Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 251736

Area Forecast Discussion
National Weather Service Des Moines IA
1236 PM CDT Mon Sep 25 2017

.SHORT TERM.../Today through Tonight/
Issued at 348 AM CDT Mon Sep 25 2017

The primary concerns today will include both temperature and precip
trends, but confidence in details is not great in either regard. On
the synoptic scale the long wave trough continues to slowly move
toward the MO Valley with primary short waves across northern CO and
another currently less active wave advancing through the base in AZ.
Extended areas of mid level kinematic forcing above stronger
frontogenetical response continue to produce widespread showers and
storms from the high Plains into the Upper MS Valley. Both of these
lifting mechanisms should persist through today, but appear to
weaken with time into tonight. Effects from the latter should still
be sufficient for waves of weak convection however.

There could still be some surface based convection into peak heating
however due to what will be a wide temp range across the forecast
area. Temps will change little NW while readings SE are expected to
rise well into the 80s. Radar data has also been showing very
shallow bores/waves propagating through the shallow inversion
overnight. GOES 16 Nighttime Microphysics RGB imagery has also noted
small patches of low clouds behind these features and will decrease
temp confidence and could also eventually provide a focus for peak
heating convection, increasing differential heating. Although
MLCAPEs may reach 1000 j/kg, storms should stay below severe limits
because of the lack of effective shear. General precip amounts will
not be overly high, but both precipitable water and specific
humidities remain anomalously high so with steady moisture transport
and a favorable mean wind for training there could be locally
moderate to heavy rains.

.LONG TERM.../Tuesday through Sunday/
Issued at 348 AM CDT Mon Sep 25 2017

The deep upper trough that has been parked over the west for the
past several days is finally shifting to the northeast, though it
will take until Tuesday night to be far enough east to have no
impact on central Iowa. At the surface, high pressure will build
into the state ending any threat of precip by late in the day.
Showers and perhaps an isolated storm is still possible during the
morning through mid afternoon Tuesday due to some weak forcing as
the upper trough moves by.

Tuesday night through the rest of the week, high pressure at the
surface combined with a trough developing across the Northern Plains
will keep a cool and dry forecast in place.  Expect highs in the mid
60`s to mid 70`s through Friday.  A trough over the Great Lakes will
deepen into a low dropping a shortwave across WI and MN which will
impact northeast Iowa but other than a reinforcing shot of cool air,
precip should remain north and east of the forecast area.  Ridging
will move overhead for the weekend keeping upper 60`s to lower 70`s
in place for Iowa with dry conditions thorugh Saturday night. Sunday
into Monday a deep trough will push across the area bringing the
next chance for showers and possibly some thunderstorms.  Model
consensus is absent in handling the evolution and movement of this
trough so chance pops for some showers is all I have going at this
point. Any potential warmup ahead of this feature will likely be
negated by cloud cover and eventually showers so highs only moderate
to the lower to mid 70`s.


.AVIATION.../For the 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1233 PM CDT Mon Sep 25 2017

Showers and storms in the vicinity of a slow moving front will
keep tempo and/or prevailing MVFR or lower cigs in place over
northwest and north central Iowa through the afternoon hours. The
front will sag further east later today and overnight, which will
bring the chance for more widespread TSRA to central Iowa
including KDSM. High confidence in IFR or lower cigs across most
of the area by tonight through Tuesday morning. The potential for
low cigs and precip are lower in southeast Iowa through the
current TAF period, so any flight restrictions remain absent from
the KOTM forecast through 12z Tuesday.




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