Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 182002
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
302 PM CDT Sat Mar 18 2017

.SHORT TERM...(This evening through Sunday)
Issued at 302 PM CDT Sat Mar 18 2017

Skies have been slow to clear with residual low-level moisture
remaining trapped under an inversion. However, visible satellite
trends have indicated gradual erosion of the cloud shield this
afternoon, and this trend should continue through this evening as
the upper ridge moves overhead.

The surface high will pass over the area tonight with winds
picking up from the south on Sunday ahead of an approaching cold
front. Stronger warm advection will allow temps to climb well into
the 40s and 50s. Clouds will be on the increase, and GFS/NAM low-
level RH fields do indicate increasing low-level moisture, so some
lower clouds could lift northward across the area later in the
day, in addition to some increasing high clouds. This could impact
temps, but either way, it should be a much milder day. Strong
850-700 mb warm advection will contribute to a steepening
inversion, which could limit mixing, but winds will likely still
gust above 20 mph with the tightening gradient, and near 30 kt
winds at the top of the mixed layer. Did include a very low chance
for precip late in the afternoon across southeast MN into western
WI as low- level warm advection strengthens, perhaps aiding in
sufficient moistening to produce a shower.

.LONG TERM...(Sunday night through Saturday)
Issued at 302 PM CDT Sat Mar 18 2017

The cold front will sweep through the region Sunday night. Model QPF
remains scant with the stronger forcing associated with the upper
wave remaining well to the north. With relatively weak forcing, NAM
forecast soundings struggle to reach deep saturation. So, at this
point not much more than perhaps a few showers/sprinkles are
expected. There continues to be a signal for some elevated
instability extending into northeastern IA and southwest WI through
the evening with very steep lapse rates rooted above 700 mb, but
expect the higher chances for elevated convection would be more focused
along the nose of a low level jet across southern Iowa. It`s not
out of the question there could be a rumble or two of thunder,
though.

For the early into middle portion of next week, very dry Canadian
high pressure will build southward across the Upper Midwest. Highs
on Monday will remain mild in the 40s and 50s ahead of a secondary
cold front that will swing through Monday night or Tuesday,
cooling temps down into the mid-week period. A weak shortwave
trough and associated frontogenetical circulation is expected to
progress eastward from the central plains on Tuesday ahead of the
surface high, with any precip expected to remain south of the
area. Model soundings indicate very dry low-levels through mid-
week, with daytime RH dropping below 30 percent, especially
across western WI. With high pressure approaching, winds should
remain on the lower side, though, in the 10-15 mph range. See the
fire weather section below for more on potential fire weather
concerns.

Late in the week, a return to more unsettled weather is expected.
Upper level ridging will give way to southwest flow aloft with
shortwave energy ejecting out of the Desert Southwest Thursday night
and Friday aiding in surface cyclone development across the high
plains. Lots of variability remains with this system, so
confidence is low on how this system will play out yet. The 18.12Z
ECMWF/GFS differ on the evolution/timing of the system, with the
GFS quicker and farther north compared with the ECMWF, which
closes off an upper low well to the south over the weekend. This
system has the potential to produce widespread precip over the
area late this week, but details are not resolvable yet at this
range.

&&

.AVIATION...(For the 18Z TAFS through 18Z Sunday afternoon)
Issued at 1150 AM CDT Sat Mar 18 2017

Cigs: persistent mvfr deck across the area. Satellite imagery
showing some holes starting to develop with only a small push east
with the back edge. Probably need to get into axis of ridge/high
pressure over the Plains to clear the clouds east of KRST/KLSE,
which looks closer to 00z. That said, more holes could develop and
expand - so will monitor and update as needed. For now, hung onto
the mvfr cigs a bit longer.

The ridge quickly shifts east of the area Sunday morning, with
models pointing to an increase in low level rh for the afternoon -
and a likely return to mvfr cigs.

VSBY/wx: no concerns anticipated until later Sunday afternoon-night.
Sfc front/low level warm advection looks to move west-east across
the region during this time, and with that increase in low level RH,
a few showers look likely. Could be more scattered in nature though,
so impacts at the taf sites isn`t assured.

Winds: winds will go light vrb/southeast tonight, but increase
quickly Sunday morning as the ridge leaves and the pressure gradient
tightens. Temp inversion (thanks to warming a loft) could keep most
of the potential gustiness in check Sunday afternoon.

&&

.FIRE WEATHER...
Issued at 302 PM CDT Sat Mar 18 2017

Drying weather Monday-Wednesday, Fire Weather Concerns Possible.

Low minimum relative humidities are expected across much of
western into central Wisconsin Monday through Wednesday as a dry
canadian airmass settles over the region. Monday and Tuesday
afternoon relative humidity values could fall into the 20s with
teens possible by Wednesday. North to northwest winds of 10 to 15
mph are expected on Monday and again on Tuesday. Winds look to be
a little lighter for Wednesday as high pressure slides overhead.

Across southeast Minnesota and northeast Iowa, conditions won`t be
as dry but we will still see afternoon relative humidity values
bottom out in the 30 to 40 percent range on Monday and Tuesday,
with drier conditions expected on Wednesday when afternoon
relative humidity values will fall into the 25 to 35 percent
range.

&&

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

$$

SHORT TERM...JM
LONG TERM...JM
AVIATION...Rieck
FIRE WEATHER...Wetenkamp/JM



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