Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 262043

Area Forecast Discussion
National Weather Service Des Moines IA
343 PM CDT Tue Sep 26 2017

.SHORT TERM.../Tonight through Wednesday/
Issued at 343 PM CDT Tue Sep 26 2017

A robust upper level shortwave was noted on water vapor imagery
near the ND/Canadian border at 20z. An attendant surface low was
located near the UP of Michigan with a cold front stretching
southwest through Missouri and into the southern Plains. In the
wake of the front, the entire CWA has experienced low stratus
clouds and cool northwest winds through most of the day. Height
rises and large scale subsidence on the back side of departing
shortwave trough will work to clear skies out late this evening
and overnight. Surface high pressure sliding through the central
Plains tomorrow will keep Iowa under northwesterly flow and in a
weak CAA regime. Despite 850mb temperature anomalies of -4C to
-6C, widespread sunshine and diurnal boundary layer mixing to
 near 800mb will still yield near average afternoon highs in the
 upper 60s to near 70.

.LONG TERM.../Wednesday night through Tuesday/
Issued at 343 PM CDT Tue Sep 26 2017

The 500 mb trough crossing our region today will leave behind a
closed low over the southwest U.S., which will slowly drift
northward later this week. Meanwhile a large gyre will have been
established over Hudson Bay, with cyclonic flow extending down
across the Great Lakes region. From Wednesday night through Friday
this will leave Iowa beneath modest northwesterly flow aloft,
with high pressure dominating at the surface. This will result in
several days of cool, dry, early autumn weather across the region.

A compact but robust shortwave will round the base of the northern
cyclonic flow regime and move over Minnesota and Wisconsin on
Thursday night and Friday, pushing a reinforcing cool front
through Iowa followed by a large and even colder high pressure
area building over the Great Lakes region. This should bring
temperatures down to their lowest levels of the season so far on
Saturday morning, but lows will probably remain in the 40s, with
perhaps some upper 30s in our northeastern counties.

By Friday the aforementioned robust trough will be moving away to
our east, scouring out the aforementioned cyclonic gyre and giving
way to riding aloft over Iowa and the Midwest. Meanwhile our old
friend, the stranded 500 mb low that had moved north from the
southwestern U.S., will be drifting around over the Rockies of
Wyoming and Montana, while a powerful cyclone will be approaching
the northwest U.S. coast from the Pacific. As this cyclone moves
ashore over the weekend, it will partially absorb and push ahead
of it the old, weakened closed low, which will then manifest as
the leading shortwave of an ever-deepening northern stream trough.
The interaction of these systems is leading to increased
variability in long-range model solutions for the overall
evolution and progression of the large-scale trough as it moves
eastward across the northern U.S. in the latter part of the
weekend into early next week. What is clear is that there will be
a generally increasing POP trend from about Sunday into Monday,
possibly with a decent degree of warming ahead of the trough, and
that when the large-scale trough does finally swing through it
should bring good rain chances around Monday or Monday night,
along with another surface front. However, the airmass behind that
front will be more Pacific than Canadian in origin and minimal
cooling is expected.

Once the early week 500 mb trough rockets by, it will be replaced
by deep layer ridging across most of the U.S. that should bring a
pronounced warm period to Iowa from Tuesday or Wednesday through
most of next week, though temperature and humidity levels are
unlikely to rise as high as this past weekend.


.AVIATION.../For the 18Z TAFS through 18Z Wednesday afternoon/
Issued at 1215 PM CDT Tue Sep 26 2017

MVFR to IFR cigs will be the primary aviation weather concern
through the current TAF period. Most model guidance is in
sound agreement with steady improvement to VFR cigs occurring from
west to east across the area between 00-06z. Widespread VFR
conditions will then prevail through the remainder of the
overnight hours through Wednesday morning.





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