Area Forecast Discussion
Issued by NWS La Crosse, WI
FXUS63 KARX 191935
Area Forecast Discussion
National Weather Service La Crosse WI
235 PM CDT Wed Oct 19 2016
.SHORT TERM...(Tonight thru Thursday night)
Issued at 235 PM CDT Wed Oct 19 2016
Main fcst concerns this period: clouds and cooling temperature thru
18z data analysis had a secondary cold front from north of KGRB into
northern IA working slowly SE across the region. Temps ahead of the
front early this afternoon again in the 60s to near 70 while behind
the front most were in the 40s/50s. A rather widespread lower cloud
deck lagged the cold front, eroding on its SE side with diurnal
warming/mixing ahead of it. Weak 700-500 FN convergence band of
clouds/sprinkles across southern WI/northern IL was weakening as a
stronger shortwave moving across Neb was focusing lift further west.
19.12z model runs initialized well. Solutions reaching a tightening
consensus as the Neb shortwave digs into/thru the mid MS valley
tonight/Thu, working to carve out a rather deep trough over the
eastern CONUS by 00z Fri. Weaker NW flow energy to drop into the
Upper Midwest tonight/Thu, keeping rather deep NW flow aloft over
the region thru Thu night. Confidence is good this period on the
cooling temps, but the lower level moisture/clouds may prove
In the short term: low level cold advection spreads across the area
tonight in the wake of the cold front and continues Thu. 850mb temps
in the 0C to -4C range for Fri night. Canadian high pressure behind
the front builds into the northern plains tonight, with the ridge
axis ending up near a KDLH-KDSM line at 12z Fri. Lows trending near
normal tonight as the cooler airmass settles in. However, higher-
res/WRF models hang onto the post-frontal lower level moisture/
clouds near 925mb and spread it SE into the fcst area tonight. This
not as critical for lows tonight with N-NW gradient winds of 5-10mph
looking to keep the boundary layer stirred a bit. Will stay with the
model consensus on the cloud cover tonight/Thu. Mixed 925mb temps
only supporting highs in the upper 40s to mid 50s Thu, a category or
2 below normal and quite a change from the temps of this past
The weak, secondary NW flow shortwave energy keeps cyclonic flow
aloft over the region Thu night. Models also wanting to bring some
925-850mb moisture SE across the area Thu night, and certainly
plenty of lower moisture/clouds from Man to Alb to advect SE into
the area. With the sfc ridge axis/light winds over the area, any
cloud cover Thu night could have as much as a 10F to 15F impact on
lows, and the threat of frost/freezing temps around 12z Fri. Until
the cloud details for Thu night are better known, will stay near a
blend of the guidance lows and the mainly patchy to areas of frost
.LONG TERM...(Friday through Wednesday)
Issued at 235 PM CDT Wed Oct 19 2016
For Fri thru Sat night: main fcst concern this period is the warming
Model runs of 19.12z in good agreement for rising hgts/longwave
ridging aloft to build into the region thru the period, favoring
stronger of the earlier runs with the ridging by Sat/Sat night. Fcst
confidence this period is good this cycle.
The period dominated by lower level SW flow/warm advection as the sfc
ridge axis moves east and low pressure slowly develops in the
northern plains. One weaker shortwave to ripple thru the flow and
across the area Fri night. This with some increase of moisture in
the 850-700mb layer. Lower level warm advection progged to be weaker
Fri night, with bulk of it looking to go into warming as 850mb temps
rise about 4C between 00z-12z Sat. May yet need a small -SHRA chance
or sprinkle mention Fri night but will leave grids dry for now.
After another cooler day Fri, 925mb temps back in the 9C to 13C
range by Sat afternoon, with highs Sat already trending back above
normal. For now stayed with a blend of the guidance lows/highs for
Fri thru Sat night.
For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances Tue/Wed.
Medium range models of 19.00z/19.12z in reasonable agreement for a
shortwave to slide southeast across Lk Superior Sun, then for hgts
to rise/ridging aloft to build across the Upper Midwest Sun night
thru Mon night. Agreement remains good for the ridge axis to pass
Tue with a transition to southwest flow aloft by later Tue into Wed.
Larger differences by Wed with the ECMWF more progressive with
stronger longwave troughing into the north-central CONUS on Wed.
Fcst confidence is average to good Sun/Mon then average by Wed.
Weak sfc low/front to slide across the region Sun. Airmass looking
too dry for precip with this feature and main dynamics pass well
north/east of the fcst area. High pressure to settle across the
region for Sun night thru Mon night under the ridging aloft. By
Tue/Wed, even as GFS/ECMWF get there by different routes, both
increase the moisture/lift/instability into the area ahead of low
pressure approaching from the plains and shortwave energy ejecting
out of the western US troughing. Increasing SHRA/TSRA chances Tue
into Wed reasonable at this point. Model/ensemble consensus of
highs/lows near normal Sun-Wed also looking well trended for now.
.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1236 PM CDT Wed Oct 19 2016
An expansive MVFR cloud deck was found over most of MN west of a
line from KDLH-KRWF. This area was slowly shifting east behind a
cold front with a more solid coverage of OVC south of KSTC.
Northwest flow in the lower levels behind the cold front will try to
usher this cloud deck in over the TAF sites. Sunshine has allowed
deeper mixing today at both KRST/KLSE and thus as the air mass
shifts southeast...the air mass will become a bit less-saturated in
the lowest levels.
Therefore it makes for a tough forecast if this layer will be
saturated enough to form clouds. Satellite over the past hours is
indicating some break up to more cellular clouds over SD/MN which
favors dissipation of the cloud. Have introduced an MVFR SCT cloud
layer to trend toward the clouds moving in later today and tonight.
Some MVFR cloud around overnight seems the way to go.