Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
1156 PM CDT Fri Aug 14 2020

...Updated Aviation Discussion...

.DISCUSSION.../Tonight through Friday/
Issued at 256 PM CDT Fri Aug 14 2020

Summary...

Primary forecast concerns will be over the next 6 to 12 hours with
the potential for strong to severe storms, including damaging winds
and hail as threats. More details in the sections below.

The remainder of the forecast will remain on the quieter, a bit
cooler, and much less humid side with northwest upper level flow
prevailing and only periodic chances for a shower/storm or two.

Today through Tonight...

Mid and upper level troughing continues to work its way across the
Dakotas and into Minnesota this afternoon, including a surface low
reflection sliding NE currently positioned in the ND/SD/MN tri-state
border region, yielding a surface cold front stretching back to the
S/SW. Ahead of the cold front, a remnant elevated outflow/gravity
wave continues to kick off scattered showers and the odd lightning
strike or two, but being away from the bulk of the forcing and
support will continue to predominantly struggle as it continues into
western Iowa. Back along the approaching front, deep convection has
begun to initiate in the Sioux City and will be the area to watch.
Across the state, established southerly flow kept the region on the
warm and humid side with temperatures in the mid 80s and dew points
regularly into the low 70s.

Ultimately, the question is whether or not we have enough support
for severe storms late this afternoon and evening. Let's take a
look...

With the warm and humid conditions established, and a degree of
moisture pooling ahead of the front, MUCAPE values in excess of 4000
J/kg and MLCAPE values in excess of 2500 J/kg respectively have been
established across W/NW IA by mid afternoon today. Generally,
forcing will not be in question either with a potent cold front,
attached to the parent surface low north of the region, that will
traverse the state through the evening. Shear profiles are at least
marginally supportive with effective shear depictions remaining in
the 30-35 kt range. The better veering profiles reside north of the
area, yielding environmental 0-3km SRH values around or less than
150 m2/s2, especially as you work south. Areas of steep lapse rates
in excess of 8 deg C/km continue to suggest at least some large hail
potential, especially should storms efficiently tap into available
shear. Strong winds may also be seen with drier low-mid levels and
DCAPE values in 1000 J/kg.

Working against severe potential are depictions of convection ahead
of the better/most supportive 0-3km and 0-6km shear profiles and
shear vectors losing orthogonal position through the event.
Additionally there are continued depictions of a stronger EML within
HRRR family than originally anticipated by overnight and early
morning synoptic and hi-res models, suggesting more of a broken
line of convection versus traditional QLCS.

Overall, anticipate broken line segments of convection to work
their way across the state through the late afternoon and evening
hours, with the best severe potential over the northwest third of
the area, lining up well with existing SPC Day One Outlook. A few
cells may be able to efficiently take advantage of the
anticipated effective shear profiles and yield periods of severe
hail and winds. Otherwise, expect majority of the activity to be
sub- severe. As the convection and front work into central Iowa,
waning instability will reduce the severe threat further,
especially on the hail side, but stronger winds could continue
into the evening.

While severe threat into central and eastern Iowa will reduce
through the evening, given the continuing recovery efforts from
the Monday derecho, recovery crews and the public will want to
keep an eye out on vulnerable trees, power poles, etc... as they
may be susceptible to additional damage from sub-severe winds, let
alone severe winds, given their already weakened nature.

Saturday through Friday...

Expected weather becomes much more comfortable, and predominantly
quiet through the rest of the forecast. Why? A large 500mb high will
develop over the SW US and result in amplified western CONUS ridging
that will yield prevailing NW flow over the region. Other than
periodic shortwaves yielding lower end shower/storm chances at
times, a dry forecast will prevail much of the time.

&&

.AVIATION.../For the 06Z TAFS through 06Z Saturday night/
Issued at 1158 PM CDT Fri Aug 14 2020

Line of weak convection moving across the state with the
advancement of a cold front. Winds will shift northwest and gust
to around 20-25 knots for a short stint. There looks to be a brief
period of MVFR visibility at ALO due to some heavy rain.
Otherwise, VFR conditions are likely through the TAF period.

&&

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

&&

DISCUSSION...Curtis
AVIATION...Podrazik

Hazardous Weather Outlook
National Weather Service Des Moines IA
1028 PM CDT Fri Aug 14 2020

IAZ004-005-015-023-024-033-034-044-045-057-160330-
Emmet-Kossuth-Palo Alto-Pocahontas-Humboldt-Sac-Calhoun-Crawford-
Carroll-Audubon-
1028 PM CDT Fri Aug 14 2020

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time.

IAZ006-007-016-017-025>028-035>039-046>050-058>062-070>075-
081>086-092>097-160330-
Winnebago-Worth-Hancock-Cerro Gordo-Wright-Franklin-Butler-Bremer-
Webster-Hamilton-Hardin-Grundy-Black Hawk-Greene-Boone-Story-
Marshall-Tama-Guthrie-Dallas-Polk-Jasper-Poweshiek-Cass-Adair-
Madison-Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-Monroe-
Wapello-Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis-
1028 PM CDT Fri Aug 14 2020

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Tonight

Thunderstorms are possible through 3 am tonight. A few storms in
northern Iowa may be strong to severe, with damaging winds and
hail as the primary threats.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not expected at this time, though any
hazardous weather reports is appreciated.

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
150 AM CDT Sat Aug 15 2020

IAZ073-083-084-150715-
Clarke IA-Lucas IA-Warren IA-
150 AM CDT Sat Aug 15 2020

...Small hail possible and brief period of heavy rain...

At 148 AM CDT, Doppler radar was tracking a strong thunderstorm 10
miles southeast of Avenue Of The Saints Event Center, or 11 miles
south of Indianola, moving southeast at 35 mph.

Penny size hail and winds up to around 30 mph will be possible with
this storm.

Rainfall amounts up to around one inch will be possible with this
storm.

Locations impacted include...
Lucas and Woodburn.

LAT...LON 4123 9375 4129 9362 4106 9340 4098 9357
TIME...MOT...LOC 0648Z 326DEG 30KT 4120 9364

Ansorge

Special Weather Statement
National Weather Service Des Moines IA
221 AM CDT Sat Aug 15 2020

IAZ083-084-095-150800-
Clarke IA-Lucas IA-Wayne IA-
221 AM CDT Sat Aug 15 2020

...Small hail possible...

At 220 AM CDT, Doppler radar was tracking a strong thunderstorm 12
miles east of Osceola, moving south at 20 mph.

Penny size hail and winds to around 30 mph will be possible with
this storm.

Rainfall amounts up to around one inch will be possible with this
storm.

Locations impacted include...
Humeston, Lucas, Woodburn and Derby.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

&&

LAT...LON 4090 9356 4090 9360 4111 9364 4115 9351
      4089 9335 4086 9356
TIME...MOT...LOC 0720Z 340DEG 19KT 4106 9355

Ansorge