Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 192049
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
349 PM CDT Tue Sep 19 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 349 PM CDT Tue Sep 19 2017

Synoptic pattern aloft continues to be defined by a broad
longwave trough with several embedded shortwaves over the western
two-thirds of the conus. One such shortwave was noted on water vapor
passing through WY/MT, which will quickly eject over the northern
Plains tonight and pivot northward into Canada. At the surface, a
cold front extends through the central Dakotas into the western High
Plains. The front will progress eastward through the forecast area
tonight into Wednesday. Shower and thunderstorm activity is likely
along the frontal axis as it tracks out of the Dakotas, but how far
south and east this activity will be supported is in question due to
the presence of a strong capping EML. Several hi-res models continue
to advertise some convection in north central Iowa late tonight into
early Wednesday morning, mostly north of Hwy 20. Any threat for
severe weather should be limited due to weakening instability
overnight, but can`t rule out the potential for a few strong wind
gusts given DCAPE values around 1000 J/kg. Otherwise, southerly
winds and abnormally humid conditions will keep temperatures very
mild overnight with lows in the 60s to near 70.

The aforementioned cold front will push through central Iowa
tomorrow morning and afternoon. A much drier air mass will advect in
behind the front, bringing a brief reprieve from the unseasonably
high humidity. A strong capping inversion will likely inhibit
convection along the front until late in the day, at which point the
front will be starting to stall out near the southeastern periphery
of the forecast area. Maintained mention of slight chance pops in
our far southeastern zones to cover any potential for late afternoon
development, but confidence is low at this time due to strong
capping aloft and uncertainties in frontal position. For any storm
that does develop, strong instability and steep mid-level lapse
rates will promote the potential for hail and strong wind gusts.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 349 PM CDT Tue Sep 19 2017

Will see renewed warm advection on Wednesday night along with
increasing moisture advection as front begins lifting northward.
The increased theta-e advection and associated destabilization may
lead to the development of isold/sct convection across
southern/central Iowa during the night and have continued Pops
during this time, mainly south of Highway 30. The warm sector will
slide quickly northward on Thursday with very warm readings
returning along with unusually high moisture values for late
September.

The upper pattern will amplify into the late week and weekend with
a large western trof and equally impressive eastern ridge. Not
much movement is expected as both Jose and Maria edge up the
western Atlantic. Iowa will mostly be in the warm sector during
this time frame with much above normal temperatures from Friday
into Sunday along with the humid conditions persisting. With
little forcing and the atmosphere essentially capped, the threat
of precipitation will be minimal during this time. By late in the
weekend into early next week, ridge in the east will finally
begin to shift eastward and weaken allowing the western trof to
advance as well. The front that will be in the Plains through the
weekend will advance toward Iowa on Sunday night into Monday with
heightened chances for storms during this time. Once the boundary
slips through, temperatures will be cooler into the middle of next
week.

&&

.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1233 PM CDT Tue Sep 19 2017

Widespread fog/low stratus from this morning has dissipated,
giving way to mainly VFR conditions over central Iowa early this
afternoon. Some terminals may still experience temporary MVFR cigs
through 20z, but otherwise VFR conditions will prevail through the
remainder of the day.

Low level wind shear will become an issue tonight due to strong
southerly winds developing between 1000-2000 feet AGL. In
addition, MVFR cigs and scattered -TSRA are possible in north
central Iowa Wednesday morning. A return to VFR conditions area
wide is expected by mid-morning as a cold front pushes east
through the state.

&&

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

&&

$$

SHORT TERM...Martin
LONG TERM...Cogil
AVIATION...Martin



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