Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182036
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
336 PM CDT Sat Mar 18 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 336 PM CDT Sat Mar 18 2017

Surface ridge currently moving into western Iowa as low ceilings
dissipate in the east with drier low level air arriving. Hints of
warm advection already showing up with mid level clouds in
portions of western Iowa. The warm advection will increase
overnight as the surface ridge slides to the east and southeast
surface winds intensify. Coolest temperatures tonight will likely
occur late this evening to shortly after midnight prior to the
increase of southeast winds and ahead of advancing clouds.

The warm advection intensifies into Sunday morning as low level
thermal gradient builds into Iowa with 850mb thermal ridge into
western Iowa by afternoon. Highs will be much warmer than today
although some cloudiness keep the overall highs in check with
soundings indicating low level moisture trapped beneath a rather
substantial inversion.  Above the inversion, a nice elevated mixed
layer is in place and the overall soundings appear to limit the
threat of showers/storms through the afternoon with the better
threat arriving after this period, sometime in the evening.

.LONG TERM.../Sunday night through Saturday/
Issued at 336 PM CDT Sat Mar 18 2017

On Sunday night a shortwave trough will move across southern
Canada, with its tail end extending down across the upper Midwest
and pushing a modest cool front southeast across Iowa overnight. A
weakly unstable airmass ahead of the front will provide the
impetus for shower and thunderstorm development across our
southeastern counties and have continued POPs to this effect.
Given the degree of shear in place overnight, there is some
potential for strong to severe thunderstorms in the form of
elevated hailers. This is well outlined by SPC outlooks.

From Monday through Tuesday night we will be under the influence
of a series of successive high pressure areas sliding across the
Minnesota/Wisconsin area, with persistent dry northeasterly low-
level flow. Brisk, more or less zonal flow aloft may provide one
or two impulses with associated weak vertical forcing, however
low-level dry air will mitigate precipitation chances for the most
part. The exception will be on Tuesday when there is stronger lift
associated with a passing shortwave aloft and forecast soundings
indicate saturation will occur mainly across about the southwest
half of our forecast area. Have maintained chance POPs
accordingly, but any precipitation should be light and mostly in
the form of a cool rain with no real impact anticipated.

The weather pattern over our region will become more active in the
latter half of the week. A longwave 500 mb trough will come ashore
over the Pacific U.S. coast late Wednesday and then deepen as it
progresses eastward. Initially this will place Iowa beneath
southwesterly flow aloft, with the early week surface high moving
off to our east and southeasterly low-level flow allowing for some
moisture return. This will likely lead to light warm air advection
regime precipitation at times from late Wednesday into Thursday.
Thereafter, prognostic model solutions diverge rather quickly. The
12Z GFS maintains a broad but open trough moving more quickly
eastward, crossing Iowa on Friday, while the ECMWF continues to
depict the development of a closed low pressure center aloft,
slowing the digging the system with the low eventually moving over
the southern half of Missouri on Saturday. In any event
precipitation will become increasingly likely from Thursday
afternoon through Friday, and possibly into Saturday if the slower
solutions hold. However, the type of precipitation varying from
snow or a wintry mix to rain and even thunderstorms, remains very
much in question.

&&

.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1234 PM CDT Sat Mar 18 2017

Low stratus will continue to push east this afternoon as surface
ridging enters western Iowa. Once these low ceilings exit later
this afternoon, any subsequent ceilings will be VFR and generally
at or above 8kft. Surface winds will become light by this evening
and switch to southeast overnight then increase quickly by midday
Sunday. Visibilities will remain unrestricted for the duration of
the forecast.

&&

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

&&

$$

SHORT TERM...Cogil
LONG TERM...Lee
AVIATION...Cogil



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