Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 180856

Area Forecast Discussion
National Weather Service La Crosse WI
356 AM CDT Thu May 18 2017

.SHORT TERM...(Today and tonight)
Issued at 356 AM CDT Thu May 18 2017

Main fcst concerns this period are lingering SHRA chances focus is
on severe potential this afternoon into this evening.

06z data analysis a 995mb low just SW of KEAU. This near/under the
500mb low, with the nearly vertically stacked system lifting
steadily NE. Bulk of the instability has been pushed out of the area
as cooling/dry gradually wraps around the south side of the low. The
deep cyclonic circulation was still producing -SHRA across much of
MN/WI, with most of this north/west of the low as expected. Abundant
low clouds north/west of the sfc low as well.

Model runs of 18.00z initialized well. Models in good agreement as
the mid level low/wave quickly exits east this morning and hgts
rise/shortwave ridging builds in later today/tonight. Short-term
fcst confidence is good this cycle.

For the short term, the main mid level trough/shortwave exits this
morning, but a trailing vort lobe and a 925-700mb trough are going
to linger over the area into the early evening. Some deeper moisture
(sfc-650mb) and 850-700mb FN convergence linger over mainly the NE
end of the fcst area in the trough axis thru the afternoon. Will
trend grids with a slower exit of the trailing -SHRA eastward and
out of the fcst thru the late morning and at least early afternoon
hours. Main surge of sfc-925mb cold advection pushes south across
the area this morning. A lower strato-cu deck to spread across the
area with this, along with the 10-20mph north winds driving it.
Highs today to be near/just after sunrise, then fall thru the
mid/late morning as the strong low level cold advection spreads
across. Today is going to feel cold/raw compared to the past several
days. Consensus highs, between 14z-01z, mostly in the 50s quite

The lower level trough exits early this evening, with drier NE to
east anticyclonic sfc-850mb flow spreading across the area. Lower
clouds looking to quickly clear from NE to SW during the evening.
This along with a dry BL airmass and lighter NE winds should allow
for strong radiational cooling once the clouds decrease. However,
thickening high level moisture/clouds ahead of the next mid level
low coming thru the central Rockies is progged to quickly spread NE
across the region later tonight. Appears there may not be a very
long window of clear/mostly clear skies over the area tonight. Some
lows may actually be around midnight, then steady/slowly rise late
tonight as thickening cirrus spread in. Stayed with the blend of
guidance lows (below normal) for tonight, with those lows dipping
into the mid 30s across Clark/Taylor ctys. Added patchy/areas of
frost to grids in the north end of the fcst area. With the cloud
trends for tonight and what could be a short period for the stronger
radiational cooling, will pass any potential need for a frost
advisory to day crew.

.LONG TERM...(Friday through Wednesday)
Issued at 356 AM CDT Thu May 18 2017

For Friday thru Saturday night, main fcst concerns this period are
SHRA/TSRA chances and rain amounts and the cooler temperatures.

Model runs of 18.00z in reasonable agreement as the mid level low
over CO at 12z Fri lifts into SD by 12z Sat then to near the MN/ONT
border by 12z Sun. Some detail differences in this low/trough,
especially by Sat/Sat night, but this not unexpected with the
amplified flow pattern. Fri thru Sat night fcst confidence is
average to good this cycle.

With the mid level low ejecting into the central plains Fri, the
deep layered S/SW flow ahead of it quickly spreads the higher PW
airmass in the central/southern plains back northward into the
region. Lead warm advection `wing` of lower level thermodynamic
forcing spreads NE across the fcst area Fri. Combined with the
increasing/deepening moisture, SHRA chances quickly spread NE across
much of the fcst area Fri. This forcing/lift pass N/E of the fcst
area Fri evening, with the deeper layered forcing/lift and
divergence aloft ahead of the mid level low/trough to begin
providing the lift by later Fri evening and thru much of Sat. Due to
the model detail differences, stayed with the consensus precip
chances for Fri thru Sat, peaking in the 60-80% range much of Sat.
By Sat night as the sfc-mid level low/trough lift into northern
MN/lk Superior, the lower level flow becomes more SW to NW advecting
in both drier and cooler air. Consensus SHRA chances already
diminishing from SW to NE Sat evening are reasonable. With the
recent widespread and locally heavy rains, plenty of concern for
how much rain this next system will produce across the area.
Latest model blend produces mostly 2/3 to 1 inch across the area.
Some potential for this amount of rain to cause additional hydro
concerns, but model continue to produce this rain over the course
of a 24 to 36 hour period vs. within a few hours. One more
question this period is how much instability may work into/over
the fcst area. The consensus has steadily increased some MUCAPE
near or into the SE end of the fcst area Fri night/Sat. This
generally remains 500 j/kg or less. For now will continue a small
TSRA chance across mainly the south 1/2 of the fcst area later Fri
night thru Sat. Blend of guidance highs/lows for Fri-Sat night,
as much as 10- 20F below normal for highs Fri look good at this

For Sunday thru Wednesday (days 4 to 7): main fcst concerns this
period are SHRA chances mainly Mon/Tue and generally below normal
temperatures under broad mid level troughing. Confidence in the
larger scale feature of the upper troughing is good, but rather low
in the details of shortwaves rotating thru this flow and across the

Sunday should trend as a dry day behind the strong trough from
Fri/Sat continues to lift well north of the region. Sunday to also
trend cooler as low level cold advection spreads in behind the
departing trough. Some models have 850mb temps near/below 0C over
the fcst area on Sun. Reasonable consensus for some warm advection
and moisture/instability return ahead of a stronger NW flow
shortwave to drop into the region Mon. Evolution of this wave beyond
Mon is where model differences increase. Some 20-30% SHRA/TSRA
chances much of Mon night-Tue night OK for now. Even the lesser
model agreement by Tue/Wed would keep a cool northerly flow aloft
over the region into the middle of next week. After a bit of a warm-
up ahead of the wave on Mon, temps back below normal for Tue/Wed
are looking well trended.


.AVIATION...(For the 06Z TAFS through 06Z Thursday night)
Issued at 1110 PM CDT Wed May 17 2017

The low pressure system responsible for the shra/ts of Tue will be
exiting northeast overnight, sliding over northern Mich by 18z Thu.
Scattered showers will persist around the TAF sites through Thu
morning as a result, but don`t anticipate a thunder threat at this
time. Should get into some IFR/MVFR cigs as the low moves through,
improving later toward Thu evening. Winds will swing to the north as
the low passes - by 12z Thu - and then more northeasterly for Thu




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