Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 152038

Area Forecast Discussion
National Weather Service Des Moines IA
338 PM CDT Fri Sep 15 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 338 PM CDT Fri Sep 15 2017

Tonight...A low and associated deep upper trough will begin to pivot
northeast.  This will drive a shortwave across the region tonight
and push an elongated surface low and associated cold front into
northeast MN and southeast NE.  Ahead of this boundary will be a wave
of theta-e advection and some weak to moderate lift.  This should be
sufficient to have showers and storms.  However, instability is only
modest and shear is negligible so the better storms will remain west
of the forecast area tonight with showers/isolated storms expected
to dribble across the area.  The HRRR handled things today the best
and leaned towards that model for tonight though all models point to
only weak showers/convection tonight.  The NAM is the most bullish
and mainly across the west.  I`ve gone with that main thinking but
tempered pops/qpf to be more in line with the HRRR.

Saturday...The aforementioned cold front will slowly make it`s way
across the state Saturday into Sunday.  By 00Z Sunday the front
should only be through about the Western third of the state.  The
best forcing and shear lag the front somewhat and there is some
uncertainty as to what the morning showers and cloud cover will do
to the instability so overall, not expecting much during the day but
may see showers with isolated storms.  By afternoon...especially
late afternoon into Saturday night, we should see enough sun and
destabilize enough that storms should develop along/behind the
frontal boundary, some of which could be strong to severe. Shear is
mostly unidirectional so large hail with maybe some damaging wind
would be the threat going into the evening.  Strongest storms should
hold off til the evening but west of highway 169 would be of concern
late in the day.

.LONG TERM.../Saturday night through Friday/
Issued at 338 PM CDT Fri Sep 15 2017

Increased kinematic forcing into Saturday evening should help any
afternoon convection to increase along the boundary across western
into central Iowa. Sufficient shear exists for some organization
and a convective line is likely to develop by early evening.
Some severe weather will occur in the evening with main threats
from damaging winds and large hail. The convection will spread
quickly southeast overnight as surface front moves through the
state with relatively dry and cooler conditions for much of
Sunday. However, upper flow will persist from the southwest and a
return to warm advection is expected by Sunday night along with
increasing low level moisture transport. Elevated convection is
expected to spread northward into the state overnight into Monday
morning with best threat in central and southern areas.

Thereafter, warm sector will spread northward into the state as
large western trof develops. As this trof deepens and matures, a
surface boundary is expected to move into the state toward the
middle of next week. Both Euro and GFS indicate a period of active
weather with periods of thunderstorms and the possibility of heavy
rainfall. Temperatures will remain generally at or above normal
with southerly flow into the state for much of the week.


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1247 PM CDT Fri Sep 15 2017

Showers across northern Iowa falling apart as the low level jet
weakens.  VFR conditions are expected, even across TAF sites with
showers(mainly KMCW).  Winds still expected to increase this
afternoon despite the slower wind speeds currently.  Aft 09Z forcing
increases across the west and will spread east through 18z bringing
a chance for showers/isold storms mainly north and west.  vfr
conditions still forecast with that round of precip as well. Surface
flow will be southerly through the period.





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