Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 202003

Area Forecast Discussion
National Weather Service Des Moines IA
303 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

.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 18Z TAFS through 18Z Thursday afternoon/
Issued at 1230 PM CDT Wed Sep 20 2017

Main concern for the latest TAF package is the potential for MVFR
or low cigs and scattered TSRA. As of 17z a low cloud deck was
observed on satellite along and ahead of cold front sagging
through central Iowa. Cigs between 2000-3000 feet are possible at
KDSM and KALO through the next couple hours. -TSRA will be
possible in the vicinity of the front late this afternoon and
evening, potentially impacting KOTM. Confidence is not high enough
in TSRA coverage to include mention in the TAF at this time, but
trends will be monitored.

Another round of low stratus is likely Thursday morning as the
aforementioned front retreats northward. High confidence in MVFR
or lower cigs at KOTM and KDSM, however confidence in ceiling
height and coverage diminishes further north.





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