Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 221733

Area Forecast Discussion
National Weather Service La Crosse WI
1233 PM CDT Wed Mar 22 2017

.SHORT TERM...(Today and tonight)
Issued at 338 AM CDT Wed Mar 22 2017

Main fcst concerns this period are cloud/temperature trends through
tonight, potential small precip chance late tonight

06z data analysis had a 1038mb Can high centered near the MN/Ont
border, building steadily SE toward Lk Superior. Winds quite light/
calm over the north half of the region near the high center, with
temps in the single digits and teens. further south, light N/NE
gradient winds of 5-10 mph keeping the BL stirred and temps from
bottoming too far as the colder airmass settled into the region.
Temps across the south half of the region remained mostly in the mid
20s to mid 30s. Further west, sharp east edge to the mid cloud deck
early this morning, lining up rather well with the 700mb ridge axis.

No issues noted with 22.00z model initializations. Solutions very
similar for today/tonight as ridging aloft, ahead of stronger
troughing moving into the western Conus, builds east over the Upper
Midwest. Some detail differences, especially with highs today but
short-term fcst confidence is generally good this cycle.

For the short term, the cool/dry Can high will slide SE across the
western great lakes today, with the fcst area on its western edge
and winds becoming SE by afternoon. Coldest of the low level airmass
is over the area this morning, with 925mb temps recovering to
the -1c to -4c range this afternoon. These 925mb temps support
highs mostly in the upper 30s/lower 40s. With the building mid
level ridging, the 700mb ridge axis with its warm advection and
moisture/clouds is progged to build east with it, reaching the
west side of the fcst area late this morning then the east side
around 00z. High level moisture/cirrus spill thru the mid level
ridge axis as well. After a mainly sunny morning, clouds increase
thru the afternoon with mostly cloudy/cloudy skies tonight as
925-700mb warm advection increase/deepens over the area. May yet
need a small -SN chance mainly across the north end of the fcst
area later tonight, with some of the stronger 850-700mb warm
advection and on leading edge of the 700mb moisture transport.
GIven the very dry sub-cloud sfc-800mb layer and rather shallow
nature of the moisture aloft, left fcst dry thru tonight. Blend
of the guidance lows for tonight looks to be in the ballpark.

.LONG TERM...(Thursday through Tuesday)
Issued at 338 AM CDT Wed Mar 22 2017

For Thursday thru Friday night: main concerns this period are rain
chances and rain amounts for what continues to shape up as a rather
damp couple days.

22.00z models in good agreement for ridging to move across the
region Thu/thu night, ahead of strong mid level low to eject into
the western plains by 12z Fri. This low to then migrate slowly east
across the central plains Fri/Fri night, becoming somewhat detached
from the main flow as it does. As one would expect, plenty of
between model and run-to-run differences where the mid level low is
at 12z Fri and 12z Sat. Fcst confidence is good that this period
will be wet, but lower on where and how much rain will fall.

Initial low level airmass over the fcst area at 12z Thu remains
quite dry, with the increasing sfc-700mb saturation spreading NE
across the fcst area thru the day. Initial moisture Thu is on the
shallow side mostly the sfc to 850-700mb. The initial dry air
limited the precip chances until later Thu morning, when the
column/BL would be warm enough for -RA as the dominant precip type.
Any mixed precip types early Thu morning would be quite light, if
they occur at all. Models have trended lighter with QPF over the
area Thu, which makes sense give the shallower moisture over the
area. Precip part of Thu may end up more of a -DZ chance but will
leave it as -RA for simplicity/continuity at this point. Moisture/
lift increase/deepen Thu night/Fri as a warm advection/deformation/
F-Gen band sets up. Where this sets up remains somewhat uncertain as
models have been north and south on the latitude of this the past
few days. Trend is also narrower with the resulting precip band Thu
night/Fri, with small between model and run-to-run shifts looking to
have significant impacts on where the heavies rain may end up
falling. For now will stick with the model/ensemble consensus, which
places the higher QPF rather squarely east-west across the fcst
area. Again, confidence is not high on this placement as later model
runs could easily shift it north or south. As the system becomes
more closed off Fri night, main band of -RA trending to sink south
across the area closer to the wrapped up sfc-500mb circulation.
Consensus for now produces 3/4 to 1.25 inches of rain across the
fcst area Thu thru Fri night/Sat, but upside potential remains for 2
inches or more at locations where the main deformation/F-gen band
would set up Thu night into Fri night. Models still indicating some
MUCAPE in/ around the area Thu night/Fri but soundings showing a
capping layer above, even lifting 850-800mb parcels. Will continue
to leave TSRA mention out for now as coverage looking isolated and
will be tied to where the deformation/F-gen band sets up.

For Saturday thru Tuesday (days 4-7): main fcst concerns this period
include continuing/lingering -RA chance much of the period.

OVerall fcst confidence this period is average. With the mid MS
valley circulation becoming detached from the main flow, it is
progged to stick around and impact the area Sat/Sat night, until it
is finally kicked out by the next stronger shortwave trough coming
thru the Rockies. This lingers/continues -RA chances in the 50-90%
across much of the fcst area Sat/Sat night. Rain chances finally
diminish on Sun as some weak sfc-mid level ridging would build into
the Upper Midwest. However, some rain chances looking to continue
into Mon as both a northern stream shortwave and the kicker wave
coming out of the Rockies would be partly phased as they move across
the mid MS valley to US/Can border. These waves generally generally
progressive with rising hgts over the region - ahead of yet more
strong troughing/shortwave energy coming thru the Rockies.
Consensus temps near/above normal for Sat-Tue quite reasonable.


.AVIATION...(For the 18Z TAFS through 18Z Thursday afternoon)
Issued at 1233 PM CDT Wed Mar 22 2017

Main taf concerns through the taf period are southerly winds
increasing tonight...then ceilings lowering into MVFR after 12z
Thursday. High pressure over eastern Wisconsin builds east of the
area and allows pressure gradient to tighten across the area late
this afternoon and evening. This will increase the winds to 10 to
15 knots...possibly to around 20 knots after 12z
both RST and LSE taf sites this evening/tonight. With the
southerly winds...low levels of the atmosphere will begin to
saturate after 09z Thursday and ceilings are expected to lower to
MVFR at both RST/LSE taf sites after 14z Thursday. The increasing
moisture will also provide a small chance for light rain mainly
west of the Mississippi river after 15z Thursday...however the
higher chances will occur after 18z Thursday. For now...will leave
mention of light rain out of tafs.


Issued at 338 AM CDT Wed Mar 22 2017

Dry/cool Canadian high pressure will build SE across the great lakes
today, with the area on the western edge of the high. Red Flag
weather conditions are not expected due to lighter winds.

Relative humidity values this afternoon are expected to drop
into the 20 to 30 percent range along and east of the Mississippi
River. Central and northern Wisconsin could see values fall into the
teens. With the center of the high nearby, the sfc winds are going
to be light out of the east/southeast at 5 to 10 mph, and clouds
will be increasing in the afternoon. Precipitation chances return
later Thursday and Thursday night with some widespread rains
expected Thursday night into Saturday.


Issued at 338 AM CDT Wed Mar 22 2017

Region getting into a wet period, with an extended period of rain
looking to spread across the area from later Thu into Sat night.
The Thu-Sat night system looks anomalously moist, with PW values
some 2 to 3 std deviations above normal. Easily potential for 3/4
to 1.25 inches of rain across the area, locally up to 2 inches or
more. The heavier rain band does look fairly narrow, so hard to
pinpoint where higher amounts are most likely at this time. Small
shifts in the latitude of this band could easily mean the
difference between a rainfall total of 3/4 inch or 2+ inches. With
soils wet and minimal E-T this time of year, a good share of the
rainfall is likely to runoff and end up in our water ways. Within
bank rises are expected but cannot rule out the potential for
minor flooding on some rivers, depending on where the heavier band
of rainfall would occur.




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