Area Forecast Discussion
Issued by NWS Des Moines, IA
FXUS63 KDMX 241140
AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
640 AM CDT Sat Sep 24 2016
Issued at 410 AM CDT Sat Sep 24 2016
Main concern in the short term will be the increasing threat of
showers and thunderstorms by late in the period. Currently...
boundary located from near Sioux City to near Des Moines and on to
Ottumwa. Low clouds and areas of fog have become quite
widespread north of this front and this should persist through
sunrise as moisture remains trapped near the surface underneath
the frontal inversion. However, the inversion will begin to erode
after sunrise with an increase in mixing as winds intensify ahead
of approaching system. Therefore, will see the low clouds
dissipate from south to north during the morning with some
sunshine in most areas by midday. However, this will be short
lived as front approaches from the west by late in the period.
Instability axis will strengthen through the Missouri river valley
by late afternoon along with some increasing shear as wind fields
strengthen. Convection is expected to form/intensify by mid/late
afternoon just west of the forecast area and push into western
areas by the end of the period. Shear and instability will be
modest and the severe threat looks somewhat limited although some
large or some damaging wind remains possible. Other concern will
be from heavier rainfall, especially after the washouts that have
occurred the past couple of nights. However, the bulk of rainfall
will occur into the next period and will be addressed then. This
will be the final day of temperatures nearly 15 degrees above
normal ahead of the approaching front.
.LONG TERM.../Tonight through Friday/
Issued at 410 AM CDT Sat Sep 24 2016
The cold front and rain chances tonight into Sunday were the
primary focus of the extended period. Deep upper level trough and
associated strong upper low pressure looks to enter the Central
and Northern Plains this evening and transition across the region
through Sunday afternoon. A sharp cold front will across the CWA
overnight tonight towards 12z Sunday. Models are actually in
fairly good agreement with the fropa across Iowa along with the
decent CAA behind the upper low Sunday. Frontogenetical forcing
increases over the west past 00z Sunday and transitions eastward
throughout the night with the fropa. Soundings are nearly or
completely saturated throughout the column ahead of the front
with good moisture transport into the state tonight as well. The
PWATs tonight continue to range from 1.5 to 2.0 inches and warm
layer cloud depths top 4000 meters over the central and south.
Plus, there is slightly better QG forcing over the south to
southwest and this is the location likely to see a bit higher QPF.
Still, the flood-ladened north could see up to an additional inch
tonight only adding to the ongoing issues. The positive twist is
that many of the rivers have crested or will have crested prior
the rains tonight. If the Shell Rock to Cedar River basins do
receive some heavy rain, the rivers will likely experience a more
secondary crest than adding to the crest that is already flowing
downstream. That all being said, flash flooding still poses a
significant threat tonight and a watch might be needed if the QPF
maximum is shifted further north.
The cold front will quickly dry out the atmosphere between 12-15z
Sunday and pops quickly diminish during this time across the east.
Increased winds during the afternoon with the decent CAA and
surface pressure gradient. Some mid level stratocu may develop
within the CAA regime in the afternoon inhibiting the deep mixing.
Still, gusts to 25-30 mph are likely.
Monday, a secondary push of cold air on the backside of the upperlevel
low centered over the Great Lakes region. Strong and deep mixing
develops during the afternoon behind the decent PV anomaly with
winds atop the mixed layer ranging from 35-40 knots, especially
over northern Iowa. Although a very dry atmosphere will be in
place Monday, combined with the strong winds, normally there would
be some minor fire weather concerns for harvesting. However,
since much of the state has been wet lately, the fire weather
threat remains very low.
Tuesday through Friday...A large upper level ridge of high
pressure dominates the weather pattern and provides a much needed
extended period of dry conditions and near normal temperatures.
.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 637 AM CDT Sat Sep 24 2016
Low stratus and areas of fog are expected early in the Taf period
at most locations with moisture trapped under a frontal inversion.
This should begin to mix out as winds increase today ahead of
approaching system. However, prior to mixing will see IFR
conditions at most locations improving to VFR by 18Z. Showers and
thunderstorms are expected to cross central Iowa tonight with a
frontal boundary with MVFR to IFR conditions in and around the