Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 190333
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
933 PM CST Wed Jan 18 2017

.UPDATE...
Issued at 933 PM CST Wed Jan 18 2017

A small area of low stratus clouds and fog lingering in far south
central Iowa at sunset has expanded northward this evening, with
visibilities falling to 1/4SM or lower at times in the fog. There
is some question as to whether the southern edge of the
fog/stratus bank, which lies just south of the IA/MO border at
this time, will advance northward overnight. However, satellite
imagery shows that this edge has been stalled for the last few
hours and a surface ridge is moving in from Missouri, which should
lighten low-level winds even further. Seeing little opportunity
for improvement and the rapid expansion of low visibilities in the
last hour or two, have issued a dense fog advisory through mid-
morning Thursday. Additional expansion of the advisory is very
possible overnight depending on observation and satellite trends.

&&

.SHORT TERM.../Tonight through Thursday/
Issued at 318 PM CST Wed Jan 18 2017

Several elements of concern into Thursday ranging from fog tonight
into increasing PoPs east later tomorrow. The current weather has IA
in fairly weak flow ahead of the KS upper low with neutral to weak
forcing in minimal warm advection. The near term concern will be
lingering low level moisture over our cold, wet ground with steady
weak southerly flow and little change of airmass through the night.
20Z temp/dewpoint depressions were <9F across central IA and often
<5F so it likely will not take much for a hybrid radiation/advection
fog event tonight with skies now mainly clear. Have added fog
wording to most locations tonight and although surrounding office
consensus was not there for a headline as of yet, would not be
surprised to see dense fog occur somewhere across the forecast area
much like last night. The concern will be whether the southerly low
level flow and associated mixing will be strong enough to preclude
widespread 1/4sm visibilities from forming.

The fog and especially stratus should be slow to dissipate into
Thursday with the attention then turning to precip chances. One of
the weak waves embedded within the deep moisture fetch that brought
heavy rains to TX overnight will be wrapped northward into the MS
Valley as the KS upper low oozes eastward. Although there will be
little baroclinicity, increasing moisture and weak kinematic forcing
in lowering static stabilities may bring showers to eastern sections
and possibly some thunder during the afternoon. The chances of
clearing are much less with the increasing moisture tomorrow so
highs will likely be below persistence.

.LONG TERM.../Thursday night through Wednesday/
Issued at 318 PM CST Wed Jan 18 2017

Thursday Night through Monday...Confidence Medium

Main concerns through the period will initially be several weak
short waves into the weekend with a stronger system expected to
impact the region by Tuesday into Wednesday. With the prospect of
more storms than breaks between them it is likely that clouds will
be favored through the period over bright sunny days. With that in
mind there will be some lowered confidence on afternoon high
temperatures...though diurnal spreads in temperatures will be
lessened at the same time. Models in general agreement Thursday
evening with regard to best lift over the area...which should remain
east of I35. With that light rain and the potential for fog
development overnight across the region once again.  Winds will
remain in the 5 to 10 mph range which will assist in some fog
development along with higher dewpoints through the day...and
eventual rainfall.  With the H500 low lifting north Friday morning
little change in airmass is expected on Friday...so some fog may
linger in the morning hours. As well...cloud cover will be tough to
break as another wave approaches from the southwest into Friday
night/Saturday. High temperatures will once again be hindered by
general lack of sunshine...though continued warm air advection will
keep temperatures on the mild side for Saturday.  GFS guidance has
risen the high to the 40s to mid 50s in the south...but considering
the potential for lingering clouds will shade toward the slightly
cooler Euro guidance over the south half.  As Friday nights system
pulls north...some lingering light rain/drizzle may remain in
northern Iowa Saturday afternoon. Late Saturday night a trough
should push east through the area with slightly cooler and drier air
moving into the region Sunday. Little sensible weather is anticipated
into Monday as a narrow ribbon of high pressure works east into
the region.

Tuesday and Wednesday...Confidence Low

Currently the medium range models diverge by Tuesday with little if
any agreement in features due to timing and evolution of H500 upper
level features.  The Euro and GEM both suggest a weaker less
amplified long wave trough while the GFS is deepening a stronger
storm over the Central US.  Obviously this has a significant impact
on the forecast and for now...have leaned toward the stronger GFS
for now.  Temperatures should remain mild next week...at least in
the 30s to 40s as a zonal flow is expected to continue.  If the GFS
is more correct...then colder air should briefly follow.  Overall
however...there is little in the way of extreme cold yet rotating
into Canada next week which should keep our area milder for now.

&&

.AVIATION.../For the 00Z TAFS through 00Z Thursday evening/
Issued at 552 PM CST Wed Jan 18 2017

VFR conditions currently prevail across most of the area. However,
patches of IFR stratus and BR persist over northeast and south
central Iowa. As a surface ridge moves up from Missouri overnight,
stratus and fog should redevelop and expand over the terminals
roughly from southeast to northwest, especially after midnight,
then largely persist through the day on Thursday. This will result
in widespread IFR or lower conditions by early Thursday morning
and into the afternoon. 00Z TAFs have made best attempt at timing
deterioration of ceilings and visibility, but adjustments and
amendments are anticipated. Some light showers are also possible
on Thursday afternoon, but confidence at this range is too low to
include in the TAFs yet.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM CST Thursday for IAZ036>038-
046>050-058>062-070>075-081>086-092>097.

&&

$$

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



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