Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 211705
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1204 PM CDT Tue Mar 21 2017

.SHORT TERM...(Today and tonight)
Issued at 325 AM CDT Tue Mar 21 2017

Main fcst concerns this period are the cooler temperatures.

Data analysis at 06z had a broad 1037mb Can high over central Saskat
with ridging to the SE into the Upper Midwest. A weak low was moving
east across Ont, dragging a cold front south into MN/NW WI. The
leading cooler/drier airmass was into northern IA/southern WI, with
the MN cold front leading a stronger, secondary surge into the
region. Some temps in the area at 06z already showing 30+F of
diurnal cooling from the Monday highs. This even with some thin
sct-bkn cirrus filtering over much of MN/IA/WI early this morning.

No issued noted with 21/00z model initializations, with solutions
quite similar for today/tonight as ridging builds over western NOAM
and increasing NW flow sends a cooler Can high SE into the Upper
Midwest. Short-term fcst confidence is good this cycle.

For the short-term, the stronger secondary surge of low level cold
advection drops quickly south across the area thru this morning,
sending 925mb temps below 0C by 18z. Low level cold advection
continues thru tonight, with the coldest of the low level airmass
progged over the area around 12z Wed. This with the center of the
sfc high over Lk Superior and weak ridging south across WI at 12z
Wed, for what should be rather light to calm/decoupled winds late
tonight/early Wed morning. Perhaps a few lower strato-cu clouds as
the cold front slides south across the area this morning, otherwise
clouds today/tonight looking limited to some cirrus from time to
time, mainly across the SW half of the fcst area today. Favored a
blend of the guidance highs today/tonight, though did trend
toward colder of guidance lows along/NE of I-94 tonight.

.LONG TERM...(Wednesday through Monday)
Issued at 325 AM CDT Tue Mar 21 2017

For Wednesday thru Thursday night: main fcst concerns this period
are slowly warming temperatures and rain/isolated TSRA chances by
Thu night.

Model runs of 21.00z in decent agreement Wed/Wed night as the
pattern progresses and the western ridging builds east into the
central Conus. However, this agreement doesn`t last as SW Conus
troughing moves into the central/southern plains by 12z Fri.
Nam/GFS/Gem in reasonable agreement on the mid level low position
near the OK panhandle at 12z Fri while ECMWF is more progressive.
Overall trend Thu/Thu night favors slower of the earlier runs with
this trough/energy. Fcst confidence is good Wed/Wed night, trending
average by Thu night.

Progressive pattern pushes the Can high east of the area thru
Wed/Wed night. However models progging an increasing inversion near
925mb over the area thru Wed night as stronger 925-850mb warm
advection spreads across the area. This while the sfc-950mb flow
remains SE out of the cold Can high to our east. BL airmass does
look to modify in place, for Wed highs trending be similar to those
of Tue. However rather consistent signal among models for an
increase/thickening of mid/high clouds over the area Wed, especially
during the afternoon and for Wed night. Will stay with the
consensus highs/lows Wed/Wed night. Some SE sfc winds and a cloud
blanket looking to keep Wed night lows near the normals.

Deeper moisture/increasing lift ahead of the system crossing the
southern Rockies into the central/southern plains spreads NE across
the area Thu/Thu night. With the slowing trend on the main mid level
low/trough, the bulk of this, and higher rain chances, now looks
to arrive Thu night. Favoring the slower consensus (vs. the faster
outlier looking ECMWF) Some small precip chances in the west end
of the fcst area mainly later Thu morning and consensus lowering
of rain chances to 20-50% for Thu afternoon look good. Column
still cold enough for possibility of some sleet or -SN early Thu
morning but appears precip chances really won`t move into the area
until later in the morning when either the column or BL temps
will be warm enough for precip as -RA. Moisture continues to
increase Thu night with PW values in excess of an inch progged
into the area. This with broad, deep layered ascent in/over/above
the lower level warm frontal/ baroclinic zone over the region.
Deeper of the moisture/lift and higher of the precip production
progged over the NW half of the fcst area Thu night. Even with
some model detail differences, mainly rain chances as high as
70-90% over the NW half of the fcst area Thu night look good.
Bigger differences and lower confidence is on the amount of
rainfall Thu night, and will there be a few TSRA or not. Some weak
MUCAPE progged over the area later Thu night when lifting
saturated 850-800mb parcels. Cannot rule out an isolated TSRA or
two late Thu night/Fri morning but will leave mention out of the
fcst grids for now. Column will be warm enough for precip as -RA
Thu night, but BL may be cold enough over the far NE end of the
fcst area for a period or two of -FZRA. This looking minor as BL
temps/lows Thu night may well be too cold under low level warm
advection/thickening clouds.

For Friday thru Monday (days 4-7): main fcst concerns this period
are rain chances much of the period, especially Fri thru Sat night,
rain totals thru Sat night.

Day 4-7 fcst confidence is generally average this cycle. Medium range
model runs of 21.00z in reasonable agreement as the mid level low
migrate east across the plains Fri/Fri night then across the Mid MS
into OH valley Sat/Sat night. Though there are plenty of detail
differences, the trend Fri-Sat night has been slowing and south with
the sfc-mid level low track. This keeps the area in the deformation
band forcing/lift north of the circulation, and in periodic -RA much
of the Fri-Sat night period. Actually decent consensus that column
will be warm enough that most if not all precip from Fri thru Sat
night will be -RA. Thu night thru Sat consensus rain totals from the
21.00z runs in the 0.75 to 1.25 inch range, but this has been highly
variable the past few runs. Loose consensus for weak sfc-500mb
ridging to build into/across the region Sat night into Sun night,
for a decrease of rain chances and a generally dry fcst by Sun/Sun
night. Parade of stronger shortwave troughs coming out of broader
longwave troughing over the western Conus continues into early next
week, with the next one looking to lift across the central plains
Mon. Lots of model variability by Mon but the small consensus -RA
chances to start next week OK for now.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday Afternoon)
Issued at 1204 PM CDT Tue Mar 21 2017

VFR conditions will prevail as high pressure builds in tonight with
drying trend. Passing mid/high level clouds with short wave moving
through mid Mississippi River valley will continue to clear heading
into the evening, but warm air return regime late in forecast period
could develop more mid/high level clouds.

&&

.FIRE WEATHER...Today through Wednesday
Issued at 325 AM CDT Tue Mar 21 2017

Dry Canadian high pressure builds into the region for today
thru Wed. Red Flag weather conditions are not expected due to
lighter winds.

Look for minimum afternoon humidity values today to drop into the
25 to 35 percent range along and east of the Mississippi river.
This along with north to northwest winds of 10 to 15 mph, gusts to
20 mph today.

Wednesday is expected to be the driest day when afternoon
relative humidity values look 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. The good
news is that winds will 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.

&&

.HYDROLOGY...
Issued at 325 AM CDT Tue Mar 21 2017

The end of the work week looks to turn wet, with rainfall totals
of 0.75 to 1.25 inches expected from Thursday night through
Saturday. Locally higher amounts are certainly possible during
this longer period of higher rain chances, but confidence is on
the low side for the occurrence and placement of any higher
rainfall amounts. At this time, the overall 0.75 to 1.25 inch
amounts during a 2+ day period would mainly create some within
bank rises on area streams and rivers.

&&

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

$$

SHORT TERM...RRS
LONG TERM...RRS
AVIATION...Shea
FIRE WEATHER...RRS
HYDROLOGY...RRS



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