Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 211208
AFDDMX

Area Forecast Discussion
National Weather Service Des Moines IA
705 AM CDT Mon Aug 21 2017

.SHORT TERM.../Today through Tonight/
Issued at 409 AM CDT Mon Aug 21 2017

Difficult forecast for the next 24 hours or so. There is a lot
going on and any one thing can impact the forecast. This morning`s
convection is the result of warm advection and a low level jet
moving across the state. These storms should slowly shift east
through the day and dissipate. They are slow moving however and
have dumped several inches of rainfall across west central Iowa.
There should be a lull from later morning into the afternoon but
cloud cover and the eclipse will have an impact on temps so high`s
today are difficult to figure. The eclipse will impact the temps
for a short time but if clouds linger and don`t allow the sun,
once it returns to warm things up, then we could be considerably
cooler than what I have going. In general I knocked highs down
about a category or so from guidance.

Later this afternoon and tonight a cold front will push through
the state in conjunction with a pretty strong trough sweeping from
the Plains into the Great Lake. We will have sufficient
instability around as well as shear and a frontal boundary as well
as boundaries left by the morning storms. Storms are expected to
re-fire late afternoon and tonight but really could develop in a
more widespread regime than just the cold front. Heavy rainfall
and severe storms are possible especially this evening and large
hail and damaging wind are the main threats but a few tornadoes
are also possible. By 12Z Tuesday the front should be through all
but the far southeast part of the state.

As for the eclipse viewing...it is not looking great for Iowa
since there will be quite a bit of cloud cover and moisture
surging north ahead of the cold front. So even if sun does occur,
it will likely pop more clouds.

.LONG TERM.../Tuesday through Sunday/
Issued at 409 AM CDT Mon Aug 21 2017

Compared to the short term, the long term is relatively benign.
The Great Lakes low and upper trough moves east as a ridge builds
over the Rockies. The cold front that will push across the state
through Monday night will keep some lingering clouds/showers going
on Tuesday morning but should be done by early morning. We will be
in northwest flow for much of the week so temps will be
comfortable and we should be dry for the most part until the
weekend. Anytime we are in NW flow, it is difficult to pick out
the potential for and timing of weak shortwaves that tend to move
through the flow but these usually do not result in much precip
for us. One such shortwave may possibly impact the state on
Thursday. The Euro and Canadian models bring light precip into the
northwest Wednesday night/Thursday morning then fizzle out any
precip chances during the day while the GFS is a little more
robust keeping light precip across the north through the day.
Model blends held a slight chance across the northwest early
Thursday. Since the impact is minimal and confidence in any one
solution is low at this point, these low PoP`s were left in
there.

A more significant wave moves across the region over the weekend
bringing a somewhat better chance for showers and isolated storms.
Again the GFS is way more bullish on precip than any of the other
models so there is low confidence PoP`s left in the grids for the
weekend. Highs will be a few degrees below normal from mainly
mid-week through the weekend but so will humidity levels. All in
all not a bad stretch coming up once we get past Tuesday morning.

&&

.AVIATION.../For the 12Z TAFS through 12Z Tuesday morning/
Issued at 630 AM CDT Mon Aug 21 2017

Ongoing convection will end by 17Z with a break over TAF locations.
Areas of MVFR/IFR conditions will prevail through that time then VFR
conditions will occur.  A cold front will move across the state from
22Z through the rest of the period.  Winds will switch to the NW
behind the front and strong storms will develop along/ahead of the
front.  MVFR conditions will exist once again with local IFR
conditions.

&&

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

$$

SHORT TERM...FAB
LONG TERM...FAB
AVIATION...MJB/FAB



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