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Area forecast discussion 
National Weather Service Des Moines Iowa
1156 PM CDT Monday Apr 20 2015

Short term /tonight/...
issued at 302 PM CDT Monday Apr 20 2015

Main challenges overnight will be temperatures. Early am surface
analysis shows three distinct cold fronts...with the first having
passed through the region Sunday...the second in far southern Iowa
at 12z and a third over southern Minnesota at 12z which will slowly edge
south overnight tonight as the deepening lows over Lake Michigan
eventually merge into one system near Hudson Bay Canada by 06z
Tuesday. This will pull the final front south into Iowa by Tuesday.
Meanwhile...strong winds of this afternoon will decouple tonight and
cumulus field caused by the h850 cold air advection will dissipate
with time...leaving the area generally clear to partly cloudy. Drier
middle level air beginning to move into western Iowa this afternoon with
showers on the decrease. Clearing will allow for some radiational
cooling and temperatures to drop into the 30s overnight over all of
the forecast area. Despite the low minimums expected...gradient
winds are likely to remain near or above 10 kts which will keep any
threat of frost to a minimum. Third cold front arriving across the
north by 08-10z will create near adiabatic conditions again...
promoting mixing and stronger winds which will keep boundary layer
mixed toward morning. Ground moisture should also inhibit frost
formation tonight...and not occur until winds become light enough to
cause lows near or under 30 degrees...near the middle to end of the

Long term /Tuesday through Monday/...
issued at 302 PM CDT Monday Apr 20 2015

The large atmospheric gyre developing over the Minnesota/Canada
border today will slide over the Great Lakes region on Tuesday and
eventually over upper New England and southeastern Canada later in
the week. This will keep US beneath a persistent but gradually
weakening northwesterly flow regime...leading to continued cool
weather and mostly dry conditions for the remainder of the work

On Tuesday...a shortwave impulse rounding the large cyclone will
move over Iowa and push through a surface reflection in the form
of a renewed wind surge and reinforcement of the cold air in
place. Most of the prognostic models are depicting a band of
clouds and light rain developing along this feature as it moves
through...especially in the afternoon and evening...and the GFS
and NAM soundings show this possibility as well with the
development of a pretty decent albeit high based saturated layer.
Given the prevalence of light showers and even some tiny hail
earlier today...believe there is a decent chance of a lower
magnitude repeat tomorrow and have introduced slight probability of precipitation and rain shower-
mention from about 20z-03z Tuesday afternoon and evening. In any
event...the rain will be of little consequence and struggle to
even produce measurable precipitation. However...the clouds
associated with this well as the renewed shot of cold
air advection behind the reinforcing wind surge...will help to
hold temperatures down and have cooled off the maximum temperature forecast
accordingly. Winds will be pretty strong behind the
surge...especially across our northern and northwestern
counties...and another Wind Advisory may become necessary for
Tuesday. This will be closely monitored on tonights update shifts.

By Tuesday night as the large gyre finally begins to move off to
our east/ will allow a large cold surface high
pressure ridge to spill down across the upper Midwest and Iowa for
a couple of days. This will result in a notable diminishing of
surface winds...but will also clear out the clouds and allow for
more efficient radiational cooling during the overnights. This
potential should be maximized on Wednesday night/Thursday
morning...then the ridge axis will be nearby and most areas will
see their coldest temperatures of the week. This will likely
result in the consideration of frost/freeze headlines at some
point...or possibly multiple points...this week.

By Thursday night the influence of the departing cyclone over Iowa
will be virtually none...but a much weaker middle/upper level trough
will be approaching from The Four Corners region. The subsequent
evolution of this feature has been the object of much hand
wringing by the long range models over the last few days...with
considerable inconsistency and variability in what they want to do
with the low as it moves up over the Midwest. This situation has
not improved today...and have stayed close to previous forecast
and model consensus for the weekend forecast. In general the most
likely scenario is showers and a few thunderstorms mainly across
the southern half of the state between Friday night and Saturday
night...but to state anything with more detail than that at this
point would be folly. The good news is that none of the solutions
result in much possibility of hazardous weather across Iowa. The
bad news is that none of them bring much warming either.


issued at 1153 PM CDT Monday Apr 20 2015

Cyclonic circulation over the taf sites tonight through Tuesday.
Winds will remain decoupled tonight...with winds increasing
rapidly late tonight north and around sunrise central and south.
Isolated showers expected Tuesday afternoon. Clearing with
diminishing winds Tuesday night.

Outlook...high pressure will build into the state with light winds
and VFR conditions through the balance of the work week.


Dmx watches/warnings/advisories...


Short term...rev
long term...Lee
aviation...MS Apr 15

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