Area Forecast Discussion
Issued by NWS La Crosse, WI

Home | Current Version | Previous Version | Graphics & Text | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42
FXUS63 KARX 181727

Area Forecast Discussion
National Weather Service La Crosse WI
1227 PM CDT Mon Sep 18 2017

.SHORT TERM...(Today and Tonight)
Issued at 315 AM CDT Mon Sep 18 2017

A couple focuses of note for shower/storm chances today. First is an
area of low level warm advection/moisture transport that the various
meso models take from the central plains into southern MN and and
then across northern WI later this afternon/evening. The other would
be a shortwave moving east/northeast across IA and northern Ill.
Forcing between these two features is weak and somewhat nebulous.
Going to paint the higher rain chances to the north and south, where
the forcing is more evident, lowering a bit between these. Some
lightning with current convection over ks/ne, but little if any
instability indicated in this neck of the woods, keeping the storm
threat low.

.LONG TERM...(Tuesday through Sunday)
Issued at 315 AM CDT Mon Sep 18 2017

The GFS and EC are still in lock-step when it comes to the overall
upper level pattern - carving out a long wave trough over the
western U.S. with ridge building across the east. This pattern looks
to hold for the better part of the week as hurricane Jose (off the
east coast) inhibits any/much eastward progression. In this
scenario, shortwaves would kick out of the mean trough, spinning
northeast across the plains/upper midwest. How far east, and impacts
locally are not entirely clear.

Both the GFS and EC lift a 500 mb shortwave across the northern
plains Tue night/Wed, then into southern Canada for Wed night. The
bulk of the upper level forcing looks to stay well north of the
local area, but an assocaited sfc front should slide west-east over
the region. Forcing along the boundary looks marginal at this time,
but it will have a feed of low level moisture to aid in convective
chances - likely better chances overnight Wed. The NAM is dry as it
brings it in from the west during the day Wed, while the GFS is a
bit more optimistic with firing showers/storms along it. The
Canadian and EC show a little potential for pcpn during the day, but
generally favor Wed night for the main threat locally. Should have a
decent amount of instability to work with - even during the
overnight, but the deeper shear will lie to the northwest. That
said, the NAM and GFS point to 30-35kts of 0-3 km shear at 00z,
which would certainly aid storm development. Could see a few
stronger storms in the late afternoon/evening and its a period that
bears watching.

Stepping back a bit, there is a warm front with this system too, and
it moves northeast across WI Tue night. Some low level warm air
advection, and probably enough forcing to result in a band of sct-
bkn convection on the leading edge of the front/warming.

For Thu, the models favor lifting the Wed night front northward as a
warm front as the west coast trough continues to deepen. The EC then
favors dry conditions into the weekend, keeping convection anchored
over the plains as it brings Jose into the east coast. The GFS is
more pessimistic (or optimistic depending on your view point),
driving shortwaves into the upper level ridge and keeping Jose in
the Atlantic. The result is small rain chances locally, although
the GFS too keeps the widespread chances just to the west.

Nothing clear cut this week, with Jose having a sizable influence in
how the pcpn chances play out for the local area. That said, trends
seem to be favoring a more dry and warm end to the week. Can`t rule
out nearing 90 degrees again for some locations.


.AVIATION...(For the 18Z TAFS through 18Z Tuesday afternoon)
Issued at 1227 PM CDT Mon Sep 18 2017

Currently watching scattered -shra extending from northwest WI
through southeast MN and central IA. These -shra will continue to
work northeast across the area into tonight with VFR conditions
expected into early this evening. Tonight`s ceiling/visibility
forecast becomes pretty tricky given departing -shra later this
evening. Moist lower boundary layer left over from rainfall
combined with very light surface winds set up favorable conditions
for fog/stratus development. However, lingering MVFR cloud cover
tonight should mitigate widespread fog/lower stratus development.
So, this forecast is very conditional in nature. Anymore clearing
than expected of the MVFR cloud cover will lead to more favorable
conditions for more widespread fog and stratus. Keeping
conditions MVFR after 03z at KRST and 05z at KLSE, but this will
have to be monitored very closely this evening and overnight.
Conditions expected to become VFR again after 14z as warm air
advection/mixing takes place.




LONG TERM....Rieck
AVIATION...DAS is the U.S. government's official web portal to all federal, state and local government web resources and services.