mon,
for some SW BL winds for mixing. Model soundings showing mixing to
near 925mb both Sun/ Mon afternoons. Sunday temps looking to take
some time to recover from some chilly lows in the morning. Blend of
guidance highs Sun in the upper 20s to upper 30s reasonable at this
time, but potential to sneak out a 40 or two in the MS/WI river
valleys. Monday continues to look like a lone day with well above
normal highs. With the continued low level warm advection, 925mb
temps climb into the +4C to +7C range by Mon afternoon. These 925mb
temps and S to SW gradient winds of 10-20 mph for mixing, the blend
of guidance highs mostly in the upper 40s to lower 50s (5F to 10F
above normal) for Mon looking good.

For Tuesday thru Friday (days 4-7): main fcst concerns this period
are temperatures.

Guidance remains consistent with a fairly quiet extended period.
Following the warm day on Monday, an upper level trough axis and
surface cold front move through the area on Tuesday. With dry air
ahead of this front, anticipating any associated precip will remain
closer to the surface low and north of the forecast area. 925 mb
temperatures in the -10 to -8 C range will move in behind the front,
giving highs in the upper 20s to 30s on Tuesday and Wednesday.
Models diverge a bit on Wednesday night, with the 17.12Z GFS holding
onto the cooler air for Thanksgiving (highs in the mid 20s to mid
30s) and the 17.12Z ECMWF bringing in warmer air (highs in the 40s).
Tend to side with the ECMWF, as it has remained much more consistent
in the past few days, but given it is still a ways out and has
relatively no impact, will stick with the blend.


.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1135 PM CST Fri Nov 17 2017

The front has pushed through both airports late this evening and
once it did, it allowed the lower visibilities from the fog to
improve. The fog is continuing to reduce the visbility into the 2
to 4 mile range and this will slowly improve through the night as
slightly drier filters and as the north winds increase to around
10 knots. By early to mid morning Saturday look for the fog to be
completly dissipated. Ceilings will also slowly improve with MVFR
expected by mid morning and then going to VFR as the clouds
scatter out. It will be a little gusty Saturday afternoon,
especially at KRST, as steep low level lapse rates allow some mid
20 knot gusts to mix down to the surface.




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