Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1151 PM CDT Wed Jul 18 2018

...Updated for 19/06Z Aviation Discussion...

.SHORT TERM.../Tonight through Thursday/
Issued at 252 PM CDT Wed Jul 18 2018

This will be an active timeframe as upper low intensifies as it
moves from the northern Plains into the upper Midwest. Initially,
gradient of deeper moisture and instability lies just to the south
and west of the forecast area this afternoon. Flow has been nearly
parallel with this gradient and therefore little progression into
Iowa has been noted today. This will begin to change into the
evening and overnight as the flow starts to veer with the approach
of the shortwave. Thunderstorms are forecast to develop in the
instability axis west of the state late this afternoon and
evening, dropping southeast into the instability during the
evening. Meanwhile, as the theta-e advection begins to intensify
this evening across Iowa, showers and thunderstorms will develop
and gradually spread east overnight. Model solutions have
continued to slow the overall speed of the system and eastern
areas my stay dry much of the night prior to arrival of rainfall
around sunrise. Severe threat continues to be rather minimal given
weak deep layer shear with only an isolated threat existing. Heavy
rain also remains possible given the moisture transport into the
state overnight although recent drier conditions will limit the
biggest threats to urban areas.

Main surface front will slide slowly through the state on Thursday
as surface low drops southeast along the boundary. The north and
east will remain cloudy for much of the day along with on and off
showers and storms through the day with moderate to strong mid
level kinematics moving into the northeast. Locations near the
boundary by afternoon will have some concern for severe weather as
overall shear is forecast to be higher on Thursday given proximity
of upper low and seasonably strong upper jet moving into the
central United States. Insolation in the warm sector will support
decent instability by afternoon with fairly good turning in the
low levels of the hodographs. While large hail and damaging winds
will remain the primary concerns, the tornado threat will also
exist near the boundary during the afternoon hours. Spatially,
areas east of I35 and south of Highway 30 have the best threat for
the strongest convection. Biggest concern for heaviest rainfall
will also be east of I35 in northeast Iowa where a few different
rounds of rainfall are expected through Thursday evening.

.LONG TERM.../Thursday night through Wednesday/
Issued at 252 PM CDT Wed Jul 18 2018

Lingering storms are expected across the northeast half of the
forecast area on Thursday evening with precipitation diminishing
overnight as forcing gradually subsides. Subsidence will increase
during the night and into Friday over much of the state with
decent cold advection. The steep mid level lapse rates near the
upper low on Friday afternoon should lead to some shower
development across eastern portions of the forecast area but
western locations should remain dry with the surface ridge just
west of the area. Thereafter, conditions will be relatively quiet
into the middle of next week. Only fly in the ointment will be a
boundary sliding through the state on Monday with the threat of a
few storms. Temperatures will be seasonal or even slightly below
normals, especially later next week as a stronger push of Canadian
air occurs toward the end of the work week.

&&

.AVIATION.../For the 06Z TAFS through 06Z Thursday night/
Issued at 1151 PM CDT Wed Jul 18 2018

SHRA/TSRA are forecast to move into the area tonight with increasing
warm advection. MVFR to IFR conditions are expected in the vicinity
of the thunderstorms with MVFR ceilings after the precipitation
ends. Surface winds will remain from the southeast but will slowly
increase tonight.

&&

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

&&

SHORT TERM...Cogil
LONG TERM...Cogil
AVIATION...Zogg

Hazardous Weather Outlook
National Weather Service Des Moines IA
338 PM CDT Wed Jul 18 2018

IAZ004-015-023-024-191100-
Emmet-Palo Alto-Pocahontas-Humboldt-
338 PM CDT Wed Jul 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

River Flood Warnings remain in effect for parts of the West Fork
Des Moines River. Please refer to the National Weather Service
website at weather.gov/desmoines for further details.

Thunderstorms are expected tonight with minimal severe weather
threat. Locally heavy rainfall is the main concern.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Thunderstorm chances linger into Thursday. Localized heavy
rainfall will be possible with a marginal threat for severe storms
with large hail and damaging winds.

Additional non-severe thunderstorm chances return again Monday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not anticipated at this time.

IAZ005>007-016-017-025>028-033>039-044>047-057>059-070>072-
081>083-092>094-191100-
Kossuth-Winnebago-Worth-Hancock-Cerro Gordo-Wright-Franklin-
Butler-Bremer-Sac-Calhoun-Webster-Hamilton-Hardin-Grundy-
Black Hawk-Crawford-Carroll-Greene-Boone-Audubon-Guthrie-Dallas-
Cass-Adair-Madison-Adams-Union-Clarke-Taylor-Ringgold-Decatur-
338 PM CDT Wed Jul 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms are expected tonight with minimal severe weather
threat. Locally heavy rainfall is the main concern.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Thunderstorm chances will continue into Thursday. Localized heavy
rainfall will be possible with a marginal threat for severe
storms with large hail and damaging winds.

Additional non-severe thunderstorm chances return again Monday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be possible Thursday.

IAZ048>050-060>062-073>075-084>086-095>097-192045-
Story-Marshall-Tama-Polk-Jasper-Poweshiek-Warren-Marion-Mahaska-
Lucas-Monroe-Wapello-Wayne-Appanoose-Davis-
338 PM CDT Wed Jul 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorm chances return to the region this evening and tonight.
A few strong storms are possible. Locally heavy rainfall is also
possible.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday

Thunderstorms are expected on Thursday. There is a slight risk of
severe storms on Thursday afternoon with large hail, damaging
winds and isolated tornadoes all possible. Localized heavy rain is
also expected.

Additional non-severe thunderstorm chances return again Monday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation will be possible Thursday.

Special Weather Statement
National Weather Service Des Moines IA
314 AM CDT THU JUL 19 2018

IAZ062-075-190915-
Poweshiek IA-Mahaska IA-
314 AM CDT THU JUL 19 2018

...Thunderstorms With Locally Heavy Rain and Lightning...

At 310 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near New Sharon to near Sigourney. Movement
was very slow to the northeast at 10 mph.

Rainfall amounts up to three inches will be possible with these
storms. Ponding of water and possible urban runoff may briefly cause
street flooding near Barnes City and Coal Creek over the next hour
or two. Rainfall rates have been over 1.5 inches per hour.  If
rainfall continues over the same area localized flooding may occur.

Locations impacted include...
Montezuma, New Sharon, Deep River and Barnes City.

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.

Frequent cloud to ground lightning is occurring with these storms.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 4151 9241 4133 9241 4147 9275 4165 9256
      4155 9230 4151 9230
TIME...MOT...LOC 0810Z 211DEG 9KT 4153 9266 4134 9222

REV