Hail and Damaging Winds in Iowa, September 18, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
633 AM CDT Tue Sep 18 2018

.SHORT TERM.../Today through Tonight/
Issued at 344 AM CDT Tue Sep 18 2018

A more active pattern will emerge today into tonight as the
westerlies drop a bit farther south through the Midwest today.
Surface front currently resides near the IA/MN border early this
morning with the warm sector in place across Iowa. Elevated
convection has developed across central Minnesota near the 850mb
boundary with additional convection rolling from central into
eastern South Dakota this morning. The CAM models have been overly
aggressive with bringing precipitation into the state overnight.
The NAM and Euro have been a bit better in keeping pcpn north of
the state for much of the night but eventually sagging pcpn south
into Iowa after daybreak. This seems more logical given recent
radar trends as convection from South Dakota will likely arrive in
northern Iowa after 12Z along with additional convection in the
warm advection zone in MN dropping south as well. Models drop this
convection southeast through northern and eventually eastern Iowa
by early afternoon. Confidence in the evolution of convection
beyond this time is tenuous at best.

All models increase the low level moisture transport this evening
into the overnight into far northern IA and southern MN.
Convection is likely to break out late afternoon/evening in SE
South Dakota and eventually move east southeast in the mean flow
across northern Iowa tonight. Given the high precipitable water,
relatively deep warm cloud layer and generally not overpowering
updrafts, warm cloud processes appear likely in rain formation
with efficient production. Heavy rainfall appears likely across
the north tonight with training possible along and north of the
surface front which will be stretched across the state. WPC has
this area highlighted as well for the potential of excessive
rainfall later tonight and a watch for flash flooding may be
needed for this time. Any severe threat would likely be during the
evening and primarily from large hail and possibly a strong wind
gust.

.LONG TERM.../Wednesday through Monday/
Issued at 344 AM CDT Tue Sep 18 2018

The main synoptic boundary will be lifting northward early on
Wednesday primarily into Minnesota with much of Iowa in the warm
sector. This should take the main focus for storm development to
the north of the state, although lingering convection on Wednesday
morning is likely in the north. Another unseasonably warm day is
expected along with dewpoints near 70 which may cause issues for
outdoor sports and activities and will highlight this potential in
various products. The bulk of thunderstorms on Wednesday night
will be north of the state into Minnesota where the boundary will
reside and the low level moisture transport will be focused
although any deviation of the frontal position to the south would
put northern Iowa in a worse position.

Surface wave will move quickly north on Thursday through Minnesota
and Wisconsin with cold front trailing through Iowa. Airmass will
remain warm and humid ahead of the front producing one more day of
well above normal temperatures across the southeast half of the
forecast area. Winds should be strong from the southwest ahead of
the boundary with around 40 kts at the top of the mixed layer so
near advisory winds may occur ahead of the front. Convection is
expected to develop ahead of the boundary by early to mid
afternoon as the capping eventually weakens. Decent instability of
1500-2500 J/KG with a strong unidirectional wind profile should
produce some strong linear convection with the threat of large
hail and damaging winds. This convection will slide east by
Thursday night with much cooler air arriving as a Canadian high
slides into the Midwest. This will provide relatively quiet
conditions through the weekend with warming temperatures again by
Sunday. Convection appears likely again by later Monday into
Tuesday of next week as the next boundary arrives.

&&

.AVIATION.../For the 12Z TAFS through 12Z Wednesday morning/
Issued at 632 AM CDT Tue Sep 18 2018

Will see increasing coverage of showers and thunderstorms in
northern and eastern Iowa this morning with some precipitation
lingering into the afternoon. This will produce some ceilings and
MVFR to IFR conditions in and around thunderstorms, however much
of the area will remain VFR through the day. Additional
thunderstorms are expected across Iowa tonight, especially the
northern half with restrictions to visibility and ceilings
expected, especially after 06Z. Surface winds will be quite
variable with a front nearby as well as thunderstorms.

&&

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

&&

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

Hazardous Weather Outlook
National Weather Service Des Moines IA
405 AM CDT Tue Sep 18 2018

IAZ004>007-015>017-023>028-033>039-190915-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-Hardin-Grundy-Black Hawk-
405 AM CDT Tue Sep 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

Thunderstorms are forecast today and tonight, with the potential
for a few strong to severe storms and locally heavy rainfall and
associated water issues. The main severe weather risk will be for
large hail.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Chances for thunderstorms are expected to continue Wednesday and
Thursday. The risk Wednesday will be best over far northern Iowa,
then more widespread Thursday. There will be the potential for
strong and severe storms Thursday, including the potential for
damaging winds, large hail, and periods of heavy rainfall.

.SPOTTER INFORMATION STATEMENT...

Any reports are welcome, though spotter activation is not
anticipated at this time.

IAZ044>050-057>062-070>075-081>086-092>097-190915-
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-
405 AM CDT Tue Sep 18 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

A few thunderstorms may move through portions of central and
eastern Iowa through the afternoon and evening, with a marginal risk
for strong to severe storms. The main risk will be for hail and
periods of locally heavy rainfall.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday

Storm chances will remain across northern Iowa Wednesday, then
spread across the state Thursday. There will be the potential for
strong and severe storms Thursday, including the potential for
damaging winds, large hail, and periods of heavy rainfall.

.SPOTTER INFORMATION STATEMENT...

Any reports are welcome, though spotter activation is not
anticipated at this time.

CURTIS