Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181133
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
633 AM CDT Wed Apr 18 2018

.SHORT TERM.../Today through Tonight/
Issued at 355 AM CDT Wed Apr 18 2018

Confidence: Medium - Due to Ptype and Possible Narrower band of
Heavy Snow.

Impressive late season winter storm beginning to hit the region
now with area of thunderstorms firing in the strong lift/warm
air advection now entering western Iowa. On the back side of the
system over Nebraska at 08z...snow is falling with vsby already
down to 2 to 4 miles. Overall the storm is progressing as
expected...with some changes to the axis of higher qpf slightly
farther south. Most of the models are tracking the best lift
between Highway 20 and Highway 18 overall and this is supported by
the the synoptic and mesoscale ensemble members of the HREF
winter graphics at SPC. Interestingly enough the max ensemble
snowfall forecast is in excess of 10 inches across the same
location. The structure of the system has not changed with a
1000mb sfc low expected to be near Columbia MO by 12z today...35
to 45 kt inflow into a strong upper level thermal boundary
promoting continuing tremendous lift/unstable conditions well back
into the cold sector. The degree of dynamic cooling should
quickly overcome any liquid precipitation with thereafter 1 or
more inch snowfall rates this morning into the afternoon in the
corridor of heaviest snowfall as the system translates east. H850-
H700 forcing is also being enhanced by the strong upper level low
tracking east today. Isentropic analysis shows a lingering trowal
into the afternoon northeast where after a peak production of 3
to 6 inches of snow occurs between 12-18z...another 1 to 2 inches
of snow may pile onto the northeastern counties in the afternoon
hours. Currently the maximum snowfall in our forecast looks to be
in a corridor from Bremer to Wright Counties where the trowal in
the afternoon may contribute to slightly more snowfall there.
Farther south we will still battle temperatures early
today...changing over to near freezing between 10-13z as more
light rain begins to fall. For headlines...have added Sac and
Calhoun to the warning as snow over the north of these counties
may exceed 6 inches. Along the Highway 30 corridor south to
I80...some adjustments in snow totals today may be needed if
cooling is delayed by several hours or a. more narrow banding of
heavier precipitation occurs. The nature of this storm and the
fast movement is adding to some uncertainties in the
forecast...even at this stage of the event. Another area of
concern is the far north over Winnebago/Worth/Hancock and Cerro
Gordo counties where there may be a sharp cutoff in precipitation
yet this morning or early afternoon...leading to less snow in
those locations. At this time it is too close to call and will opt
to err on the side of caution given some minor uncertainties in
how the ongoing convection may affect the evolution of the system.
Winds today will be strong enough to cause some blowing snow and
drifting ...but significant blowing and drifting is not
anticipated. It cannot be stressed enough that with snowfall rates
of 1 or more inches per hour later this morning into the
afternoon across the warning areas...travel will be hazardous due
to half mile or less visibility in falling snow - especially on
higher volume roads such as highways and interstates. Highs today
will remain in the lower 30s over the north with mid to upper 30s
in the south. Tonight the system will depart the region with
headlines expiring in the evening hours if not sooner once
conditions improve over those impacted areas. Lows tonight will
fall into the mid 20s north to the lower 30s in the south.

.LONG TERM.../Thursday through Tuesday/
Issued at 355 AM CDT Wed Apr 18 2018

Temperatures were the main focus of the extended period with a
welcomed quiet and dry weather pattern setting up across the
region. Leaned toward a blend of the ECMWF and GFS as they remain
in good agreement through the weekend into early next week.

Thursday into Friday...introduced mention of record lows across
the northern portion of the CWA and near record lows over the
south for Friday morning. Temperatures should drop off
dramatically Thursday night into Friday morning with the surface
high centered itself directly over the state b/t 06-12z Friday.
Perfect radiational cooling setup with very dry air mass, clear
skies, and strong subsidence over Iowa by Friday morning. Plus,
with much of the snow pack over northern Iowa is likely to aid in
the cooling. Winds do begin to veer to the south across wester
Iowa towards sunrise Friday morning as the surface high shifts
east. Confident the lows will already have occurred prior to the
weak return flow.

Saturday into Tuesday...The strong surface high pressure continues
to ward off the next system to the south of the state on Saturday.
A gradual warming trend is likely to begin Saturday even with
the decent easterly flow and mid-level cloud cover over the CWA.
Sunday and Monday look to be warm with plenty of sunshine and weak
mixing each day. Finally some actually spring-like weather late
this weekend into early next week. GFS and ECMWF bring a cold
front through the state during the day Tuesday with the ECMWF much
drier with the FROPA. Very little, if any, instability with this
front and thus have low confidence in severe weather potential.

&&

.AVIATION.../For the 12Z TAFS through 12Z Thursday morning/
Issued at 633 AM CDT Wed Apr 18 2018

Conditions expected to deteriorate through day with IFR/LIFR cigs
cross north at KMCW/KFOD/KALO aft 15z and MVFR to IFR at
KDSM/KOTM aft 16z. Scattered thunder/thundersnow expected north
with visby to 1/2sm at times. Farther south sleet/ra/snow mixture
at KDSM with -sn aft 16z through 22z. Conditions improve 00 to 06z
with wx...and cigs from 06 to 12z. /rev

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 1 AM CDT Thursday for IAZ047>050-
059>062.

Winter Weather Advisory until 7 PM CDT this evening for
IAZ044>046-057-058.

Winter Storm Warning until 7 PM CDT this evening for IAZ004-005-
015-023-024-033>035.

Winter Storm Warning until 1 AM CDT Thursday for IAZ006-007-016-
017-025>028-036>039.

&&

$$

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



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