Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 140446
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1146 PM CDT Fri Oct 13 2017

.SHORT TERM...(Tonight through Saturday night)
Issued at 215 PM CDT Fri Oct 13 2017

Main short-term fcst concerns are SHRA/TSRA chances thru the period
and rain amounts Sat/Sat night.

18z data analysis had a weak cold front from NE WI to near KPDC into
south-central IA. Two bands of SHRA/TSRA occurring near the front at
mid-day. The southern band across central into east-central IA is on
the northern edge of the 850mb moisture transport and in an axis of
stronger 850mb theta-e convergence with MUCAPE of about 500 j/kg.
The northern band from north-central IA into western WI is near the
northern edge of the 700mb moisture transport in an axis of 700-
500mb FN convergence with up to 250-500 j/kg MUCAPE in IA. Precip in
the bands lifting NE while the bands themselves propagate east.

13.12z model runs initialized well. Solutions similar in lifting the
trough north of Lk Superior at mid-day quickly NE tonight. Models
continue to have a nice, tight consensus on the shortwave trough to
dig into the central Rockies tonight, then into/across the Upper
Midwest Sat night. Trend favors faster of the earlier runs with this
feature. Short-term fcst confidence is good this cycle.

In the short term, deeper layered frontogenetical forcing, 300-310K
isentropic lift, 850-700mb moisture transport/theta-e convergence
responsible for the SHRA in/around the area this afternoon weakens
and moves off this evening. Some weaker lift and deeper moisture
remains over the south end of the fcst area later this evening into
early Sat morning. A diminished trend of SHRA chances expected this
evening, with an overall lull in rain chances/coverage by late
evening into early Sat morning. Lift deepens/increases and spreads
north across the fcst area thru Sat as the Rockies trough ejects
into the plains and a sfc-805mb wave begins to lift NE along the
frontal boundary across southern IA/northern IL. Increasing moisture
spreads north as well, with PW values of 1 to 1.75 inches (north
to south) over the fcst area by 00z Sun. Trend of 80-95% rain
chances spreading north across the fcst area by Sat afternoon and
thru Sat evening look good. Bulk of the CAPE remains south of the
fcst area Sat/Sat night near/south of the frontal boundary, but
some signal for as much as 250 j/kg MUCAPE to spread across the SE
1/2 to 2/3 of the fcst area by 00z Sun. This plus the strength/
depth of the lift ahead of the approaching shortwave, some TSRA
chances across much of the fcst area by later Sat afternoon into
Sat evening are well trended. With passage of the sfc-700mb
low/trough, the lower level thermo-dynamic forcing and deeper
moisture quickly exits quickly after 06z Sun with decreasing SHRA
chances.

Tighter SFC-850mb pressure gradient spreads across the area after
midnight Sat night, along with the stronger of the 925-850mb low
level cold advection and subsidence. Later Sat night into Sun
morning to be a period of brisk/gusty NW winds behind the departing
low. NW winds as strong as 15-25 mph with some gusts of 30-40 mph
looking well trended.

.LONG TERM...(Sunday through Friday)
Issued at 215 PM CDT Fri Oct 13 2017

For Sunday thru Monday night: main fcst concerns this period include
lingering rain chances and brisk winds Sun morning, frost potential
late Sun night/early Mon morning.

Model runs of 13.12z in good agreement for the trough/shortwave to
quickly move east of the region Sun morning and for a secondary NW
flow shortwave to quickly drop into/across the region Sun/Sun night.
Reasonable agreement for broad ridging/rising hgts to build across
the region Mon/Mon night. Fcst confidence for Sun thru Mon night is
generally good this cycle.

The quickly moving shortwave trough Sun morning also quickly takes
any remaining deeper moisture and lift with is as the mid level
trough axis passes and hgts briefly rise. Any lingering -RA/-SHRA
chances over the NE end of the fcst area would mainly be in the 12-
15z period. Strongest of the sfc-850mb cold and advection and
tightest pressure gradients/strongest winds are over the area thru
about mid morning Sun. Gradients relax with a return of some low
level warm advection (ahead of the NW flow shortwave) already by 18z
Sun. Previous shifts increased the winds a bit Sun morning with the
subsidence and model soundings showing mixing to around 900mb and
will continue this trend. Main impact of the NW flow shortwave
approaching Sun looks to be to spread some 850-700mb moisture/clouds
into the area Sun afternoon, limiting diurnal warming. Sun trends to
be a cool day, with the consensus highs in the upper 40s to mid 50s
looking reasonable. A cool Sunday sets the stage for a cool Sunday
night. Coolest of the low level airmass is over the area Sun with
some 925-850mb warming Sun night as lower level westerly flow brings
warm advection to the area. Lows will be chilly Sun night, but with
potential for some clouds to linger Sun evening, then the warm
advection and some gradient BL winds later Sun night, appears the
bulk of the frost threat will mainly be in the colder low laying
areas along/NE of I-94. Quiet and warming under ridging and warm
advection for Mon/Mon night.

For Tuesday thru Friday (days 4 to 7): main fcst concerns this
period are warming temperatures.

ECMWF/GFS model runs of 13.00z/13.12z in reasonable agreement for
broad ridging/zonal flow near the US/Can border across NOAM Tue/Wed,
then for the flow to buckle Thu/Fri with redevelopment of western
CONUS troughing and eastern NOAM ridging. Fcst confidence in the day
4-7 time-frame is average to good this cycle.

This period to be dominated by west to SW mid/upper level flow and
high pressure over the Upper Midwest as the storm track is shifted
north into Can. Airmass over the region remains on the dry side Tue-
Fri. Dry trend for Tue-Fri looks good. Bigger concern for Tue-Fri is
the warming lower level temps in what is looking to be rather
persistent low level warm advection under the ridging/SW flow aloft.
NAEFS 850mb temps 0.5 to 1 std deviation above normal for Tue-Thu
then 1 to 1.5 std deviations above normal for Fri. Consensus highs
in the 60s this period with perhaps a few lower 70s by Fri, (some 5F
to 10F above normal) appear well trended at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1146 PM CDT Fri Oct 13 2017

High pressure remains just north of the area, with earlier
clearing allowing for rapid fog development at LSE while RST
remains clear. Expect dense fog to remain in place for LSE through
about sunrise, though it may lift a little before then as winds
start to increase after 10Z/5 AM. RST should remain VFR with
mainly clear conditions the next few hours.

That all changes through late morning and this afternoon with low
stratus working back north ahead of approaching low pressure.
Expect a transition back to MVFR and eventually IFR for all areas
by late afternoon, along with periods of showers and drizzle. Some
thunder remains possible at LSE, but confidence not there to
include just yet.

&&

.HYDROLOGY...Saturday Into Saturday Night
Issued at 215 PM CDT Fri Oct 13 2017

Locally heavy rainfall is possible Saturday into Saturday night,
mainly over the SE end of the fcst area closer to the frontal
boundary track of the sfc low. Rain amounts of 1 to 2 inches appear
possible Sat/Sat night near/SE of roughly a Fayette IA to Richland
Center WI line. Given the expected duration and rain rates, mainly
within bank rises are expected on rivers and streams. If this amount
of rain would fall in a short period of time, some localized, minor
flooding would be possible in parts of far NE IA and far SW WI.

&&

.ARX WATCHES/WARNINGS/ADVISORIES...
WI...Dense Fog Advisory until 7 AM CDT Saturday for WIZ029-032>034-
     041-042.

MN...Dense Fog Advisory until 7 AM CDT Saturday for MNZ088-095-096.

IA...Dense Fog Advisory until 7 AM CDT Saturday for IAZ009-010.

&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...Lawrence
HYDROLOGY....RRS



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