Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211727

Area Forecast Discussion
National Weather Service Des Moines IA
1227 PM CDT Sat Apr 21 2018

.SHORT TERM.../Today through Tonight/
Issued at 408 AM CDT Sat Apr 21 2018

Little sensible weather is anticipated through the period. The
closed low and associated short waves continue to rotate through
the Rockies this morning. The parent low will drift south and east
into OK and AR by Sunday keeping much of the deeper forcing,
moisture and associated precipitation to our south. Most models do
depict a short wave rotating into IA later today, but it will be
weak with little thermodynamic or kinematic support. Forecast
soundings show plenty of mid level moisture but equally dry
conditions below, which is also shown by ECMWF and NAEFS 500mb
specific humidities well through the 90th percentiles but only in
the low single digits by the time you get down to 850mb and
925mb. A few convection allowing models do try to generate light
precip/sprinkles but with weak UVM and low level dry air will
leave the forecast dry. There will be some radar echoes, but
precip staying aloft. Any potential for this at all should exit by
Sunday with some patchy lingering mid and high cloudiness.

Temperatures should continue to slowly moderate into the weekend,
especially north with further snow melt, but the flow regime,
cloudiness and steady easterly flow will keep any appreciable
rebound at bay. Temps should be just a shade above persistence
today and then several degrees warmer into Sunday but still just
below normal.

.LONG TERM.../Sunday through Friday/
Issued at 408 AM CDT Sat Apr 21 2018

The weather pattern through this period will be equally inactive
with temperatures still at or below normal and the potential for
precip seeming to be trending downward, at least through midweek.
The flow will be rather weak early next week with little forcing
or moisture keeping the forecast dry until at least Tuesday. The
00z GFS seems to be the strongest/wettest solution with fairly
deep lift driven by kinematic and frontogenetical forcing Tuesday
into Tuesday Night and lingering somewhat into Sunday. This
solution seems to be on the high end however with the GEFS, CMC,
ECMWF deterministic and ensemble solutions all recently weaker.
Chance PoPs still persist but if present trends continue the
potential may diminish further. The 00Z ECMWF has little forcing
below 3km and no thermodynamic component and only weak forcing

Beyond this the forecast looks dry before the flow strengthens
once again and drives a northern stream trough into the northern
and central Plains and Great Lakes Thursday and Friday. This will
bring spotty precip chances around Thursday, and as opposed to
Tue/Wed, the potential may be increased somewhat with now better
agreement among the 00z deterministic runs and ensembles. If
trends continue, PoPs may increase by afternoon. Temp wise, the
flow leading up to this is near normal at best with the stronger
NW flow leading to another shot of readings even further below
normal to end the work week.


.AVIATION.../For the 18Z TAFS through 18Z Sunday afternoon/
Issued at 1227 PM CDT Sat Apr 21 2018

Main challenge this period will be fog potential across northern
Iowa and relaxing winds this evening. Models showing some clearing
possible north aft 02z with potential for fog development north.
Some uncertainty remains and will only lower vsby to MVFR at this
time at KFOD/KMCW. Cigs expected to remain VFR unless thicker fog
develops north. Winds should relax to under 10kts by 23z...but
remain near 8 kts or so much of the period. Mid level bkn cigs aoa
10kft most areas through 15z with thinner areas expected
thereafter. /rev




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