Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182343
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
543 PM CST Sat Nov 18 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 301 PM CST Sat Nov 18 2017

Surface low pressure that began the morning over east central
Missouri has moved rapidly ENE into Ohio this afternoon. Much of
the cloud cover has passed east of Iowa this afternoon except for
a small area of 3-4 kft stratus that has moved into northern IA
associated with another wave of cold advection arriving. Gusty
northwest winds will remain near to slightly below advisory
criteria through the remainder of the afternoon. Colder
temperatures will settle into the state overnight though
temperatures will not bottom out with breezy northwest winds
continuing and a near mixed out boundary layer remaining into
place. That said, lows should dip into the upper teens north to
the low to mid 20s south.

High pressure ridge will pass across the state on Sunday and will
lead to a transition to west/southwest winds by the afternoon and
the return of warm advection. High temperatures should outperform
guidance values with good mixing and dry air in place. Temperatures
may warm even more should an abundance of sunshine persist as
expected.

.LONG TERM.../Sunday night through Saturday/
Issued at 301 PM CST Sat Nov 18 2017

Overall, fairly high confidence that it will be mainly dry through
Thanksgiving Day over central Iowa while temperatures will swing
from above normal Monday to below normal Tuesday and Wednesday and
should return to above normal for Friday.

Trough of low pressure that brought rain showers to central Iowa
Friday night into Saturday morning will be over eastern Canada/US by
Sunday night with ridging and northwest flow over our region. A
shortwave trough will drop out of the Yukon/Northwest Territories
provinces with surface low pressure cresting the Canadian Rockies
and moving quickly eastward across the Canadian prairies. A surface
cold front associated with this low will race southeastward through
the Dakotas on Monday while surface high pressure moves eastward
across the Southeast US. This will provide for breezy winds from the
southwest and 850mb temperatures will peak between 7 and 9C by
Monday afternoon over central Iowa. High temperatures were increased
a degree or two, but restrained from raising temperatures too much
given expected high clouds. Cross sections over central Iowa show
high relative humidity values above 400mb. The GFS is the most
robust with RH values greater than 85% while the NAM is generally
above 60%. Therefore, bumped up sky cover to account for this high
level cloudiness.

The cold front will move through the area Monday night into early
Tuesday with precipitation staying well north of the area closer to
the mid-level dynamics. Strong cold air advection with gusty winds
will begin behind this front on Tuesday as a 1028mb high pressure
drops out of Canada. High temperatures Tuesday and Wednesday and
low temperatures Tuesday night/Wednesday morning will average
about 10 degrees below climatological normals.

Another shortwave trough will drop into the Great Lakes/southeast
Canada with a weak boundary passing through the area. No
precipitation is expected as mid-level riding will already be
pushing into the area. This will spell milder temperatures by
Thanksgiving Day closer to climatology. By late in the week and
early next weekend, an area of upper level energy will be
approaching the area having traveled from its current Saturday
afternoon location over eastern Russia southward through Alaska
lingering over the north Atlantic before beginning to move eastward
across southern Canada/northern US states. Before this arrives,
warmer air is expected with 850mb temperatures of 12-14C and
southwesterly surface flow with more westerly mid-level flow. Even
milder air may be possible by Friday with the GFS showing 60s
over the southwestern half of Iowa. The 00z ECMWF also had 60s,
but the 12z run is about 10 degrees lower. Adding to the
complexity of high temperatures on Friday, GFS and ECMWF
soundings show some moistening in the upper levels around 300mb
later in the day with perhaps low level clouds moving into
northern Iowa given saturation around 700mb. Still could see a
warmup before any clouds arrive so blended equally the
Superblend/12z GFS/12z ECMWF to account for a slightly warmer
solution. If the clouds don`t materialize and models stabilize in
their run to run solutions, temperatures may need to be boosted
more than currently forecast.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 543 PM CST Sat Nov 18 2017

Brief high MVFR/low VFR ceilings may affect KALO and KOTM through
01z, but otherwise there is high confidence in VFR conditions with
nothing beyond patchy high cloudiness Sun. NW winds will diminish
early this evening before gradually becoming W then SW throughout
the day Sun.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...Donavon
LONG TERM...Ansorge
AVIATION...Small



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