Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 261734
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1234 PM CDT Mon Jun 26 2017

...Updated for 18z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 343 AM CDT Mon Jun 26 2017

Currently a shortwave is passing from NW to SE across the state. A
weak frontal boundary is also tied to this wave as a surface high
pressure builds into the state.  MUCAPES compared to the models
yesterday are somewhat less...around 350 J/Kg compared to around 500
that they were advertising yesterday.  As a result, precip is mainly
in the form of showers although a weak storm or two does pop up from
time to time.  The shortwave will pass through by late morning and
the weak boundary will slow as it gets into southern Iowa.  Still
quite a bit of lift going on across southern Iowa by 18Z and by this
time instability increases to around 1200 J/Kg.  This should be
sufficient to see a few more storms across southern Iowa by late
morning.  The boundary is expected to take it`s time crossing into
Missouri by late afternoon and while forcing and instability begin
to wane, there should still be enough to keep  a few showers/storms
going.  I generally left isolated shower/storm wording going across
the far south through the afternoon.  Temps should be about like
they were yesterday...perhaps a degree or two cooler depending on
cloud cover.

Tonight the aforementioned surface high slides across the forecast
area bringing light winds and clearing skies to the state.  This
will allow temps to fall into the 40`s and 50`s...quite cool for
late June, but comfortable.

.LONG TERM.../Tuesday through Sunday/
Issued at 343 AM CDT Mon Jun 26 2017

High pressure will be exiting to the east on Tuesday allowing for
return flow to become established across Iowa through the day. The
day will begin with the atmosphere devoid of much moisture with
PWATS less than one half inch. It will take time for the column to
saturate despite modest theta-e advection commencing by the
afternoon, the main impact during the day will be increasing high
level clouds. The low level jet and the best moisture transport
will be focused to the west Tuesday evening over eastern Nebraska.
This will be the region for thunderstorm development. The low
level jet will veer after midnight into western Iowa as a strong
short wave arrives and this will allow the activity to shift east
into central Iowa late Tuesday night into Wednesday morning.
While convection may bubble through the day and limit surface
instability, destabilization should develop west towards the
Missouri River ahead of a surface boundary. SBCAPES in excess of
4000 J/kg and MLCAPES of 3000-4000 J/kg. Proximity soundings
suggest the potential for strong updraft acceleration and a
tremendous amount of CAPE through the -20C to -30C range which
would support very large hail production. Deep layer shear of 35+
kts would support supercell development which would further
support the potential for very large hail through cycling of hail
through the updraft. Low level shear and helicity values and lower
LCL values near 750 meters thanks to 70 degree dew points
advecting into the region. These storms would have an attendant
damaging wind threat with signals for strong cold pool development
with 0-3 km theta-e difference of 30-40C. Finally, a threat for
heavy rainfall with PWATS near 1.75 inches and the potential for a
few rounds of storms passing over similar areas.

The thunderstorm activity may linger through much of Wednesday
night with the instability axis remaining across central Iowa as
the boundary slows and eventually stalls near the IA/MO border.
The boundary should lift back north into southern Iowa on Thursday
as an area of low pressure lifts into Kansas. This would bring
another chance for thunderstorms and the potential for severe
weather in vicinity of the boundary. A short wave trough will
arrive Thursday night and will spark more thunderstorms through
the region before ending Friday morning. Heavy rainfall will again
be possible. Will really need to monitor Wednesday through
Thursday night for severe weather and heavy rainfall.

The closed low that will persist over Alberta and Saskatchewan
through much of the week will drop southeast Friday and will be
over the Northern Plains on Saturday. This will place Iowa in the
trough axis and bring drier PWATS back to the region.
Precipitation will still be possible but the threat for severe
weather and heavy rainfall will be much lower than the mid week
activity. High temperatures through the period will generally be
in the upper 70s to the low to mid 80s. Potentially warmer
Wednesday and Thursday pending convective debris and boundary
location.

&&

.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1234 PM CDT Mon Jun 26 2017

Morning showers and widespread clouds have pushed south of the
state while scattered cumulus have developed in their wake. Per
observations, the lowest clouds bases range from 2000 to 3000
feet at a few places. With these clouds progressing through any
given site in an hour or so, have gone ahead with VFR conditions
through the TAF period. There may be periodic, brief periods of
MVFR ceilings, but not enough confidence in timing the impact at
the terminals. By this evening, clouds will clear and winds will
become light as high pressure passes through the region.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
NONE.
&&

$$

SHORT TERM...FAB
LONG TERM...Donavon
AVIATION...Ansorge


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