Area Forecast Discussion
Issued by NWS Des Moines, IA

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741
FXUS63 KDMX 202345
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
645 PM CDT Wed Sep 20 2017

.SHORT TERM.../Tonight through Thursday/
Issued at 303 PM CDT Wed Sep 20 2017

As of 20z a cold front was slowly sagging east through the CWA,
evident on radar and surface obs stretching northeast from near
Creston, through the DSM metro, to just west of Waterloo. Northwest
winds behind the front have ushered in a much drier airmass with
dewpoints falling into the 50s. Warm and humid conditions continue
out ahead of the front. Increasing instability and convergence along
the frontal axis has triggered a few updrafts in northeast Iowa. A
strong mid-level cap will likely limit the intensity and coverage of
this activity through early evening. Hi-res models advertise more
convective potential after 00z further south along the front into
south central and southeast Iowa. Again, widespread coverage should
be limited by the cap and weak height rises aloft. The unstable
airmass and 20-30 kts of effective layer shear may be enough to
support a few strong to severe storms, capable of producing hail and
strong wind gusts.

The cold front stalls out over our southeastern zones tonight. Rich
theta-e low level flow lifting over the boundary will maintain the
potential for a few showers and storms through Thursday morning,
mainly south of Interstate 80. The boundary retreats northward as a
warm front tomorrow morning in response to mid-level height falls
over the high Plains. The reprieve from the unseasonably high
humidity will be short lived as upper 60s/low 70s dewpoints advect
back northward through the state. A hot and humid afternoon is
expected with heat indicies in the low to mid 90s. Despite the
development of a highly unstable airmass, a strong cap and lack of
focus for convergence should suppress any late day convective
potential.

.LONG TERM.../Thursday night through Wednesday/
Issued at 303 PM CDT Wed Sep 20 2017

First half to two thirds of the longer term will be dominated by
the amplifying upper pattern. The deepening western will lead to
an large upper ridge across the eastern United States. Will see
surface front enter the Plains on Thursday and becoming nearly
stationary as upper flow becomes nearly parallel with the
boundary. This remains the case until Monday when the boundary
finally slips eastward into the state. In the meantime, deep
southerly flow is expected across Iowa from Thursday night into
the weekend. Being in the warm sector, forcing will be limited
with a very warm to hot airmass in place for the state.
Temperatures will be well above normal with highs in the mid to
upper 80s with even some 90s on Friday into Saturday and likely
Sunday as well. In addition, moisture will flow north into the
state with dewpoints into the mid 60s to even lower 70s. Heat
index values well into the 90s if not approaching 100 are expected
on Friday if not into the weekend as well.

The front will begin to approach western Iowa late Sunday and
gradually move into the state on Monday. Increased convergence and
lift along the front will help to spread showers and storms into
the forecast area during the day. Both GFS and Euro have continued
to slow the progression of the boundary with thunderstorms chances
persisting well into Tuesday before gradually moving eastward by
the end of day 7. Temperatures will cool to normal or below by
the middle of next week as the boundary passes through the state.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 645 PM CDT Wed Sep 20 2017

A boundary over central Iowa will move slowly southeast overnight
before stalling. Expect light winds in vicinity of the boundary
with northerly winds over northern Iowa. A few thunderstorms are
possible across Iowa through this evening and may be possible in
vicinity of KOTM but threat is look enough to not mention at this
time. A band of MVFR to local IFR stratus is possible over central
and northern Iowa late tonight into early Thursday in advance of a
warm front lifting back north. The stratus should clear by the
afternoon. Expect gusty south winds as the boundary passes
through.

&&

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

$$

SHORT TERM...Martin
LONG TERM...Cogil
AVIATION...Donavon



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