Area Forecast Discussion
Issued by NWS Des Moines, IA

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960
FXUS63 KDMX 240555
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1155 PM CST Mon Jan 23 2017

.SHORT TERM.../Tonight through Tuesday/
Issued at 351 PM CST Mon Jan 23 2017

Weak ridging was in place over the central US as troughs of low
pressure were found over the West and East Coasts this afternoon.
The trough over the West Coast will move toward the Central US
tonight into tomorrow. As this occurs, surface low pressure will
develop and move from western Kansas Tuesday morning to near the
Kansas/Nebraska/Missouri intersection by tomorrow evening. Ahead
of the low, moisture will be drawn northward, clouds will thicken,
and precipitation will spread from western Iowa in the morning to
eastern Iowa by late in the afternoon. Over far southern parts of
the CWA, mainly rain is expected as the low pressure brings
warmer air into the low levels. Over central Iowa, a brief period
of snow or sleet may mix in tomorrow morning before all rain is
expected. Farther north, temperature profiles will support mainly
snow and this is where a Winter Storm Warning/Winter Weather
Advisory is being advertised. There could be a brief burst of
precipitation as a band of frontogenesis moves from southwest to
northeast across the forecast area mid-morning to mid-afternoon.
In addition, the snow initially will be wet and heavy with snow
ratios ranging from 7:1 to 9:1.

.LONG TERM.../Tuesday night through Monday/
Issued at 351 PM CST Mon Jan 23 2017

Ongoing winter storm was the main focus through the extended.
Models have finally come into much better agreement with timing
and location of the surface low and the moderate to heavy snow
band Tuesday night into Wednesday. Continued to lean closer
towards the ECMWF as the 23.12z GFS, NAM, SREF are more in line
with this more consistent solution.

Tuesday night through Wednesday night...The surface low looks to
be centered over northeast Kansas into northwest Missouri by
around 00z Wednesday with the 500mb low located northwestward into
central to eastern Nebraska. The surface low will continue to
track across Iowa from south-central to east-central Iowa and
finally push into the Great Lakes by midday Wednesday. The pivot
point looks set up across northwest portions of the forecast area
and this location is likely to receive the heaviest snow amounts
by Wednesday as the deformation zone sets up overnight tomorrow
night.

The initial snow band looks to continue to advect northwards,
likely be close to the Minnesota border, with the Theta-E
advection and isentropic lift ahead of the surface low Tuesday
evening. This band will be see the strongest forcing within the
dendritic layer combined with some negative EPV present which
potentially could release CSI for a brief stint Tuesday evening.
In addition, steep mid-level lapse rates hint towards some
instability and even the GFS/NAM bring in low MUCAPE values over
central to southern Iowa. Certainly a potential for thundersnow,
but left out mentioned due to the low confidence in the location
and duration. Regardless, if thunder does occur, this will enhance
the snowfall rates considerable for a short duration during the
evening hours Tuesday.

A lull in the large scale forcing as the dry slot begins to punch
into much of the forecast area late tomorrow evening before the
surface low moves far enough east to allow the deformation zone to
set up over the west to northwest b/t 06-12z Wednesday. Soundings
do lose a bit of ice introduction along the southern edge of the
band which may lead to a brief duration of freezing drizzle/rain.

From 06z to 18z Wednesday, the forcing doesn`t match up within the
DGZ and the snowfall rates look to drop off dramatically. However,
there is plenty of large scale forcing to keep snowing for a long
duration.  By this time the strong pressure gradient winds develop
past 09z Wednesday and persist throughout the day. So with the
falling snow and already fallen snow, blowing snow and visibility
restrictions are highly likely. Thus, with the moderate to heavy
snow and the strong winds, confident to upgrade to a Winter Storm
Warning for the northern tiers of the forecast area. Lingering
blowing snow and snow showers are possible over the northeast
Wednesday evening.

Thursday through Saturday...A series of weak clipper systems bring
reinforced shots of cold air and at least some cloud cover during
this time frame.  There is some weak lift available that may cause
some flurries and/or snow showers Thursday, Friday evening, and
again throughout the day Saturday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1155 PM CST Mon Jan 23 2017

Little change to previous package. As storm increases through the
area cigs will drop to LIFR over much of the north with -SN vsby
between 1/2 to 1sm across the north aft 00z at KFOD and KMCW.
Elsewhere RASN will result in slightly higher vsby with IFR cigs.
Winds increase from the east southeast at 12 to 22 mph aft 16z
through 22z.  /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from noon Tuesday to 6 PM CST Wednesday for
IAZ004>007-015>017-023>027-033>035.

Winter Weather Advisory from noon Tuesday to 6 PM CST Wednesday
for IAZ028-036>039-044>047.

&&

$$

SHORT TERM...Ansorge
LONG TERM...Podrazik
AVIATION...REV



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