Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 212015
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
315 PM CDT Thu Sep 21 2017

.SHORT TERM.../Tonight through Friday/
Issued at 315 PM CDT Thu Sep 21 2017

Warm front making nice progress across Iowa.  Early afternoon had
the boundary just north of Highway 20.  The front will move into
southern MN this evening putting all of the forecast area in strong
warm advection.  Lows tonight will be around what normal highs are
for this time of year with readings in the low to mid 70`s. Expect a
southerly wind to maintain through the night at 10 to 15 mph.

On Friday we will mix to at least 850mb so winds will be pretty
strong and gusty...in the 20 to 30 mph range.  The good mixing will
also mean that temps will warm up quite a bit.  I nudged highs at
least 2 to 3 degrees above guidance and this may not be warm enough.
Heat indices in the mid afternoon hours will likely hit 100 or so in
a few places. The wind will help but it will be a hot day.

As far as the overall pattern...we will remain stagnant with a low
and front parked out west in South Dakota and Nebraska bringing
rainfall to those areas while over Iowa we will be in between the
low to the west and the high to the east, while under the influence
of an upper ridge.  We will remain hot and breezy going into the
weekend with little threat of rain.


.LONG TERM.../Friday night through Thursday/
Issued at 315 PM CDT Thu Sep 21 2017

Highly amplified pattern will be in place at the start of longer
term with deep southerly flow across the Midwest. Frontal boundary
will be located in the Plains with upper flow nearly parallel
with the front. Therefore, not much movement/or only slow movement
is expected with the front for several days leaving Iowa in the
warm sector during the weekend. 850mb temperatures will be
approaching +20C over the weekend and with good mixing, should
lead to surface temperatures of around 90 degrees in most
locations through Sunday. The combination of these warm
temperatures and dewpoints approaching 70 will have heat index
values in the upper 90s to nearly 100 during the afternoon
timeframe. Highs will also be near record levels, especially on
Sunday where readings around around 90 across the area.

Frontal boundary begins to approach on Monday as upper low across
the western United States slowly moves east. Lift will increase
with the approach of the trof with showers and thunderstorms
becoming more likely by late Monday into Tuesday as the front
provides sufficient convergence for development. However, system
overall is weakening through the early part of the weak and wind
fields/lift will become less with time. Therefore, coverage and
intensity may not be quite a good as previous forecast, but most
locations will likely see at least a little rainfall. Much cooler
temperatures are expected by the end of the period as Canadian air
moves into the Midwest behind the system.

&&

.AVIATION.../For the 18Z TAFS through 18Z Friday afternoon/
Issued at 1255 PM CDT Thu Sep 21 2017

Warm front to quickly lift through northern/northeastern TAF
locations through 00Z. Any MVFR cigs remaining should quickly become
VFR.  Brisk winds will occur through the period from the surface up
through 3 or 4Kft.  For now I took out the wind shear from the TAF`s
because winds will be strong from the surface up and pretty
unidirectional. Not sure there will be a 20kt difference. This
will need to be monitored and may be put back in, in later
forecasts, especially KMCW and KALO.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...NONE.

&&

$$

SHORT TERM...FAB
LONG TERM...Cogil
AVIATION...FAB



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