Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212359
AFDDMX

AREA FORECAST DISCUSSION
National Weather Service Des Moines IA
559 PM CST Sat Jan 21 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 343 PM CST Sat Jan 21 2017

The main concern will be fog and stratus trends yet again tonight.
Any forcing with the short wave that brought scattered showers to
northern IA earlier today has exited and any lift associated with
the NM/TX wave should remain to our south through the period. The
deeper moisture may just brush the far southeast corner around
12z, but doesn`t appear worthy of PoPs at this point.

Fog wise, restricted visibilities linger along and northeast of an
Estherville to Cedar Rapids line and this will be the area of
highest confidence for significant fog restrictions once again so
have issued another Dense Fog Advisory for these locations through
Noon. Fog is likely farther south and west as well with
temp/dewpoint depressions still quite low heading toward sunset,
and an expansion could very well be needed into the night. The
winds will be become light into early Sunday morning until the NW
low level flow gradually strengthens which should improve
conditions by afternoon.

.LONG TERM.../Sunday night through Saturday/
Issued at 343 PM CST Sat Jan 21 2017

Increasing probability for snow Tuesday night into Wednesday was
the main forecast concern over the extended period. Leaned
slightly closer to the ECMWF as it remains consistent with its
respective solution, even though the GFS is in fair agreement
with the ECMWF.

Sunday night into Monday...Weak upper level ridge builds into the
region Monday while a surface low begins to develop over the
central Rockies during this time frame. Cloud cover will remain
across much of the forecast area Monday as mid to upper level
moisture persists.

Tuesday into Wednesday...The aforementioned surface low continues
to advance eastward throughout the day Tuesday with an upper
level trough strengthening and digging into Kansas and Missouri
Tuesday night into Wednesday. A fairly potent 500mb vort max
punches into southern Iowa by around 00z Wednesday with a stout
upper level jet placing the state within the left exit region. The
surface low looks to be centered over southern Iowa into northern
Missouri by late Tuesday night placing much of the forecast area
within the CAA. A decent deformation zone develops on the backside
of the surface low in west-central to northern Iowa by around 06z
Wednesday. The ECMWF and NAEFS are in good agreement with the
slower timing with the GFS roughly 6 hours faster with moving the
surface low east. Increased pops across the north Tuesday night
into Wednesday.  The deep moisture returns by the late afternoon
Tuesday into Tuesday evening with the strongest forcing within the
dendritic layer during this time as well. Looks to be a break in
the precipitation across the west-southwest by around 06z
Wednesday before the CAA develops towards Wednesday morning. For
now, appears moderate snow amounts across the northern portions of
forecast area are likely with lighter accumulations further
south as the transition from rain to snow doesn`t occur until
past 06z. Thus, the south to southeast will remain mostly rain,
with a rain/snow mix towards the end of the storm Wednesday
morning. The other concern is blowing snow, especially from 12-18z
Wednesday when additional light snow is anticipated. Increased
winds Wednesday as winds atop the mixed layer are near 30 knots.
The deep saturation moves east by 18z Wednesday and thus the bulk
of any snow accumulation will be from 03z to 15z Wednesday.

Thursday and Friday...cooler temperatures are expected late in the
week with a weak clipper system punching across the forecast area
Thursday. This looks to provide a reinforcing shot of cold air
plus some flurries and snow showers possible over northern Iowa.
May need to extend pops further south if ECMWF/GFS continue this
trend.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 559 PM CST Sat Jan 21 2017

MVFR cigs/vsbys expected again tonight with IFR conditions across
Northern TAF locations.  It`s possible the lower conditions could
set up further South and this will need to be monitored.  Sfc wind
will become N to NW and increase after 15Z.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until noon CST Sunday for IAZ004>007-016-017-
025>028-037>039-050.

&&

$$

SHORT TERM...Small
LONG TERM...Podrazik
AVIATION...FAB



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