Area Forecast Discussion
Issued by NWS Des Moines, IA

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195
FXUS63 KDMX 191140
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
540 AM CST Mon Feb 19 2018

.SHORT TERM.../Today through Tuesday/
Issued at 324 AM CST Mon Feb 19 2018

Very active period with broad southwest flow aloft and frontal
boundary positioned across the state. Front has progressed to
Mason City to Atlantic to Nebraska City early this morning with
temperatures now around 20 in northwest Iowa. South of the front,
temperatures remain mostly in the 40s with dewpoints climbing as
moisture returns. Returns are increasing on radar along with
precipitation reports from automated sites in western Iowa. Will
continue to see the expansion of precipitation with strong warm
and moisture advection over the next few hours ahead of an
approaching shortwave. Anticipate deteriorating conditions in
advisory area with expansion of freezing drizzle and light
freezing rain. Meanwhile, showers and even a few storms remain
likely along and east of the boundary this morning as weak
instability moves into the state.

Forcing is expected to wane in the west by late morning and expand
east northeast through the afternoon. Therefore, much of the
accumulating precipitation is expected to end by this afternoon
over much of the forecast area. There will be some remaining
patchy freezing drizzle behind the boundary with the threat of
scattered showers and a few storms near the front into the
afternoon. Icing amounts across the north today will likely be
around 0.10" with some amounts near 0.15".

A renewed push of warm advection and moisture surge will occur
tonight across much of Iowa ahead of the next shortwave. Much of
the forecast area will see freezing drizzle/freezing rain with an
impressive warm layer above shallow polar airmass. If anything,
this push is a bit slower than initially thought with the bulk of
precipitation falling from 06Z-12Z across the forecast area. There
will be a tight gradient on the east side between fzra/ra near a
Tama-Knoxville-Lamoni line. Just west of this line, amounts of
0.20" are expected and will impinge upon 0.25" by the end of the
event and will have to be cognizant of warning potential. For now,
have gone with an advisory for much of the area tonight into
Tuesday morning. Forcing will be subsiding quickly on Tuesday with
the passage of the shortwave and precipitation will be confined to
the far southeast by midday as drier air enters the state.

.LONG TERM.../Tuesday Night through Sunday/
Issued at 324 AM CST Mon Feb 19 2018

The early week winter storm will move away on Tuesday, yielding a
brief quiet but cool period from Tuesday night into Wednesday night
as a high pressure area moves across the region. However, a broad
500 mb trough will remain anchored across the western U.S. keeping
Iowa beneath southwesterly steering flow aloft. By Wednesday night
the next storm system will approach through the flow, in the form of
a compact shortwave generating a reflective surface cyclone that
will pass through the region Thursday and Thursday night. This will
result in precipitation in or near our forecast area, but
precipitation type is very much in question due to dry air layers in
the column. Forecast soundings have shown a high degree of
variability with respect to saturation depth and degree of ice
crystal introduction but it seems likely we will see some
combination of rain, snow, and freezing rain/drizzle within our
forecast area. The details of timing and location with this
precipitation is very uncertain at this time but we will be
monitoring it over the next several days. Later in the week we will
see a repeat of this pattern with another brief break around
Friday/Friday night followed by yet another modest storm system
around Saturday as much of the energy in the western U.S. trough
finally kicks out and crosses the Midwest. This will likely yield
more precipitation around Saturday, followed by somewhat quieter
weather toward the latter part of the weekend and into early next
week. In terms of temperature the southwesterly flow will prohibit
any large temperature swings from Thursday onward, with highs
generally in the 30s and 40s and lows in the teens and 20s. Thus the
time of day will also play a large role in precipitation types with
both the Thursday and Saturday systems.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 540 AM CST Mon Feb 19 2018

A large area of very low stratus will envelop the area for most or
all of the TAF period, with prevailing IFR or lower conditions. It
is possible DSM and OTM could come up just into MVFR for a few
hours this afternoon, but otherwise all terminals will be in the
tank. Light rain/freezing rain will also affect the area this
morning, then again tonight, with light ice accumulations at
times. Wind direction will be variable with a frontal boundary
moving slowly across the area, but winds will be less than 12
knots for the most part.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...

Winter Weather Advisory from 9 PM this evening to noon CST
Tuesday for IAZ027-028-037>039-048>050-059>062-070>074-081>084-
092>094.

Winter Weather Advisory until noon CST Tuesday for IAZ004>007-
015>017-023>026-033>036-044>047-057-058.

&&

$$

SHORT TERM...Cogil
LONG TERM...Lee
AVIATION...Lee



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