Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 180238
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
838 PM CST Fri Nov 17 2017

.UPDATE...
Issued at 837 PM CST Fri Nov 17 2017

Changed course from previous thinking with <=1/2sm visibilities
becoming more widespread and potential impacts with weekend of
pre-holiday travel starting. This prompted the Dense Fog Advisory
issuance. Inverted trough/front has reached DSM metro and
conditions should start improving several hours after its passage
as cooler/drier air follows. This will likely result in headline
being cancelled early NW, but potentially also extended in time
SE.

UPDATE Issued at 639 PM CST Fri Nov 17 2017

Fog will remain a concern through the evening hours and have opted
for an SPS vs a headline. The fog will certainly be dense in
spots, but variable in time and space. Visibilities were still
several miles SE and improvement is expected through the evening
central and NW as the inverted trough moves through NW-SE and
surface winds becoming northerly.

&&

.SHORT TERM.../Tonight through Saturday/
Issued at 336 PM CST Fri Nov 17 2017

Confidence: Medium

Still some ptype concerns regarding possible overnight changeover to
rain/snow mix or completely to snow prior to ending Saturday
morning.  16z subjective sfc analysis shows high building southeast
into the Northern Plains. A north/south cold front and weak waves
just entering west/northwest Iowa now and a more significant wave
over eastern Wyoming/eastern Colorado/western Kansas. The southern
waves will combine overnight and tap into moisture plume at H850/sfc
which have been riding northeast toward the Central Plains in the
past 24 hours. Meanwhile...rather mild air has also accompanied the
southerly flow with temperatures in the 60s/70s over southeast NE
and Kansas respectively.  The models are in general agreement for
the southern stream to take control...pushing the cold front south
through the region overnight.  There are very minor differences in
frontal placement by 06z with slightly more significant differences
in both frontal/sfc low placement by 12z. The Euro/NAM are now the
farthest south models by 12z...with the GFS slightly farther north.
All three models continue to suggest a deformation axis on the west
side of the system by 12z...but wit
The remainder of the forecast looks on track and relatively quiet.
Quickly on Sunday warm air will return to the region as an Alberta
Clipper races across Southern Canada...pulling mild air back into
Iowa through Monday.  Highs are anticipated to rise into the 40s to
mid 50s Monday afternoon.  A cold front will drop south with much
colder temperatures with stronger northwest winds Tuesday. With the
Gulf of Mexico shut off for much of next week...little moisture will
have any opportunity to return north to the plains. The next quick
clipper moves east Wednesday night into Thursday across southern
Canada once again resulting in little sensible weather here with
continued ridging aloft.  By Friday H850 temperatures will rise to
the lower teens...with milder air both Thursday night into Friday.
Highs Friday should reach the 40s to 50s once again. h the GFS farther north the
colder air aloft has a better chance of getting ingested into the
system prior to exiting...resulting in a better opportunity for
either a rain/snow mix or complete changeover to light snow aloft
but the GFS also holds onto milder sfc temperatures longer
...resulting in less overall snow reaching the ground (top down
considerations). The NAM is suggesting stronger cold air advection
through the column and hints at complete changeover across the south
by 5-6am...exiting southeast by 15-17z.  The Euro is slower today in
bringing the cold air into the system as well as with the farther
south track...keeps the deformation zone and higher precipitation
production/forcing south of our area. SREF soundings suggest a
changeover is at least likely as far south as Des Moines. Given the
amount of uncertainty and the nature of the this forecast...the fine
details will be ironed out in the next forecast package. For now
will continue to lean toward a mix on the backside of the front with
little if any accumulations.  The warm ground today and push of mild
air ahead of the system will help mitigate any accumulations that
might occur. Lows tonight will fall into the lower 30s north into
the upper 30s in the south. Late tonight and tomorrow will be windy
with winds quickly coming up across the area once the system begins
to deepen to our south. Clouds will also be the rule for much of
tomorrow morning gradually clearing in the afternoon hours. Outdoor
activity participants by late afternoon will notice the 20 F to
lower 30 F wind chills over our area between 21-00z as well.

.LONG TERM.../Saturday night through Friday/
Issued at 336 PM CST Fri Nov 17 2017

The remainder of the forecast looks on track and relatively quiet.
Quickly on Sunday warm air will return to the region as an Alberta
Clipper races across Southern Canada...pulling mild air back into
Iowa through Monday.  Highs are anticipated to rise into the 40s to
mid 50s Monday afternoon.  A cold front will drop south with much
colder temperatures with stronger northwest winds Tuesday. With the
Gulf of Mexico shut off for much of next week...little moisture will
have any opportunity to return north to the plains. The next quick
clipper moves east Wednesday night into Thursday across southern
Canada once again resulting in little sensible weather here with
continued ridging aloft.  By Friday H850 temperatures will rise to
the lower teens...with milder air both Thursday night into Friday.
Highs Friday should reach the 40s to 50s once again.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 639 PM CST Fri Nov 17 2017

Widespread IFR/LIFR conditions are in place across the forecast
area and TAFs sites due to a combination of low visibilities,
low ceilings and drizzle. Conditions will change little through
midnight toward KOTM, but visibility and drizzle improvements
will gradually occur NW-SE for other TAF locations as an inverted
trough and front moves through IA tonight as winds become
northerly. At least IFR ceilings will linger longer behind the
wind shift, but a similar NW-SE improvement should occur in
ceilings early Sat with VFR conditions area wide by late morning.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until midnight CST tonight for IAZ005>007-016-
017-024>028-034>039-044>050-057>062-070>075-081>086-092>097.

&&

$$

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


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