Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182118

National Weather Service Des Moines IA
318 PM CST Wed Jan 18 2017

.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 some fog may
linger in the morning hours. As 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 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 18Z TAFS through 18Z Thursday afternoon/
Issued at 1133 AM CST Wed Jan 18 2017

The forecast area is split by an extreme gradient in conditions at
18z with VFR/clear skies west and a quick drop to LIFR fog and
stratus east. The fog and stratus should gradually improve to MVFR
into early this evening, but with persistent weak southerly flow
over the cold, wet ground in IA expect a return to IFR/LIFR to
blossom across much of central IA into the night with only slow
recovery in the morning.




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