Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 282336

Area Forecast Discussion
National Weather Service Des Moines IA
636 PM CDT Sun May 28 2017

...Updated for 29/00Z Aviation Discussion...

.SHORT TERM.../Tonight through Monday/
Issued at 311 PM CDT Sun May 28 2017

Mixing will continue through around 23z-00z before decoupling and
should expect the gustiness to diminish by this time frame. A few
fair weather cu have develop this afternoon but expecting them to
go away as well with the sunset. Large upper level low pressure
looks to remain over Ontario and provide northwest flow aloft
through tomorrow. The GFS and even the RAP suggest a weak
shortwave to quickly transition across the state overnight tonight
towards 12z Monday. Mainly looking to bring a few clouds past 06z
and possibly a stray shower. However, left forecast dry attm due
to the lack of significant moisture available tonight. Another,
slightly strong shortwave, moves through Minnesota by tomorrow
afternoon. This system has a better chance of providing showers
across northern Iowa and much more cloud cover across the forecast

.LONG TERM.../Monday night through Sunday/
Issued at 311 PM CDT Sun May 28 2017

From Monday night through Wednesday Iowa will be beneath broad
cyclonic flow surrounding a gyre over Ontario. This will keep
temperatures slightly below normal, while clouds and a few light
showers may affect our northeastern counties at times, but with no
real impact. From Wednesday night through Friday night, as the
gyre moves slowly eastward, we will enter a more zonal flow regime
punctuated by several shortwave impulses moving overhead. The
first of these will come late Wednesday night and Thursday, with a
quick return of southerly low-level flow and instability along
with a slug of forcing, supporting the development of showers and
thunderstorms. Several prognostic forecast models indicate the
potential for a complex of thunderstorms moving across southern
Iowa or northern Missouri on Thursday morning, and while the
synoptic scenario supports this generally, confidence in
timing/location being pinned down that specifically is not very
high at this range. Nevertheless have included likely POPs, even
though the intensity inferred from some of the QPF output may be
too aggressive. Meanwhile the southerly flow established in the
latter half of the work week will allow for notable warming on
Thursday and Friday, with temperatures likely climbing into the
lower 80s across much of the area. Accordingly, destabilization on
Thursday afternoon may allow for additional thunderstorms late in
the day, with a repeat on Friday, though most of the forcing will
be diurnal/thermodynamic with no strong dynamic systems foreseen.

Next weekend the long range models diverge rapidly and
significantly, with the EC for example indicating modest deep layer
ridging and dry weather, while the GFS indicates a return of
cyclonic flow with two fairly robust troughs moving overhead and
prolonged thunderstorm chances. This discrepancy stems from their
respective evolutions of a mid/upper level trough coming ashore over
the Pacific northwest coast late this week. While the depiction of
this evolution in the EC appears more consistent with climatology,
have maintained low POPs in deference to the very wet solution of
the GFS.


.AVIATION.../For the 00Z TAFS through 00Z Monday evening/
Issued at 636 PM CDT Sun May 28 2017

VFR conditions will prevail through the valid TAF period. Line of
SHRA/TSRA will enter northwest IA by 01Z and continue movg SE. Have
introduced VCSH at KFOD and KMCW. Localized wind gusts to 30-40 kt
are psbl with even the SHRA. Otherwise the only concern is mixing
Monday afternoon with gusts of 20-30 kt psbl.





SHORT TERM...Podrazik
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