Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 190016
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
716 PM CDT Tue Jul 18 2017

.SHORT TERM...(This Evening through Wednesday Night)
Issued at 218 PM CDT Tue Jul 18 2017

High Impact Weather Potential: Several bouts of thunderstorms remain
possible from this evening through mainly Wednesday night. Can`t
rule out an isolated severe storm into this evening, though the
greater risk for severe could come toward Wednesday night with
increasing signals for a potential wind-producing MCS crossing the
region.

Today`s attempt at a convective forecast is brought to you by the
words "uncertainty" and "good luck". As discussed at length for the
past few days, forecast confidence regarding any one convective
solution the next few days (into Saturday really) is quite low,
owing to lots of questions regarding the placement of best
instability as well as a wavering frontal boundary which will serve
as a focus for convective activity. Some good news at least -
today`s forecast so far is behaving somewhat as expected, with an
earlier convectively-enhanced vort lifting from south-central
Minnesota into northwest Minnesota, as a trailing "cold" front
continues to advance southward toward the area, itself driven by a
ridge-running shortwave over northern Minnesota. After earlier
activity just skirted our western areas, starting to see some
convective elements develop on the southern flank of that shortwave,
with additional scattered convection expected to expand along the
front as it lays out across the area. Mixed layer CAPE isn`t
anything great - generally about 1300 J/kg give or take - and
both low level and deep layer shear aren`t terribly impressive
either (maybe 20 knots). As such, continue to believe we`re
mostly looking at a pulse-type environment, with a lower end risk
for a rogue severe storm producing mainly wind and perhaps some
smaller hail (pretty warm aloft for anything too large).

Things still look to quiet down tonight into Wednesday as the
frontal boundary likely gets pushed down into at least northern if
not central or southern Iowa (the more widespread the storms today,
the farther south it should be). Can`t rule out something bubbling
up overnight as our airmass remains rather moist, though overall
forcing is quite weak. Maybe we could sneak something into northern
Iowa on the edge of the strongest capping building north from the
central Plains, though again confidence isn`t too great.

Of greater confidence is Wednesday being a mainly dry day with
bubble high development across the region and likely an influx of
lower dew points for many areas. Focus then shifts back into
north/south Dakota late in the day with persistent hints of another
shortwave working through that area, and encountering a quite
unstable airmass. Given the pattern with a large heat dome just to
our south (well capped off of course) and expanding instability
northward with time coupled with an uptick in modest mid level flow,
really wondering if we may see a much more organized MCS roll east
out of North Dakota during the afternoon. Should that occur, Corfidi
vector propagation would suggest a gradual southward turn into our
direction in between the capped environment to the south and nil
instability to the north. This bears watching if storms can get
organized for a potential damaging wind risk.

.LONG TERM...(Thursday through Tuesday)
Issued at 218 PM CDT Tue Jul 18 2017

Unfortunately, forecast confidence only continues to decline into
late week and Saturday (maybe Sunday too?) with lots of questions
regarding the position of our focusing frontal boundary. Still like
the idea of a mainly dry day on Thursday as we again see bubble high
development over the region. Thereafter, there remains a pretty good
signal for another round of convection across the area sometime on
Friday, though timing/placement concerns remain, with another bout
possible toward Saturday with guidance hints of a stronger wave
sliding by just to the north. While overall confidence on the
specifics is low, the risk for repeated rounds of heavier rain is
certainly there, though we`ll have to see how our first two rounds
of storms unfold through Wednesday night. One bit of good news
perhaps is that much of Sunday and into early next week continue to
look on the much quieter and potentially cooler side as low level
Canadian ridging expands across the region.

&&

.AVIATION...(For the 00Z TAFS through 00Z Wednesday evening)
Issued at 716 PM CDT Tue Jul 18 2017

Scattered thunderstorms will continue across the region through
around 04Z. High pressure then builds in tonight with some patchy
fog possible. Conditions could lower to MVFR near any
thunderstorms this evening and in BR and possible fog after 06Z.

&&

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

$$

SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...Wetenkamp


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