Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 172305
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
605 PM CDT WED AUG 17 2016

.SHORT TERM...(Tonight through Thursday night)
Issued at 315 PM CDT Wed Aug 17 2016

Main fcst concerns this period are valley fog potential tonight,
SHRA/TSRA chances Thu night and severe threat with TSRA, warm temps
Thu.

Data analysis at 18z had a broad but weak ridge of high pressure
over the region. visible satellite imagery showing cumulus with
more vertical extent early this afternoon mainly over the north end
of the region. WV imagery showing a weak shortwave over southwest
MN, but this feature has little to work with and most of the
central/south parts of the region with no to sct diurnal cumulus.
Rather moist airmass in place over the south 1/2 of the region with
dew points in the mid 60s to lower 70s where corn/soybeans dominate
the landscape. This moisture plays into valley fog potential tonight.

Model runs of 17.12z initialized quite well. Models in good
agreement on troughing to move across central Can and tighten up the
mid level flow over the region. Westerly flow continues to tighten
Thu as stronger energy begins to swing around the south side of the
trough along the US/Can border. Other shortwave ripples in the flow
remain across central MN/northern WI Thu. Models in tighter
agreement THu night as the troughing digs into MT/ND and flow over
the region becomes W-SW. Trend favors more progressive of earlier
runs with the northern plains/southern Can troughing Thu/Thu night.
Short term fcst confidence is generally good this cycle.

In the short term, sfc ridge axis remains near/over the area tonight
along with some very weak mid level shortwave ridging, for what
should be clear/mostly clear skies and light winds in the sfc to at
least 850mb layer. NAM and its variants the odd-model-out in
bringing a weak shortwave with deeper moisture across northeast IA/
southwest WI tonight, while other models take this more across
central IA into northern IL. NAM develops sct SHRA/TSRA over the
central/south parts of the fcst area this evening into Thu morning.
With minimal cumulus across IA and much of MN/WI early this
afternoon, will side with the Non-NAM consensus and leave tonight/
Thu morning dry.  The clear/mostly clear skies/light winds/moist
boundary layer then favorable for late night/early morning
radiational (valley) fog formation. Some increase of the sfc
gradient over the area late tonight, but this looking too little/too
late to have much impact on the valley fog potential. Short-term/
aviation forecaster has already beefed-up/added BR/FG to the fcst
grids in the 08-14z timeframe Thu morning. Low level warm/moisture
advection increase into the area Thu, ahead of the sfc-850mb low/
front advancing across the plains. Higher dew points over IA to be
pushed northward into the fcst area along with mixed 925mb temps
indicating highs in the mid-upper 80s. Thu to be a summery day with
many afternoon heat indices in the 90-95 range.

Model soundings indicating varying degrees of capping over the area
Thu. With little in the way of forcing signals over the area Thu and
the potential capping, left Thu dry. By Thu evening the sfc low
pushes into SW MN with a sfc-850mb warm frontal boundary taking
shape near the MN/IA state line. Moisture transport/theta-e
convergence of a 1.5 to 1.75 inch PW airmass over this boundary
along with increasing sfc-700mb FN convergence near the boundaries,
SHRA/TSRA expected to develop north of the warm front. If the
position of the sfc-850mb warm front is good, SHRA/TSRA chances Thu
night would mainly be over the north half of the fcst area.
Continued with highest precip chances (30-60%) Thu night north of I-
90. With plenty of CAPE expected over the area Thu evening along
with increasing wind shear aloft, a few TSRA Thu evening could be
strong to severe. See SWODY2 for more details. If TSRA train/repeat
over any one location, locally heavy rains also possible over the
north end of the fcst area.

.LONG TERM...(Friday through Wednesday)
Issued at 315 PM CDT Wed Aug 17 2016

For Friday thru Saturday night: main fcst concerns this period
SHRA/TSRA chances centered on Fri night/Sat, severe threat centered
on Fri evening, colder temps moving in Sat.

17.12z models show improving agreement Fri/Fri night on the
troughing to dig into the northern plains/Upper Midwest. Trend is
toward a sharper/stronger trough/shortwave somewhere near eastern
SD/NEB. Can GEM fastest, NAM slowest with this at 12z Sat. Model
consensus then sweeps this energy thru the region Sat, with the
longer wave trough continuing to deepen over the region Sat night as
secondary shortwave energy drops into it. Trend is for a deeper
longwave trough over the region by 12z Sun. Given the shortwave
timing detail differences at 12z Sat, fcst confidence this period is
average this cycle.

Stronger forcing continues over the west/north parts of the fcst
area Fri morning, slowly translating southeast with the sfc-850mb
trough/front. Feed of 1.5 to 1.75 inch PW airmass continues into the
convergence/lift with ahead of the lower level trough/low/front.
Deeper shear looks to continue increasing Fri/Fri night, but ongoing
convection/debris clouds may limit some of the diurnal warming/CAPE
development for this period. Severe threat per SWODY3 may be more
conditional, pending a break and a period of sunshine for diurnal
warming Fri. Bigger threat here may be heavy rain threat with
potential for repeat/training TSRA near/ahead of the sfc-850mb
trough. Continued with 60-75% SHRA/TSRA chances FRI night.

By Sat, models differ on where/how much low level cyclogenesis
occurs under the deepening mid level trough. Nam strongest furthest
west with this, Can-GEM weakest/furthest east. These differences
have an impact on strength/location of a potential deformation band
of precip to wrap around what would be a slower moving system. With
what would be the 1.5 to 1.75 inch PW airmass to feed into the
deformation band, depending on model, Sat could be a very wet/dreary
day. For now stayed with the model/ensemble consensus of 40-70%
precip chances Sat morning, diminishing Sat afternoon. Not much for
CAPE indicated over the area by Sat, and reduced TSRA mention to
slight chance Sat then removed TSRA mention from Sat night as
stronger low level cooling/stabilization spread in. Used a blend of
the guidance highs/lows Fri/Fri night/Sat night. Highs Sat tricky.
If NAM solution trends more correct for a rainy/dreary day,
consensus highs in the fcst grids for Sat may be as much as 10F too
warm. For now stayed with the consensus on Sat until a tighter
consensus is reached.

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period include lingering SHRA chances Sun, shot of early fall-like
temps Sun/Mon, returning SHRA/TSRA chances by Wed.

Medium range model runs of 17.00z/17.12z in good agreement sun for
strong/cool mid level troughing to be swinging across the Upper
Midwest. Trend is deeper/strong with this trough on Sun. Good
consistency for the trough axis to move east of the region Mon, with
weak shortwave ridging aloft over MN/IA/WI by Tue. Models still in
decent agreement Wed for broad west-SW flow over the north-central
CONUS, ahead of/south of stronger troughing over Man/Saskat. Fcst
confidence for Sun-Wed is average to good this cycle.

Model consensus has the low from Sat departing on Sun, but wrap-
around moisture and deep, cold cyclonic flow may squeeze out a few
SHRA, especially over the east side of the fcst area. Small SHRA
chances mainly east of the MS river Sun looking OK at this point.
Coolest of the low level airmass set to slide across the area Sun.
Even with some sunshine, mixing to the 925mb temps only results in
highs in the upper 60s/lower 70s, around 10F below normal and more
link normal for Sept 20th. A cool night Sun night with the sfc ridge
axis moving overhead for light winds/clear skies. Northwest flow
aloft continues Mon but the sfc high moves off with a return of south
flow/warm advection. Mixed 925mb temps Mon produce highs in the low-
mid 70s. Warming continues Tue into Wed as the ridging builds across
then flow becomes W-SW with temps returning to near normal by Wed.
By Tue/Wed the lower level moisture axis/plume spreads back into
the area along with an increase of CAPE. Small SHRA/TSRA chances
in the Tue thru Wed reasonable at this time. Model/ensemble
consensus of highs/lows for Sun-Wed looks well trended for now.

&&

.AVIATION...(For the 00Z TAFS through 00Z Thursday evening)
Issued at 605 PM CDT Wed Aug 17 2016

Weak high pressure in the low levels will remain over the area
tonight. This should provide clear skies and light winds with a
good setup for fog formation overnight. The 17.18Z NAM and 17.21Z
RAP forecast soundings show surface saturation occurring at both
sites late tonight into early Thursday morning. With a deep light
wind profile, should be able to get a good valley fog event with
IFR conditions with MVFR to IFR visibility reductions elsewhere.
Once the fog dissipates Thursday morning, the rest of the period
looks to have VFR conditions.

&&

.HYDROLOGY...Thursday night into Saturday
Issued at 315 PM CDT Wed Aug 17 2016

Confidence on heavy rainfall on the lower side, but the potential
exists for another round or two of locally heavy rains from Thu
night into Sat. A slower moving system is set to move into/across
across the Upper Midwest during this time. PW values in airmass
looking to be in the 1.5 to 1.75 inch range. Winds aloft increase,
so overall, SHRA/TSRA during this time expected to be progressive.
However, there is the potential for TSRA to train/repeat Thu night
into Fri night as the trough/front slowly moves in, then for a
slower moving band of SHRA/embedded TSRA to pivot across the area
Sat as a deeper low would drift across the region. Widespread 0.75 to
2 inch rain amounts from Thu night thru Sat night reasonable, but
if the training or pivoting band occur, local amounts of 2 to 4
inches with the threat of water/flooding issues are possible.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION...04
HYDROLOGY....RRS



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