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Area forecast discussion 
National Weather Service Des Moines Iowa
1245 PM CDT sun Aug 30 2015

issued at 902 am CDT sun Aug 30 2015

Little improvement in fog over the area at this time so have
extended the advisory. Conditions over Polk have actually now
dropped down again after improving last hour...may be due to some
northwest advection of entire area of fog over the past hour. Will
handle with Special Weather Statement...for the next couple of hours. Otherwise...
expect slowly improving conditions toward noon

Update issued at 651 am CDT sun Aug 30 2015

Determining favorable areas for dense fog formation has become
problematic with it now difficult to discriminate between existing
stratus which has precluded fusibility dropping significantly and
and similar areas that have developed due to more shallow stratus
and radiation fog. However 1/4sm observation density made it
fairly clear that an advisory was needed across southern Iowa. Expect
conditions to improve by 9am or so...although confidence in
improvement timing is low.


Short term /today/...
issued at 320 am CDT sun Aug 30 2015

Fog and temperature trends will be the main concerns early this morning.
Wet soils from recent heavy rains and light winds have made for
favorable radiation fog conditions where skies have cleared early
this morning. The only thing holding widespread dense fog back
appears to be patchy lingering stratus per recent visible/fog satellite
imagery. Thus have opted for an Special Weather Statement in lieu of a dense fog headline
for the time being. Even under clear skies in the Des Moines area
observations here at the office and Airport have not
deteriorated...and visibilities also remain up from Newton to
Knoxville under stratus. Thus went Special Weather Statement rather than trying to draw a
patchwork headline.

Have also lowered high temperatures a category or two from previous
forecast. Recent verification has shown a warm bias and cannot see
any appreciable warming today with lingering stratus/fog and winds
not increasing until late. GFS and its associated MOS also continues
its recent warm bias so stayed closer to cooler NAM MOS.

Long term /tonight through Saturday/...
issued at 320 am CDT sun Aug 30 2015

Little has changed in overall forecast thinking tonight. The 500
mb synoptic pattern for the coming week is characterized by a
large warm area of high pressure across the southern and eastern
U.S. And longwave troughing across the northern rockies and
Pacific coast. The forecast challenge is identifying to what
extent shortwaves ejecting out of the northwest trough will be
able to Dent the ridge and nudge it southward. If the ridge holds
strong...then we will remain hot and dry for the coming week. If
the shortwaves are able to flatten the ridge out...then it will be
a bit less warm and we will have intermittent thunderstorm
chances. Long range models have generally trended more toward the
latter scenario since last night...and as a result low probability of precipitation are
now being carried from late Tuesday night through Thursday...and
again next weekend.

Other than the aforementioned intermittent low probability of precipitation...the work
week will feature mild late Summer weather with highs in the 80s
and lows generally in the middle to upper 60s. Little change has been
made to the forecast temperatures...except to lower them by a
degree or two around midweek concurrent with the probability of precipitation and
associated weakening of the ridge. The hottest days will likely
come at the end of the week as the ridge builds back in
overhead...especially around Friday and Saturday. There is pretty
good agreement between the European model (ecmwf) and GFS that a more robust 500 mb
trough will then move eastward along the U.S./Canada border and
push a cool front across Iowa late in the weekend. This system is
responsible for the chance probability of precipitation at the end of the long term
forecast and will likely result in cooler weather just beyond a
week from now.


issued at 1245 PM CDT sun Aug 30 2015

Main concern this period continues to be low stratus and fog
redevelopment. Heating above the boundary layer is beginning to
erode the stratus/fog layer beneath with more widespread return to
VFR through 00z. With ridge of high pressure moving slowly
east...conditions will improve a bit tonight though over the
east/southeast especially IFR ceilings/fog may again develop after 10z
at kotm/Kalo with kotm the most likely. Winds will remain light
becoming more south with time remainder of today and light south
tonight. Met guidance has a better handle on situation past few
nights than mav. /Rev


Dmx watches/warnings/advisories...


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

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