Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 141136
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
636 AM CDT Mon Aug 14 2017

.SHORT TERM.../Today through Tonight/
Issued at 405 AM CDT Mon Aug 14 2017

Thunderstorms are currently ongoing over north central Iowa mainly
along and north of Highway 20. These storms are ahead of a surface
boundary that has moved into northwest Iowa and are supported by
strong theta-e advection along the nose of a veering low level jet
that is pointing into the state. Forcing is further enhanced by a
strong short wave that is morning into southern Minnesota. The
boundary will gradually settle southeast today. Much drier mid-
level air will filter into the state in the wake of the boundary
with the aid of subsidence behind the short wave. The
thunderstorms will settle southeast and should be mostly out of
the forecast area by 15z. Modest instability does return this
afternoon as the mid-level moisture recovers some but it will be
difficult to activate the instability. Any chances for
precipitation should remain south of the boundary and over
southern Iowa though no overly enthused about chances across that
region. Severe weather chances will be low though an isolated
severe storms is possible across the south.

Low thunderstorm chances will remain over southern Iowa tonight.
Have reduced pops through this period and suspect that much of the
state will have little to no rainfall overnight. Other concern
will be fog potential tonight across northern Iowa. Light winds,
recent precipitation and the potential for good radiational
conditions will enhance the fog potential. Similar to Sunday, high
temperatures today will be dependent on amount of sunshine but
generally highs will be in the mid 70s to mid 80s.

.LONG TERM.../Tuesday through Sunday/
Issued at 405 AM CDT Mon Aug 14 2017

The main focus of the extended period was on rain chances Tuesday
and Wednesday. Leaned toward a blend of the ECMWF/NAM Tuesday
into Wednesday and then closer to the ECMWF through the rest of
the forecast package.

Tuesday into Wednesday...GFS seems to be the outlier on Tuesday`s
convection as it develops convection along the stalled boundary
over southern Iowa early Tuesday morning and lingers across the
southern two-thirds of the forecast area to around 18z. NAM and
ECMWF delay the convection associated with an upper level
shortwave until the afternoon and leaned toward trimming off pops
across the eastern and northern sections of the forecast area
Tuesday morning to coincide with this solution. Plenty of dry air
to contend with from the surface high pressure remaining planted
over the Great Lakes before decent theta-e advection pushes across
the forecast area past 18z Tuesday. The parent surface low
remains on track to move across the state late Tuesday night into
Wednesday. A fair amount of isentropic upglide and moisture
transport along the 305-315K isentropic surface into the state b/t
03z to 18z Wednesday. The GFS suggest forecast soundings across
central Iowa become deeply saturated by around 12z Wednesday,
while the NAM/ECMWF are a bit slower to saturate the column until
around 18-21z. The GFS/ECMWF show a secondary 500mb shortwave
trough trailing behind the surface low Wednesday evening providing
additional lift for rain through 06z Thursday. The best potential
for long duration rainfall across the forecast area looks to be
from past 09z to 21z Wednesday. Precipitable water values around
2.0 inches and warm layer cloud depths range from 4000-4500 meters
during the aforementioned time frame. Although growing confidence
in widespread and potentially long-duration rainfall Wednesday,
certainly not a drought buster for the drought stricken locations
in Iowa. The QPF amounts of 0.25 to over an inch widespread are
possible by Wednesday evening.

Thursday through Sunday...Surface high pressure builds into the
region Thursday and Friday providing some clear skies and dry
conditions. This high pressure looks to dominate the weather
pattern through the weekend with the GFS and ECMWF suggestions a
couple very weak shortwaves to try and penetrate the surface
high, but higher confidence to go dry for much of the weekend with
a significant amount dry air in place.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 635 AM CDT Mon Aug 14 2017

Scattered thunderstorms will move across portions of the area
early before ending. MVFR cigs will be in the vicinity of storms
this morning before a return to VFR conditions. A few
thunderstorms may occur in vicinity of KOTM this afternoon and
evening. Areas of fog possible tonight, mainly over northern
Iowa. Expect a wind shift from the north over northern Iowa today
as a boundary settles south.

&&

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

$$

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Donavon



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