Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180623
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
123 AM CDT Thu Aug 18 2016

.SHORT TERM.../Tonight/
Issued at 344 PM CDT Wed Aug 17 2016

Main forecast concern was focused on isolated
thunderstorm potential this afternoon across the east and patchy fog
possible over the north and east towards Thursday morning.  There is
some weak moisture convergence over central to southeast Iowa with
MUCAPE values in the 3000-4000 J/kg range and weak effective shear
(around 25 knots) that may spawn an isolated storm or two in the
southeast before sunset. Otherwise, CU field should diminish with
the sunset and surface high pressure remains in control of the
pattern. This should allow for some patchy fog over the north and
east past 09z Thursday as radiational cooling looks to develop.

.LONG TERM.../Thursday through Wednesday/
Issued at 344 PM CDT Wed Aug 17 2016

Overview...Long term period can be broken down into three windows.
Namely warm to hot and humid conditions Thursday and parts of
Friday, possible severe weather and heavy rainfall Friday afternoon
into Saturday, and cool conditions Saturday through mid-week.

Thursday into Friday...The message has not changed much with regards
to the warmth and humid conditions. On the heels of a warm advection
wing, 850 mb temperatures should have little issue rising from the
mid to upper teens currently to the 22-24 C range across much of the
central and western portions of the state by Thursday afternoon. As
a result, have widespread upper 80s to pockets of low 90s
continuing. Tied in with a bit of moisture return, dew points will
creep into the mid 70s in some locations, boosting heat index values
to around 100 at times in the afternoon/evening hours. The warmth
will continue into portions of Friday, primarily the southeast half
of the state, as 850 temps around 20 C will linger ahead of the
impending front and associated precipitation.

Friday afternoon into Saturday...Models remain in pretty good
agreement with regards to the onset, entering northwest Iowa late
morning to early afternoon and moving southeast through the
overnight hours. Currently, a severe threat remains possible, though
supportive factors are not outstanding. Mixed layer CAPE will be
around 2000-2500 J/kg, but with shear around 25 kts, overall wind
profiles directionally uniform out of the southwest, and freezing
levels possibly approaching 14kft, hail should be of minimal threat.
Strong to damaging winds will certainly be possible with any MCS
development, but mean wind and storm motions parallel to the front
may minimize organization and the overall threat.  Heavy rain
continues to remain a threat with warm cloud layer depths in excess
of 11-12 kft, PWats around 1.5 to nearly 2 inches, relatively slow
frontal progression, and storm motions nearly parallel to the front
will lend to areas possibly seeing 2 inches or more. Currently, do
not believe flooding issues will arise given relatively dry past few
days and crop coverage helping limit runoff. While models agree
well on the onset, at least the NAM lingers precipitation chances
over the state into the afternoon Saturday. Given it is a fairly
significant outlier with regards to surface low placement versus
EURO/GFS/UKMET, have trended pops towards them a bit more,
reducing POPs to chances in the 12-00z time frame Saturday.

Saturday evening through mid-week...Period significantly cools off
behind front and will be relatively dry. Temperatures Saturday
and Sunday will be on the cool side with highs only in the 70s,
before a bit of upper level ridging warms temperatures into the
low 80s. Models do continue to suggest a weak shortwave moving
through overnight Sunday possibly triggering a few showers. While
it cannot be completely ruled out, have removed POP mentions for
the time being with a normally aggressive GFS the only model
depicting QPF. Stronger wave continues to be signaled by EURO/GFS
Tuesday night/Wednesday, though details for any severe potential
remain uncertain given how far out.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 112 AM CDT Thu Aug 18 2016

Clear skies...light winds...and residual boundary layer moisture
is leading to patchy fog as of 06z. The terminals most likely to
be impacted are KMCW/KALO/KOTM...although this fog could spread
to all terminals towards daybreak. Due to the shallow nature of
the fog...expect that vsbys will bounce around between VFR and
IFR...with even LIFR at times. Fog should dissipate by mid-morning
with VFR conditions expected the remainder of the day with just
some diurnal cumulus or high cirrus clouds. Light and variable
winds overnight will become southerly by Thursday afternoon.

&&

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

$$

SHORT TERM...Podrazik
LONG TERM...Curtis
AVIATION...Fowle



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