Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 150516

Area Forecast Discussion
National Weather Service La Crosse WI
1115 PM CST Thu Dec 14 2017

.SHORT TERM...(Tonight thru Friday night)
Issued at 255 PM CST Thu Dec 14 2017

Main short-term fcst concerns are flurries tonight then returning
-SN chances thru Fri into Fri night.

Data analysis at 18z had high pressure centered over eastern IA. WV
imagery showed a few shortwaves rippling SE thru the NW flow, a
couple over Neb and another over eastern Man/western Ont. Even with
the high pressure, low clouds covered much of MN/northern WI and
western/central IA, with flurries leaking from the clouds across
much of MN and northern WI. This in deeper cyclonic flow, a cold low
level airmass and ahead of the Man/Ont shortwave. Clouds/no clouds,
mid-day temps across the region generally near normal for mid Dec.

No issues noted with 14.12z model initializations. Solutions quite
similar as the Man/Ont drops across the NE half of the region
tonight, then as hgts rise Fri/Fri night and lower level warm
advection spreads across the region. Short term fcst confidence
remains good this cycle.

In the short term, with plenty of flurries upstream today and the
shortwave to drop across tonight, continued/increased the sct
flurry/-SN mention across much of the fcst area, especially this
evening near/east of the shortwave track. Tapered/ended flurries
overnight as the shortwave passes and lower level flow becomes
neutral or anti-cyclonic behind the passing wave. Freedom from
-SN chances across the fcst area looking short-lived, as mdt/strong
925-700mb warm advection and 850-700mb FGEN quickly spread a round
of lower level thermodynamic forcing, ahead of low pressure and a
warm front moving across the northern plains, later Fri and Fri
night. Stronger/deeper of the lift looking to come across Fri night.
Models showing moisture takes a bit to arrive and initial forcing/
lift works to saturate the column. By the time this occurs the main
forcing/lift has set up across mainly the NE 1/3 to 1/2 of the fcst
area Fri night into Sat morning. On the easier side, column looking
to be cold enough with ice in the cloud tops for any precip Fri
afternoon into Sat morning as snow. Will leave Fri afternoon -SN
chances across the NE half of the fcst area in the 20-40% range
(column saturation issues), then raised snow chances Fri night into
Sat morning across the NE end into the 50-70% range, with potential
for an inch or so of accumulation across the far north end of the
fcst area. Blend of the guidance lows/highs for tonight thru Fri
night look good at this time.

.LONG TERM...(Saturday through Thursday)
Issued at 255 AM CST Thu Dec 14 2017

For Saturday thru Sunday night: main fcst concerns this period are
exiting -SN chances Sat morning and warmer weekend temperatures.

14.12z model runs in good agreement for hgts to continue rising with
shortwave ridging to build across the Upper Midwest Sat/Sat night,
ahead of strong troughing digging thru the Rockies. Models show
improving agreement on this troughing as it would eject into the
plains Sun, then as the northern portion would move into the Upper
Midwest and weaken Sun night. Fcst confidence in the Sat-Sun night
period is generally good this cycle.

Forcing/lift for the Fri night -SN across the NE side of the fcst
area exits rather quickly Sat morning, as drier 925-700mb air from
the central plains spreads across much of the area. Bulk of any Sat
morning snow chance to be in the 12-15z period. Some low level
warming continues Sat as a weak low lifts into the area. Lower
levels to be drier for Sat but considerable moisture around 500mb
looks to remain over the area. Even so, model soundings showing
mixing to near 925mb with mixed 925mb temps supporting highs as much
as 10F above normal. Consensus highs Sat from the mid 20s north
(more clouds/snow cover) to around 40F south (less snow cover/
clouds) look good. Seasonably mild temps to continue Sat night into
Sun night but highs Sun trending a few degrees cooler than Sat as
some weak low level cold advection behind a lead wave and out of the
plains/Rockies trough spreads across the area. 850-700mb layer
remains rather dry Sat afternoon thru Sun night with the flow
generally off the central and western plains. Any precip chances Sat
night thru Sun night looking to be quite minor, even as the
weakening mid level trough/shortwave moves in. This depicted well by
any consensus -RA or -SN chances these periods at 20% or less.

For Monday thru Thursday: main fcst concerns this period are warmer
temps early next week then precip chances by Thu.

Medium range model runs of 14.00z/14.12z in reasonable agreement for
a weakening mid level trough axis to pass early Monday, with
northern stream shortwave energy to then drop in and carve out a
broad trough over the great lakes for later Mon into early Wed.
Model differences then begin to increase on how fast the next
troughing into the Rockies by late Wed moves east toward/into the
Upper Midwest for Wed night/Thu. Fcst confidence average to good Mon-
Tue then below average by Thu.

A warmer day Mon, with high in the mid 30s to near 40 (about 10F
above normal) across much of the fcst area. Temps trend cooler for
Tue/Wed but not much as colder air behind the shortwave dropping
into the great lakes is more of a glancing blow. By later Wed as the
Rockies trough begins to eject, 925-700mb warm advection starts to
over-run the retreating high/cold dome as the next low ahead of the
Rockies trough develops into the central/ northern plains. Plenty of
between model and run-to-run differences on location/timing of a
warm frontal/FGEN/isentropic lift band, and resulting precip to
develop into/across the region Wed night/ Thu. Not a lot of
confidence given the model variability/ensemble spread in the day 7
time-frame, but wed night/Thu consensus precip chances in the 20-50%
range OK for now.


.AVIATION...(For the 06Z TAFS through 06Z Friday night)
Issued at 1115 PM CST Thu Dec 14 2017

Cigs: probably the most challenging part of the forecast as
observations, upstream cigs, and models all suggesting periods of
mvfr and vfr (within around 1kft of each other) through Fri night.
Latest GOES16 Fog/Stratus satellite imagery has a pretty good handle
on where the backedge of the current mvfr cigs lie - and if trends
hold - should be through KRST before 06z and KLSE by 07z. Then, it
looks mostly VFR into Fri afternoon, with a drop back into MVFR
looking more likely for KLSE than KRST. That said, confidence
remains shaky on when/if the back and forth between flight
categories will occur at KRST/KLSE. Will lean on current trends with
model consensus to try and narrow down most likely time periods.

WX/vsby: upper level shortwave sliding across the region tonight,
bringing areas of light snow/flurries but little if any vsby
reductions. Don`t expect any accumulations as a result. Will
continue a mention for tonight at both TAF sites. Another shortwave
trough and a good surge of low level warm air advection push in for
later friday afternoon/night. Models still in good agreement with
developing a narrow band of snow from central mn through central wi,
northeast of the TAF sites. That said, might be enough lift for at
least some flurries of light vfr snow farther south - and will
continue mention.

Winds: should hold westerly through Friday, a swing to more
southerly Friday night.




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