Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 170842

Area Forecast Discussion
National Weather Service La Crosse WI
342 AM CDT Thu May 17 2018

.SHORT TERM...(Tonight and Thursday)
Issued at 342 AM CDT Thu May 17 2018

Main short-term fcst concerns are any SHRA/TSRA chances today and
warm temperatures.

Data analysis at 06z had high pressure holding across the Upper
Midwest. A weak frontal boundary was located near KMTW to KMFI to
north of the twin cities and was moving south across MN/WI. Front
ushering some late modified cooler air into northern WI/MN, with
mainly some sct mid clouds behind it. Further south, a mid level
low/shortwave near Kansas City was responsible for spreading some
cirrus across the south end of the region. A mild May night with
early morning temps in the mid 50s to mid 60s across much of

No issues noted with 17.00z model initializations. Solutions
similar at the mid/upper levels today/tonight with general rising
hgts/ridging across the Upper Midwest. However, models have their
lower level detail differences with potential to impact the sensible
weather today. Short-term fcst confidence is average to good this

For the short-term, model consensus drops the front S/SW today to
near or just west of the MS River in the fcst area. The front does 2
things, 1) ushers some cooler low level air into the NE side of the
fcst area and 2) provides a focus for some dew point pooling this

After a sunny, warm day Wed, 925mb temps progged to cool as much as
5C to 8C from those of Wed afternoon over the NE end of the fcst
area. Depending on model, 925mb temps near/NE of I-94 may only
support highs in the upper 60s to lower 70s. 925mb temps west of the
MS River near to a couple degrees cooler than those of Wed.
Generally favored warmer of guidance highs across the area today.
However will have to temps across the south parts of the fcst area
as shortwave spinning SE across MO today may spin some thicker
cirrus northward across the southern 1/3 to 1/2 of the fcst area
thru much of the day.

The boundary provides the focus for some lower level convergence and
dew point pooing this afternoon. GFS raises sfc dew pts as high as
the mid 60s near/west of the MS river (near the boundary) and
convects this airmass on the boundary this afternoon into early this
evening. Nam has the same idea, with sfc dew pts into the lower 60s
near the boundary. Not sure where models are getting this moisture
in drier SE flow out of the Can high dropping into the great lakes.
Airmass is on the dry side above the BL and evapo-transpiration is
just getting started. Will side with the drier models on sfc dew
points today (consensus in the 50-55F range along the boundary) and
leave the fcst for this afternoon/evening dry. Gradient SE winds of
5-10 mph tonight to keep any radiational fog to a minimum and will
leave any fog mention out of the grid-set.

.LONG TERM...(Friday through Wednesday)
Issued at 342 AM CDT Thu May 17 2018

For Friday thru Saturday night: main fcst concerns this period are
mainly SHRA chances later Sat/Sat night and temperatures.

Model runs of 17.00z in rather good agreement as shortwave energy
ejects out of the mid MS valley Fri/Fri night and a large piece of
the western CONUS trough moves into the central plains. Models
remain in decent agreement as the shortwave energy and troughing
slowly lift NE into Neb/IA Sat/Sat night, trending toward slower of
earlier model runs. Fcst confidence this period is average this

High pressure centered over the great lakes to remain in control of
the area`s weather Fri/Fri night as the shortwave energy lifts out
of the mid MS valley. Little more than perhaps an increase of clouds
by Fri night expected. Another warm day Fri with mixed 925mb
supporting highs in the upper 70s/lower 80s. Shortwave ridging aloft
between the exiting and approaching shortwave troughs builds over
the area for Sat. A weak sfc trough as part of the plains system
slides into the area Sat. With the slowing of the mid/upper level
trough lifting out of the plains, moisture return and arrival of
stronger forcing/lift are delayed. Instability/CAPE remains rather
minimal as well. Sat may well end up dry across the entire fcst area
but a small -SHRA chance across the NW side mainly in the afternoon
is OK for now. With the slower arrival of the plains system and less
precip in the fcst, favored warmer of the guidance highs (more in
the mid/upper 70s) for Sat. Plenty of detail differences by Sat
night with speed/track of the sfc-mid level low/trough/shortwave
energy which have impacts on the rain chances across the area.
Consensus of higher rain chances SW to lower NE Sat night looks
good. Little in the way of CAPE progged across the area Sat night
and limited any TSRA mention this period to the south end of the
fcst area.

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances and cooler temps Sun into Mon.

Medium range model runs of 17.00z in general agreement for a slower
passage of the plains trough/energy Sun/Mon but do have their timing
differences. ECMWF faster, Gem slower. Models offering better
agreement for broad troughing over the great lakes/Upper Midwest
into the middle of next week but as expected there are detail
differences with shortwave thru this flow. Fcst confidence in the day
4-7 period is average this cycle.

Timing differences with shortwaves are more or less expected in the
day 4-7 period and this one fits the bill. The details of when and
how the plains shortwave energy/trough moves across the region
Sun/Mon remains uncertain and could still end up in the Mon/Tue
timeframe. For now, little more to do than stay with the model/
ensemble consensus until the details sort themselves out. Present
trend of highest SHRA/TSRA chances Sun afternoon/night is reasonable
but could still change if one of the faster or slower solutions is
more correct. Cooler temps Sun/Mon are looking well trend with more
cloud cover and potential precip Sun afternoon into Mon. Trend of
drier/warmer Tue as a weak ridge of high pressure would build in
behind the passing low/shortwave is reasonable at this time. Some
small -SHRA chance Wed as a potential shortwave drops thru the broad
cyclonic mid level flow over the region reasonable as well.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1145 PM CDT Wed May 16 2018

VFR conditions set to stick around the next 24 hours. Still
watching a back-door "cold" front working southwest through
central Wisconsin at this late hour, with that feature expected to
slide westward across the area toward or after sunrise, sending
winds easterly at 8-10 knots through the coming daylight hours.
Will likely see some passing cirrus clouds at LSE into the
afternoon with a touch of cumulus around RST, though don`t believe
we`ll see enough moisture to spark off any showers out that way.




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