Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 231152

Area Forecast Discussion
National Weather Service Des Moines IA
552 AM CST Thu Feb 23 2017

.SHORT TERM.../Today through Tonight/
Issued at 408 AM CST Thu Feb 23 2017

Plenty to discuss this morning, but unfortunately the confidence in
details has not increased much even as the storm nears.  The system
in question continues to advance through UT/ID with mid level
kinematic forcing already spreading snow along the NE/SD border
toward the MO River. Farther south warm/theta-e advection is also
increasing ahead of southern stream influence with clouds and precip
developing over KS. Our current cold advection regime will quickly
flip to warm advection this morning, especially aloft, realizing the
lingering baroclinic zone already in place with increasing lift and
frontogenetical response. This will likely spread the current KS
precip across southern IA and eventually help expand precip farther
north as well with help from the approaching mid level forcing.

Precip should then blossom quickly this evening, especially across
the northern 2/3rds of the state. Very strong thermodynamic forcing
will develop with noted theta-e advection and frontogenesis
resulting in plenty of elevated convection. Increasing mid level
lapse rates will lead to MUCapes well through the hundreds as the
surface low approaches the MO Valley. With estimated effective shear
/1-6km/ quite high would not be surprised to see some elevated weak
supercells produce some hail so fully agree with the current SPC
marginal outlook.

Unfortunately this leads to diminished confidence in winter weather
as well. The dry slot driving this instability will surge into Iowa
overnight which may cutoff ice introduction and this will also line
up with marginal temps aloft and at the surface to fully flip the
precip type to all snow. Latent heat release from the convection may
also be problematic with precip distribution and temps. Thus it may
only be the far northern counties that see sufficient confidence in
cooling for significant snow through 12z. A wintry mix including
potentially some freezing drizzle could occur in the freezing line
transition zone with plenty of shear and turbulence driving light
precip under the dry slot. Forcing through the dendritic growth zone
along the IA/MN border as depicted by model 300K surfaces is quite
impressive however with thundersnow not out of the question looking
at a band of negative EPV aloft in cross sections. So overall
confidence is not great, but unfortunately significant winter
weather is <24 hours out so the warning decision needs to be made.
Thus have turned on Blizzard Warnings for 5 counties toward the MN
border starting at 6pm. The tightening surface gradient looks to
support sustained winds of 25 mph by this time with gusts to 35 mph
plus. This combination of lift and instability may result in 3-7
inches by 12z toward the MN border.

.LONG TERM.../Friday through Wednesday/
Issued at 408 AM CST Thu Feb 23 2017

Blizzard conditions over far northern Iowa will continue and the
transition to snow will progress to the southeast during Friday.
This forecast favors more of a GFS and ECMWF solution as the NAM
is too far to the northwest and the Canadian has persistently been
slow and farther south. The latest run of the MPAS, while not
included in the forecast package, also favors a southern solution.
Across northern Iowa, snow will be heavy at times throughout
Friday as strong frontogenesis and omega pass over the area,
especially during the first part of the day. With increasing winds
from the northwest and the falling snow, blizzard conditions are
expected which will have significant impacts on travel. Sustained
wind speeds were increased from initial guidance as they are
typically underdone in such dynamic system. Wind gusts were also
increased as winds at the top of the mixed layer are in the 40 to
45 knots range and this will yield wind gust of at least 30 to 35
knots at the surface. Added counties to the Blizzard Warning south
of the previous watch starting at 6am Friday as wind gusts around
30 knots and falling snow along with the shift in the heaviest
snowfall southward. Also added a Winter Weather Advisory tier of counties
to the south of the Blizzard Warning for the snow and blowing
snow, but gradient could be fairly tight on the southern edge. By
Friday evening, snow along with blowing snow will continue over
much of central Iowa. Snow will gradually end after midnight as
the back edge of the storm departs the area. There still could be
areas of blowing and drifting snow Saturday morning as winds from
the northwest will be brisk.

As for snow totals, parts of far northern Iowa should expect 12"+
as they will be under the influence of deformation zone for a
prolonged period. Snow totals will taper off to the southeast with
areas south of Highway 30 and east of I-35 receiving 2 inches or
less as snow will be limited to what falls on the back side of the
low pressure.

Beyond Saturday, confidence is low as multiple fast-moving short
wave troughs pass over the region with models handling thermal
profiles differently. Have kept things simple at this point with
just rain/snow.


.AVIATION.../For the 12Z TAFS through 12Z Friday morning/
Issued at 551 AM CST Thu Feb 23 2017

Stratus moved north to south through IA overnight with IFR or MVFR
conditions drifting either side of 1000 ft. Conditions should
begin improving somewhat to MVFR or just into VFR into midday as
cooler, drier air begins to filter into the state on brisk N
winds and a warm front approaches from the south. Ceilings and
visibilities should deteriorate again into the night however as
precip increases. This could be a wintry mix north with showers
central and south. Either could see periods of thunder but
confidence in timing at any particular location is not great
enough to include quite yet. KMCW may see restrictions due to snow
and blowing late late in the period. Conditions may even dip to
LIFR but did not want to be that aggressive in the last few hours.


Winter Weather Advisory from 6 AM Friday to midnight CST Friday
night for IAZ026-027-036-044>047-057-058-070.

Blizzard Warning from 6 PM this evening to midnight CST Friday
night for IAZ004>007-015.

Blizzard Warning from 6 AM Friday to midnight CST Friday night
for IAZ016-017-023>025-033>035.



LONG TERM...Ansorge
AVIATION...Small is the U.S. government's official web portal to all federal, state and local government web resources and services.