Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 182326
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
626 PM CDT Mon Sep 18 2017

.SHORT TERM...(This evening through Tuesday)
Issued at 218 PM CDT Mon Sep 18 2017

GOES water vapor imagery shows a mid-level trough moving slowly
northeast out of the Central Plains. Isentropic lift and moisture
transport forcing from this wave was producing a mass of scattered
shower activity from northwest WI into southeast MN and central IA.
Lightning/thunder has been very limited and mainly confined to areas
south of Highway 20 in IA. Otherwise, temperatures this afternoon
were in the middle 60s to the lower 70s.

For tonight, the aforementioned mid-level trough will continue to
lift northeast across the area, exiting and taking scattered showers
activity with it by sunrise. Will keep an eye out for fog
possibilities tonight as the wave departs and skies become partly
cloudy. There is a lot of uncertainty on how much clearing we get
tonight behind the wave and this will dictate how much fog we`ll
get. Thinking areas west of the Mississippi would be most
susceptible given earlier clearing. Will put some patchy mention in
for now but will have to be watched closely. Lows tonight could be
tricky as well given cloud cover uncertainty. Right now, looking for
lows mostly in the lower/middle 50s.

South/southeasterly flow and warm air advection kicks in for Tuesday
ahead of low pressure/cold front moving into the Northern/Central
Plains. Under part to mostly sunny skies, plan on highs reaching
well into the 70s or several degrees above seasonal norms.

.LONG TERM...(Tuesday night through Monday)
Issued at 218 PM CDT Mon Sep 18 2017

The next chance for rain will arrive Tuesday night and continue
into Wednesday, but there are concerns that this system could end
up doing a split and missing parts of the area. The upper level
flow is expected to become more amplified as the large trough
over western Canada shows little eastward progression while energy
coming in off the Pacific causes it to dig south over the Rockies
as large scale ridging builds north from the Gulf of Mexico into
the eastern Great Lakes behind Jose and ahead of Maria. Before
this amplification of the flow occurs, a short wave trough coming
around the upper level low will cross the northern Rockies Tuesday
and then track northeast across the Dakotas and northern
Minnesota before moving into Ontario Wednesday. With this track,
both the 18.12Z NAM and GFS keep the best pv advection in the
500-300 mb layer well to the northwest of the local area. This
system will push a cold front in Tuesday night and through much of
the area Wednesday and this will be the focus for the rain
chances, providing there will be enough forcing to generate some
showers and storms. There should be a band of frontogenesis
associated with the front, with both models showing this being
weak in the 850-700 mb layer. However, the GFS is stronger with
the frontogenesis in the 1000-850 mb layer than the NAM. A band of
strong low level moisture transport will develop Tuesday night
with this being focused on locations to the north ahead of the
short wave trough. As the whole system moves east, the convergent
side of the moisture transport axis looks to start moving into the
area Tuesday evening and continues into early Wednesday morning
before showing the diurnal weakening after sunrise with the
strongest portion of the remain transport axis north of the area.
This moisture transport will start to strengthen again Wednesday
afternoon, but by that time the area will be west of the transport
axis. The warm air advection ahead of the front looks to produce
a large swath of general 2 to 3 ubar/s of up glide on the 305K
isentropic surface Tuesday night into Wednesday morning before
this becomes a little more concentrated across the south
Wednesday afternoon. As a result of all of this, expecting the
most widespread showers and storms to remain northwest of the area
with more scattered activity along the front as it moves across
the area. By Wednesday afternoon, a split in the activity could
develop with one to the north with the short wave trough and
second to the south with the isentropic up glide. For now, plan to
go with general 30 to 50 percent rain chances from Tuesday night
into Wednesday evening with the cold front as it slides across the
region.

The timing of the cold front would place it over the eastern
sections of the area for Wednesday afternoon during the peak
heating of the day. The best deep layer shear will remain behind
the front but the 0-3 km shear should be in the 30 to 40 knot
range. The big question will be how much CAPE will be available.
With showers and storms possible during the morning, how long
these and the associated cloud cover linger, will determine how
much CAPE there will be. Presently, the models suggest there will
be 1500-2000 J/Kg available ahead of the front during the
afternoon. If this does occur, this would be enough to support a
severe threat.

Once this system moves through, the flow will continue to amplify
with some differences between the 18.00Z ECMWF and GFS on how far
north the ridge axis will extend. This difference will affect the
expected path of short wave troughs rotating around the western
upper level low. The ECMWF has the ridge axis extending farther
north and takes the short wave troughs farther to the north and
west which keeps them well away from the region. The GFS does not
extend the ridge as far north and has a bit more
southwest/northeast tilt to it which allows the short wave troughs
to move across the Northern Plains into the Upper Midwest. With
these differences, the ECMWF would keep the region dry while the
GFS shows some small rain chances for the area. With the
uncertainty on what will happen with the ridge, will keep the
small rain chances in the forecast per what the GFS is showing.

By the end of the weekend into the start of next week, the
expectation is that the upper level ridge will start to shift east
to put itself into a position north of Maria as it approaches from
the southern Atlantic. This eastward shift should then allow the
long wave trough to begin shifting east out of the Rockies
allowing the short wave troughs to be farther east. This should
increase the general forcing over the area while pushing a weak
cold front into the region. This will allow for an increase in the
rain chances starting Sunday and continuing into Monday.

&&

.AVIATION...(For the 00Z TAFS through 00Z Tuesday evening)
Issued at 626 PM CDT Mon Sep 18 2017

A tricky cloud and possible fog forecast through the overnight
hours. At the moment, the last vestiges of showers continue to
work through the region, with widespread clouds in place, lowest
west of the Mississippi River where MVFR ceilings will continue at
RST into late evening. There remain hints from near-term guidance
(and satellite trends) that we may see some breaks develop in the
cloud cover later tonight, and if that indeed does occur, fog
development is likely.

For the moment, the greatest risk for dense fog looks to be at
RST where it`s not out of the question that visibility will dip to
1/4SM before sunrise. Farther east at LSE, confidence is very low
in how fast clouds will thin out, and we will also be dealing
with entrenched drier air in place. It did rain at both locations,
so if LSE can clear, some fog is also possible there. Improvement
should come for all areas through the morning hours Tuesday, with
a return to VFR conditions but also some gustier southeast winds
into the afternoon.

&&

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

$$

SHORT TERM...DAS
LONG TERM...04
AVIATION...Lawrence



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