Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 201125

Area Forecast Discussion
National Weather Service La Crosse WI
625 AM CDT Tue Jun 20 2017

.SHORT TERM...(Today and tonight)
Issued at 322 AM CDT Tue Jun 20 2017

Early this morning, water vapor imagery showed a strong upper level
shortwave trough diving southeast across Minnesota. Showers forced
by both the upper wave/exit region of a 300 mb jet streak and the
associated zone of mid-level frontogenesis/warm advection ahead
of the wave will move into southeast Minnesota/western Wisconsin
before exiting towards noon. Although instability is weak, a few
thunderstorms are possible as this system works through, given
the strong forcing/frontogenesis with the wave.

Weak high pressure will build across the region behind this system
for this afternoon and tonight with skies clearing through the
afternoon and evening. Highs will continue to be seasonably cool
today with temperatures climbing to near 70 over central Wisconsin,
with generally 70s expected elsewhere. With mostly clear skies and
light winds tonight, temperatures should fall into the upper 40s to
low 50s in most areas, although the typical cool spots in central
Wisconsin could see lows into the low/mid 40s. Some fog development
is possible late tonight with clear skies,light winds and moist
ground from recent rains.

.LONG TERM...(Wednesday through Monday)
Issued at 322 AM CDT Tue Jun 20 2017

The surface ridge will pass to the east on Wednesday ahead of an
approaching surface cold front/trough. Much of the day should be
dry as low-level moisture initial is slow to increase. However, a
few showers are possible as low-level warm advection strengthens.
However, the potentially impactful weather is expected Wednesday
night as 850 mb moisture transport really increases with a
strengthening low-level jet and strong low-level warm advection.
At the same time, steepening mid-level lapse rates will increase
atop of the nocturnal boundary layer inversion, yielding
increasing elevated instability. Would expect increasing
thunderstorm coverage Wednesday evening in this environment.
Strong/severe storms would be possible with effective shear around
30-40 kts. Even though the stronger moisture transport quickly
shifts east of the area late Wednesday night, locally heavy
rainfall is possible as storms organize.

The cold front will be slow to push southward as it becomes
oriented nearly parallel to the mid-level flow. It may take until
shortwave energy dives into the northern plains late this week
before the front gets pushed southward completely through the
area. Until this occurs, showers/storms will remain possible near
the boundary Thursday into early Friday, with 20.00Z model
guidance continuing to focus these chances near and south of I-90.
Will have to watch to see where exactly the boundary ends up on
Thursday, though. With richer boundary layer moisture continuing
to advect northward into the boundary, sufficient
instability/shear could still support strong/severe storms with
locally heavy rainfall.

A brief return to dry weather is expected Friday with seasonable
temps. However, heading into the weekend an upper longwave trough
is expected to be established through the Great Lakes region.
Under northwesterly flow aloft ,seasonably cool temps will return
for the weekend with highs generally in the 60s to mid 70s.
Embedded shortwave troughs in this regime will bring chances for
showers and possibly thunderstorms to the region, dependent on
timing of the waves, and with some diurnal enhancement possible
under cold air aloft.


.AVIATION...(For the 12Z TAFS through 12Z Wednesday morning)
Issued at 549 AM CDT Tue Jun 20 2017

Scattered showers with a few isolated thunderstorms will continue
to move through the region this morning and into the early
afternoon. There has been a recent slight increase in lightning
activity with ongoing showers/storms, but it is still neither
widespread nor frequent enough to include in TAFs. VFR conditions
are maintained through the period, but could see a brief drop to
MVFR with any heavier shower/storm that directly impacts the TAF
sites. The rain should delay mixing at both sites, though KRST may
begin to gust to 18 kts by late morning/early afternoon as precip
exits the region. Diminishing cloud cover will then be seen
through the afternoon and into the evening. Fog could become a
concern for Wednesday morning, with visibilities in the 4 to 5 SM
range for KLSE, but confidence isn`t high enough to include in
this issuance. It is also possible that a bit more dense fog
develops in the Mississippi River Valley, so those with Wednesday
morning flight plans will want to monitor the fog forecast




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