Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182025
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
325 PM CDT Fri Oct 18 2019

.SHORT TERM.../Tonight through Saturday night/
Issued at 319 PM CDT Fri Oct 18 2019

Summary...Gusty conditions will begin to ease over the next couple
of hours with the loss of diurnal heating/mixing. While surface
pressure gradient will ease a bit as well, surface winds likely to
remain around 10 to 15 mph through much of the evening and overnight
as a surface trough slides into and through the state. As the core
of the surface low continues to develop and pass through south-
central Canada, it will drag a cold front through the state
overnight as well as precipitation.

Tonight through Saturday Night...Fortunately, the "cold air" with
this system is nothing to write home about, more on that in a
second. Ahead of the boundary, theta-e advection and general
isentropic lift should be sufficient to bring about precipitation
across central and eastern portions of the state late overnight and
through the morning. Synoptic and hi-res guidance continue to depict
little to no instability, so have continued with previous shifts in
keeping thunder mentions out. An isolated rumble of thunder or two
may be possible, but of little consequence. System appears to be a
bit slower than previously advertised and with ample dry air in
place, models have trended precip slower. As a result, have also
slowed POP build up and exit this evening and tomorrow respectively
by a couple of hours.

Back to temperatures, with relatively mild conditions in southern
Canada and the recent/ongoing development of the parent surface low,
only a minor cool bump in temperatures will be experienced.
Overnight lows, aided by cloud cover, many will remain mild in the
mid 40s to low 50s deg F from west to east. Daytime temperatures
Saturday will not hit the 70s deg F seen today/Friday, but should be
able to reach into the low to mid 60s deg F under sunshine behind
the front and 850 mb temps in the upper single digits Celsius. One
caveat for eastern/southeast portions of the CWA though will be how
quickly or slowly cloud cover exits. Given the slower appearance,
pushed temps a couple degrees cooler with expected cloud cover in
the area longer.

Winds will also be much lighter Saturday with the relaxing surface
pressure gradient, yielding northwesterly to westerly winds around
10 mph for most. Clear skies and light winds Saturday night will
yield decent radiational cooling, dropping temperatures down into
the upper 30s to low 40s.

.LONG TERM.../Sunday through Friday/
Issued at 319 PM CDT Fri Oct 18 2019

During the day Sunday it should be a nice day for much of the
state. Weak upper ridging will be shifting east as a deep trough
over the Northern into Central Plains begins to shift east. All
models develop a closed low with this system though the GFS is
faster to do so and begins to lift the low a little further north
and east than the other models. Uncertainty between the models
still remains as the Euro is slower and at least for
now...maintains a negative tilt to the trough as it comes through
Iowa. This model had been flip flopping on whether the trough
would come through positively tilted or not. Other models were
more consistent in bringing a quick shot of precip Sunday night
then dry slot us. The Euro keeps a prolonged area of precip with
moderate qpf across central and eastern Iowa Sunday night and
north central to east central Iowa Monday. Modest instability for
this time of year with strong lift across central Iowa with this
very dynamic system Sunday evening so confidence is high in
seeing fairly widespread showers and a few thunderstorms. Timing
is a problem though. Given the time of day even with the modest
instability and strong shear, severe threat may be limited.
Previous forecast did not mention any severe threat and with
models not changing much, I will not be messaging anything
different than the previous forecast had.

Strong cold advection will take place Monday through Tuesday as the
system pulls out but given the different solutions, precip chances
range from dry Monday night to far northern Iowa getting clipped
with a rain to snow or mixed rain and snow period as the colder air
surges in.  After weekend highs in the 60s to around 70, highs in
the upper 40s to 50s will be the rule by Tuesday.  These highs
combined with expected strong winds will make for a raw start to the
week.

By Wednesday the upper low lifts off into Canada bringing a more
zonal flow to the region and moderating temps but another shortwave
appears to be digging over the Plains which will move through the
state sometime in the Thursday/Friday timeframe.  Timing differences
between the models are due to the GFS digging the trough deep into
the Southern Plains while the Euro and Canadian, not as deep and a
little more progressive. The GFS also taps better moisture with a
focus more over southern/southeast Iowa while the Euro has less
moisture and is more widespread across Iowa ahead of the passing
of the trough axis. After the brief warmup Wednesday, colder temps
are back in the picture for late week with mainly 40s expected by
Friday.

&&

.AVIATION.../For the 18Z TAFS through 18Z Saturday afternoon/
Issued at 1240 PM CDT Fri Oct 18 2019

Main concerns within the TAF period continue to be strong winds,
LLWS, and precipitation overnight. Sustained winds 20 to 30 kts
and gusts 30 to 40 kts will continue until close to 00z. KOTM
least likely to see highest end winds. Above surface winds will
remain around 40 to 45 kts after surface winds ease some this
evening, giving rise to LLWS wind shear concerns, as mentioned at
all sites except KOTM. Concerns then turn toward VCSH/SHRA
potential after 06z. Fortunately little to no TS expected,
limiting impacts. Potential for MVFR ceilings during most vigorous
SHRA, but confidence limited in aerial extent so have left out
mentions at this time. Lastly, toward end of the period, winds
will shift northwesterly, but much lighter, under 10 kts, versus
today.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM CDT this evening for IAZ004>007-015>017-
023>028-033>039-044>049-057>060-070>072-081-082-092-093.

&&

$$

SHORT TERM...Curtis
LONG TERM...FAB
AVIATION...Curtis


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