Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 172306
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
606 PM CDT Thu Aug 17 2017

...Updated for 00z Aviation Discussion...

.SHORT TERM.../Tonight through Friday/
Issued at 358 PM CDT Thu Aug 17 2017

Bottom Line up Front...
Widespread showers and thunderstorms are expected across the DMX
CWA Friday. A few storms could be strong to possibly severe, with
large hail and damaging winds the primary threats.

Rest of today...
20z Visible to water vapor imagery clearly picking up on well-
defined, closed low spinning over Wisconsin. Subsidence and dry
air entrainment aloft wrapping in behind this low gradually
clearing skies out from west to east across Iowa. On the heels of
this system, another upper low is coming off the northern Rockies
and is slated to propagate eastward...making it into NW Minnesota
by 12z Fri.

Tonight...
Fog potential will be tricky. Clearing skies help set-up for
radiative fog. However, mid-level clouds ahead of the second
system are modeled to be in northern Iowa by around the 09z-12z
timeframe that is ideal for any radiational fog development. Going
back to the source region, the models do initialize well with
these mid-level clouds, so must buy on this solution...and have
left fog out of mention for tonight.

Heading into Friday...
The second system will be impacting the upper Midwest. An
attendant, weak, boundary draped southward from the sfc reflection
low will pass through Iowa during the afternoon Friday, serving
as a focusing mechanism for convection. General consensus is
around 1500-2000 J/kg 0-6km MUCAPE for tomorrow, with very steep
low- level lapse rates. DCAPE values over 1500 j/kg put strong
winds into play... however given vertical profile, storms will
likely remain elevated. Given elevated nature/high LCL heights,
tornado threat is nil.

.LONG TERM.../Friday night through Thursday/
Issued at 358 PM CDT Thu Aug 17 2017

Bottom Line up Front...
Temperatures warm back up Sunday through Tuesday as heat index
values during this time should generally be into the 90s across
the DMX CWA. Cooler and drier conditions look to move in from the
middle of next week and beyond. Lastly, viewing conditions for the
solar eclipse on Monday are beginning to look dicey.

Sunday through Tuesday Night...
Broad longwave trough digging through Alaska and western Canada
associated with an upper low that is going to be pushing across
the northern portions of the country through the weekend. This
setup ...in conjunction with high pressure slated to be over the
upper Midwest Saturday...will help thermal ridge to build well
into Canada. As the pattern shifts eastward, thermal ridge to come
crashing down over Iowa Sunday into Tuesday...with 850mb temps to
push into the +20C to +24C range across Iowa.

Meanwhile, a broad shortwave is progged to push across the Dakotas
on Monday. Ahead of the sfc low, a warm front looks to position
itself somewhere near the IA/MN border Sunday night into Monday.
Growing consensus on moisture transport aided by a 30 to 40 kt LLJ
feeding into this boundary throughout Monday. As we have learned all
summer, definitely a lot of time for boundary to shift position
between now and Monday, but possibility of at least junky stratocu
across Iowa looking more plausible... so have upped cloud cover
forecast for Monday.

Wednesday and beyond...
Large high pressure to set up over the intermountain west will
support decent CAA for the remainder of the week...dropping 850mb
temps back to the +10C to +15C range...similar to today...and drying
out Iowa.

&&

.AVIATION.../For the 00Z TAFS through 00Z Friday evening/
Issued at 608 PM CDT Thu Aug 17 2017

Scattered to broken clouds with bases between FL020 and FL030 have
prevailed over north central Iowa terminals this afternoon. This
has meant some terminals have bounced between VFR and MVFR
ceilings this afternoon. Expect these clouds to gradually lift
and move out of north central Iowa with VFR conditions at all
terminals by mid-evening. Gusty northwest will decrease as well
this evening. An area of clouds over western South Dakota is
expected to reach Iowa after midnight tonight. A few models are
hinting at fog over north central Iowa, but with this cloud cover
and overall low probability have left out mention of any
visibility reduction. The aforementioned clouds will also bring a
chance for precipitation and have continued/added VCSH at the
northern terminals. Confidence was not high enough at this time to
include any mention of showers or storms beyond early afternoon.

&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Ansorge



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