Area Forecast Discussion
Issued by NWS Des Moines, IA

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000
FXUS63 KDMX 170554
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
1154 PM CST Mon Jan 16 2017

.UPDATE...
Issued at 403 PM CST Mon Jan 16 2017

Areas of dense fog have developed rapidly across southern Iowa
with visibilities crashing in the last half hour or so. The dense
fog will likely persist until the low pressure center moves away
later tonight. A dense fog advisory has been issued accordingly,
for areas south of the winter weather advisory, through midnight.

&&

.SHORT TERM.../Tonight through Tuesday/
Issued at 330 PM CST Mon Jan 16 2017

The primary concern will be the exit strategy as this storm finally
pulls away. The potent short wave/PV anomaly over eastern KS/NE late
this afternoon and associated surface low just downstream will both
track across northern MO reaching IL by 06z. Deformation and DPVA,
and frontogenetical forcing and warm advection to a lesser extent,
continue to drive a lobe of rain and freezing rain northward through
northern IA and will be the last of any significant ice
accumulations. The latest reports suggest a quarter to third of an
inch widespread from Highway 20 south to the MO border with icing
just below those values north by the time this precip ends. Although
the moisture depth of the deformation zone will persist through the
evening, much of the lift and deeper moisture will have exited the
forecast area by later this evening leaving behind mainly
intermittent drizzle or freezing drizzle depending on surface temps
and dew points. There could also be some very minor snow and/or
sleet accums north where the deeper moisture and ice introduction
lasts longer. Light rain showers may also affect southeast IA this
evening associated with the lower static stability and kinematic
forcing of the upper low. This should be nothing more than a cold
rain however with no additional icing.

Thus with the event winding down and the surface wet bulb freezing
line now just north of Highway 20 have downgraded the Ice Storm
Warning to Winter Weather Advisory along and north of an Audubon-Des
Moines-Tama line where some icing remains and additional drizzle or
freezing drizzle is possible. Farther south road conditions have
returned to normal with little if any additional ice accumulations
anticipated. There will be some re-freeze potential overnight, but
with plenty of low level moisture and low cloudiness this will be
very slow to occur.

The precipitation will be over by 12z but low level moisture and
stratus will remain in place through the broad cyclonic flow behind
the system keeping skies mainly cloudy. With little sun and neutral
to cold advection, temps should rebound little into Tuesday with MOS
likely overzealous, especially the warmer NAM, and highs closer to
raw model values. The only sunshine may occur northwest by late
afternoon.

.LONG TERM.../Tuesday night through Monday/
Issued at 330 PM CST Mon Jan 16 2017

From Tuesday night through Wednesday night weak ridging will allow
for moderating temperatures, with highs on Wednesday reaching the
mid-40`s across southern Iowa. With the rain that fell today
working its way through the basins and thawing ice in the warmer
temperatures, there may be some minor flooding and even ice jams
later in the week. We will be monitoring this potential closely.

By late Wednesday a somewhat unusual 500 mb pattern will set up
with a large cutoff low pressure center blocked and nearly
stationary over the southern high plains and poking up toward
Iowa. Meanwhile an energetic longwave trough will be coming ashore
over the Pacific U.S. coast. This trough will progress eastward
and interact with the cutoff low ahead of it. The trough will also
be followed by an even stronger gyre setting up just off the
Washington coast and carving out deep cyclonic flow across the
western U.S. by late in the week. With several lobes of energy
ejecting out of this trough and interacting with the waves
preceding them, long range prognostic models are having difficulty
resolving the details of these interactions and resulting sensible
weather across our forecast area. However, with flow remaining
generally southwesterly aloft the mild temperatures should persist
through at least the end of the week. We will also likely see
fairly persistent cloud cover from around Friday through the
remainder of the forecast period along with intermittent
precipitation chances.

Most of the systems producing precipitation near Iowa will
probably just glance off our area and provide spotty and light
QPF overall through Saturday. There has been a somewhat consistent
signal for the strongest mid-level low pressure system moving to
our south and southeast around Sunday or Monday, leading to higher
precipitation chances and the possibility of more significant
accumulation. At this range thermal profiles are unreliable, but
given the expected temperatures it would appear that most
precipitation will fall as rain during the long term forecast
period, possibly mixing with snow at times especially toward the
end of the Sunday/Monday system.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1152 PM CST Mon Jan 16 2017

Messy aviation fcst as LIFR and IFR conditions run prevalent
throughout the TAF period. Challenge will be when to lift vsbys
to IFR. Went with TEMPO groups as cigs should hover near 500ft for
many sites. Tuesday challenge will when sites lift to MVFR during
the afternoon period and if they will reach VFR, or if HZ
conditions will prevail overnight from moisture from pvs system.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 6 AM CST Tuesday for IAZ038-039-049-050-
061-062-074-075-085-086.

Winter Weather Advisory until midnight CST tonight for IAZ015-
023-024-033>037-044-045.

Winter Weather Advisory until 3 AM CST Tuesday for IAZ004>007-
016-017-025>028.

Dense Fog Advisory until midnight CST tonight for IAZ060-073-084-
096-097.

&&

$$

UPDATE...Lee
SHORT TERM...Small
LONG TERM...Lee
AVIATION...Kotenberg


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