Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 131603

Area Forecast Discussion
National Weather Service Des Moines IA
1003 AM CST Mon Nov 13 2017

Issued at 1003 AM CST Mon Nov 13 2017

Dense fog has improved across the south and has expanded some
north to along and just north of the hwy 20 corridor this morning.
The fog is expected to persist through mid-day, thus a portion of
the advisory has been cancelled (south) and expanded/extended in
time (north). Low level moisture to remain high today into
tonight, with models suggesting the fog to lift into a low stratus
and spread across the entire forecast area through this
afternoon/early evening from west to east. Have a drizzle mention
tonight with fairly deep low level saturated area and waa expected
across the area. Will be assessing through the day if the drizzle
mention needs to be added into the evening hours as well.
Otherwise have increased cloud cover through the day/evening and
will have to look at temperatures as well. Have nudged them down a
touch already, but will look at the trends through mid-day before
making any further changes.


.SHORT TERM.../Today through Tonight/
Issued at 358 AM CST Mon Nov 13 2017

For today, a surface high over eastern Iowa will continue to push
east and as it does a southerly flow will increase/deepen over the
state.  There are several forecast concerns for today.  First,
satellite imagery shows stratus lifting/pivoting out of Missouri
which will spread up into most of Iowa today.  Ahead of this
stratus, the richer low level moisture is advancing across parts of
eastern Iowa as well as southern and western Iowa.  Dense fog has
become an issue across the southeast half of Iowa and as the flow
pivots around the departing high, the fog will become less of an
issue northeast but more of an issue across the west.  Clouds
especially and to some extent the fog, were adjusted some to try and
match conditions but the fog will be more problematic this morning
lingering longer in lower lying areas.  A dense fog advisory is in
place for locations that will see near zero visibility at times this
morning. The next question for today will be what impact will the
clouds have on temps? Despite being in weak warm advection,
deepening moisture through the day will mean increasing clouds which
will have a negative impact on temps this time of year.  In general
I went with the the lower guidance and nudged temps down a degree or
so from there.

For tonight, the surface high departs further east allowing a broad
surface trough to push across the state through Tuesday.  Tonight
southerly flow will deepen in the lowest layers of the atmosphere
along with moisture thus low clouds will develop/thicken over the
state.  This will keep temps up but overnight several weak
disturbances will ride through the flow providing some weak lift.
After 06Z we are saturated through at least 2Kft and with the weak
lift in place, we should see drizzle develop mainly over the eastern
half of the state.

.LONG TERM.../Tuesday through Sunday/
Issued at 358 AM CST Mon Nov 13 2017

Low level Gulf moisture will continue to advect into Iowa on Tuesday
and expect widespread stratus and some drizzle for much of the day.
A stout elevated mixed layer (EML) featuring very dry air situated
between 850-600 mb. Warm advection through the moist layer across
the southeast half of the forecast area is expected to be sufficient
for good collision coalescence and drizzle formation.  The layer is
more stable over the northwest and this region should have the
stratus thinning during the day, lessening the drizzle potential.
South winds should be breezy enough to limit fog potential and CAMs
are focusing the fog farther west into portions of eastern Nebraska
and South Dakota where the pressure gradient is weaker. The high
temperature forecast may still be too optimistic considering the
cloud potential on Tuesday. Generally expect highs in the upper 40s
to mid 50s with these highs occurring late in the afternoon as the
higher dew points arrive. Have removed thunder chances for Tuesday
night as the EML remains in place though a little more moisture will
be available, therefore have transitioned the drizzle to rain.

High pressure will settle into the region Wednesday into Wednesday
night and the upper flow transitions to weak ridging. As alluded to
in previous forecast discussions, Thursday will be cooler with
surface winds originating from the departing surface high pressure.
Still the potential for a strong storm system to develop late in the
week as a vigorous PV Anomaly ejects out into the Midwest. There
still remains a fair amount of uncertainty regarding the spatial and
temporal evolution of this system. The most likely impact to Iowa
will be strong and gusty winds Friday into Saturday. A few
thunderstorm reaching into southern Iowa and a brief transition to
light snow may be possible as well. Winter impacts look low at this
time drier air arrives with the cold advection and ending
precipitation chances.


.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 535 AM CST Mon Nov 13 2017

Stratus and fog to create IFR conditions or worse from KALO to KDSM
to KDNS and south.  While vsbys improve after 16Z, stratus may lift
briefly from mid/late morning through early afternoon then re-
develop and become even lower after 00Z.  Deepening moisture will
also aid in the development of drizzle and fog again tonight through
the end of the TAF forecast period and beyond.


Dense Fog Advisory until noon CST today for IAZ023>026-033>039-



LONG TERM...Donavon
AVIATION...FAB is the U.S. government's official web portal to all federal, state and local government web resources and services.