Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 162341
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
641 PM CDT Sat Sep 16 2017

.UPDATE...
Issued at 641 PM CDT Sat Sep 16 2017

Convection has proven fickle once again so far late this afternoon
(what`s new recently?), with a real lack of any activity noted
along a cold front butting up against the I-35 corridor the past
few hours. Despite OK instability (MUCAPE around 1000-1500 J/kg),
overall forcing isn`t very strong, with the main upper shortwave
skirting to the northwest, leaving mainly weakish frontal
convergence and perhaps an uptick in moisture transport/warm
advection ascent to help us out the next few hours. Not ready to
give up just yet, but trends aren`t exactly encouraging either.
Could be one of those cases where we see things take off after
sunset as the low level jet tries to increase overall mass
convergence along the front, but we shall see.

&&

.SYNOPSIS...
Issued at 127 PM CDT Sat Sep 16 2017

Afternoon GOES Water Vapor imagery showing a vigorous mid-level
trough rotating northeast from the Dakotas into northwest MN. This
was driving a surface cold front eastward into the area. Current
radar was showing scattered showers and a few storms working
northeast into the area from IA in 850mb moisture transport ahead
of the front.

&&

.SHORT TERM...(This evening through Sunday)
Issued at 127 PM CDT Sat Sep 16 2017

For tonight...main focus will be on convection along/ahead of the
cold front pushing through the area. Given,the main mid-level pv
support rotating northeast through northern MN and better CAPE
pool remaining south of the area over southern IA/MO, thinking
(and latest CAMs confirm), showers/storms will be fairly scattered
in nature along the front as it pushes east through our area.
CAPE will be waning as this convection moves into/through the area
with better bulk shear post-frontal. As such, not expecting any
severe storms but cannot rule out an isolated stronger storm or
two that may produce gusty winds of 30-35 mph and maybe some pea-
size hail. Cooler air filtering into the area after midnight will
drop temperatures into the 50s and lower 60s.

Cooler/drier air filters into the area Sunday as high pressure
builds in. After some morning clouds, skies will be mostly sunny
by afternoon with highs more typical of mid-September in the
middle 60s to middle 70s.

.LONG TERM...(Sunday night through Saturday)
Issued at 127 PM CDT Sat Sep 16 2017

The high will begin to move off to the east Sunday night as a weak
mid-level trough ejects northeast out of the Central Plains. This
will bring increasing mid=level clouds along with a slight chance
of showers across northeast IA. This trough will continue to move
northeast across the area Monday for scattered showers along with
a slight chance of a few storms across portions of northeast
IA/far southwest WI. Otherwise, looking cooler than normal with
the cloud cover and scattered rain showers. Plan on highs only in
the middle 60s to the lower 70s.

For Tuesday through Saturday, southwest flow aloft will continue
to produce periodic showers and storm chances. Temperatures are
expected to be slightly above normal through this period.

&&

.AVIATION...(For the 00Z TAFS through 00Z Sunday evening)
Issued at 641 PM CDT Sat Sep 16 2017

Not the highest confidence forecast over the next 6 hours
regarding precipitation chances for RST and LSE. Still intently
watching radar trends to the west along an approaching cold front,
though despite available instability, weak overall forcing has
failed to produce much just yet. There is a window just after
sunset where we may see showers and storms blossom, but with
greatly reduced confidence, have pulled back on the mention of
thunder for both locations at this time.

Instead, we may see scattered showers and a few rumbles of thunder
through about 06-07Z at RST/LSE, ending as the front passes.
Shallow colder air spilling in behind the front spells likely MVFR
ceilings, with increasing hints that RST will also see a stint of
IFR ceilings right behind the front for a few hours (along with
maybe a little drizzle). Improvement should come late tonight into
Sunday as much drier air works into the area, as winds shift from
the south to northwest behind the front tonight, averaging 7-12
knots into Sunday afternoon.

&&

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

$$

UPDATE...Lawrence
SYNOPSIS...DAS
SHORT TERM...DAS
LONG TERM...DAS
AVIATION...Lawrence



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