Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 221949
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
249 PM CDT Fri Sep 22 2017

.SHORT TERM.../Tonight through Saturday/
Issued at 247 PM CDT Fri Sep 22 2017

Little change to the weather is expected during the short term
period. Very warm temperatures will continue into Saturday.
Although heat index values will be slightly lower on Saturday than
today, it will feel similar to today for all practical purposes.

Went with a Heat Advisory for today per collaboration with
neighboring offices as well as with partners. The Heat Advisory
will expire at 7 pm this evening. Another Heat Advisory is not
anticipated for Saturday given slightly lower heat index values--
perhaps an SPS instead. Will defer to later shifts for the final
decision.

.LONG TERM.../Saturday night through Friday/
Issued at 247 PM CDT Fri Sep 22 2017

Summary... Excitement in the long term through the beginning of
the week entails extreme heat, for this time of the year, through
Sunday and storms/frontal passage Monday/Tuesday leading to more
seasonable conditions to finish the week.

Saturday night through Sunday night... Upper level ridging will
remain planted over the eastern half of the CONUS as a deep trough
continues to slowly dig its way into the Four Corners region.
With remnants of Post-Tropical Cyclone Jose and Hurricane Maria
situated in the Atlantic, it will be tough sledding with regards
to breaking down the eastern CONUS ridge. Tendency over the last
few runs has been to slow down its breakdown and subsequent exit,
and would not be surprised if the trend continued. What that means
for Iowa will be much above normal temperatures sticking around
through Sunday and much of Monday for the eastern third to half of
the state. While warmest 850 mb temps will be seen prior to the
start of the long term window, they will remain warm around 18 deg
C, so highs should once again reach into the upper 80s Sunday.
Fortunately, dew points should relax a few degrees into the low to
mid 60s, keeping heat index values around 90. Regardless, still
uncomfortable for this time of the year so caution should be
exercised for those outside. Record highs will also likely be at
risk with relatively low hanging fruit in the upper 80s at many
locations Sunday.

Early Monday through Friday... Precipitation and cooler
temperatures will the be name of the game. A cold front will
slowly progress into northwest IA late Sunday night and slowly
traverse the state before exiting Tuesday. Little concern for
severe weather with only modest MUCAPE values around 700-1000 J/kg
continuing to be depicted and lackluster 0-6 km bulk shear under
20-25 kts. As one would imagine, best shear right along the front,
but working against any isolated severe weather is unidirectional
parallel flow to the front. Would yield a larger threat for
sustained moderate to heavy rainfall with training storms and the
slow moving nature of the front. Certainly cannot rule out areas
receiving 2-3+ inches from late Sunday night through Tuesday,
especially with PWats 175-200% of normal for this time of the
year. Best area looks to be western areas with best moisture
transport and convective potential throughout the night.
Fortunately, given duration and general dry conditions across most
areas, no concerns for flooding or other hydrologic issues. Even
southeast IA, which received 6+ inches of rain earlier this week,
should have little issue absorbing upcoming rains. Post cold
front, surface high pressure will build in with predominantly
westerly to northwesterly flow aloft. Temperatures will be much
more seasonal with highs in the upper 60s to low 70s and lows in
the mid 40s to low 50s.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1125 AM CDT Fri Sep 22 2017

There is high confidence in VFR conditions through the period
with nothing beyond patches of mid level cloudiness. South winds
will persist and be gusty during peak mixing of the day. Some low-
level wind shear is possible at the KMCW terminal this evening and
is indicated in the TAF.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 7 PM CDT this evening for IAZ004>007-015>017-
023>028-033>039-044>050-057>062-070>075-081>086-092>097.

&&

$$

SHORT TERM...Zogg
LONG TERM...Curtis
AVIATION...Zogg



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