Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 101942
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
242 PM CDT Tue Oct 10 2017

.SHORT TERM...(Tonight thru Wednesday night)
Issued at 242 PM CDT Tue Oct 10 2017

Main short-term fcst concerns include continuing SHRA chances with
the passing low tonight and small -RA/-DZ chances Wed night.

18z data analysis had a dry high pressure ridge axis from Lk
Superior to eastern SD/Neb. The SE half of the region was being
impacted by plentiful moisture circulating around a sfc low in SE
MO thru a mid level low over NE KS. SHRA were falling over much
of the east half of IA into south-central MN, where the lift/
moisture are able to overcome the dry low level influence of the
high. Coolest mid-day temps were over parts of IA where clouds
and/or SHRA have been most persistent.

No issues noted with 10.12z model initializations. Models in rather
good agreement as the mid level flow pattern remains rather
progressive across the northern CONUS this period. short-term fcst
confidence is generally good this cycle.

For the short term, the mid level low over NE KS/NW MO moves to near
southern Lake MI by 12z Wed. Deformation band of moisture/lift
wrapped around the N and NW side of the sfc-mid level low continues
to translate across roughly the SE half of the fcst area this
evening, weakening and exiting overnight as the sfc-mid level trough
axis passes and hgts start to rise/high pressure builds in. SHRA
chances trended 50-90+% over the SE half of the fcst area this
evening, then diminishing/exiting SE overnight look good. Short-term
forecaster is watching radar trends and hi-res/CAMs models and will
make any late needed changes to SHRA chances/coverage thru tonight.

Even with sfc-mid level high pressure build in, model soundings/x-
sections showing considerable moisture in the 925-800mb layer to get
trapped in/under the subsidence inversion. This especially over the
west/south 3/4 of the fcst area. 925mb warm for Wed, with the mixed
925mb supporting highs in the upper 50s-mid 60s,. Blend of the
guidance highs for Wed is right in this range, however where skies
stay mostly cloudy to cloudy, these highs may easily be 5F too warm.
Moisture deepens a bit Wed night, mainly over the west 1/3 or so of
the fcst area. This with an increase of warm advection/weak
isentropic lift in the 925-700mb layer. Signal among a few models
for the depth of moisture/weak lift to be enough to produce some
-DZ/light sprinkles west of the MS river later Wed night. For now
included a sprinkle mention late Wed night across much of SE MN/NE
IA.

.LONG TERM...(Thursday through Tuesday)
Issued at 242 PM CDT Tue Oct 10 2017

For Thursday thru Friday night: main fcst concerns this period are
SHRA chances with the passing front Fri/Fri night, warmer temps
ahead of the front Fri.

Model runs of 10.12z in reasonable agreement as a lead wave out of
the western NOAM troughing lifts across south-central Can thru the
period. Fcst confidence for Thu thru Fri night is average to good
this cycle.

Moisture/lift signal from late Wed night translates across the fcst
area Thu. However, inversion above the moisture layer (about 850mb)
strengthens during the day with the depth of moisture more in the 1
to 1.5 KM range, near the minimum for -DZ. For now will leave -DZ/
sprinkle mention out of Thu, but Thu could still be a grey/OVC day
for much of the fcst area with the moisture/saturation stuck
under the stout inversion. 925mb temps for Thu similar to those of
Wed, but given cloud potential and the inversion, appears column
is not likely to mix that deep. Leaned toward cooler of guidance
highs Thu.

Trough passing across south-central Can drags a cold front into/
across the area Fri/Fri night. This into what should be a rather
moist airmass being pulled north on persistent 925-700mb SW flow
thru the plains much of Wed into Fri. By late Fri, PW values across
the area ahead of the front in the 1 to 1.5 inch range, 1 to 2 std
deviations above normal. Front looking to be accompanied by some mdt
sfc-500mb FN/QG convergence. By late Fri and Fri night, lifting
saturated 750-700mb parcels results in some weak CAPE. Left any TSRA
mention out of Fri afternoon/evening for now.  With the strength/
depth of the lift and the higher PW airmass, consensus SHRA
chances in the 50-80% range centered on Fri afternoon/evening as
the front would pass appearing well trended. Stronger 925mb
warming indicated over the area ahead of the front on Fri, much
more in some models than others. BL winds for mixing on Fri not
all that strong, and there is the continued cloud cover issue. If
one of the warmer models is more correct and the SE end of the
fcst area sees some sunshine Fri, sites like KPDC, KOVS, KLNR
could reach 80F. Given the model differences and concerns with
cloud cover, Friday`s consensus highs mostly in the 60s to lower
70s K at this time.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are rain chances Sat/Sun and highs/lows thru the period.

Medium range model runs of 10.00z/10.12z in rather good agreement on
the troughing over the Rockies at 12z Sat, then diverge on the
timing/ strength as the trough/shortwave energy moves across the
plains and Upper MS valley thru the weekend. General agreement for
rising hgts and quiet NW to W over the region early next week. Fcst
confidence for days 4-7 is average this cycle.

On Sat, low pressure strengthens in the central plains in response
to the troughing coming thru the Rockies. Moisture from the Friday
system (PW values 1 to 1.5 inches) is barely pushed out of the area,
with the plains low quickly pulling it back northward Sat. Consensus
SHRA chances spreading north across the area Sat, increasing to 60-
80% by Sat night looking well trended. MUCAPE over thru Sat night
progged to remain just south of the fcst area. May yet need a TSRA
mention over the south 1/3 of the fcst area Sat afternoon/evening
but will leave it out for now. Model timing differences by Sun
have impacts on the sensible weather: how long SHRA chances would
continue thru Sun into perhaps Sun evening. With no one model
favorite and average confidence, some smaller consensus SHRA
chances into Sun afternoon/evening will have to be for now. Strong,
deep layered drying indicated for Mon/Tue, for what are trending
to be a couple of mostly sunny, dry and seasonable days to start
next week. Blend of the guidance highs/lows, near to a little
above or below the normals, for Sat-Tue appear reasonable. At this
time there does not appear to be any major threat of frost/
freezing temps for the area thru at least the middle of next week.

&&

.AVIATION...(For the 18Z TAFS through 18Z Wednesday afternoon)
Issued at 1259 PM CDT Tue Oct 10 2017

Low pressure passing through IA/northern IL will continue to spread
pockets of rain northward into the area tonight, mainly south of I-
90. It looks like KRST may be missed by the rain but KLSE may be in
and out of -shra. Otherwise, mainly VFR conditions expected into
this evening, but will have to watch for some MVFR cloud cover at
KLSE after midnight and at KRST toward sunrise.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....DAS



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