Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
604 am CST Sat Dec 7 2013
Short term /today/...
issued at 354 am CST Sat Dec 7 2013
Main concern today remains temperature and wind chill trends. Any initial
effects of advancing western Continental U.S. Trough will not reach Iowa until very
late in the period...and that would be nothing more than increasing
middle cloudiness. Raw GFS temperatures are still too warm so temperatures will be
based more on NAM...MOS...rap and other high res runs. There is
still some spread in these values however and somewhat lower
confidence than yesterday. Less sunshine and mixing with increasing
clouds and diminishing winds may not allow as great of a rebound so
followed a blend more than anything. Wind Chill Advisory is
somewhat marginal at the moment...but with temperatures continuing to fall
and winds not diminishing until later this morning changes make
little sense based on public perception of sensible weather.
Long term /tonight through Friday/...
issued at 354 am CST Sat Dec 7 2013
Forecast concern was focused on the long duration snow event
beginning late tonight and lasting into the evening hours on
Sunday. Models are in good agreement with timing and track
location of middle-level shortwave to push across the state Sunday
and leaned toward a blend of the sref/nam12 probability of precipitation and temperatures.
Tonight through Monday...broad isentropic lift and weak moisture
transport developed tonight past 03z Sunday over the western to
central portions of the state...then spreads east by 06z. NAM
sounding at crl completely saturates by around 09z Sunday...while
the GFS and sref roughly an hour or two slower. However...should
see some light snow b/T 06-09z time frame as isentropic lift looks
to be enough to help the column saturate a little quicker than
when the models are anticipating. So kept mention of likely probability of precipitation
across the west b/T 06-09z but with little accumulation. Dsm to
mcw become completely saturated close to 12z...while alo to otm
should see snow by 15z.
All soundings during the event have a deep dendritic
layer...basically from the surface to near 600mb but lift within
this sgz is very weak as Omega only ranges from 2 to 4 ubar/S.
06.21z sref probability dendrite growth zone greater than 100mb is 100
percent over much of the forecast area but the probability of 1 inch/hour
snowfall rate is less than 10 percent...and that is located over
far northwest Iowa. The snow ratios increase to 20 to 22:1 and
possibly a little higher across the north during the bulk of the
event. Expecting light fluffy snow flakes with Little Water
content as precipitable water is at or less than 0.25 inches from
12z Sunday to 00z Monday.
No major changes with ongoing snow forecast and still
anticipating a widespread 2 to 4 inches central to north...with
generally 2 inches or less in the south to southeast. Certainly
cannot rule out a few isolated locations receiving 5 inches with
the light fluffy snow. In addition...winds will not be much of a
factor so felt confident to hold off on issuing any headlines
at this time. Isentropic lift and deep moisture move east Sunday
night...and decreased probability of precipitation slightly across the west. Went
completely dry after midnight Sunday night.
Tuesday through Friday...a reinforced shot of cold air filters into
the state on Tuesday resulting in a very cold night again for Tuesday
night. With the fresh snowpack over the area...tweaked lows down
a degree or two. Otherwise...the forecast will be continued cold
and dry through the remainder of the forecast period with high
pressure persisting over the region.
issued at 604 am CST Sat Dec 7 2013
VFR conditions anticipated through the period...outside of
possibly just reaching MVFR at kfod late. Although skies are
presently clear...clouds will very gradually thicken and lower west-east
into tonight as rockies system approaches. This will eventually
lead to snow and restrictions sun...especially after 12z...but
only reach western Iowa and potentially kfod toward daybreak. Winds will
be fairly light...north-northwesterly becoming Ely late.
Wind Chill Advisory until 9 am CST this morning for Black Hawk-