Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 292022

Area Forecast Discussion
National Weather Service Des Moines IA
322 PM CDT Fri Jul 29 2016

.SHORT TERM.../Tonight/
Issued at 321 PM CDT Fri Jul 29 2016

A large but slowly weakening trough will move off to the east
tonight, with a surface ridge building in from the northwest
late. As a result the isolated showers that have affected our
northern and eastern counties today will fade out and move off to
the east while winds gradually become light and variable.
Temperatures will be fairly cool again as even at the time of
this writing they are only in the upper 60s and lower 70s. The
other forecast question of the night is what will happen with
cloud cover trends. A large stratocumulus shield wrapped around
the departing trough has affected most of Iowa through today,
resulting in the aforementioned cool temperatures, and has shown
only a little breakup in satellite imagery. This is more
reminiscent of a fall/winter event in which a cloud shield that
persists through sunset generally persists into the following
morning, at least, especially given the light to calm winds moving
in overnight. Have trended the forecast this way as it appears
likely that at least a good chunk of the area will remain affected
by these clouds through the night, which is supported by forecast
soundings and some deterministic output.

.LONG TERM.../Saturday through Friday/
Issued at 321 PM CDT Fri Jul 29 2016

The weather will remain cool and quiet this weekend. Saturday will
be similar to today but the light showers and thicker cloud cover
should slowly continue to translate eastward, allowing fore some
spotty sunshine and slightly warmer temperatures. On Sunday the
surface ridge will move off to the east, allowing for a return of
southeasterly breezes and a continuation of the slow warming

From Sunday night through the middle of next week the synoptic
weather pattern will be dominated by a building deep layer central
U.S. ridge, with characteristic generally southerly low-level
flow. This will be a classic setup for the development of another
spell of hot and humid weather across Iowa. However, once again
this period will be tainted by at least two opportunities for
thunderstorms. The first will come on Sunday night into Monday in
the form of a band of warm air advection moving from southwest to
northeast across the state. The second will come on Monday night
into Tuesday morning as a mid-level shortwave overtops the ridge
and may generate an MCS somewhere over the region with some severe
weather potential. If these storms do cross our forecast area,
they may leave lingering effects that would dampen daytime heating
somewhat on Tuesday. Therefore, have held high temperatures for
the early part of next week in check due to the uncertainty
associated with these thunderstorm chances. On Wednesday this is
less of a concern and have maintained temperatures in the low to
mid 90s, a little above consensus guidance. This may still be too
cool, but with the probability of dewpoints rising well into the
70s again the high humidity may mitigate temperature rises a bit.

A deeper trough moving across the region will bring a cold front
sweeping across Iowa from Thursday into Friday, likely accompanied
by more thunderstorms and followed by dry and cooler weather at
the end of next week.


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1234 PM CDT Fri Jul 29 2016

MVFR ceilings will affect most of Iowa this afternoon. Satellite
imagery indicates that the cloud shield has been slightly
expanding today, but ceiling heights have also been slowly
rising. It is likely that the clouds will persist through tonight
and into Saturday morning, but it remains uncertain whether
ceiling heights will rise to VFR by sunset or not, and either way,
how much they may lower again overnight. For now have advertised
VFR ceilings after this evening but uncertainty is high and this
will be reevaluated for subsequent TAF issuances. A few showers
will also affect the area this afternoon, but any impacts will be
too minimal and brief to include in the TAFs other than at ALO
where they may be a bit more frequent.





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