Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 300445
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1145 PM CDT FRI JUL 29 2016

.SHORT TERM...(Tonight thru Saturday night)
Issued at 251 PM CDT Fri Jul 29 2016

Main fcst concerns this period: lingering small SHRA/TSRA chances
mainly southeast of KLSE this evening and Sat afternoon, valley fog
potential late tonight/Sat morning.

18z data analysis had low pressure over IN/OH with an inverted
trough to its northwest into northern IL/eastern IA. East-northeast
flow around this low and a 850-700mb low just south of the fcst area
continued to wrap plentiful sfc-700mb moisture westward across the
fcst area. Convergence in this lower level cyclonic flow, aided by
the 500mb trough axis and a shortwave moving across eastern IA,
continued to produce sct SHRA/TSRA across southern WI/northern
IL/eastern IA. Temps once again on the seasonably cool side,
especially where the thicker clouds/SHRA have been more persistent
across parts of southern WI/northern IA.

29.12z models look to have initialized quite well. Solutions similar
as one mid level trough axis exits east of the area late this
afternoon/early this evening and weak shortwave ridging builds over
the area tonight. The shortwave ridging is very fleeting/temporary
as the next bundle of shortwave energy moves into MN Sat then across
the area Sat night, once again lowering hgts across MN/WI. Trend is
toward a tighter consensus of the earlier runs. Short-term fcst
confidence is good this cycle.

For the short term, passing mid level trough axis, weak shortwave
ridging aloft and loss of diurnal heating/mixing should put an end
to the sct -SHRA this evening. However, the 925-700mb circulation
only moves to near Chicago by 12z Sat. This keeps the fcst area
under generally easterly flow at these levels. RH progs and mid
afternoon satellite imagery indicate plenty of moisture/clouds to
advect westward into the fcst area in this flow. Clouds looking to
be persistent and problematic over much of the fcst area tonight
into Sat. Increased cloud cover tonight over the central/south parts
of the fcst area. Given the clouds, left any mentions of radiational
valley fog around 12z Sat out of the grids. Plenty of lower level
moisture remains over the fcst area Sat, with a bit of an axis of
higher 925-850mb moisture near the MS river during the afternoon.
Models progging 500-1000 J/KG MUCAPE builds over the area in this
moisture axis. This with the next shortwave coming across MN Sat
afternoon. Removed small precip chances from Sat morning with
minimal instability. Inherited a small -SHRA/TSRA chance for much of
the fcst area Sat afternoon and continued/expanded these a bit as
cannot rule out an isolated SHRA/TSRA or two during the peak heating
of the day. Shortwave passes Sat evening with some rise of hgts
toward Sun morning. Sfc-850mb ridge axis builds in Sat night with
models indicating drying in the sfc-700mb layer in the anti-cyclonic
low level flow. Late Sat night/early Sun morning setting up to be
the more favorable night for radiational valley fog with models
showing winds less than 10kts thru 600mb. Added patchy to areas of
fog to grids for the 09-13z period Sat night/Sun morning. Used a
blend of the guidance lows for tonight thru Sat night.

.LONG TERM...(Sunday through Friday)
Issued at 251 PM CDT Fri Jul 29 2016

For Sunday thru Monday night: main fcst concerns this period include
valley fog potential around 12z Sun, SHRA/TSRA chances late Sun
night thru Mon night.

Good agreement among 29.12z models for hgts to rise/ridging aloft to
build into the Upper Midwest Sun thru Mon as troughing finally
pushed out of the region as hgts fall along the west coast and
troughing moves across southern Can. Lesser agreement on the details
but by Mon night hgt falls a bit across the region as the central
Can trough moves into western Ont and weak shortwave energy south of
the trough is pushed into the region. Fcst confidence is generally
good in the Sun thru Mon night period.

Dry, quiet, seasonable weather expected Sun into Sun night as sfc
high pressure remains over the western great lakes and ridging aloft
builds into the region. With what should be plenty of sunshine on
Sun, mixed 925mb temps supporting highs in the upper 70s to lower
80s, near the July 31 normals. Sfc-700mb low pressure/troughing
(response to the troughing moving across Can) begins to move across
the northern plains Sun night, with stronger low level warm
advection/moisture transport moving into MN/IA thru the night. Trend
is slower with this reaching the fcst area, and only carried a small
SHRA/TSRA chance across the southwest end of the fcst area toward
Mon morning to blend with neighboring grids. As the Can trough
continues east Mon/Mon night the lower level thermo-dynamic forcing/
moisture transport/moisture plume slowly spread east across MN/IA/
WI, as do increasing SHRA/TSRA chances. Highest chances Mon night as
the sfc-700mb trough/front would approach/move into the fcst area.
This trough/front weakens thru Mon/Mon night as the main trough
lifts into western Ont, so consensus SHRA/TSRA chances in the 30-40%
range by Mon night reasonable for now. With continued lower level
warm advection, 925-850mb temps continue to warm. Depending on how
much sunshine we see on Mon, highs to start the work-week could be
back above normal. For now stayed with a blend of the guidance
highs/lows for Sun thru Mon night.

For Tuesday thru Friday (days 4 to 7): Main fcst concerns this
period include SHRA/TSRA chances much of the period, very warm/humid
conditions around mid-week.

Medium range model runs of 29.00z/29.12z in reasonable agreement for
a shortwave trough to be passing Tue with rising hgts/stronger mid
level ridging to build across the Upper Midwest Tue night into Wed
night. Surprisingly good agreement on a stronger trough to move east
along the US/Can border Thu/Fri and flatten the ridging/lower hgts
over the region for later in the week. With rather good model
agreement thru Fri, fcst confidence in the day 4-7 period is above
average this cycle.

Passing shortwave trough Tue pushes the weakening sfc-700mb trough
from Mon/Mon night into/across the area. This with a CAPE axis and
plume of PW above 1.5 inches near/ahead of it. With this trough/
boundary near/moving thru the area, consensus 20-40% SHRA/TSRA
chances on Tue looking OK. Perhaps some lingering SHRA/TSRA Tue
evening, but much of the Tue night thru Wed night period should
trend dry with the area between troughs and under the building
ridging aloft. With a very warm/moist airmass remaining over the
area cannot completely rule out a small SHRA/TSRA chance this period
but appears airmass much of the Tue night-Wed night period will be
mdt-strongly capped. Model detail differences on the warmth of the
925-850mb airmass over the area Wed into Thu. Deeper mixing of the
warmer of the models could result in highs Wed/Thu well into the
90s, along with dew points in the 70s and heat indices climbing to
near/above 100. Time will tell, but the potential exits for a couple
of warm/muggy days Wed/Thu. Shortwave trough tracking along the
US/Can border Thu/Fri drives a stronger sfc-700mb trough/front
toward/across the area in the later Wed night thru Thu night period.
Timing differences among the models, but that more-or-less expected
in the day 6/7 time-frame, but and increase of SHRA/TSRA chances
later Wed night into Thu well trended for now. Fri would trend
drier/cooler, more comfortable behind the front. Model/ensemble
consensus highs/lows and precip chances for Tue-Fri in the ballpark
at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Saturday night)
Issued at 1145 PM CDT Fri Jul 29 2016

Low confidence forecast overnight. Cloud cover has dissipated more
than expected, especially for areas mainly east of the MS River.
While forecast consensus is to build clouds back across much of
the region overnight, latest satellite trends would suggest less
cloud cover. If this clearing trend continues, an adequately deep
light wind layer and ample low level moisture lends itself to
valley fog impacting KLSE. If fog develops, visibility could
rapidly drop below 1SM between 30.10Z and 30.13Z. However, given
low confidence, will only add BCFG SCT003 at this time and
closely monitor observations.

At KRST, higher confidence that cloud cover will hold tough
through the night, resulting in only some MVFR visibility
restrictions. Beyond 31.13Z and through the rest of the period,
expect VFR conditions at both TAF airfields with periods of VFR
cloud cover and light east-northeast wind.

&&

.ARX Watches/Warnings/Advisories...
WI...None.
MN...None.
IA...None.
&&

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...ROGERS



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