Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190942
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
342 AM CST Thu Jan 19 2017

.SHORT TERM.../Today through Tonight/
Issued at 343 AM CST Thu Jan 19 2017

06Z Water vapor imagery picking up on well-defined upper level low
parked over the plains. This low will be main driver for continued
fog/haze potential and light rain potential across the DMX CWA.

Fog potential...
Confidence: Medium

Dense fog ongoing across much of central to eastern portion of the
CWA. Somewhat classic conditions for advection fog present as fcst
soundings show decoupled boundary layer and low to moderate shear.
With IA northeast of our upper low, warm air advection clearly
present...which will likely help maintain fog/haze into this
afternoon. Fog will be fighting radiative process during the
afternoon, but soundings in solid agreement with keeping low ST in
fcst at a minimum. As a result, have begun to nudge temps down vs
guidance.

It appears to be very plausible for fog to fill in once again
tonight as the upper low tracks across western Iowa. Similar
processes in play tonight as this morning. Comparatively, it seems
differential heating/WAA and dynamic processes not as strong tonight
as this morning, so cannot guarantee need for headlines at this
time...although certainly cannot rule out either.

Light rain potential...
Confidence: Medium-High

For starters, warm layer temps reach +7C across all of CWA,
eliminating wintry precip potential and keeping all precip in the
form of rain. Low-level moisture/thermal advection may support
showers primarily across eastern Iowa during the day today. Weak mid-
level q-vector convergence and kinematic forcing on eastern fringes
of low should provide barely sufficient support for precip. Overall,
setup not very impressive and think QPF may be overdone.

.LONG TERM.../Friday through Wednesday/
Issued at 343 AM CST Thu Jan 19 2017

Broad western trof will be in place for much of the forecast
period with the mean trof axis shifting into the central United
States by the end of the period. The initial upper low will be
lifting out of the state on Friday morning but will be follow
quickly by the next shortwave ejecting out of the southwest United
States. Forcing associated with this will produce some light rain
by late Friday in southwest Iowa. As this system moves north on
Friday night into Saturday morning, saturation will deepen across
the state with light rain becoming likely in northern Iowa
overnight. However, amounts will remain light as system lifts
north into Minnesota on Saturday.

There will be a significant lull in the threat of precipitation
from midday Saturday into early next week with subsequent
shortwaves remaining south of Iowa. The airmass over the state
will not be particularly cold for January and temperatures will
remain above normal into early next week. Both GFS and Euro are
indicating a decent shortwave approaching the state on Tuesday.
Strong warm advection and increasing moisture combined with good
isentropic lift should be sufficient for precipitation to
overspread the state on Tuesday and continue into Tuesday night as
strong mid level dynamics cross Iowa. Likely to see snow in the
north with rain or snow farther south. Colder temperatures will be
in store by the end of the period with increasing cold advection
on backside of the departing system on Wednesday.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1128 PM CST Wed Jan 18 2017

LIFR and VLIFR conditions are spreading across the area and will
prevail for most of the night. There is some question as to
whether the southern edge of the fog/stratus field, which remains
anchored just south of the IA/MO border, will progress northward
overnight or not. Satellite trends and climatology indicate that
it will not and have trended that way in the TAFs. During the day
on Thursday there will only be modest improvement as a surface
ridge builds across the area and low-level moisture continues to
increase and become trapped beneath an inversion. As a result, low
MVFR or IFR conditions will likely persist for much of the day.
Ceilings and visibilities will then lower again after sunset
Thursday night.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CST this morning for IAZ017-
025>028-035>039-046>050-058>062-071>075-082>086-093>097.

&&

$$

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



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