Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1239 PM 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 18Z TAFS through 18Z Tuesday afternoon/
Issued at 1236 PM CDT Mon Aug 21 2017

TAF forecast a mess as round one of showers and thunderstorms to
impact KMCW in the next few hours... with more widespread showers
and thunderstorms arriving overnight. Thunderstorms should be very
widespread in coverage and periods of heavier rain may reduce
visibility to 3 miles or less. Have left IFR TEMPO groups from
vsby out of TAFs for now. These may need to be added in during
next update. Thunderstorm timing will obviously need to be
refined as well.

&&

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

SHORT TERM...FAB
LONG TERM...FAB
AVIATION...Kotenberg

Hazardous Weather Outlook
National Weather Service Des Moines IA
403 AM CDT Mon Aug 21 2017

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-220915-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-
Boone-Story-Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-
Poweshiek-Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-
Clarke-Lucas-Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-
Appanoose-Davis-
403 AM CDT Mon Aug 21 2017

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Periods of thunderstorms will occur today and tonight. There is a
marginal risk for hail and strong wind with the storms this
morning, along with heavy rainfall that could lead to localized
flooding.

There is a slight risk for severe storms late this afternoon and
tonight. All modes of severe weather is possible along with heavy
rainfall which could lead to localized flooding. Very large hail
is possible across much of central Iowa especially north of
Interstate 80 and west of Interstate 35. Damaging wind and a few
tornadoes are possible across all of central Iowa.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed this afternoon and evening.

FAB

Special Weather Statement
National Weather Service Des Moines IA
943 AM CDT MON AUG 21 2017

IAZ023-024-034-035-211545-
Humboldt IA-Calhoun IA-Webster IA-Pocahontas IA-
943 AM CDT MON AUG 21 2017

...Gusty Winds and Hail Possible over Northwest Iowa...

At 940 AM CDT, Doppler radar was tracking a strong thunderstorm near
Rockwell City, moving northeast at 20 mph with another strong storm
exiting Buena Vista County into Pocahontas County.

Nickel size hail and winds in excess of 40 mph will be possible with
these storms.

Locations impacted include...
Fort Dodge, Pocahontas, Rockwell City, Dakota City, Humboldt, Lake
City, Manson, Laurens, Gowrie, North Twin Lake, Fort Dodge Regional
Airport, South Twin Lake, Dayton, Stratford, Pomeroy, Fonda,
Coalville, Rolfe, Badger and Otho.

LAT...LON 4290 9397 4247 9397 4247 9394 4222 9393
      4221 9485 4247 9486 4248 9488 4290 9492
TIME...MOT...LOC 1440Z 243DEG 15KT 4244 9453

Small

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
957 AM CDT MON AUG 21 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0913 AM     HEAVY RAIN       1 NE AUDUBON            41.72N 94.92W
08/21/2017  M5.30 INCH       AUDUBON            IA   FIRE DEPT/RESCUE

&&

BSS

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1028 AM CDT MON AUG 21 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0700 AM     HEAVY RAIN       5 NW WALL LAKE          42.31N 95.16W
08/21/2017  M3.30 INCH       SAC                IA   COCORAHS

            COCORAHS OBSERVER REPORTED 3.30 INCHES OVER
            THE LAST 24 HOURS.

&&

PRELIMINARY LOCAL STORM REPORT
NATIONAL WEATHER SERVICE DES MOINES IA
1030 AM CDT MON AUG 21 2017

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0655 AM     HEAVY RAIN       1 SE AUDUBON            41.71N 94.92W
08/21/2017  M3.17 INCH       AUDUBON            IA   AWOS

            AUDUBON AWOS RECORDED 3.17 INCHES OVER THE
            LAST 24 HOURS.

&&

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Special Weather Statement
National Weather Service Des Moines IA
139 PM CDT MON AUG 21 2017

IAZ004-005-015-211945-
Palo Alto IA-Kossuth IA-Emmet IA-
139 PM CDT MON AUG 21 2017

...Strong Storms Moving across Northern Iowa...

At 137 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Okamanpedan Lake to near Five Island Lake
to Silver Lake. Movement was northeast at 55 mph.

Dime size hail and wind gusts up to 50 mph will be possible with
these storms.

Locations impacted include...
Emmetsburg, Okamanpedan Lake, Iowa Lake, Five Island Lake, Union
Slogh, Swan Lake, Silver Lake, Ingham Lake, Armstrong, Bancroft, Swea
City, Burt, Titonka, Ringsted, Fenton, Lakota, Lone Rock, Ayrshire,
Ledyard and Cylinder.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with these storms, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

These storms may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

&&

LAT...LON 4350 9397 4325 9397 4324 9398 4296 9491
      4301 9491 4316 9472 4350 9465
TIME...MOT...LOC 1837Z 241DEG 46KT 4347 9461 4319 9467 4306 9490

Small