Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131726

Area Forecast Discussion
National Weather Service Des Moines IA
1226 PM CDT Sun Aug 13 2017

...Updated for 18z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 401 AM CDT Sun Aug 13 2017

Two areas of precipitation currently ongoing with one across
southeast North Dakota and into west central Minnesota and the
other over eastern Nebraska and into far west central Iowa. Each
region is focused along the northern edges of two low level jet
maxes and regions of strong theta-e advection and greater
instability. The convection over eastern Nebraska has been
weakening over the last several hours with cloud tops warming and
lightning activity diminishing. CAM solutions have not been
initializing well overnight and remain too aggressive with
precipitation. Most solutions continue to focus over northwest
Iowa where no activity is occurring. Furthermore, the current
region of precipitation is moving east into a more stable and
drier air mass as the large surface high pressure remains
entrenched through the Mississippi Valley and Lake Michigan
region. Expect a continued degradation in activity over the next
several hours. Still uncertainty on how far east the precipitation
will reach this morning as the low level jet does veer into the
state. Overall most locations are expected to stay mostly dry
though a period of brief light rain could reach east towards
Interstate 35 before ending. Better instability arrives tonight
and should have renewed convection over northwest Iowa that will
gradually settle into central Iowa.

.LONG TERM.../Monday through Saturday/
Issued at 401 AM CDT Sun Aug 13 2017

The main forecast concern was focused on periodic precipitation
chances through mid-week. Leaned toward a blend of the ECMWF/GFS
throughout the forecast period even though the NAM seems be in
good agreement with these two through Tuesday. Overall, no
significant changes to the ongoing forecast.

Monday into Tuesday...weak upper level shortwave trough cuts
across Minnesota during the day Monday and brings some decent
forcing over northern Iowa. No significant deep moisture, but
enough in the mid- levels (b/t 850-700mb) to provide the threat of
elevated convection Monday. Low confidence in any heavy rain
threat, at least widespread across the forecast area. Boundary
lays up west to east across the state early Tuesday with another
upper level vort max coupled with some 700-500mb theta-e advection
looks to be enough for a few storms along and northern of Highway
30 to Minnesota border. Weak CAPE available, but enough present
to warrant mention of thunder early Tuesday.

Tuesday night into Wednesday...surface low pressure, 500mb
shortwave trough, and associated cold front look to bring
increasing rain chances late Tuesday night into Wednesday. Latest
ECMWF run suggest Iowa will be split between two 500mb vort maxes
placing the higher QPF amounts in Minnesota and Missouri but still
enough across the state to keep with higher chance to likely
pops. The GFS beginning to show this trend with the main vort max
cutting across Minnesota during the same time and then a second
shortwave trailing behind by Wednesday afternoon. Regardless of
solution, Wednesday appears to be the state`s best chance of any
widespread convection and rainfall.

Thursday into saturday...trough moves east and allows for surface
high pressure to build into the state Thursday into Friday.
ECMWF/GFS have a shortwave that tries to push into western Iowa by
Friday morning but with the fairly strong surface high and plenty
of dry air in place, low confidence with any pops Thursday night
into Friday. Thus, decreased or removed them entirely during this


.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1228 PM CDT Sun Aug 13 2017

VFR conditions have prevailed at the TAF sites with scattered
showers moving across central Iowa this morning and early this
afternoon. These showers may brush KALO and have mentioned it for
the afternoon hours. Also cannot rule out a rogue shower
affecting KMCW or KFOD, but have not mentioned at this time due to
lower confidence. Have continued VCTS later tonight at the
northern terminals and this is also where lower ceilings will
arrive toward sunrise Monday with MVFR restrictions. It`s possible
these lower ceilings will reach farther south and east, but
confidence is lower given SREF guidance maintains the best chance
for lower ceilings over northwestern and north central Iowa.




SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Ansorge is the U.S. government's official web portal to all federal, state and local government web resources and services.