Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 160829
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
329 AM CDT Wed May 16 2018

.SHORT TERM...(Today and tonight)
Issued at 329 AM CDT Wed May 16 2018

Main short-term fcst concerns are warm temps today and potential for
radiational fog late tonight.

06z data analysis had high pressure centered over the great lakes
with ridging westward across southern WI to Neb. Winds were light
and skies were clear across most of the Upper Midwest with the high
nearby/overhead. Sfc dew points were on the lower side and along
with the light winds/clear skies most early morning temperatures
were on the comfortable side in the upper 40s to lower 60s.

Model runs of 16.00z initialized with some slight differences this
cycle. Given the rather quiet weather across the region this period
the minor differences at the start have minimal impact on the
sensible weather as hgts aloft rise over the region. Short-term
fcst confidence is good this cycle.

In the short term, The high moves little today, with another day of
plentiful sunshine and light winds for the area. Model soundings
showing mixing to 850-800mb with the deeper mixing and drier air
aloft lowering afternoon dew points into the mid 40s to lower 50s.
This for another warm and relatively comfortable day. 925mb temps
today progged about 2C warmer that those of Tue, for highs today
mainly in the 80-85F range looking good. Bit of a re-enforcing shot
of high pressure tonight, for continued generally clear skies.
Another night of strong radiational cooling under clear/mostly clear
skies and light winds. Dew points recover a bit tonight and will
continue with a mention of patchy late night fog in the river
valleys and normally cooler, low-laying areas along/NE of I-94. With
stronger radiational cooling again tonight leaned toward cooler of
guidance lows in the upper 40s to mid 50s.

.LONG TERM...(Thursday through Tuesday)
Issued at 329 AM CDT Wed May 16 2018

For Thursday thru Friday night: main fcst concerns this period are
the continued warm temps thru the period and SHRA/TSRA chances Fri
night.

16.00z models showing some consensus for a weakening shortwave to
ripple across the north part of the region Thu, with shortwave
ridging over the region for Thu night/Fri. Hgts to start falling Fri
night as the western CONUS troughing moves east into the plains.
Detail differences on pieces of energy ejecting out of this trough
and NE into the Upper Midwest Fri night. Fcst confidence is Good Thu
thru Fri then lowers a bit for Fri night.

High pressure remains over the area Thu then is gradually pushed
east of the area as low pressure develops in and move across the
central/northern plains. Shortwave moving across northern MN Thu
brings with it some increase of moisture as southerly flow on the
back side of the retreating high deepens/increases. This with
potential of 500-1000 j/kg MUCAPE Thu afternoon. Enough moisture/
instability/lift that a few fcst models produce a few SHRA/TSRA in
central MN/NW WI Thu afternoon into Thu evening. Small consensus
SHRA clips the NW end of the fcst area Thu afternoon and will leave
this be for now. Pressure gradient tightens over the area for Thu
night, precluding any late night radiational fog development.
Tighter gradient and diurnal mixing to near 850mb on Fri for some
brisk south to SE winds. Trend of models is a bit slower with the
sfc/lower level trough/front to move into the region Fri night, and
now keeps the bulk of the increase of moisture/moisture transport
and lift west of the fcst area thru 12z Sat. Trend of all but the
west end of the west end of the fcst dry Fri night is reasonable.
With 925mb temps progged in the 18-20C range across most if not all
of the fcst area Thu/Fri, favored warmer of guidance highs mostly in
the upper 70s to lower 80s both days.

For Saturday thru Tuesday (days 4 to 7): main fcst concerns this
period are SHRA/TSRA chances and cooler temps Sat thru Sun.

Medium range model runs of 16.0z in reasonable agreement for a
stronger piece of the western CONUS troughing to lift NE into/across
the mid/upper MS valley Sat into Sun. Trend favors slower/stronger
of earlier model runs with this feature with more sensible weather
impacts across the region into Sun. Loose agreement for quasi-zonal
flow over the central/eastern CONUS Mon/Tue with models differing on
the shortwave details thru this flow and across the region on days
6/7. Fcst confidence is average Sat/Sun the below average Mon/Tue.

Stronger/slower trend of models with the shortwave energy/trough to
impact the region Sat/Sun now brings more of a sfc-700mb low
into/across the region for the weekend. This trend of solutions
brings the main rain chances across the fcst area Sat afternoon into
Sun morning. Trend is also for less CAPE over the area, especially
for Sat night Sun. Stayed with the model/ensemble consensus for Sat-
Sun for now but if latest trends are more correct, much of Sat may
well end up dry with precip chances needing to be raised for Sunday,
with SHRA chance needing to be added for Mon. Given the lesser
confidence only minor changes, for better blends with neighboring
grids, made to the 16.00z day 4-7 consensus grid-set. Trend of cooler
temps Sat and especially on Sun, then back to above normal for
Mon/Tue does look good.

&&

.AVIATION...(For the 06Z TAFS through 06Z Wednesday night)
Issued at 1154 PM CDT Tue May 15 2018

VFR conditions in place now and will remain the next 24 hours as
high pressure dominates the region. Winds will remain under 8
knots with mainly clear skies expected as well. Can`t rule out a
touch of ground fog for some of the deeper river valleys through
sunrise but given deep mixing that occurred Tuesday afternoon,
don`t anticipate seeing any of this at RST or LSE.

&&

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

$$

SHORT TERM...RRS
LONG TERM....RRS
AVIATION.....Lawrence



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