Area Forecast Discussion
Issued by NWS La Crosse, WI

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FXUS63 KARX 202343
AFDARX

Area Forecast Discussion
National Weather Service La Crosse WI
643 PM CDT Sun Aug 20 2017

.SHORT TERM...(This Evening through Monday Night
Issued at 315 PM CDT Sun Aug 20 2017

High Impact Weather Potential: Several rounds of showers and
thunderstorms are still expected to impact the area through Monday
night. Depending on how things unfold, there is a small risk for a
few strong to severe storms, perhaps late tonight but particularly
Monday evening south of Interstate 90. Heavy rainfall is also
possible pending evolution of storms, though the overall weather
pattern is rather progressive.

A rather quiet day out there weather-wise, with subsidence in the
wake of an earlier convective complex crossing Iowa holding things
in check. Earlier clouds have been replaced by some sunshine for
many areas, with dew points creeping upward through the 60s to
near 70 and resulting in increasing CAPE values toward 2k J/kg or
better. For the moment, however, there`s just not much of any
forcing to work on that instability, though a cold front is
working east out of central Minnesota. We`ll be watching that
boundary the next few hours with possible scattered convective
development, though help aloft is rather paltry as is both low
level and deep layer shear, with the better shear in the post-
frontal environment. Additionally, per RAP forecast soundings,
things look increasingly capped given a lack of better boundary
layer moisture, so we ultimately may not see much of anything
through this evening.

The setup changes to a more favorable one overnight as that frontal
boundary washes out while a redeveloping warm front sets up across
central Iowa. Per multi-model consensus, looking like a pretty good
shot of low/mid level moisture transport will impinge upon that
boundary later tonight, suggesting convection should blossom east
out of northwest Iowa with time. Confidence in just how far north
coverage will reach isn`t the highest but the greater risk will
be south of I-90 closer to the nose of best low level jet
convergence. Uptick in effective shear toward 30 knots may allow
some degree of storm organization, with maybe a risk for some
damaging winds and maybe hail over mainly Iowa given pretty decent
elevated mixed layer overspreading the area. With that said, will
really have to watch trends into the evening as persistent
convection into northern MO might be hinting that our forcing
boundary will end up farther south.

We may then take a break for much of Monday in the wake of any
morning convection as another push of subsidence works into the
area, particularly if overnight storms get a little better
organized. With that said, will have to closely watch the warm front
and where it ends up, as some convection could bubble along that
boundary through the day. Gut feeling based on the pattern is that
the front will get shunted southward into central or even southern
Iowa, so we may well end up dry with potentially some breaks in the
cloud cover for eclipse viewing time.

Another ramp-up of better moisture transport through the late
afternoon and overnight hours should deliver a second bout of
showers and storms for much of the area, helped by the passage of a
better shortwave. Slightly better shear profiles could support a
severe threat for a few hours Monday evening, particularly south of
I-90 once again. Also, the setup remains conducive for some brief
heavy rains with any of this convection the next 36 hours with PWAT
values pushing 2 inches and deep warm cloud depths, though the
progressive nature of any storms should (in theory at least) keep
things in check from a flooding perspective. Additionally, guidance
continues to trend faster and faster with the exit of convection by
Monday night, as high pressure and much drier air make a very quick
approach before sunrise Tuesday.

.LONG TERM...(Tuesday through Sunday)
Issued at 315 PM CDT Sun Aug 20 2017

A return to some great late summer weather remains on track
Tuesday into at least Thursday as broad Canadian high pressure
works through the region, in concert with upper troughing. Looking
like another stretch of below normal temps along with dew points
mixing through the 40s and low 50s, and some morning lows likely
to push 40 degrees (if not a touch colder) for some of the
traditional colder locales and lots of upper 40s-lower 50s
elsewhere. Not bad at all for late August!

Thereafter, strengthening return flow into Friday and the weekend
suggests we will eventually pick up additional precipitation
chances, perhaps initially on Friday with the onset of better
thermal and moisture advection, and then toward later Saturday or
Sunday with hints of a stronger upper trough and cold front crossing
the area. Of course, exact timing remains to be seen, but the
overall trend of near to continued below normal temperatures looks
to stick through the weekend.

&&

.AVIATION...(For the 00Z TAFS through 00Z Monday evening)
Issued at 643 PM CDT Sun Aug 20 2017

A cold front extends from northern WI through southern MN as of
20.23Z. Will be watching an area of weak convection along this
front for possible strengthening as it drops southeastward this
evening. KRST/KLSE could be impacted, but confidence in timing and
more so convective coverage/additional development is low, precluding
mention in the 00Z TAFs. Better chances for more widespread showers
and storms exists overnight and will keep previous forecast
timing for VCTS. Additional convective activity is possible late
in the period, but will focus on the first 12 hours for now.
Barring heavier showers/storms, ceilings should remain above 3000
ft agl through the period with no restrictions to visibility.
Winds will be light and if not variable, favor a southerly
direction at KLSE and a northwesterly direction at KRST through
tonight, then shifting back to the south tomorrow.

&&

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

$$

SHORT TERM...Lawrence
LONG TERM...Lawrence
AVIATION...Rogers



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