Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 110006
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
606 PM CST Sat Feb 10 2018

.SHORT TERM.../Tonight through Sunday/
Issued at 312 PM CST Sat Feb 10 2018

...Confidence: Medium to High

Another system tracking toward Iowa tonight. Confidence increasing
regarding placement of higher snowfall axis...but still some minor
uncertainty regarding available moisture. This system will have
less moisture to work with than last nights nearly .30 to .35 qpf
that occurred in the southeast portion of the forecast area. In
general models kicking out between 0.10 and 0.15 in the heavier
axis with the main axis running along the Iowa-Missouri border.
The farther north you go the lighter the expected qpf/snow amounts.
Overall snow ratios still favorable for 16.5:1 from 00 to 06z
over the southwest with similar values for the southeast 16.4:1
increasing a bit western areas as slightly colder air settles into
the column between 06-12z. Last night sfc temperatures over the
areas of heaviest snowfall ranged from about 7 to 9 F during the
period. Snow ratios with last nights event were near 20:1 in many
cases. Tonight with temperatures holding in the lower teens through
the event south though similar aloft...the slightly lower snowfall
ratios appear appropriate. Totals still looking like 1 or so in
the metro along the I80 corridor to 2 to 3 in the far south. The
synoptic models are still a couple of hours faster with the onset
over the southwest than the Hires models but overall the timing of
the event still resides in the overnight. Will be going with headlines
in the far south for the snow tonight mainly due to the late evening
nature and potential for some travel impacts early in the morning
upon near sunrise though most of the precipitation should be over
by then. Overnight shift may expire some of the western portion
earlier if conditions improve there. Lows tonight will drop into
the upper single digits north by morning and with clearing northwest
into central Iowa by sunrise tomorrow...lows northwest should
drop to about 0 to 5 below.

Sunday will see ample sunshine with southwest flow aloft ahead of a
stronger cold front. H850 temperatures moderate by afternoon from
the lower teens below zero at 12z up to -4C south/-9C north by 00z
Monday. Despite the snow cover the bright sun and southwest flow
will result in a decent day over the region. Though model output
varies considerably with the MET guidance about 15 degrees warmer
than the Euro and the GFS splitting the difference...we should rally
about 8 to 10 degrees above today in most cases. For most of the day
sunshine will prevail...though a few clouds may increase over the
northwest by 00z Monday.

.LONG TERM.../Sunday night through Saturday/
Issued at 312 PM CST Sat Feb 10 2018

Summary...Snow looks to be in the rear view mirror, at least for a
few days as a predominantly quiet and warmer period lies ahead.
Temperatures will warm predominantly into the mid 20s to mid 30s,
peaking a few degrees warmer Wednesday.

Sunday Night through Wednesday...Weak cold front will move through
as high pressure slides by north of the area, keeping temperatures
on the cold side of normal Monday in the teens to mid 20s. As high
pressure moves east, WAA returns by Mon night/Tue. Along with a vort
max sliding through the mid/upper flow, light precip may be possible
over the northwest/northern reaches of the state. Working against
precip formation is a sizable low-mid level dry layer that will take
time to saturate. GFS has backed off previous runs, tending to keep
the area dry now, but multiple model soundings indicate near-
saturation up to 550 mb or so. Would not take much to eek out light
precip/snow. Have held off upping any POPs with the only model that
depicted precip previously, GFS, trending drier. By Wed,
WAA/southerly flow will see a boost as surface pressure gradient
tightens between surface low tracking to the north and surface high
over the SE US. A number of guidance attempting to bring mid/upper
40s as far north as Highway 30, but extremely hard to believe at the
moment for a couple reasons. Main one being, moisture surge with
southerly flow over snow pack will likely lead to widespread areas
of fog over the southern third to half of Iowa, preventing any
tapping into the warm nose aloft. That and the fact that it is
extremely difficult to get much more than low 40s over an
established snow pack. Current highs possibly if not likely 5
degrees or more too warm from at least I-80 south, however minimally
touched at this point with minimal tangible impact on the forecast
and allowing for models to show consistency going forward a run or
two.

Thursday through Saturday...Guidance divergence grows at this point,
primarily struggling with the evolution of upper level flow and
surface reflections. Essentially impossible to put strong confidence
in one solution over another, electing to leave the blend
predominantly alone. Things to watch as forecast evolves include
Thur/Thur night and Sat, where Euro and Canadian respectively depict
snowfall chances while other guidance at those times do not. Overall
confidence in this window is low.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 606 PM CST Sat Feb 10 2018

A band of light to moderate snow will develop across southern Iowa
overnight, reducing visibility to IFR at OTM and perhaps DSM at
times, mainly between about 05Z and 10Z. Conditions will rapidly
improve as the snow ends on Sunday morning, with VFR returning by
around 12Z. Further north only VFR conditions are forecast
through the TAF period.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until 6 AM CST Sunday for IAZ075-083>086-
092>097.

&&

$$

SHORT TERM...Curtis
LONG TERM...REV
AVIATION...Lee



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