Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Des Moines Iowa
540 am CST Sat Feb 6 2016
..updated for 12z aviation discussion...
Short term /today/...
issued at 349 am CST Sat Feb 6 2016
Rather benign short term forecast in terms of sensible weather. At 06z
Sat...surface trough passing over County Warning Area. Freezing fog over Sioux Falls County Warning Area
slowly advecting eastward. Given visibility field and current rate of
advection at 06z Sat...fog may clip our northwest to northern County Warning Area by
sunrise...but visibility not expected to drop much below 2 to 3 sm for
any prolonged period of time. Forecast soundings show solid inversion
persisting through Saturday...even into Sunday morning across the
northern to northeastern County Warning Area. Have bumped up cloud cover and
moderated daytime "heating" slightly as a result. 850mb temperatures on Friday
were in the -4c to -7c range. For Sat... 850mb temperatures -1c to -3c so
Sat maximum temperatures 5 to 10 degrees warmer for Sat vs Friday seems
Long term /tonight through Friday/...
issued at 349 am CST Sat Feb 6 2016
The long term forecast is punctuated by the potential for
blizzard conditions across northern Iowa from Sunday afternoon
through Monday morning.
At 500 mb...a robust shortwave trough will sweep across the
Dakotas and Minnesota on Saturday night...probably closing off as
a low centered over northern Minnesota by Sunday morning.
Subsequent reinforcing shortwaves will expand this low into a
large gyre over the Great Lakes with a deep longwave trough
carving out across the entire eastern half of the Continental U.S. For the
first half of the work week. A western U.S. Ridge forming in
response will eventually work its way eastward toward our region
around the end of next week.
In the lower levels and at the surface...southwest flow and weak
ridging this evening will keep things pretty mild and dry tonight.
On Sunday a surface low pressure center will develop rapidly
across northern Minnesota in response to the strengthening upper
level system...with its attendant trough sweeping from northwest
to southeast across Iowa around Sunday afternoon and evening. A
strong surface pressure gradient coupled with intense winds aloft
and the development of a seasonally deep mixed layer will promote
a significant increase in winds as the trough moves through. By 6
PM Sunday forecast soundings for Mason City from both the GFS and
NAM indicate a mixed layer extending up to nearly 800 mb with
winds at the top blowing at 45 to 50 knots. Given the forcing
associated with the upper level system and the meager but
positive instability associated with the forecast soundings...it
is likely that we will see a scenario in which snow showers form
within and environment characterized by deep and turbulent
boundary layer mixing. Combined with the deep snow pack in place
across northern Iowa...generally averaging between 7 and 14
inches...this creates an ideal set up for blizzard conditions and
the blizzard watch remains in effect. In fact the watch has been
expanded into five more northern Iowa counties where a deep snow
pack also exists...and also extended through noon Monday as it
appears there will be lingering snowfall on Monday morning along
with what could be the strongest winds of the entire event. In
short...travel conditions will become very hazardous once the
strong winds and snow showers begin late Sunday.
By Monday afternoon and evening the snow will depart and winds
will diminish as the large stacked low pressure system moves off
over the Great Lakes. The remainder of the forecast is then
generally quiet. A weaker and quick moving system may produce
some light snow over parts of Iowa on Wednesday...but solutions
for that system are contingent on the evolution of the earlier
week storm...and in any event it does not appear that any
significant accumulation is likely and winds will be fairly weak
around Wednesday so no hazardous impacts are anticipated at this
issued at 535 am CST Sat Feb 6 2016
Current ceilings and visibilities should effectively hold through Sat
morning. Removed widepsread LIFR wording from pvs taf package.
Gradual VFR from kfod to kdsm and to kotm may need tweaking on
timing. Kept Kalo and kmcw out of VFR. Kalo and kmcw will likely
need at least IFR Sunday night as blowing snow very possible.
blizzard watch from Sunday afternoon through Monday morning for