Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 220454
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1154 PM CDT Sun Aug 21 2016

...Aviation Update...

.SHORT TERM.../Tonight/
Issued at 312 PM CDT Sun Aug 21 2016

General subsidence will occur tonight with surface ridging sliding
through the state this evening. A dry cool airmass will accompany
the high with temperatures quickly falling this evening with the
light winds and clear skies. Temperatures will likely steady out
during the early morning hours as winds come around to the south
as the ridge passes to the east, although readings will already be
well below normal. There may be some patchy fog as well although
much of the low level moisture will likely go into dew formation.

.LONG TERM.../Monday through Sunday/
Issued at 312 PM CDT Sun Aug 21 2016

From Monday night into Tuesday a ridging pattern overhead and
southerly low-level flow will promote a steady warming trend and
mostly clear skies. Meanwhile a compact mid-level cyclone will
meander slowly eastward across southwestern Canada. This low
center will pass just north of North Dakota on Tuesday night and
reach International Falls by late Wednesday. Two shortwave
impulses rounding the base of the associated cyclonic flow will
move over Iowa late Tuesday/Tuesday night and again late
Wednesday. On Tuesday night the first impulse will provide broad
forcing for ascent within an airmass characterized by modest
instability and high moisture content with PWATs nearing 2 inches.
Moisture transport and warm air advection aloft will be aided by
the development of a low-level jet during the night, resulting in
a marginal severe weather threat and a tangible potential for
heavy rainfall. However, recent heavy rain events have produced
little to no impact in terms of flooding due to antecedent
conditions and the state of vegetation across the region, so any
threat of flooding Tuesday night into Wednesday remains ambiguous.

Behind the initial impulse a surface cool front will sag
southeastward into Iowa and provide the focus for redevelopment of
thunderstorms on Wednesday afternoon and evening as the second
impulse approaches aloft. Deep-layer shear values remain weak,
generally predicted in the 15-25 kt range, but depending on the
effects of overnight convection and resultant overturning
sufficient destabilization may occur during the day Wednesday to
allow for some strong updrafts during peak heating and there may
be some severe weather threat.

The cool front will push through by Wednesday night, ushering in
another period of quiet and pleasant weather to end the work week,
with a surface high pressure area sliding across the region around
Friday. Looking ahead toward next weekend it appears that a broad
mid-level trough may set up over the Pacific Northwest, placing
Iowa beneath modest west southwesterly flow. This pattern would be
generally favorable for more rain and thunderstorms, but
predicting any details in that scenario at this range is not
feasible and widespread chance POPs will be maintained in the
forecast.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1128 PM CDT Sun Aug 21 2016

VFR conditions prevailing through period, with main concern being
strong and gusty winds possibly affecting airport operations.
Southerly wind gusts approaching 25 to 30 kts likely at KFOD/KMCW
with 20 to 25 kts at KDSM/KALO/KOTM. Gusts will ease around
23-00z with loss of diurnal heating, with remaining sustained
southerly winds around 8 to 12 kts across sites.

&&

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

$$

SHORT TERM...Cogil
LONG TERM...Lee
AVIATION...Curtis



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