Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 071817

Area Forecast Discussion
National Weather Service Des Moines IA
117 PM CDT Fri Aug 7 2020

...Updated for 18Z Aviation...

.DISCUSSION.../Today through Thursday/
Issued at 310 AM CDT Fri Aug 7 2020

Southerly flow has returned to Iowa with high pressure now centered
over Michigan. The overnight southerly winds have kept temperatures
warmer than the past several night with readings in the mid 60s to
low 70s currently. The forecast area has remained dry thus far
overnight. A few thunderstorms have started to develop over south
central Minnesota in a zone of strong theta-e advection and along an
instability axis. An MCS is ongoing over central South Dakota and
continues to trek eastward along the aforementioned instability axis
and with that axis expected to lift northward over the next several
hours, the chance that complex reaches the forecast area is low.
Warm advection aloft will continue to move into central Iowa today
and will promote weak sfc based capping situated between 750-800 mb.
Some cumulus development below that capping is possible but the
chance for any updrafts punching through the weak capping is low
leading to a mainly dry and warmer day.

A split low level jet will develop tonight with one area focusing
into central/northern Minnesota and the other into northern
Missouri/far southern Iowa. As has been discussed that past forecast
discussions, there remains a moisture and forcing discontinuity in
addition to formation of a elevated mixed layer resulting in warming
and drying aloft. The deterministic ECMWF continues to most
aggressive and likely over zealous on a thunderstorm complex
developing over South Dakota and then following the corfedi vectors
and instability axis into Iowa. A vast amount of dry air above 850
mb across northern Iowa should make it difficult for any complex to
sustain into Iowa and while a lower likely scenario, not impossible.
A more probable scenario would be a few showers and storms in
vicinity of the speed converge regions of the split low level jet,
leaving most of Iowa dry.  Southern Iowa does have a brief period of
better moisture and vertical ascent continuity that could promote a
few storms.

The elevated mixed layer (EML) will become more pronounced on
Saturday and into Sunday and will greatly limit any surface based
convective chances. Temperatures both days will be warmer with highs
in well into the 80s to lower 90s. A few mid 90s could occur on
Sunday along with heat index values approaching 100.  There is some
moisture and instability above the EML late Saturday night and
Sunday as a short wave passes across the area that could promote
scattered highly elevated thunderstorms. Another chance arrives
Sunday night into Monday as a boundary settles south through Iowa. A
potential MCS may occur during this period with the focus still
appearing to be northern Iowa. Some severe weather would be possible
with damaging winds the primary threat.

A fast moving high pressure system will follow Monday night into
early Tuesday then will be replaced by the return of southerly
surface flow. Zonal/westerly flow aloft will have several embedded
impulses that will pass through Iowa and will keep the persistent
low thunderstorms chances in the forecast. Despite these numerous
periods of precipitation chances in the forecast, the chances for
meaningful precipitation in the recently declared D2/D3 drought
regions of central and southern Iowa look low at this time. A
western long wave trough will amplify by mid to late next week and
will lead to ridging that should nudge the thermal ridge closer to
the Missouri valley and perhaps temperatures approaching 90 once


.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1254 PM CDT Fri Aug 7 2020

Mainly VFR conditions, with local MVFR CIGs at times, expected
through this evening. Some non-severe storms are still possible
overnight, especially north, which is reflected in drop to MVFR
CIGs overnight. More widespread thunderstorm chances tomorrow
morning so have added VCTS to KFOD, KMCW, and KALO after 10z and
keep MVFR conditions through the rest of the period. CIGs could
drop to IFR at times tomorrow morning but have left out of the
TAFs at this time and will continue to monitor with future
forecast updates.




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