Damaging Winds and Heavy Rain in Iowa, August 26 to 27, 2018

Area Forecast Discussion
National Weather Service Des Moines IA
333 PM CDT Sun Aug 26 2018

.SHORT TERM.../Tonight through Tonight/
Issued at 326 PM CDT Sun Aug 26 2018

Several waves of theta-e advection have been lifting north across
Iowa today resulting in thunderstorms this morning and a scattered
showers this afternoon. A broad cloud shield remains in place and
has limited surface based instability though some partial sun is
beginning to occur over western Iowa. A strong short wave is
lifting northeast through northwest Kansas and into south central
Nebraska this afternoon. The kinematic forcing associated with
this system will begin to arrive into western Iowa by late this
afternoon and into the early evening. This energy combined with an
increasing low level jet and another wave of theta-e advection
will further destabilize western into central Iowa and should
result in thunderstorm development. Should have a few strong to
severe storms with this activity in the 23z-03z timeframe as a few
vigorous updrafts develop, mainly north of Highway 30 in central
Iowa. The primary threat initially will be large hail and damaging
wind gusts. The environment does suggest the potential to evolve
into a QLCS event with an attendant isolated tornado threat.
Sufficient 0-3 km CAPE for enhanced stretching, good effective SRH
in the 200-300 range and 0-3km shear vector of 30-35 kts would
further support the potential as the event moves east.

There should be a brief break in the activity mid to late evening
before additional storm become possible over central Iowa between
06-09z. At this time, an severe threat with this round appears to
be minimal.

.LONG TERM.../Monday through Sunday/
Issued at 326 PM CDT Sun Aug 26 2018

A couple of forecast concerns in the early part of the work week
with hot and breezy conditions Monday, severe chances in the north
Monday night, and then widespread storm chances on Tuesday into
Tuesday night. A brief cooldown anticipated on Wendesday and
Thursday. Leaned closer to a NAM/GFS blend for Monday and then
closer to an ECMWF/NAM blend Tuesday.

Monday into Monday night...shortwave looks to track across
Minnesota, much further north than the latest ECMWF run predicts,
late in the afternoon into the evening.  Mainly have pops confined
north of Highway 20 Monday afternoon before the shortwave and
front shifts southeast Monday night. Strong instability develops
throughout the day, but with forecast soundings suggesting a
strong capping inversion, expecting storms to likely hold off
until late in the afternoon/early evening over far northern Iowa.
It's more likely to see storms remain in southern Minnesota in the
vicinity of the front. If storms do enter northern Iowa, the
shear is strong but remains nearly unidirectional and any cells
that develop will likely develop into a MCS type feature and
quickly push east. The LLJ increases late Monday night into
Tuesday, but the upper level forcing wanes and have low confidence
with storm chances past 06z Tuesday. Also on Monday, south to
southwest flow increases and will lead to a breezy day and hot day
over much of the CWA. Nudged up winds and gusts through the
afternoon as the pressure gradient and mixing increases.

Tuesday, the front looks to be oriented from southwest to
northeast Iowa waiting for the next 500mb shortwave trough to
shove it southeast by late Tuesday night. The timing of the
shortwave remains critical for any severe storm chances. There is
enough instability that develops southeast of front and the
GFS/NAM suggest decent shear available. However, the upper level
support of the aforementioned shortwave looks to finally move into
the state late Tuesday night and instability weakens considerably.
Thus, per the disorganization/timing of the Tuesday system, low
confidence with any widespread severe weather attm.

Wednesday through Saturday, much cooler weather remains consistent
to move into the state Wednesday and even Thursday as surface high
pressure dominates the weather pattern. Late in the week, the
pattern appears to become active again by Friday into Saturday
with both the ECMWF and GFS suggesting a shortwave to cut across
the region during this time.

&&

.AVIATION.../For the 18Z TAFS through 18Z Monday afternoon/
Issued at 1236 PM CDT Sun Aug 26 2018

Scattered showers and isolated storms early this afternoon. Expect
additional storms to develop by late afternoon and into the
evening. Expect a break then some potential for additional
activity towards 06z. Have not accounted for this activity in TAFs
due to lack of confidence. Low level wind shear also expected
overnight. Gusty southwest winds will develop Monday morning.

&&

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

&&

SHORT TERM...Donavon
LONG TERM...Podrazik
AVIATION...Donavon

Hazardous Weather Outlook
National Weather Service Des Moines IA
350 PM CDT Sun Aug 26 2018

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-272100-
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-
350 PM CDT Sun Aug 26 2018

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...This afternoon and tonight

Thunderstorms are expected to redevelop over central and west
central Iowa late this afternoon and into the early evening. A
few severe storms are anticipated with large hail the primary
threat along with a few damaging wind gusts. An isolated tornado
may occur. The best chance for severe weather is along and north
of Highway 30.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

Thunderstorms are possible once again Monday into Monday evening.
There is a threat for a few severe storms over mainly northern
Iowa. Large hail is the primary threat though isolated damaging
wind gusts and isolated tornadoes remain possible. A few severe
storms are possible again on Tuesday in central into southern
Iowa.

Another chance for storms arrives late Thursday night into Friday.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed at various times today through
Tuesday. The potential today is from 6 pm through 11 pm.

Special Weather Statement
National Weather Service Des Moines IA
724 PM CDT Sun Aug 26 2018

IAZ004>007-015>017-023-024-270200-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-
Including the cities of Estherville, Algona, Forest City,
Lake Mills, Northwood, Manly, Emmetsburg, Garner, Britt, Kanawha,
Mason City, Clear Lake, Pocahontas, Laurens, Rolfe, Fonda,
Gilmore City, and Humboldt
724 PM CDT Sun Aug 26 2018

...Gusty Winds Possible over Northern Iowa This Evening...

Showers and a few thunderstorms from Sioux City east along the
Highway 20 corridor will be capable of producing wind gusts of 40
to 50 mph as they move northeast at 35 to 40 mph this evening.
Isolated gusts to 55 mph or more are possible. These showers,
storms and gusty winds may affect locations from Sac City and
Rockwell City north and east toward the Minnesota border through
the evening hours.

A 58 mph gust occurred at the Sioux City Airport with the passage
of this precipitation.

Small