Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 181733

Area Forecast Discussion
National Weather Service Des Moines IA
1233 PM CDT Wed Oct 18 2017

...18z Aviation Update...

.SHORT TERM.../Today through Tonight/
Issued at 347 AM CDT Wed Oct 18 2017

There will be some features of interest through the period, but
these will result in little change in sensible weather conditions.
In the immediate time frame, a fairly decent PV anomaly continues to
drift down the MO River Valley as shown in water vapor imagery and
objective model analysis. Moisture is so limited however that there
is little to show for it beyond a small patch of cirrus.

A much more sizable trough and forcing/subsidence couplet will track
across the northern Plains and southern Canada into the Great Lakes
today and tonight however. Although there will be deep, coupled
thermodynamic and kinematic forcing it too will be quite moisture
starved with nothing beyond more cirrus this far south. The only
weather of note will be the associated slow moving weak frontal
passage into tonight. This will stall along the IA/MO border by
12z with slightly cooler temps tonight, especially north.

Forecast soundings suggest MOS is again on the low side with the
high end of raw model guidance more appropriate versus 00z RAOBs and
what occurred yesterday. This would be at or just above persistence.

.LONG TERM.../Thursday through Tuesday/
Issued at 347 AM CDT Wed Oct 18 2017

Not much has changed in the larger scale with upper ridging passing
through the region through Friday producing dry conditions and
unseasonably warm temperatures that will be gradually warmer each
day through Friday.  By Friday upper 70`s will be likely across the

Friday night through Saturday evening, a deep trough advances out of
the Rockies with a surface low and frontal boundary advancing
through the Plains Friday night and through the Upper Midwest on
Saturday.  The system is pretty progressive so we will not see
lasting precip.  Precip is questionable Friday night as we will be
in warm advection ahead of the upper trough but soundings are
showing some dry air to overcome before we see much in the way of
precip.  By Saturday especially in the afternoon, this will not be
the case as deeper moisture moves in by 18Z.  We will see
thunderstorms by Saturday afternoon with moderate to heavy rainfall
possible as PWATS jump to an inch and a half in the afternoon.  While
there are still a lot of model runs between now and Saturday,
instability with this system jump to 1500 J/Kg or more in the
afternoon with 40kts or so of deep shear.  While it`s questionable
whether we realize all of the instability, I think models have been
consistent enough to at least mention the possibility of strong

After the front and upper trough pass temps cool back into the 60s
for Sunday and Monday as another upper trough passes through the
Northern Plains into the Great Lakes.  Models diverge here as to how
deep this trough will be and whether we get an precip from this.
Right now if we did see precip, it would mainly impact the east.
However by Tuesday a second shot of colder air comes down with the
passage of the trough and this will knock highs down into the 50`s
for Tuesday and Wednesday with lows in the 30`s tuesday night.  So
definitely seeing some cooler temps in the longer term that could
potentially remain with us beyond the period if the pattern of a
ridge to the west and a trough over the Great Lakes with the Upper
Midwest in broad northwest flow comes to pass.


.AVIATION.../For the 18Z TAFS through 18Z Thursday afternoon/
Issued at 1227 PM CDT Wed Oct 18 2017

VFR prevailing through yet another TAF period. Winds to continue
out of the S/SW during the daylight hours with gusts around 20kts
at times. Sustained winds and gusts will subside quickly at
sunset and eventually turn out of the northwest, but light, as a
weak front moves through. Winds will return back out of the S/SE
tomorrow morning as surface high slides by overnight as well.




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