Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 191747
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1147 AM CST Mon Feb 19 2018

.UPDATE...
Issued at 845 AM CST Mon Feb 19 2018

Updated forecast to reflect areas of fog over portions of
southwest and eastern Iowa. Fog tied to lighter winds and low
pressure near Des Moines at this time...with moisture convergence
along the frontal boundary and trough sitting across the region.
The low will slowly move northeast this morning resulting in a
shift of the thicker fog to near the Highway 20/30 corridor mainly
east of I35. /rev

&&

.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
t500 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 t
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 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1147 AM CST Mon Feb 19 2018

FZDZ expected to continue across the north with some dz/rain
showers in the east/south this afternoon. VSBYS and CIGS very low
in the LIFR to IFR categories for much of the period. Some lighter
precip expected into the evening, if not even a small dry period
with widespread freezing precipitation expected late tonight into
early Tuesday AM. KOTM will remain above freezing with mainly rain
showers expected. Some instability aloft could result in thunder
at nearly all sites, but uncertainty too much to mention at this
time.

&&

.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.

&&

$$

UPDATE...REV
SHORT TERM...Cogil
LONG TERM...Lee
AVIATION...Beerends



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