Area Forecast Discussion
Issued by NWS La Crosse, WI

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179
FXUS63 KARX 162020
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
220 PM CST Thu Nov 16 2017

.SHORT TERM...(Tonight thru Friday night)
Issued at 220 PM CST Thu Nov 16 2017

Main short-term fcst concerns are precip types/chances later tonight
thru Fri night.

16.12z model runs look to have initialized well and offer quite
similar solutions for tonight thru Fri night as shortwave energy
comes thru the northern Rockies and moves into the Upper Midwest by
12z Sat. Trend tends to favor slower of earlier runs by later
Fri/Fri night when the trough is approaching. Short term fcst
confidence is good this cycle.

6.19Z observations show upper-level and surface ridging transiting
across the Upper Midwest. The associated deep-layer subsidence is
finally helping to erode the pesky low stratus that has been
plaguing the region for over a day. However, there are some
indications that some of the stratus could hang around into tonight
and even fill back in as the low-level flow becomes southeasterly.
Meanwhile, higher clouds associated with a deepening longwave trough
moving into the Pacific northwest have begun streaming over the
region. All told, mostly cloudy skies should remain the rule for the
rest of today and into tonight.

By 17.12Z, strong low-level warm air and moisture advection will
commence over the Upper Mississippi River Valley, bringing in a new
batch of low stratus along with increasing precipitation chances.
Thermal profiles indicate a brief sleet/snow potential over our far
northern counties prior to and around 17.15Z. Temperatures aloft
should then quickly rise above freezing, switching all precipitation
to liquid. The question then becomes how quickly will surface
temperatures rise above freezing. If the warming is delayed until
closer to noon on Friday, could see some minor ice accumulations
across Taylor and perhaps Clark Counties late Friday morning.

Strong isentropic lift will continue to support light rain through
Friday afternoon and into the evening as the upper-level trough
approaches the Northern Plains. Once the colder air behind the sfc-
85-mb trough starts to filter in, there may be a change over to snow
before precipitation begins to wind down late Friday night. Not
expecting any impacts at this time as by the time change over would
occur, precip amounts trend quite light and BL temps looking to
remain above 32F.

.LONG TERM...(Saturday through Thursday (Thanksgiving))
Issued at 220 PM CST Thu Nov 16 2017

For Saturday thru Sunday night: main fcst concern this period are
continuing precip chances/types Sat morning, temperatures thru the
weekend.

Model runs of 16.12z on rather good agreement on the mid level
trough axis to move across the Upper Midwest on Sat, leaving the
region under NW flow aloft for Sat night/Sun. Improving agreement
for shortwaves to flatten the mid NOAM ridging Sun night with flow
transitioning to westerly across MN/IA/WI. Fcst confidence this
period is generally good this cycle.

The SE side of the fcst area is progged to be ahead of the 700-500mb
trough axis early Sat morning. Deeper moisture and modest 700-500mb
FN convergence along with 500-300mb PV advection with the shortwave/
trough axis to still be pushing across the SE 1/3 to 1/2 of the fcst
area Sat morning. By Sat morning, model soundings showing column
generally cold enough for bulk of any precip after 12z Sat as snow.
With BL temps looking to be in the mid/upper 30s under a thicker
cloud deck Sat morning, will leave precip chances as a mix of -RA/-
SN. If it ends up more -SN, some minor accumulation would be
possible across the SE 1/3 of the fcst area Sat morning before
precip exits.

By mid day, appears the 900-700mb drying has spread SE across the
area and the bulk of the lift has exited, with the dry trend and
decreasing clouds for Sat afternoon looking good. Tighter pressure
gradient on the back side of the departing low remains over the area
Sat, for brisk NW winds and mdt low level cold advection. Temps Sat
looking to be rather steady thru the day. Low level cold advection
continues Sat night, under what should be clear to partly cloudy
skies as the sfc high builds in. Sat night continues to trend as a
colder night with lows in the teens to lower 20s. Sfc-700mb ridge
axis passes quickly on Sun, with low level warm advection already
returning by mid day Sun thru Sun night as the sfc-850mb flow
transitions to W and SW. With the source region of the lower level
airmass from the central/western plains, trend of Sunday to be a
sunny to partly cloudy day looks good. Consensus highs Sun in the
upper 20s to mid 30s OK for now but may be a category too cool if
warmer of models with the 925mb temps and deeper BL mixing are more
correct.

For Monday thru Thursday/Thanksgiving (days 4-7): main fcst concerns
this period are temperatures.

Much better agreement for the extended period in the 16.12Z GFS and
ECMWF compared to the 15.12Z guidance. The period begins with an
upper level ridge axis moving over the area with above normal
temperature on Monday, as highs will be in the 40s. An upper level
trough will then begin to amplify over Canada on Monday evening,
with the axis sliding through the forecast area on Tuesday. An
associated surface low will track along the Canadian border and
swing a cold front through the area. Any precip looks to generally
stay north, though guidance hints that some could clip the far
northern parts of the forecast area. Cooler air moves back in behind
the front, giving highs in the upper 20s and 30s on Tuesday and
Wednesday. The 15.12Z GFS and ECMWF then diverged greatly at this
point, with the GFS bringing in very cold air (highs in the teens
for Thanksgiving) and the ECMWF bringing in near-normal
temperatures. The 16.12Z guidance is in much better agreement, with
both the GFS and ECMWF showing generally dry with near-normal highs
in the mid 30s to mid 40s for Thanksgiving.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1205 PM CST Thu Nov 16 2017

The main forecast challenge this period is timing the changes of
flight categories. The general idea is for MVFR ceilings to become
VFR by late this afternoon, a return to MVFR ceilings overnight, and
a lowering to IFR between late Friday morning and early Friday
afternoon. But the exact timing of these changes remains uncertain
at this point.

Expect the current trend of gradual clearing in the massive stratus
deck to continue through the afternoon, giving way to VFR ceilings
after 16.21Z. The question then becomes how quick the MVFR clouds
fill back in overnight. Thinking this could happen by 17.06Z, but
these times will likely be adjusted in future TAF issuances. There
is also a potential for IFR ceilings to develop at KRST overnight,
which is reflected in the SCT008 group at 17.03Z. IFR clouds and
drizzle are expected to build in from the south late Friday morning
into Friday afternoon. The switch to IFR should occur at KRST first,
so included in this TAF.

Southeast winds will gradually increase to around 10-15 kt through
tonight, with gusts to 25 kt possible at KRST.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...Hollan
LONG TERM....RRS/CJA
AVIATION.....Hollan



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