Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 150344

Area Forecast Discussion
National Weather Service Des Moines IA
944 PM CST Tue Nov 14 2017

Issued at 944 PM CST Tue Nov 14 2017

Visibilities remain around 1 to 3 miles across much of the area
due to drizzle and light fog. As the boundary moves into the
state late tonight, visibilities are expected to improve. Light
rain has developed in pockets along with the drizzle and is
expected to become more widespread across the central and eastern
portions of the area through midnight, with rain ending from
northwest to southeast as the boundary moves through late tonight.
With the visibilities remaining well above 1/4 mile across the
far northeast, the dense fog advisory was cancelled with
visibilities expected to remain around 1 to 3 miles for the next
few hours before improving late tonight.


.SHORT TERM.../Tonight through Wednesday/
Issued at 322 PM CST Tue Nov 14 2017

12z KOAX RAOB sampled a strong inversion/environmental mixed
layer (EML) around 850mb with a moisture rich environment beneath
it. Early afternoon GOES-16 Day Cloud Phase Distinction RGB showed
plenty of low clouds with some passing higher clouds over the
state. Surface observations and Iowa DOT webcams show areas of fog
with visibilities reduced the most over far south central and
north central Iowa early this afternoon. These clouds and fog will
remain as well as pockets of drizzle as a strong PV anomaly moves
out of the northern Rockies late this afternoon to the western
Great Lakes by tomorrow night. At the surface, low pressure will
move from central South Dakota to over Lake Superior tonight with
a cold front moving across the state. This front will bring
increased forcing with plenty of moisture persisting beneath the
EML. The strongest omega will be within or near the top of the EML
so expect only scattered, short-lived light rain showers

By daybreak Wednesday, the front will be moving through eastern Iowa
with colder and drier air advecting into the state. Clouds will
break as the colder air erodes the inversion with mechanical mixing
also aiding in this effort. Cross sections across central Iowa from
the NAM, GFS, and ECMWF all show a decrease in depth of the high
moisture. The NAM remains aggressive with high RH values greater
than 85% below 850mb while the GFS/ECMWF have a small layer of RHs
roughly greater than 60%. This is most true over northern Iowa and
have left in few to scattered clouds. Otherwise, gusty winds from
the northwest will develop behind the front and have increased winds
several knots. High pressure will begin to ease the winds late in
the day as it starts to arrive from the Dakotas and Nebraska.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 322 PM CST Tue Nov 14 2017

./Wednesday Night through Saturday/...Confidence Medium

Main challenge will be system from Thursday through Saturday. At
this time the models are in fairly good agreement with regard to
most of the features...though suspect there may be a few minor
changes moving forward.  Before the arrival of the end of week
system...a secondary Arctic boundary will move south into northern
Iowa Wednesday evening before retreating by Thursday morning as a
weak ridge of high pressure slides east across the region Thursday.
Forecast mins Thursday morning may be impacted by clouds over the
west and may result in slightly warmer readings if mid to high level
clouds return quickly enough prior to 12z. Otherwise a northern
stream shortwave tracking into Manitoba by 06z Friday will pull the
warm front northeast with time...with milder air returning to the
region late Thursday into Friday.  Meanwhile farther west the large
scale trough digging into the west coast will amplify somewhat as it
progresses east with time. Clouds...strong warm air advection and
rain showers will increase Friday. Instability also increases during
the day over the southeast...and soundings support some elevated
convection mainly over the far southeast during the day/evening
hours as a sfc low tracks into the region. The models have been in
very good consistency regarding the flow remaining generally non
phased...or split through this event. The lack of strong cold air
advection following the first northern stream wave will allow the
southern stream system to track east with little northerly
amplification into Friday night/Saturday. Even though that is
true...the Euro and to a lesser degree the GFS bring in enough
cold air Friday night to create a brief rain/snow mix...possibly
changing briefly over to all snow as the system departs. At this
time...any impacts that might occur appear to be minor. By
Saturday afternoon both the GFS and Euro continue to show
sufficient cold air advection to keep afternoon highs rather cold
over the area. There is still some uncertainty as to how fast we
will clear out in the afternoon with the Euro slightly slower.
With that trend have kept afternoon highs closer to the lower end
of ensemble guidance at this time. Brisk northwest winds will
lower wind chills to the upper teens over the north by late any outdoor events will feel quite cold by late

./Saturday Night through Tuesday/...Confidence Medium to High

Once the system exits to the east...clearing is anticipated for at
least part of Saturday night. The past two runs of the Euro are
already showing return clouds overnight into Sunday morning due to
upstream warm air advection processes over the Dakotas. This is
happening despite an area of high pressure moving into the area.
Min temps are somewhat in question...especially if the cloud cover
returns sooner. The current forecast is tilted toward the lower
end of guidance...but may need to be raised by about 4 degrees
most areas if clouds arrive earlier in the night on Saturday. The
remainder of the forecast is quiet with high confidence on a
warming trend with virtually no precipitation expected. With a
flatter more zonal flow in store for the area Monday and
Tuesday...a recovery in forecast temperatures is in the offing.
Highs Monday and Tuesday given H850 temps in the single digits
south...should recover to the mid to upper 40s south...while the
north holds onto 30s to lower 40s for the 2 day period. The
beginning of the Thanksgiving holiday travel period (Tuesday) at
this time looks benign in the Central Plains as storms continue
along the east and west coasts but ridging high pressure continues


.AVIATION.../For the 00Z TAFS through 00Z Wednesday evening/
Issued at 532 PM CST Tue Nov 14 2017

Areas of drizzle and fog reducing visibilities across portions of
the area to around 3 miles to less than a mile in spots this
evening. Some rain showers to develop later tonight which may help
lift visibilities. Winds to shift around northwest toward early
Wednesday, with LIFR/IFR CIGS lifting or clearing out into
Wednesday morning. WIll have to monitor as the clouds could linger
longer in the north/northeast through the day Wednesday.





SHORT TERM...Ansorge
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