Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
322 PM CST Sat Dec 7 2013
Short term /tonight/...
issued at 322 PM CST Sat Dec 7 2013
Main challenge will be timing of onset of
snow overnight...which will have some impact on lows as clouds move
across the region. Current models differ on timing by about 3 to 4
hours...but generally are on board with snow beginning over western
half of Iowa between 09z and 12z...spreading east thereafter. High
pressure exiting today with ridge axis over SW Iowa at 15z now.
Models have initialized poorly this morning regarding center of high
with all models showing the high over Minnesota by 18z and farther north at
12z than subjective analysis showed.
Although there have been some differences between model inits on the
ridge axis the amount of available quantitative precipitation forecast produced by the system
continues to be consistent across the region. GFS isentropic
analysis shows that the saturation aloft will begin between 06-09z
with some residual dry air near the surface. This should result in some
virga as the column saturates then quickly see light snow and visibility
drop once the snow begins to reach the surface from 09-12z west and 12-
15z east. Overnight accums will be rather light overall with up to
about an inch over the west and from i35 west just enough to begin
accumulating or under a half inch. With the column saturating
quickly snowfall rates will pick up once the column fully saturates.
Mins tonight will fall back into the single digits to lower teens
prior to some moderation during the night as clouds and milder air
Long term /Sunday through Saturday/...
issued at 322 PM CST Sat Dec 7 2013
Focus of the long term forecast remains fixed on the widespread
snowfall event for Sunday/Sunday night. There is a high degree of
consistency among the short range models spreading snow across
most or all of our area on Sunday morning...along the northward
periphery of a weak low/inverted trough moving across Missouri.
Forecast soundings and cross sections indicate a fairly deep
dendritic layer but only modest large scale forcing...and there is
very good agreement amongst the various MOS guidance regarding
quantitative precipitation forecast. There is high confidence in widespread amounts of 2 to 4
inches...lowest in the southeast and highest in the
northwest...with a few locally higher amounts possible but not
likely. Given the cold air that has preceded and will accompany
this system the snow should be fairly light and fluffy...and light
to moderate breezes combined with bare ground across the state
should mitigate blowing/drifting of snow. Despite these
factors...accumulations of several inches with the first
widespread snowfall of the season will have travel impacts...and
the forecast of 3 to 4 inches across more than half the forecast
area justifies hoisting a Winter Weather Advisory for Sunday into
Sunday night and will issue this with the afternoon package.
The snow will end on Sunday evening with dry but cold weather
prevailing for the first half of the coming work week. The cold
temperatures will be reinforced by fresh snow pack and an Arctic
high settling across Iowa on Monday...when highs will only be in
the single digits and teens...then again on Wednesday in a similar
scenario. Lowered temperatures by a couple of degrees from
guidance/previous forecast from Sunday night through the middle of
next week due to these effects. There may be some temporary relief
around Thursday and Friday as weak thermal ridging aloft coincides
with modest southwest flow at the surface...but even this will
probably only be enough to get US up into the 20s and lower 30s by
Friday. Another cold front will then cross the region late Friday
bringing somewhat cooler temperatures again next weekend.
issued at 1151 am CST Sat Dec 7 2013
Main concern this period will be lowering conditions through the
period after 09z and through end of period. Have modified timing a
bit as lift may arrive a bit slower...by a few hours at most...
but then conditions deteriorate rapidly as ceilings drop from VFR to
IFR very quickly over a 1 to 2 hour period. Light snow will then
continue with lowering visibilities for the remainder of the period.
Though winds will not be a major concern...some increase in surface
winds and gusts should arrive about the same time as snow begins
with east-southeast winds of 10-16kts for a period from west/east Sunday.
Winter Weather Advisory from 6 am Sunday to midnight CST Sunday
night for Black Hawk-Boone-Bremer-Butler-Cerro Gordo-Clarke-
Winter Weather Advisory from 3 am to 9 PM CST Sunday for Adair-