Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 152030

Area Forecast Discussion
National Weather Service La Crosse WI
230 PM CST Wed Nov 15 2017

.SHORT TERM...(Today and tonight)
Issued at 230 PM CST Wed Nov 15 2017

The main short-term fcst concerns are cloud cover trends and

Model runs of 15.12z initialized rather well and offer quite similar
solutions (at least at 500mb) as hgts rise/shortwave ridging builds
across the Upper Midwest tonight and Thu. Even with some lower level
meso-scale (moisture/cloud) differences, short term fcst is
generally good this cycle.

15.19Z surface observations show a low pressure system over southern
Lake Superior with a cold front extending south-southwestward toward
eastern Iowa/northern Illinois. The air mass behind the front has
been characterized by steadily falling temperatures and dewpoints,
brisk northwest winds gusting over 30 mph, and persistent low
stratus trapped under a mid-level temperature inversion. The stratus
shield extends well into southern Canada, with just a few breaks
noted across central Minnesota. Given little change in air mass
through tonight and upstream observations, do not expect many breaks
in the cloud cover. This scenario is supported by model guidance of
RH fields. Afternoon water vapor imagery also shows a shortwave
trough over northern Minnesota, moving southeast. This shortwave may
produce a few sprinkles or flurries over central Wisconsin late this
afternoon. Confidence on this is low however, due to 1) no low-level
forcing and 2) very dry mid levels.

The persistent low stratus should begin to clear by late Thursday
morning as surface and upper-level ridging build into the area.
However, mid-level clouds associated with a deepening trough over
the northern Rockies will begin to move in during the afternoon,
leading to another mostly cloudy day for most locations. High
temperatures on Thursday will depend on how much clearing can occur,
but should generally range from the mid 30s to lower 40s.

.LONG TERM...(Thursday night through Wednesday)
Issued at 230 PM CST Wed Nov 15 2017

For Thursday night thru Saturday night: main fcst concerns this
period are precip chances/types later Thu night into Sat morning.

Model runs of 15.12z are in improving agreement on troughing to dig
into/across the northern Rockies Thu night/Fri, then into/across the
Upper Midwest Fri night/Sat. GFS remains a faster solution by Fri
night/Sat but overall trend favors the slower, more positive
tilt/open wave look of the ECMWF. Fcst confidence is generally good
for Thu night thru Sat night.

Much like the system from Tue/Tue night, it`s all about the depth of
moisture/saturation for the system to pass Fri/Fri night. increasing
SW lower level flow ahead of developing lee troughing quickly
spreads moisture in the 925 to about 800mb layer back into the area
by later Thu night. However this moisture stuck under an inversion
near 800mb, with the 800-600mb layer above it remaining on the dry
side. Any precip chances late Thu night into Fri morning looking to
be -DZ/-FZDZ depending on the sfc/BL temp as soundings indicating no
ice in the cloud layer and dry layer above too deep for seeder-
feeder to be effective. Fcst grids already trended toward precip as
-DZ/-FZDZ 06-18z Fri and look good. Column moisture condition
changes little for Fri afternoon, except for warming in the sfc-
800mb layer. Precip chances Fri afternoon have also been trended to
-DZ by previous shifts and look good. Signal among models for some
deeper moisture up to 700mb to move across the area Fri evening with
the sfc-700mb front/trough axis. By this time, lower column plenty
warm for precip as -RA Fri evening. Cold advection behind the
passing sfc-700mb trough cools the column enough for any lingering
post-frontal precip as -RA perhaps mixing with/changing to -SN
before ending, if there is enough depth of moisture/ice in the
clouds. Precip may end as a period of -DZ later Fri night but will
leave it as -RA/-SN for now.

Lows Thu night looking to be during the evening with steady/slowly
rising temps overnight as SW flow increase and clouds thicken. Will
have to watch for fog again later Thu night into Fri evening with
the low level moisture increase ahead of the approaching trough.
Friday could be a rather warm day with brisk south winds for mixing
ahead of the front, but appears low ovc clouds, fog and -DZ chances
will temper the warmup, much like Tue. Consensus highs mostly in the
mid 30s to mid 40s appear reasonable. Brisk NW winds and mdt low
level cold advection spread across the area for later Fri night into
Sat night. A mix of sun & clouds Sat, with skies looking to clear
Sat night. Consensus temps nearly steady and mostly in the 30s for
Sat look good at this time. A colder night again Sat night into Sun
morning under what should be clear skies and lighter winds as the
sfc ridge axis approaches.

For Sunday thru Wednesday (days 4-7): the main fcst concerns this
period are temperatures.

Decent model agreement of a fairly dry extended period under mainly
northwest flow aloft. The 15.12Z GFS hints at some light
precipitation in the northern part of the forecast area Tuesday
night, while the 15.12Z ECMWF keeps a surface low and associated
precip well to the north. Otherwise, below normal temperatures will
continue, other than on Monday when the GFS and ECMWF bring 925 mb
temperatures of 5-9 C and highs in the mid 40s, which is just
slightly above normal. At the end of the this period, the GFS begins
a transition to even colder temperatures, while the ECMWF returns to
normal or above normal.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1151 AM CST Wed Nov 15 2017

MVFR conditions are expected at KLSE and KRST through the forecast
period. Current observations show IFR ceilings at KRST. Looking at
upstream trends, these ceilings should rise back to MVFR levels by
around 15.19Z. There could be some breaks in the MVFR clouds this
evening through Thursday morning, but confidence on when and even if
this will happen is too low to include in the TAFs at this time.
Northwest winds will remain strong through the afternoon, with gusts
approaching 25 kt at KLSE and 30 kt at KRST. Winds will gradually
diminish this evening through the overnight. Look for winds to turn
southeasterly Thursday morning while remaining under 10 kt.




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