Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 310455
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1155 PM CDT TUE AUG 30 2016

.SHORT TERM...(Tonight thru Wednesday night)
Issued at 221 PM CDT Tue Aug 30 2016

Main fcst concern this period are temperatures and potential for
late night/early morning valley fog.

Data analysis at 18z had the cold front/trough already nearing the
southern border of the fcst area. Little in the way of convection
along the front as it crossed the fcst area this morning, as it came
thru the fcst area as a katabatic front. Both KDMX/KDVN (as well as
KARX) 88D VAD wind profiles had NW to NE winds 15-25kts at the 2K-3K
ft levels much of the morning over the top/well ahead of the sfc
front. Sfc dew points already dropping into the lower/mid 60s over
the north 2/3 of the fcst area, with MU/SB CAPE those areas
generally less than 500 J/KG.

Model runs of 30.12z initialized well and offer quite similar
solutions tonight thru Wed night. This as stronger NW flow aloft
develops over the region between building ridging over the northern
plains into south-central Can and troughing deepens over eastern
Can. Short-term fcst confidence is good this cycle.

In the short term: increasing NW flow in the 925-500mb layer will
continue to quickly shove the sfc trough/front south of the fcst
area this evening. With lower level drying and decrease of CAPE
rather quickly spreading across the fcst area behind the front this
afternoon, short-term forecaster already removed any lingering
evening SHRA/TSRA chances. Model soundings and time-hgt X-sections
showing a rather deep, dry airmass spreads in for tonight thru Wed
night as Can high pressure builds into the region under the
Strengthening NW flow aloft. North gradient winds around 5kt and the
drying to keep any late night/early morning valley fog to a minimum
Wed morning. Lows tonight trending closer to end of Aug normals as
the cooler/drier airmass builds in. A more comfortable day on Wed
with mixed 925mb temps supporting near normal highs in the mid/upper
70s, along with dew points in the 50s. Sfc ridge axis noses in Wed
night and will have to watch for potential of late night fog
formation headed into Thu morning. Continued light N winds late Wed
night and model soundings indicating 10-15kts of NE wind on top of
the boundary layer not overly favorable for widespread radiational
fog formation late Wed night. Left late night fog mention out of Wed
night grids for now. Used a blend of the guidance lows/highs
tonight/Wed then favored cooler of guidance lows Wed night.

.LONG TERM...(Thursday through Tuesday)
Issued at 221 PM CDT Tue Aug 30 2016

For Thursday thru Friday night: main fcst concerns this period are
temperatures and late night/early morning fog potential.

30.12z models remain in good agreement Thu thru Fri night as the
ridging aloft slowly builds east and across the Upper Midwest. Much
of the region under NW flow aloft Thu/Thu night ahead of the ridge
axis, then under SW flow aloft by Fri night as the ridge axis
passes. Fcst confidence remains good for this period.

This late-week period continues to shape up as being very pleasant
with seasonable temperatures and comfortable dew points as the
calendar turns to Sept and the summer of 2016 goes in the history
books. Thu starts out with the sfc ridge axis extending from western
Ont to KS, with the high only moving to centered over the great
lakes and the ridge axis to MO by Fri/Fri night. The sfc high and
mid level ridging provide a deep/dry airmass for the area with
mostly sunny days and clear/mostly clear nights. The sfc ridge axis
remains over the area Thu night/Fri morning for light winds. Model
soundings showing a much deeper layer of light winds over the area
at 12z Fri. Conditions more favorable for late night/early morning
fog centered on 12z Fri but given this is 3 days, did not any fog to
the late Thu night/Fri morning grids just yet. By Fri night the high
starts to slip east of the area with a tightening pressure gradient
and southeast winds 5-10kts after midnight Fri night. These winds to
keep lows up a bit Fri night and should limit the late-night fog
potential into Sat morning. 925mb temps progged to be in the 16C-18C
range Thu/Fri, for highs mostly in the low-mid 70s under sunny/
mostly sunny skies. With the deep/dry airmass, clear skies and light
winds, leaned toward warmer of guidance highs Thu/Fri, cooler of
guidance lows Thu night. Favored a blend of the lows Fri night.

Saturday thru Tuesday (days 4 to 7): main fcst concerns this period
are SHRA/TSRA chances Sat night thru Tue.

Medium range model runs of 30.00z/30.12z in good agreement for the
mid level ridging to be pushed east of the region Sat and hgts to
fall as shortwave energy lifts into south-central Can. Trend Sat is
a bit slower than previous runs. Models remain in good agreement on
the large scale features of western CONUS troughing and broad
southwest flow aloft over the north-central CONUS Sun thru Tue.  As
usual though, plenty of smaller/meso-scale detail differences with
shortwaves thru this southwest flow and across the Upper Midwest by
the Sun-Tue period. Fcst confidence good Sat then average Sun-Tue
with the smaller scale features impacting the sensible weather.

High pressure and a drier/stable lower level airmass holds over the
area one more day on Sat, even as the mid level ridge axis moves
east and SW flow aloft strengthens over the region. Even with the
increase of SW sfc-500mb flow stronger of the lower level warm
advection does not spread in until Sat night/Sun. Sat highs looking
to be in the 70s to near 80 for a nice start to the holiday weekend.
First piece of stronger shortwave energy lifting into south-central
Can late Sat/Sat night pushes the moisture/instability axis and
stronger moisture transport east into the fcst area later Sat
night/Sun. PW values progged to increase from around 0.75 inch Sat
evening to 1.5 inch or higher by Sun evening, especially over the NW
1/2 to 2/3 of the fcst area. SHRA/ TSRA chances returning to much of
the fcst area by Sun afternoon quite reasonable based on the model
consensus. SHRA/TSRA chances continue Sun night/Mon as the lower
level trough/front slowly approaches/moves into the area. Latest
model trend stalls the sfc trough/front in the region Mon night/Tue,
develops a wave of low pressure along it then lifts the wave NE
along the front into/across the region. Details far from certain,
but with a what would be plenty of forcing/lift/instability and a
very moist airmass over the area (PW values around 2 std deviations
above normal per NAEFS), Mon night/Tue could see a threat of locally
heavy rains somewhere across IA/WI/southern MN. Temps looking to
remain near normal Sun-Tue and the model/ensemble consensus has this
well trended at this time.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1155 PM CDT Tue Aug 30 2016

VFR conditions will continue at KLSE/KRST with mostly clear/sunny
skies through the period. Stronger winds atop the nocturnal
boundary layer and drier near surface air will prevent fog
formation overnight. Winds will be light from the north, generally
10 kts or less.

&&

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

$$

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


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