Area Forecast Discussion
Issued by NWS La Crosse, WI

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047
FXUS63 KARX 121938
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
238 PM CDT Thu Oct 12 2017

.SHORT TERM...(This evening through Friday)
Issued at 238 PM CDT Thu Oct 12 2017

Afternoon upper air analysis showed a ridge axis moving through
lower Michigan with surface analysis showing a surface trough and
cold front extending north to south across the Dakotas. Low clouds
remained in the area throughout the day, but continue to slowly
decay from from west to east from central Minnesota into north-
central Missouri. May see a brief break in the clouds by late
Thursday afternoon/early evening, but additional clouds will quickly
move in/develop as the cold front approaches the area from the west.
RAP BUFKIT soundings have consistently shown saturating low levels
after midnight, especially east of the Mississippi River, coupled
with decent negative omega and some isentropic lift, so have added
drizzle mention for these areas.

For Friday, 12Z guidance shows the surface cold front moving into
the northwest part of the forecast area by Friday morning, with
frontogenesis increasing across the southern part of the forecast
area during the afternoon. The best moisture transport does not
reach the area until Friday evening, so have delayed the highest
PoPs a bit, but rain still appears likely for much of the southern
half of the forecast area by late Friday afternoon. Any instability
looks to remain to the south during this time period, so have left
out thunder mention.

.LONG TERM...(Friday night through Thursday)
Issued at 238 PM CDT Thu Oct 12 2017

Wave of low pressure rides up along the baroclinic boundary laid
up across eastern IA into northern IL Friday night into Saturday
morning. Increasing moisture transport/isentropic lift over this
frontal boundary will produce more showers and a few
thunderstorms, mainly across far northeast IA/southwest WI.

Will continue to keep a close eye on Saturday afternoon/evening as
our far southeastern forecast area will be on the cusp of being in the
warm sector. Right now, looks like better CAPE stays just to our
southeast. However, if we can get a bit deeper into the warm
sector/better CAPE, we will have more than plenty Bulk Shear and
impressive hodographs for a severe storm threat. Will maintain
mention of a few stronger storms in the Hazardous Weather Outlook.
Another aspect to consider will be heavy rain threat as
precipitable water values creep up into the 1.5-2 inch range.

Attention then turns to strong/deepening low pressure coming
through the area later Saturday night into Sunday. Most models
have central pressure dropping 15-20mb in 12 hr (Sun 00-12z) as
the low lifts from northeast IA into eastern Lake MI...impressive!
Models drive a dry slot into the area as the low pulls into
northeast WI/southern Upper MI. At the same time, expecting winds
to pick up significantly from the west as the low rapidly deepens.
Could see winds sustained in the 20-30 mph range with some gusts
over 40 mph through Sunday morning, then tapering through the
afternoon as the low pulls into eastern Ontario. Cooler air also
filters in on these gusty winds with highs only in the upper 40s
to the middle 50s.

Clear/cold conditions set up for Sunday night/Monday morning as
high pressure settles in across the region. Lows will be in the
30s and looks like it will produce frost for the majority of the
area.

Looks like quiet conditions then through Thursday as a ridge of
high pressure builds over the central CONUS. Temperatures are
expected to be at or a couple degrees above seasonal normals.

&&

.AVIATION...(For the 18Z TAFS through 18Z Friday afternoon)
Issued at 1246 PM CDT Thu Oct 12 2017

MVFR conditions are expected at both sites through the period.
Some light drizzle developed, mainly along and east of the
Mississippi River, but should come to an end by mid-afternoon as
the saturated layer becomes shallower and near-surface levels dry
a bit. Otherwise, an expansive MVFR cloud deck in the 1500 to 2500
ft agl layer will remain over the area through much of the
afternoon, along with scattered lower clouds. Could see a few
breaks in the clouds and ceilings rising into VFR levels during
the late afternoon/early evening, but this would be very short-
lived as a cold front and surface trough approach from the west,
bringing in another MVFR cloud deck. Some more drizzle is expected
to form east of the Mississippi River during the overnight hours,
but confidence is low on if it will reach far enough west to
impact KLSE. Winds will shift from the southeast to westerly as
the cold front moves through the area, but should generally remain
near or below 10 kts. Shower chances also increase as the front
moves through, but the best chances look to be after 18Z, so will
leave TAFs dry for this issuance.

&&

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

$$

SHORT TERM...CA
LONG TERM...DAS
AVIATION...CA



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