Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 160832
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
332 AM CDT Mon Apr 16 2018

.SHORT TERM.../Today through Tonight/
Issued at 332 AM CDT Mon Apr 16 2018

The region is still under the influence of broad cyclonic flow as
the storm system continued to depart.  Weak forcing still in place
over eastern Iowa as the last bit of vorticity wraps around the low.
This is producing some flurries across the eastern third of the
state which should exit in the next couple of hours.  Currently
patchy clearing is occurring across the west but mid clouds upstream
will still come down across the region making for partly to mostly
cloudy skies into the morning.

We will see a brief break from the very winter-like spring over the
Upper Midwest for the next 24 hours or so as weak upper level
ridging and a weak surface high slide over the state.  We will still
be in northwesterly flow so there is no expectation of warm
temperatures by any means but at least we will not see snow.  Highs
today will only be in the lower to mid 30s north to the mid 40s
south with lows tonight in the 20s.  These kind of temperatures are
some 15 to 20 degrees off their normals for this time of year.

Cloud cover will be somewhat problematic as well...especially over
the far east and northeast.  The western half of the state should
clear off by mid to late morning and be mostly sunny for the
remainder of the day.  Over the far east and northeast there will
still be some cyclonic influence from the departing low and the
cloud shield upstream is so expansive, we will likely see mostly
cloudy skies for much of the day.  This would negatively impact
highs for that area.  I have taken this into account somewhat but
temps will need to be watched as cloud trends change.

By tonight, mostly clear skies are expected which will contribute to
the cold overnight lows forecast.  Winds across the western forecast
area will switch around to the east but this will have little impact
on lows.

.LONG TERM.../Tuesday through Sunday/
Issued at 332 AM CDT Mon Apr 16 2018

/Tuesday through Thursday/...Confidence Medium

Major storm to affect the area again from Tuesday night into
Wednesday evening.  Tuesday will remain rather pleasant with high
pressure pulling east of the region as low pressure quickly begins
to deepen over the Rockies/Kansas by Tuesday evening.  Low pressure
is expected to rapidly deepen Tuesday evening over eastern Kansas
and then quickly track into south central Iowa by 12z Wednesday
spreading an area of heavy precipitation across northwest and north
central Iowa once again.  Initially models indicate that a lack of
ice introduction aloft will result in a mixture of precipitation
northwest/north central with some freezing rain and potential sleet
from 06z to 15z in a narrow band from northwest to northeast areas
during that time.  Rapid dynamic cooling will occur with the strong
negatively tilted upper level low around 12z northwest and a rapid
changeover to heavy snow is expected generally north of Highway 3
spreading east during the morning hours.  Just south of that "area"
will be a narrow band of freezing rain from northern Sac east to
near Grundy and Bremer/Black Hawk Counties.  Here less snow will
fall with up to a tenth of an inch of ice in some cases.  At this
time the amount of qpf expected over the north is a conservative
0.65 to nearly an inch of water across the north. The resultant snow
accumulations are expected to be near or in excess of 6 inches north
with locally higher amounts possible.  This depends on storm
track...timing of ptype changeover to snow and overall qpf.  Though
some differences continue between the GFS and Euro...both models are
in agreement with dynamics and available moisture.  Overall the Euro
is preferred over the GFS with a slightly more southern placement of
the qpf max across the north. None the less...enough confidence
exists to issue a Winter Storm Watch for the northern areas late
Tuesday night into Wednesday evening. Farther south through Central
Iowa a rain or rain/snow mix is expected with little if any snowfall
accumulation based on this model run package. The peak snowfall
period will occur from 12 to 18z Wednesday over the north with
snowfall rates of nearly an inch per hour in the heavier snowfall
area. Winds are expected to pick up in the afternoon behind the low
as the system begins to move east and continue into the evening
hours.  This will likely result in hazardous travel well into the
evening over the more affected areas of northern Iowa. Unfortunately
this is coming on the heels of yesterdays event which has already
impacted the region with heavy snow.  Some adjustments to the
snowfall and ice amounts may be necessary in the next 24 hours. A
sharp cutoff from higher to lower snowfall amounts is expected with
this storm and that poses some uncertainty on the exact amounts and
placement. Once the storm exits early Thursday some sunshine and
quieter conditions are expected. The deep snowpack north will hold
highs in the 40s or colder while the southern areas may enjoy some
warmer April sunshine and warm into the 50s.

/Thursday Night through Sunday/...Confidence Medium to High

Little significant weather is anticipated this period.  The large
storm that earlier was expected to reach Iowa is now tracking
farther south.  Both the GFS/Euro suggest that a glancing shot of
light precipitation may impact portions of southern and central Iowa
with a little light rain/snow Saturday.  Highs thorugh the period
will gradually warm with 40s/50s more common.  The snowpack north
will limit, somewhat, the warming trend there.  Also - warming
temperatures mean snow melt and possible additional water entering
streams and main stem rivers.  This will have impacts on river
levels and later monitoring of river levels is needed for possible
flooding potential sometime late in the week or next weekend.

&&

.AVIATION.../For the 06Z TAFS through 06Z Monday night/
Issued at 1210 AM CDT Mon Apr 16 2018

Clouds will persist across Iowa overnight but confidence in
whether individual site ceilings will be MVFR or VFR is medium at
best as these conditions are still quite variable in time and
space. Occasional flurries east should end shortly. Clouds should
persist in the broad cyclonic low level flow into today as well,
but confidence between VFR or MVFR conditions will stay no better
than medium east. There is better confidence in VFR and clearing
from the loss of heating into Monday evening.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Winter Storm Watch from late Tuesday night through late
Wednesday night for IAZ004>007-015>017-023>027.

&&

$$

SHORT TERM...FAB
LONG TERM...REV
AVIATION...Small


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