Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 161750
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
1250 PM CDT Fri Jun 16 2017

.UPDATE...
Issued at 1159 AM CDT Fri Jun 16 2017

Severe storms on our minds.

Will be ramping up severe storm messaging for this afternoon and
tonight based on a supportive deep wind shear environment, higher
boundary layer moisture, and ongoing destabilization. 0-6km bulk
shear values in the 35-45kt range will support supercell storms on
initiation and with time, cold pool formation and a QLCS damaging
wind threat. Storms should initiate later today in the 4-5pm time
frame near I-35 based on consensus CAM guidance and convergent
dry line.

SPC 16Z update to Day 1 outlook has shifted threat further north.
This looks good based on shear but with clear skies over eastern
Iowa and morning shortwave cloudiness further north, CAPE may be
higher over NERN IA and SWRN WI and a bit more favorable.
However, temperatures are rebounding quickly west of I-35 /behind
the clouds/ and this clearing will shift into SERN MN and western
WI over next hours to help build instability.

Bottom line...time to get the word out on more severe potential
for this afternoon into evening. Large hail and damaging wind,
transitioning to mainly damaging wind system. Would think it
could become more widespread with many warnings based on wind
shear. Storms should progress west to east from ~ 4pm to 9 pm.
Best CAPE/shear overlap would be south of I-90.

&&

.SHORT TERM...(Today through Saturday Night)
Issued at 247 AM CDT Fri Jun 16 2017

High Impact Weather Potential: Thunderstorms remain possible at
times through Saturday night. There is some risk for severe storms
this evening, especially across northern Iowa and far southwest
Wisconsin, and potentially into Saturday as well, though overall
confidence remains on the lower side.

Another stretch of a pesky convective weather pattern, with
confidence (especially today) much less than this particular
forecaster would really like. Early this morning, a diffuse
frontal boundary remains draped somewhere across central Iowa,
influenced locally by outflow from earlier widespread convection,
which continues to percolate even at this early hour. Aloft, the
flow regime remains fairly flat/zonal and progressive, with that
setup expected to maintain itself through Saturday before a much
more pronounced upper wave digs through the region Saturday night,
kicking a cold front through the area and leading to some much
cooler conditions to round out the weekend.

At the moment, the last of earlier showers/storms are departing,
with our focus this afternoon and evening on the potential for
additional convective development somewhere near and along that
weaker frontal boundary. Confidence on just where said feature
ends up remains very low, with that lower confidence also based on
the idea that areas near and north of I-90 may once again mix out
dew points through early afternoon, suggesting a local minimum in
available instability. Still, it does appear we may recover
through late afternoon and evening mainly south of the Interstate
(similar to yesterday evening), with weak but persistent moisture
transport pointing northward toward the frontal boundary, which
should at least mix somewhat northward today, though potentially
slowed a little by increasing mid level cloud cover from the west.
Overall shear profiles remain similar to those from yesterday,
with much of the shear concentrated in the 0-3km range, suggestive
of some risk for a hail/wind threat, though the bigger focus may
end up being a linear MCS developing near a lee cyclone over
eastern Nebraska, which would likely propagate southeastward into
southern Iowa per Corfidi vector analysis. Per recent RAP trends,
there may also be a low end risk for a few supercells, again
similar to what we saw just to the south yesterday, but lack of
low level shear and a well-mixed environment should preclude a
tornado threat.

Much of Friday night could prove to be on the quieter side after we
lose any residual convection through late evening, with the focus
then shifting toward our incoming stronger wave later Saturday.
Looking more and more like convection with that feature will erupt
mainly just to our south and east, with a subsequent decrease in the
risk for severe storms here (some good news). Will still need to
monitor trends of course, but we may escape this one with just a
risk for some showers and isolated storms in advance of the
approaching cold front, slated to arrive Saturday night.

.LONG TERM...(Sunday through Thursday)
Issued at 247 AM CDT Fri Jun 16 2017

As well advertised for several days now, much cooler conditions are
on tap by Sunday and into the first half of next week with the
development of upper troughing over the region. From a sensible
weather standpoint, still looking like some risk for diurnally
enhanced showers each day through Tuesday, perhaps most likely on
Sunday with deeper moisture rotating across the area and maybe again
on Tuesday with hints of a better reinforcing shortwave dropping
through the region. Per multi-model consensus, high pressure and dry
weather appears likely on Wednesday, with maybe a chilly morning in
spots given clear skies, dry air, and light flow, before return flow
brings increasing chances for additional convection toward Thursday
and beyond. Of much more certainty is a trend toward much more
comfortable temperatures, with highs back in the 60s and 70s and low
down into the 50s and likely cooler in some spots at times pending
cloud and wind trends.

&&

.AVIATION...(For the 18Z TAFS through 18Z Saturday afternoon)
Issued at 1250 PM CDT Fri Jun 16 2017

VFR conditions will give way to a period of developing
thunderstorms near the KRST TAF site in the 21-23Z time frame.
Confidence is higher that storms will be developing in the VC of
KRST, but lower on if the airfield will be impacted.

Storms are expected to increase in coverage and move east with
time and therefore a TEMPO group was included in the KLSE TAF for
23-01Z. Confidence is moderate on the timing window for these
storms and the impacts at the airfield.


Morning fog is a possibility and it could be dense if clouds clear
to allow ideal radiational cooling in the post-storm environment
overnight. Light winds and fresh rains promote the fog.

Large hail and damaging winds are possible with the strongest
storms.

&&

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

$$

UPDATE...Baumgardt
SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...Baumgardt



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