Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 111755

Area Forecast Discussion
National Weather Service Des Moines IA
1155 AM CST Thu Jan 11 2018

...Updated for Near Term Trends and 18z Aviation Discussion...

Issued at 1154 AM CST Thu Jan 11 2018

Main PV anomaly and associated dry slot is centered over central
Iowa along and east of the I-35 corridor and will continue to move
to the northeast per GOES-16 imagery. Radar shows the deformation
snow band from roughly Estherville to around Carroll. This
location is a bit farther west and therefore have shifted the
highest axis of QPF a bit in that direction as well for the
afternoon. Another area of precipitation (mainly snow) continues
over the southeastern forecast area that may also have enhanced
precipitation rates. At this time, no plan to change headlines.
Snow will end this afternoon, though gusty winds will persist
causing areas of blowing snow, especially in rural areas.


.SHORT TERM.../Today through Tonight/
Issued at 313 AM CST Thu Jan 11 2018

Confidence: Medium to High

Complicated system with some adjustments to headline outline. Models
have arrived with a slight southeast shift of the main axis of
heavier precipitation. For that reason have removed some of the far
north/northwest counties from the warning due to snowfall amounts
possibly about 1 to 2 inches lower overall.  Farther south there
remains good agreement between the models for a heavier axis of snow
to set up between 12-18z across the remaining warning area. Per
previous forecasts this will be a combination of lift along the
boundary along with increasing deformation/weak trowal extending
back into southwest to northeast Iowa during the 12-18z timeframe.
Though the layer isn`t quite perfectly matched in the dendritic
layer...potential banding along the frontal boundary and just north
of it will help to increase snowfall rates during the morning into
early afternoon hours.  As expected the cold air is already getting
ingested into the system with the potential for a brief light glaze
of ice extending across the eastern counties this morning...and
lingering light snow with minor accumulations later in the afternoon
have expanded...along with our neighbors to the east/south...the
winter weather advisory through the afternoon hours.  Winds will
still be strong on the back side of the front through the morning
and afternoon hours...up to 20 to 30 mph with higher gusts at times.
Regarding the headlines...the winter storm warning has been extended
slightly southeast of the present northwest portions of
Dallas County/Adair Counties where increased snowfall and higher
winds should occur this morning and through the early afternoon. The
snowfall amounts for the system still look to be in the 3 to 5 inch
range across the warning areas with some potential for a few higher
totals.  The main concerns in the warning area remain the impacts
from the snow and wind resulting in very poor travel conditions for
the morning into the early afternoon.  By mid to late afternoon we
will again see much colder air streaming southeast along with the
snow ending east/northeast...additional light accumulations
northeast during the afternoon hours. By tonight any blowing snow
will be on the decrease with clearing skies...along with bitter cold
temperatures.  Lows tonight will drop into the 0 to 5 above range
across the southeast and from 0 to nearly 10 below across the far
northwest areas of the state.

.LONG TERM.../Friday through Wednesday/
Issued at 313 AM CST Thu Jan 11 2018

Minor changes made to the extended, with the largest change being
the removal of pops from the 06-12Z Saturday time frame.  That
shortwave doesn`t look as strong anymore and looks to diver further
to the southwest which only grazes our forecast area in the evening.

Otherwise the main story of very cold air returning remains on
track.  Highs on Friday will only be in the single digits north to
the teens south which will be the story through Saturday as the
Upper Midwest remains entrenched in a deep trough over ht e region
with a surface high centered to the north and west of Iowa.

Sunday looks to be the most interesting day of the extended. A weak
shortwave drops south of the state but will bring clouds and some
light snow to the area...mainly across southern Iowa then by late
morning a strong trough drops into the state with an associated
shortwave ahead of the main trough.  At this time models have less
than a tenth of an inch qpf with this system but given the forcing
with this and the potential for high snow ratios as cold air pours
in with the trough...we could see more than the inch or so of snow
currently forecast. Following this will be another blast of arctic

We recover from the first blast of cold somewhat on Sunday with weak
theta-e advection moving across the state.  Highs will be in the
teens and lower 20s but then for Monday and Tuesday, highs will be
in the single digits to around 10.  The trough then moves off into
the Great Lakes for Wednesday with ridging developing over the
region and moderating temps.  Highs currently forecast for Wednesday
are in the upper 20s to mid 30s but this may be optimistic depending
on how much and widespread the snow is on Sunday.  At the tail end
of the long term period models are in high disagreement but they all
indicate, at varying degrees, another system impacting the state
late Wednesday night and Thursday.  Due to low confidence and model
discrepancy, left the token PoPs the model blends put in the
Thursday period.


.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1154 AM CST Thu Jan 11 2018

Primary concerns are ceiling and visibility restrictions and gusty
winds causing blowing snow through this evening. All terminals
have switched over to snow, although brief periods of freezing
drizzle are still possible mainly at KDSM and KOTM. Conditions
will improve from widespread LIFR/IFR at midday to VFR late this
afternoon or early this evening as the snow ends from southwest to
northeast. Winds will decrease overnight with VFR conditions
prevailing through the end of the period.


Winter Weather Advisory until 6 PM CST this evening for

Winter Storm Warning until 6 PM CST this evening for IAZ007-016-



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