Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201930

Area Forecast Discussion
National Weather Service La Crosse WI
230 PM CDT Fri Oct 20 2017

.SHORT TERM...(This evening through Saturday)
Issued at 230 PM CDT Fri Oct 20 2017

Water vapor satellite this afternoon shows a couple of short wave
troughs over eastern Colorado and the Texas Panhandle. These will
lift north/northeast into Minnesota and northern Wisconsin through
Saturday morning. As they move over the area, the 20.12Z NAM
suggests they will produce some weak pv advection in the 500-300 mb
layer late tonight and early Saturday morning. With the low level
jet/moisture transport still in place ahead of the mean long wave
trough, there should be enough forcing from the waves and warm air
advection to cause some showers to form in the moisture transport
axis. Forecast soundings show the lift will be above the low level
cap with no instability present, so looking from some mid level
showers to form. Several of the hi-res meso-scale models have a band
of showers moving across portions of northeast Iowa and southeast
Minnesota in the morning and have raised the rain chances up into
the 40 to 60 percent range to account for the expected development.
Once these short wave troughs go by, there really isn`t a lot of
forcing for the rest of Saturday. The mean long wave trough will
still be well to the west, moving into the central Dakotas and
Nebraska by late afternoon. The cold front should make it into
western Minnesota and Iowa and the forecast soundings show a well
cap environment ahead of the front. With the lack of forcing and cap
in place, there may not be a lot of activity other than some stray
showers in the moisture transport axis. Have cut back on the rain
chances, especially from along the Mississippi River east into
Wisconsin as it looks like any widespread showers and storms will
have to wait for the front to arrive and break the cap.

.LONG TERM...(Saturday night through Friday)
Issued at 230 PM CDT Fri Oct 20 2017

Showers and scattered storms will increase from the west Saturday
evening, spreading eastward through the night. Instability
continues to be a limiting factor with only around 500-1000 J/kg
of MUCAPE. However, southwesterly 0-3 km bulk shear on the order
of 30-35 kts would support some potential organization of low-
topped storms. A few storms could be strong to marginally severe,
with winds the primary hazard. The time window for any stronger
storms looks to be fairly short, mainly between 7-11 pm, with the
highest chances near and west of the Mississippi River before any
storms weaken farther east through the overnight.

The cold front will push east of the area by Sunday morning,
although post-frontal showers could linger into the first half of
the day, especially across western Wisconsin, before high pressure
quickly settles in from the west. Temps will be cooler, but still
near late October norms.

A change to cooler, more seasonable weather is still on target
heading into the new work week. A stronger cold front will slide
through on Monday ahead of a deepening upper trough that will be
carved out over the Great Lakes through mid-week. After one more
mild day on Monday, temps will trend downward Tuesday/Wednesday
with near to slightly below average highs in the upper 40s to mid
50s along with breezy north winds. Lows Tuesday night could drop
well down into the 30s, with a potential frost concern. Not a
whole lot in the way of precip is expected through mid-week,
although with cold air aloft as the trough digs over the area
later Monday and Tuesday, at least low shower chances look
warranted at times, with the highest chances Tuesday across
central Wisconsin as the 20.12Z GEM/ECMWF indicate a surface low
tracking into the Eastern Great Lakes.

After a brief period of shortwave ridging, model guidance supports
another deepening upper trough across the central US late this week,
bringing a reinforcing shot of cool air. Highs by Friday may be
stuck in the 40s. Kept close to consensus pops given the model
variability, with the 20.12Z GFS showing a frontal precip band by
swinging southeastward by Thursday night into Friday and the
ECMWF mainly dry through Thursday night.


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1207 PM CDT Fri Oct 20 2017

The first concern with the forecast is with low level wind shear
for much of the overnight hours. As the mean long wave trough
swings out of the Rockies tonight, the low level jet will increase
ahead of this with the 20.12Z NAM soundings showing speeds up to
50 knots possible in this. Will continue with the trend of
including the wind shear for both sites starting right about at
sunset and then diminishing late tonight before sunrise as the low
level jet starts to weaken. The other concern is with the shower
potential Saturday morning. The NAM shows a lead short wave trough
coming out ahead of the main trough and moving from central Iowa
into northern Wisconsin. This is expected to interact with the
remains of the low level jet/moisture transport to pop some
elevated showers. This idea is supported by several of the hi-res
meso-scale models and will include a VCSH at KRST. Expecting this
activity to stay west of KLSE so will stay with a dry forecast




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