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Area forecast discussion 
National Weather Service Des Moines Iowa
647 am CDT Monday Aug 31 2015

issued at 646 am CDT Monday Aug 31 2015

Visibilities never deteriorated as expected over much of southern
and east central Iowa...only down to a few miles at the
previous advisory has been cancelled. Low visibilities do continue
in areas over the northern half of Iowa however...but they are
expected to improve shortly.


Short term /today/...
issued at 320 am CDT Monday Aug 31 2015

Much like yesterday...the main concerns are temperature and visibility
trends. Stratus coverage is less than last night with only some
token cirrus northwest. This would suggest a more uniform radiation fog
development but that has not been the case with visibilities
somewhat variable. Even though southern Iowa was hit hardest with dense fog
last night and conditions are similar...visibilities have not really
dropped much in the advisory area. Rap 1000-950mb relative humidity is actually
relatively lower in this area so advisory confidence is not great.
However confidence is equally not great that conditions will not
tank in the next several hours either so will leave headline as is
and evaluate hourly for needed changes. Lowest visibilities are
actually north so will go with keeping headline and Special Weather Statement other areas
for now.

Temperatures will be problematic once again. Model soundings are somewhat
more aggressive with mixing potential...which makes some sense with
winds a tad higher and less stratus around. However is has been
difficult warming too much over the past few weeks...especially to
GFS expectations which has shown and warm bias. Cooler NAM MOS has
been better recently and will continue to follow that lead. This
results in trimming previous forecast several degrees.

Long term /tonight through Sunday/...
issued at 320 am CDT Monday Aug 31 2015

The forecast for this week remains warm and mostly dry. A deep
layer ridge blanketing the southern U.S. Will gradually pivot
around over the eastern U.S. As a longwave trough steadily digs
down the West Coast. This will place Iowa beneath the proverbial
ring of fire for several days this week...however...the subtle
shortwaves moving through the steering flow aloft will be fairly
weak and difficult to pin down in terms of timing. Will maintain
low 20 to 30 probability of precipitation at times during the work week...mainly across
the northwest half of the forecast area...but any storms should be
of little impact. Meanwhile temperatures will climb well into the
80s each day given the warm temperatures aloft...modest south to
southwest low level flow...and mostly sunny skies forecast.

Later in the week...from about Thursday through Saturday...the
deepening western trough will shunt the large southern ridge over
toward the Appalachians and result in any subtle shortwaves
ejecting out of the trough moving more northeastward across the
High Plains and Minnesota. This results in a period of lower probability of precipitation
but slightly higher temperatures as any storms that form should
range from Nebraska up across the Dakotas and the warm airmass to
our southeast will encroach a bit more into Iowa by the end of the
week. The West Coast trough will eventually kick out across the
central U.S. And push a cold front across our region...generating
more widespread showers and thunderstorms followed by cooler
weather. However...there continues to be a considerable amount of
spread among the long range models in their solutions for this
evolution and particularly the timing of rain and frontal passage
for Iowa. Have thus stuck close to model consensus with rain
chances mostly in the Sunday/Sunday night time frames and cooler
weather likely by next Monday. These details will gradually come
into focus later this week.


issued at 646 am CDT Monday Aug 31 2015

Fog trends remain the primary concern early this morning.
Conditions vary widely at taf sites from LIFR /kmcw/ to VFR /kdsm/
but all sites should continue to improve through 15z. Although
some scattered/broken cumulus may develop during peak heating of the day...they
should be above 3k feet with VFR anticipated through the afternoon
and evening. Some patchy MVFR radiation fog may develop again into
early Tuesday morning...but not to the extent seen the past few


Dmx watches/warnings/advisories...none.



short term...small
long term...Lee

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