Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211930

Area Forecast Discussion
National Weather Service La Crosse WI
230 PM CDT Fri Oct 21 2016

.SHORT TERM...(Tonight thru Saturday night)
Issued at 230 PM CDT Fri Oct 21 2016

Main fcst concerns this period are temperatures.

18z data analysis had a ridge of high pressure from near Jame`s Bay
across WI into MO. Lower strato-cu clouds were over central/eastern
WI.  Clouds over MN/western IA were mainly above 10k ft in the mid
level warm advection south/east of a rather vigorous shortwave seen
in WV imagery near Lk Winnipeg, as the lower level airmass coming
from the central/northern plains was quite dry. Early afternoon
temps across MN/IA/WI on the cool side again today, mostly in the
mid 40s to mid 50s.

 Model runs of 21.12z initialized well and offer quite similar
solutions as ridging aloft builds over the region tonight/Sat, then
as another shortwave moves into northern MN Sat night. Short-term
fcst confidence remains good this cycle.

Tonight thru Sat night a quiet period with slowly moderating temps.
A band of mainly mid/high clouds assocaited with the Lk Winnipeg
shortwave moves east across the area tonight. This along with south
gradient winds of 4-8kts and mdt low level warm advection, lows
tonight trending 5F to 15F warmer than those of this morning. Mostly
sunny day expected Sat as 850-500mb drying spreads in. Mixed 925mb
temps Sat produce highs in the mid 50s to mid 60s (warmest NE IA).
Shortwave tracking into northern MN Sat night to induce a sfc low
near the MN/WI border by 12z Sun. Some low/mid level thermo-dynamic
forcing/lift indicated over the area later Sat night, but moisture
remains scant with bulk of the moisture/lift/clouds remaining north
of the fcst area. Lower level airmass continues to warm Sat night
and with some 4-8kt gradient winds for mixing, consensus lows
near/above normal in the mid 30s-mid 40s look good.

.LONG TERM...(Sunday through Friday)
Issued at 230 PM CDT Fri Oct 21 2016

For Sunday thru Monday night: main fcst concerns this period temps
and winds on Sunday, returning SHRA chances late Mon night.

21.12z models in good agreement tracking the northern MN shortwave
toward northern lower MI Sun, lowering hgts for Sun/Sun night.
Consistency remains good for hgts to rise/ridging to build over the
region Mon/Mon night, ahead of falling hgts/shortwave energy moving
into the western plains. Fcst confidence for Sun thru Mon night is
good this cycle.

With quick passage of northern MN the shortwave southeast Sun, the
sfc low is in eastern WI by 18z, with mdt/strong low level cold
advection spreading across the area for the later morning/afternoon
hours. 925mb temps start out Sun in the 10C to 15C range, falling to
6C to 10C by 00z Mon. Signal for a non-diurnal temp trend Sun, but
model soundings showing maximum mixing still occurs around 18z
before 925mb temps have cooled much. 925mb winds behind the
departing low increase to 25-30kts and with the mixing/cold
advection/subsidence, late Sun morning/afternoon trend breezy with
NW winds 10-20 mph gusting 20-30 mph. Gradient relaxes quickly Sun
evening with the next Can high progged to be centered over MN/IA/WI
by 12z Mon, for another cooler morning. The high brings a dry and
seasonable day to the area Mon, quickly moving east Mon night.
Lead moisture return/theta-e advection ahead of the developing
plains system arrives quickly later Mon night. First of this is
generally above 700mb, with mdt isentropic lift on the 295-315K
surfaces between 850-550mb. Question is will the moisture/lift
above 700mb be enough to overcome the drier sfc-700mb part of the
column and put rain on the ground. The small consensus -SHRA
chances later Mon night looks good for now.

For Tuesday thru Friday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances Tue thru Wed, rain amounts with
these SHRA/TSRA.

Medium range model runs of 21.00z/21.12z in pretty good agreement
bringing shortwave energy out of the west coast/eastern Pac trough
and across the plains Tue. Some detail differences Tue night/Wed as
modes bring the shortwave trough/energy into/across the mid/upper MS
valley. Bigger detail/flow differences behind this trough for Thu/
Fri as GFS/ECMWF contend with shortwaves in quasi-zonal flow across
the northern CONUS. Fcst confidence is average to good Tue/Wed then
below average Thu/Fri.

Moisture and 850-500mb QG/FN convergence set to increase across the
area Tue as the mdt/strong sfc thru mid level low/trough moves
across Neb. PW values progged into the 1+ inch range by 00z Wed,
around 1 std deviation above normal. Models show surprisingly good
agreement for the sfc low into central IA by 12z Wed then into
north-central IL by 00z Thu, putting the fcst area in the main
deformation/FN/QG forcing/lift band, near/north of the sfc-700mb
low track. Consensus precip chances increasing into the 50-80%
range for Tue night/Wed morning, then decreasing Wed afternoon/
evening with passage of the sfc-mid level trough axis quite
reasonable. Models continue to indicate CAPE will be rather
limited over the area Tue into Wed, perhaps up to 250 J/KG near
the south border of the fcst area. Will continue to limit TSRA
mention to 20% chance south of I-90 Tue afternoon thru Wed. Given
the duration, lift signal and high PW values for this time of
year, potential exists for 1 to 3 inches of rainfall across the
central/south parts of the fcst area from Tue thru Wed. Much of
the fcst area has seen an inch or less of rain since Sept 22nd,
and should be able to handle these rains falling over a 36hr
period without significant runoff/river issues. Model/ensemble
consensus temps near or a bit below normal for Tue-Fri appear


.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1232 PM CDT Fri Oct 21 2016

Main taf concern is wind shift at both RST/LSE taf sites this
afternoon/tonight. High pressure builds east of the area tonight and
allows winds to shift direction to the south this afternoon. Wind
speeds are expected to be up to 10 knots west of the Mississippi
River this afternoon into tonight. Weak upper level trough will
spread some mid to high level moisture across both taf sites this
afternoon into tonight. bases are expected to be
above 9000 feet. Expect VFR conditions to prevail at both RST/LSE
taf sites through period. With the passage of the upper level
trough...winds will shift to the west after 12z Saturday and wind
speeds will remain less than 10 knots.




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