Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182111
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
411 PM CDT Fri Aug 18 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 404 PM CDT Fri Aug 18 2017

Bottom Line up Front...
A few strong to potentially severe thunderstorms are possible
this evening primarily from central to northwestern Iowa. The
storms should generally be out of our area by midnight, and then
fog will impact the state of Iowa, including southwestern Iowa,
through sunrise.


Rest of today through tonight...
20z Visible satellite has shown generally clear skies across
northwestern Iowa throughout today...now gradually being replaced
by bubbling cu. 20z Water vapor imagery clearly shows an upper low
pushing through eastern Minnesota into northwestern Wisconsin.
Hi- res models picking up on a subtle 1012 sfc high propagating
along the IA/MN border throughout today..adding a SE component to
the wind. Meanwhile, another subtle sfc high was located over SE
Nebraska. Coupled with westerly flow aloft, this has produced a
line of slight sfc convergence oriented from NW to SE in
northwestern IA...with a veering vertical profile. Around 1500
j/KG 0-6km MUCAPE starting to become tapped into in NW IA. With
low-level lapse rates reaching 8 C/km and dcape values upwards of
1,500 J/kg, large hail and damaging winds will be the primary
threats. Challenge will be timing...once daytime heating lost, cap
will likely win out and severe potential will wane.

Strong subsidence and clearing behind this system will lead to skc
overnight. With minimal dwpt depression and potential moisture from
precip, radiational fog seems a given across much of Iowa, including
southwestern Iowa. The need for a Dense Fog Advisory cannot be ruled
out north of Highway 30. Clear skies on Saturday with another day of
near-normal temperatures likely.


.LONG TERM.../Saturday night through Friday/
Issued at 404 PM CDT Fri Aug 18 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...
Models remain consistent with overall setup. A deep low beings to
develop over the intermountain west by sunday afternoon, with a
boundary/WAA making it into southern MN by peak heating. LLJ will
start to push moisture into Iowa from west to east. This LLJ will
overspread Iowa during the day Monday. Fcst soundings respond,
showing a decent stratocu field across Iowa in the warm, moist
sector, which is very plausible. Thus, solar eclipse viewing may
be very hampered across Iowa. Have 60 to 70 percent sky coverage
in grids...which if anything, is underdone.

Severe potential exists both Sunday and Monday evenings, though
there are some negatives on each day. Sunday evening will feature
"off the charts" MUCAPE values across central to northern Iowa and
strong low-level lapse rates. Location of boundary will be key, as
this would serve as good focusing mechanism to make storms more sfc-
based versus elevated. Problem for Monday will be cloud cover and
precip potential all day...this may keep environment capped.


Tuesday and beyond...
Long-range models have accelerated CAA as large high pressure to
set up over the intermountain west. Thus, have lowered Tuesday
temperatures versus previous forecasts. 850mb temps look to drop
back to the +10C to +15C range...which is slightly below
normal...suggesting 70s and low 80s the rest of the week...with
dry air and subsidence winning out.


&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1220 PM CDT Fri Aug 18 2017

Primary forecast challenge of the latest terminal forecast
continues to be the potential for fog/visby restrictions Saturday
morning. Model guidance has come into sound agreement with the
lowest visby across north central Iowa between 09-14z, which may
drop sites into LIFR/VLIFR category at times. Fog is possible
further south toward KDSM/KOTM, although guidance suggests that
visby will likely remain at or above 2sm. This trend has been
reflected in the latest forecast. The fog will dissipate and give
way to VFR conditions by mid-morning.

Isolated to scattered TSRA activity is also possible this evening
and early overnight. The highest potential will be in north
central Iowa. A mention of TSRA has been included at KMCW after
02z. Confidence is not high at the moment enough to warrant
mention at other sites.


&&

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

$$

SHORT TERM...Kotenberg
LONG TERM...Kotenberg
AVIATION...Martin



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