Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
543 am CST Sat Dec 20 2014
Short term /today/...
issued at 343 am CST Sat Dec 20 2014
A compact short wave is currently moving across southwest Iowa. Low
level moisture advection initially resulted in areas of freezing
drizzle during the evening however middle level moisture immediately in
advance of the short wave has brought areas of ice introducing and
has switched the precipitation to very light snow at times.
Subsidence and drying behind the short wave will remove the ice
introduction and will return any precipitation back to drizzle or
freezing drizzle...depending on the surface temperatures. The most
favored areas between 6 am and noon will be along and east of
Interstate 35. A few icy patches will likely develop on roadways
through middle morning but do not expect minor ice accumulations to be
widespread enough to warrant headlines but may require an Special Weather Statement for
morning travel. Temperatures today will again be below guidance and
reliant on moisture advection with a thick cloud shield in place.
Not much change in surface dewpoints through the day will relegate highs
to near to slightly higher than fridays temperatures.
Long term /tonight through Friday/...
issued at 343 am CST Sat Dec 20 2014
Active weather pattern setting up for extended...with main
concerns coming at beginning and end of period. For beginning of
period...upper level trough will keep southerly flow and moisture
increasing across the County Warning Area. This may allow for light precipitation
in the overnight hours...especially as forcing arrives near 06z.
NAM has forcing further south...and may not be far enough south
with probability of precipitation. However...main concern for overnight and into Sunday
will be with precipitation type. Surface temperatures will drop
below freezing in the north...and with shallow low level moisture
may not see ice introduction...leading to fzdz potential.
However...soundings also indicate upper level moisture arriving
late in the overnight which may help to keep precipitation snow.
Models trying to indicate sleet...though have kept mention out
at this time with melting of any ice introduced unlikely. Therefore have
kept fzdz mention in the north/northeast...with dz
elsewhere...changing to rn/snow mix in the early morning hours
before switching back to rain in the afternoon Sunday. Given
uncertainty in precipitation type am not confident enough to issue
advisory at this time...though will need to be monitored.
Surface low will push into Iowa for late in the weekend...keeping
prolonged precipitation chances into early next week. Will see
primarily rain with system as temperatures will be above
freezing. Models indicating dry slot across much of central Iowa
for Monday...and have cut back on probability of precipitation slightly in the south
Monday. Precipitation will return with cold air as low pulls
east...which will see precipitation switching to snow Monday
night into Tuesday. May warm enough during the day Tuesday to see
a Switch Back to rain/snow mix in the south. Low will pull north
into Great Lakes...with precipitation chances lingering into
Wednesday...especially in the northeast. GFS is a bit faster with
the system departing...and may have held onto probability of precipitation too long into
Wednesday. Behind system winds will increase...and may see blowing
of any snow which is falling/fresh fallen through Wednesday which
may cause travel problems and will need to continue to monitor.
Will see a brief break in precipitation chances for late in the
week...primarily Thursday...before another system arrives for the
end of the week. There is still a lot of model uncertainty with
this late system...with GFS bringing shortwave across Friday and
European model (ecmwf) lifting surface low across Iowa. Model discrepancies appear
to be in handling of early week low pressure system...with European model (ecmwf)
being more aggressive in lifting north...while GFS lingers
helping to keep upper level trough across Iowa. Either
way...precipitation will be possible...with temperatures cold
enough for snow...and winds may be strong...capable of blowing
snow. Gain...will need to closely watch system...especially
evolution of early week low.
issued at 542 am CST Sat Dec 20 2014
Widespread IFR stratus is in place across Iowa with a few areas
dropping to LIFR. The ceilings should rise slightly today and
possibly become MVFR at times. Areas of fog and freezing
drizzle/very light snow will continue early this morning before
diminishing. The precipitation likely will return again tonight.
The wind will be out of the south to southeast during the period.