Area Forecast Discussion
Issued by NWS Des Moines, IA

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485
FXUS63 KDMX 160547
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
1147 PM CST Mon Jan 15 2018

...Updated for 06z Aviation Discussion...

.SHORT TERM.../Tonight through Tuesday/
Issued at 350 PM CST Mon Jan 15 2018

An elongated upper trough continues to spin aloft across the
region and will drop down through Iowa tonight into Tuesday. A
lobe of energy rotating around the center of the trough will drop
through Iowa tonight. This will bring increasing cloud cover
across the forecast area from north to south tonight with a chance
for flurries with the cyclonic flow aloft and saturation
increasing through the night. Have thus expanded area of flurries
to the southern border by late tonight. Clouds should start to
diminish and erode during the day Tuesday as the upper trough
begins to push east out of the region. The arctic air will
continue to sink into the state tonight into early Tuesday with
h85 temps dropping into the teens Celsius. Therefore even with
increasing clouds, temps will still drop to bitter cold values
given the strong CAA. Additionally winds will remain up with the
tightened pressure gradient, with some diminishment into Tuesday
as an area of high pressure builds into the region. Temps expected
to fall quickly in cloud free areas this evening, then level off
and fall slower with the increasing clouds and CAA. The cold temps
and gusty winds will push wind chills into wind chill warning
criteria across much of the north and western forecast area.
Therefore have opted to expand the Wind Chill Warning into the
west central/southwestern portions of the forecast area as wind
chills drop into the -30 to -35 range by late tonight into early
Tuesday morning. Will continue the headlines as is for the
remainder of the forecast area.

.LONG TERM.../Tuesday night through Monday/
Issued at 350 PM CST Mon Jan 15 2018

The long term forecast period is quiet. Tuesday night will be
quite cold again with a large high pressure area sliding by just
to our southwest. There is some uncertainty in lingering cloud
cover and very weak warm air advection aloft may keep temperatures
from falling too much late in the night, but with Tuesday
afternoon highs only in the single digits still expect
temperatures will fall below zero after dark Tuesday night. This
will lead to wind chills flirting with 20 below zero in our
northern counties at times, but the prospect of meeting the
advisory criterion is spotty and marginal and will maintain no
headlines for this period so we can concentrate our messaging in
the short term where it will be much colder and more dangerous.

As warm air advection kicks in more earnestly with a return of
southwesterly low-level flow on Wednesday, we will enter a quiet,
mostly clear and much more mild period right into the weekend.
Essentially no changes have been made to the ongoing forecast
during this time, with temperatures peaking in the upper 30s and
40s on Friday and Saturday and melting most or all of our snow
pack once again.

By Saturday a large 500 mb trough will have developed over the
western U.S., with an attendant surface cyclone developing over
the southern High Plains around the Oklahoma panhandle, and an
inverted surface trough stretching northeastward across parts of
Missouri, Iowa and Illinois. During the day Saturday clouds will
thicken near the surface trough and some light rain may fall, but
no impacts are anticipated. However, from Saturday night through
Sunday night the trough will eject eastward across our region and
deepen fairly rapidly. This will lead to increasing precipitation
chances spreading across Iowa and we now have likelies and even
some categoricals going from Sunday into Sunday night. Of course
at this range there is much uncertainty regarding precipitation
types and amounts, but the nearby location of the inverted surface
trough, as well as forecast temperatures and soundings, support a
rain/snow mix likely changing to snow later in the event as the
cyclone moves by. We will be watching this system through the
coming week as solutions coalesce.

&&

.AVIATION.../For the 06Z TAFS through 06Z Tuesday night/
Issued at 1139 PM CST Mon Jan 15 2018

TAF package remains difficult as spotty clouds wreak havoc on
categories. To put it into perspective at 0535z, KIKV had BKN011,
where KDSM had SCT014. Would think MVFR to prevail for more than
half the predominant groups, so have no TEMPO groups going.
Periodic flurries possible at all TAF sites tonight, but these
flurries should not reduce vsbys below 3 to 4 SM. Winds should
gradually diminish, becoming less than 10 kts by 00z Wed.


&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Chill Advisory until noon CST Tuesday for IAZ027-028-038-
039-049-050-060>062-073>075-083>086-094>097.

Wind Chill Warning until noon CST Tuesday for IAZ004>007-015>017-
023>026-033>037-044>048-057>059-070>072-081-082-092-093.

&&

$$

SHORT TERM...Beerends
LONG TERM...Lee
AVIATION...Kotenberg



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