Area Forecast Discussion
Issued by NWS Des Moines, IA

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407
FXUS63 KDMX 301805
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
105 PM CDT Tue Aug 30 2016

.SHORT TERM.../Today/
Issued at 400 AM CDT Tue Aug 30 2016

Another day of isolated to scattered convection anticipated
as an unstable airmass remains in place across central Iowa. To the
south, a stationary boundary is draped west-to-east across northern
Missouri and may trickle northward through the morning hours to
provide some focus for thunderstorm development in southern Iowa.
Further north, a cold front over South Dakota through central
Minnesota will continue to sag southeast throughout the day and
look to creep into northern Iowa past 21z today. Further west, a
weak mid-level shortwave looks to push into western Iowa later this
morning and provide a little better deeper forcing for ascent over
the western half of the forecast area. Models might be overdoing the
amount instability building into the state today with much of the
cloud cover looking to hang around before the cold front clears out
the atmosphere later tonight. Still, enough instability with the
better forcing over the west to southwest late this morning into the
afternoon was enough to increase pops to likely. Models are in
agreement about the random convection later this morning into the
afternoon, with the 29.18z ESRL HRRR and 30.00z NAM12 seemingly
handling the convective trends slightly better. But the ARW/NMM-WRF,
RAP, and even the HopWRF have some sort of convection through the
afternoon, especially across southern Iowa.

Confident enough to mention the severe threat is low with the lack
of any significant shear. However, may get a few more funnel clouds
today with some low level vorticity and CAPE present this afternoon,
especially near the aforementioned fronts in the north and south.
The main threat is heavy rain. PWATs across central Iowa are near
2.0 inches with warm layer cloud depths around 3500 meters and with
the lack of any significant stearing flow and very weak Corfidi
vectors, fairly high confidence in some isolated 2-3 inch rainfall
totals. However, lower confidence in the location and thus held off
on any flash flood headline at this time. Potential there for some
localized flash flooding, especially if storms impact an area hit
from Monday`s convection.

.LONG TERM.../Tonight through Monday/
Issued at 400 AM CDT Tue Aug 30 2016

Forcing will persist across southern Iowa tonight with lingering
threat of storms. However, farther north, increasing subsidence
and drier air will minimize threat north of Interstate 80 with
clearing skies moving southward overnight as drier air pushes
through the state. Large Canadian high pressure will build into
the upper Midwest on Wednesday and this will continue to influence
the weather for the remainder of the work week. Temperatures will
be cooler along with a relatively dry airmass with comfortable
conditions.

Warm advection increases by the weekend as upper ridging slides
overhead as trof moves into the western United States. A big
shortwave will pass through the trof by later in the weekend and
into south central Canada. This will push a frontal boundary into
the vicinity of Iowa by Sunday. Thereafter, the boundary will
remain near Iowa with broad southwest flow persisting. This will
act as a focus for thunderstorms development from late Saturday
into early next week with the potential for heavy rainfall and
possibly severe storms.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 104 PM CDT Tue Aug 30 2016

The initial concerns will be convection and cloud trends. Light
showers continue to drift across central and southern sections
with patchy sub 2Kft MVFR ceilings. The likelihood of thunder and
non-VFR conditions is expected to decrease through sunset as a
weak pressure trough along I-80 drifts southward through IA. Thus
only have VCSH and short term MVFR wording pending current trends
until confidence increases at any particular point. Some MVFR fog
may develop south immediately behind the frontal passage toward
daybreak, but otherwise VFR conditions are anticipated into Wed
morning.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...None.

&&

$$

SHORT TERM...Podrazik
LONG TERM...Cogil
AVIATION...Small



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