Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201633
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1133 AM CDT Sun Aug 20 2017

.SHORT TERM...(Today through Tuesday)
Issued at 257 AM CDT Sun Aug 20 2017

Regional radars this morning are showing a convective complex over
eastern Nebraska into western Iowa. A MCV is expected to form from
this complex with the 20.05Z RAP indicating this will travel
east/northeast into southern Wisconsin by early afternoon. The low
level moisture transport will weaken after sunrise but the RAP
suggests it will continue to stay focused just ahead of the MCV
which would bring it across the southern sections of the forecast
area by late morning and into the early afternoon. The RAP along
with the 20.05Z HRRR and 20.03Z CR-HRRR all bring the northern
remains of the convective complex across the southern parts of the
forecast area and have adjusted the rain chances to show the best
chances today across this area to account for this. How much
thunder will occur is still somewhat questionable as there is
little to no CAPE over the area at this time and as the clouds
spread in after sunrise, there will not be much of an opportunity
for rapid heating, so would only expect weak CAPE to develop which
should limit the thunder threat.

Once this MCV moves past the area, subsidence should set in and
limit additional development through the remainder of the
afternoon. Not willing to go totally dry at this point as another
short wave trough, currently over North Dakota, will be working
east into northern Wisconsin. This wave will push a weak cold
front into the area, which could help serve as a focus for some
additional afternoon development, but the meso-scale models and
the 20.00Z NAM and ECMWF all indicate additional development along
the front will wait until this evening. There are some
suggestions that there may be another weak short wave trough in
the zonal flow aloft that comes out of Montana and Wyoming to
interact with the front. The low level moisture transport
increases this evening in response to this short wave trough with
showers and storms forming along the front.

This activity is expected to weaken/move out of the area Monday
morning before the next round of convection starts to develop
during the afternoon. The models are now suggesting a short wave
trough coming out of the southwest U.S. monsoon flow will get
caught in the main band of westerlies and come across the region
to interact with the front. Another surge in the moisture
transport will occur ahead of this wave and cause showers and
storms to form as the frontal boundary will still be over the
area. Where the boundary is located in the late afternoon and
evening will determine where any severe threat will reside. South
of the front, ML CAPE values up to 2000 J/Kg should be in place
with 0-3 km shear around 30 knots. This should be sufficient to
support the possibility of some severe storms with damaging winds
the main threat. While some hail will likely occur, this threat
will be mitigated by the warm cloud depths of 3.5 to 4 km. A QLCS
tornado could also be a concern with 0-1 km shear values around 20
knots.

This system will finally get pushed east of the area late Monday
night into Tuesday morning as a long wave trough from an upper
level low moving across Ontario sweeps across the Upper Midwest.
Clearing will occur Tuesday afternoon in the wake of this long
wave trough and as ridging starts to build in over the region.

.LONG TERM...(Tuesday night through Saturday)
Issued at 257 AM CDT Sun Aug 20 2017

Behind the departing long wave trough, ridging will form over the
Rockies and push east out onto the High Plains through Thursday.
This ridging then looks to start weakening Friday and by Saturday
the flow looks to become northwest as troughing forms from an
upper level low crossing the Canadian Rockies. The low level
ridging looks to remain over the Great Lakes through Friday with
some model differences whether it is still over the area Saturday
or farther east. The 20.00Z ECMWF is quicker to move the ridging
east which allows some rain from the approaching trough to start
spreading over the area during the day.

&&

.AVIATION...(For the 18Z TAFS through 18Z Monday afternoon)
Issued at 1133 AM CDT Sun Aug 20 2017

Currently dealing with VFR debris cloud from a complex of shra/ts
moving across eastern IA. Otherwise, quiet across much of the area
right now. Will be watching a cold front moves slowly southeast into
the area this evening which could spark a few showers and storms.
This front is then expected to hang up in the vicinity of the I-90
corridor overnight into Monday morning. Weather models then show a
mid-level trough tracking out of the Plain and along the cold front
which should fuel scattered shra/ts activity toward morning.
Confidence is fairly low this evening for convection along the front
as it moves in and impact on KRST/KLSE. Better confidence for
shra/ts activity as that that mid-level trough moves in by morning.
Plan on including VCTS at KRST after 09z and 12z for KLSE.
Otherwise, overall VFR conditions expected.

&&

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

$$

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



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