Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 130455
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1155 PM CDT Thu Apr 12 2018

...Updated Aviation Discussion...

.SHORT TERM.../Tonight through Friday Evening/
Issued at 420 PM CDT Thu Apr 12 2018

Surface front which passed through overnight has stalled near the
Iowa/Missouri border this afternoon. Pressure falls are now
evident behind the boundary across Nebraska into Iowa as warm
advection begins to intensify. This will continue to strengthen
into the evening across the state with strong theta-e advection
moving northward. Models continue to indicate some development of
showers by late evening in central into northern Iowa in this area
of moisture advection well north of the surface front. As the
moisture surges north, elevated instability will also spread
across the state with some isolated thunder possible. By later
tonight, a shortwave and associated lift will arrive from Nebraska
which should act to enhance the convection, mainly after 2-3 AM.
Sufficient instability will be available for some hail production
with the elevated convection and a few severe storms will be
possible far north. Farther south, lower clouds will overspread
the southern portions of the state with a shower or two and
eventually some drizzle toward daybreak.

Friday will see the warm front linger near the I80/Highway 92
corridor for much of the day.  North of this front, scattered
showers and a few thunderstorms will persist for much of the day.
A very raw/brisk east/northeast wind is expected north of the
front along with low cloudiness and temperatures mainly in the
40s. In the warm sector across the south, much warmer air and
dewpoints approaching 60 degrees will be common. This will MUCapes
of 1500- 2500 j/kg by afternoon in the far south although with
large sweeping hodographs. Initial convection into the afternoon
will likely be north of the warm front where some elevated hailers
are expected. However, the primary surface based convection is
forecast to develop near the Missouri River around 21Z and race to
the east northeast at 50-60kts. Those storms that form near the
dry trof are likely to be supercellular in nature and a few
tornadoes will be possible. This prompted the expansion of the
enhanced outlook into the area for DY2 via SPC. However, the
farther east the convection gets, the less likely for tornadoes as
storms become more of a wind/hail threat. Cold air will continue
to move into the north and may begin to see some transition near
KEST in the evening, especially with the strong synoptic cooling
and enhanced precipitation rates. Rest of the area is expected to
remain liquid through 06Z Saturday.

.LONG TERM.../Friday Evening through Thursday/
Issued at 420 PM CDT Thu Apr 12 2018

Bottom Line up Front...
Potentially high-impact Saturday-Sunday time period, with
blizzard conditions possible in northern to northwestern Iowa.
Main impacts will be driven by high winds and snowfall rates, as
opposed to pure snowfall amounts.

Setup...
At 06z Sat, our sfc low should be somewhere near northeastern
Kansas to northwestern Missouri, with the parent upper low
oriented westward with height over the central Nebraska/Kansas
border. In terms of tracking this system, the ECMWF has generally
been a faster, weaker outlier... whereas the GFS has been slower
and deeper/more closed with the 990mb or so sfc low. So am
thinking a 12z consensus, with a lean towards the GFS captures the
progression of this system. With lingering
instability/convection, near the front, have held spotty thunder
through 12z Sat. North of this system, towards the Hudson Bay,
there will be a 1040mb low propagating eastward...setting the
stage for a strong pressure gradient across the Upper Midwest.

High Winds...
High wind potential looking very likely, especially along and
north of Highway 20. Run-to-run consistency has been very high,
yielding high confidence in this wind forecast. Pressure gradient
has consistently been around 3mb/40 km, with 3-hr pressure rises
of 3 to 4mb/3 hr to help remove inhibition to mixing. In northern
Iowa, sustained northeasterly winds of 25 to 35 mph, with gusts
of 40 to 50 mph looks likely. With a decent easterly component to
the winds, hazardous travel impacts could be amplified via
crosswinds for high- profile vehicles and semis. Winds will
decrease south of I-80 as the pressure gradient is far lower due
to proximity of the sfc low.

Wintry Precipitation...
Isentropic lift remains high and persistent through all of
Saturday, with strong ascent through the low-levels of the
atmosphere driven by decent frontogenetical forcing. 850mb
moisture flux convergence is strong across Iowa and wanes
throughout the daytime hours... suggesting precip intensity should
gradually diminish throughout Saturday.

The high winds, combined with snowfall intensity will be the main
drivers for potential blizzard conditions. Snowfall rates upwards of
2 inches per hour cannot be ruled out Saturday morning. With the
forecast winds, this would lead to visibility less than 1/4 of a
mile and approaching zero. Also worth mentioning thundersnow
possibility.

Sunday and Beyond...
By 12z Sunday, the low and associated precip should be east of
the DMX CWA. Road temperatures should quickly warm into the 40s
across much of Iowa, which will help melt snowfall off the roads.
Winds will likely stay decently strong, in the 15 to 25 mph range
from the north, so there may be minor travel impacts along west-
east oriented roadways.

Long-range models have been fairly consistent in dropping an upper
level low through the upper Midwest sometime on Wednesday. With
height falls dipping below 540dam, a wintry mix of precip may
accompany this system if it makes it into Iowa during the overnight
period. Highs on Wednesday should be well into the 40s and possibly
50s, so no lingering winter impacts expected. Another system may
impact Iowa late next week into next-weekend.

&&

.AVIATION.../For the 06Z TAFS through 06Z Friday night/
Issued at 1143 PM CDT Thu Apr 12 2018

Continued to delay the onset of MVFR to IFR ceilings until Friday
morning. Hires models keep northern Iowa in the low stratus deck
and shower/drizzle for much of the day. Should be a break at DSM
and OTM as the warm front nudges northward throughout the day
Friday. Introduced the strong winds by tomorrow night across the
forecast area and confident to at least see MVFR ceilings at OTM
and DSM, while the northern TAF sites are likely to remain in the
IFR/LIFR conditions for ceilings.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Kotenberg
AVIATION...Podrazik



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