Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 222138
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
338 PM CST Thu Feb 22 2018

.SHORT TERM.../Tonight through Friday/
Issued at 334 PM CST Thu Feb 22 2018

The primary concern will obviously be the evolution of the current
system. Varied wintry precip types have been ongoing across central
and southern Iowa today with mainly freezing drizzle south now and a
light snow, sleet, freezing rain/drizzle mix central. Farther north
little in the way of precip has fallen due to dry low levels, but
they are starting to saturate. Even though air temps are still at or
below freezing in most areas, freezing rain impacts have not been as
significant as anticipated with subtle SE-NW temp and moisture
advection, late Feb insolation and road temps now in the middle 30s
in many locations. The road impacts where IDOT conditions note
partial coverage parallel the sub-freezing road temps from NW Des
Moines metro farther NW. This is close to a burst of heavier snow
noted on radar from Carroll toward Fort Dodge, Clarion and Webster
City.

Looking ahead into tonight the forecast confidence remains medium at
best. Forcing is expected to expand through the MO Valley ahead of
the approaching NM/CO short wave/PV anomaly. This will have phased
thermodynamic and kinematic contributions with weak convective
elements possible as well due to low static stabilities. Thus expect
upstream precip to expand over the next several hours which is also
supported by hi res model simulated reflectivities. Part of the low
confidence will be due to aforementioned icing questions with
borderline temps, but also precip type. Soundings and upstream GOES
cloud phase products suggest ice intro may be limited due to the
approaching KS/NE dry wedge which would result in mainly moisture
below -12C and liquid precip dependent on surface temps. However
convective/showery elements may offset that somewhat with bursts of
snow or sleet possible as well. Thus have knocked back ice accums
somewhat with better snow chances north and low confidence accums of
one to two inches. Farther south ice accums < 0.10 are anticipated,
mainly on elevated surfaces, and mainly along and NW of a Lamoni, to
Newton, to Waterloo line as temps/dewpoints SE of that do not
support much in the way of additional ice accums. Do not expect road
temps to change much into the evening with plenty of clouds and
roads already wet. Thus have cancelled the advisory SE. All of the
lift should quickly exit by daybreak leading into seasonal
temperatures Friday and fair conditions.

.LONG TERM.../Friday night through Thursday/
Issued at 334 PM CST Thu Feb 22 2018

The primary forecast challenge for the period arrives Saturday and
Saturday night as the vigorous upper level system currently over the
Pacific Northwest moves towards the region. This system will
fragment into two segments and move east as an open wave.  One
fragment will move well north through Montana and North Dakota with
the southern fragment moving across Iowa. The GFS remains most
aggressive with the southern fragment closing off more quickly over
the state with a much more negative tilt and more mature deformation
zone as it moves through the state. Currently prefer the more open
wave solutions of the EC/GEM which transition to a closed low to the
north of the state, thus limiting the residence time of the system
and limiting any wrap around warm conveyor moisture and deformation
precipitation. That said, the combination of kinematic and
thermodynamic forcing will generate good vertical ascent and promote
higher precipitation rates and possibly some convective elements.
The precipitation will begin as a mix of freezing rain or snow over
southern Iowa as a warm nose lifts into the state. The precipitation
will eventually orient as mostly snow northwest with a wintry mix
possible near the surface low track and rain to the southeast. Have
dropped Saturday`s high temperatures due to saturated low levels
with readings not straying more than 1-3 degrees from dew points and
have eliminated the MOS Guidance warm bias in such situations.

The wind will increase during the period and should become gusty
from the west northwest by Saturday night. This would create some
blowing snow issues over the northwest. Despite the favored faster
progression of the system, still have a 4 to 8 inch swath of snow
mainly along and north of a Mason City to Fort Dodge to Carroll line
due to expected higher rates. Some light freezing rain may occur
along the southern fringe of this line. Have added a winter storm
watch for Saturday and Saturday night for this region to include
areas with greater than 4 inches forecast along with the wind
potential for blowing snow.  The precipitation should be ending by
midnight for most locations followed by high pressure ridging by
Sunday morning.

The upper level flow will transition to more zonal by Monday then
become southwesterly Tuesday night into Wednesday.  Do expect
precipitation mid week, however exact track of the system will
determine precipitation type due to warmer air lifting north. At
this time best chances for snow north and a transition to rain south.
High temperatures both Tuesday and Wednesday should be in the 40s
and 50s central and south.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1210 PM CST Thu Feb 22 2018

IFR/MVFR conditions due to a combination of fog and wintry precip are
in place over the southern half of Iowa and will continue
spreading northward through the afternoon. Ceilings and
visibilities are expected to lower further into the evening with
more widespread IFR and possibly LIFR conditions as the mixed
wintry precip continues. The precipitation should exit late in
the period with winds becoming more W or NW and a return to VFR.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory until midnight CST tonight for
IAZ004>007-015>017-023>028-033>039-044>049-057>061-070>073-
081>083-092>094.

Winter Storm Watch from Saturday morning through late Saturday
night for IAZ004>007-015>017-023>025-033>035-044-045.

&&

$$

SHORT TERM...Small
LONG TERM...Donavon
AVIATION...Small



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