Damaging Wind in Iowa, October 21, 2017

Area Forecast Discussion
National Weather Service Des Moines IA
627 PM CDT Sat Oct 21 2017

.SHORT TERM.../Tonight through Sunday/
Issued at 312 PM CDT Sat Oct 21 2017

The primary concern will obviously be convective trends into
tonight. Confidence is high that showers and storms will blossom
eventually due to the deep convergence and deep, strong
thermodynamic and kinematic forcing with the long wave trough
passage, but less so with respect to intensity. Storms developed in
NW Iowa around 18z and looked as if they were going to mature fairly
quickly, but since diminished. Additional convection has also
developed farther south in a zone of strong frontal convergence and
frontogenetical forcing, but this has also been somewhat slow to
strengthen. Western Iowa RAP soundings show a 2km warm layer at the
base of the EML continuing to erode and there may be a slim window
for surface based convection to mature, but it will not last long as
the front quickly undercuts anything resulting in convection being
rooted around 1km. With plenty of stratus over Iowa and the loss of
heating soon, feel much of the convection will be below severe
limits unless trends increase soon.

Shallow and deep shear parameters are both high so there remains a
small potential for supercells and brief tornadoes in addition to
hail and wind. QLCS tornadoes will be possible into the evening with
low level shear persisting, but as mentioned overnight the 0-3km
shear vectors are not oriented favorably: too far to the north vs
east and line/front orientation. Small segments would need to orient
NE to realize available shallow shear and SRH. Moderate to locally
heavy rains may eventually be a greater threat with strong low level
moisture transport ahead of the front. 00Z NAEFS precipitable water,
specific humidities and integrated water vapor transport are all
near climatological extremes. Thus expect widespread efficient rains
in 3.5+km warm cloud depths with 1” amounts over the SE half of the
forecast area possible by tonight.

Skies should clear into tomorrow as the system departs with
temperatures 5 to 10 degrees cooler than today. The biggest change
will be less humid conditions with surface dewpoints no better than
the 30s and 40s.

.LONG TERM.../Sunday night through Saturday/
Issued at 312 PM CDT Sat Oct 21 2017

The long term period begins with significant shortwave trough diving
south through the Upper Midwest early next week. This will send a
strong cold front through central Iowa during the morning and
afternoon hours on Monday. Deep unidirectional winds and efficient
boundary layer mixing behind the front should still be enough to
warm temperatures into the upper 50s and low 60s. Not expecting
anything more than isolated showers with the frontal passage as the
atmosphere is starved of moisture. A reinforcing shot of cold air
advection will lead to much cooler and blustery conditions on
Tuesday. Model guidance remained consistent to the previous forecast
with highs only in the upper 40s and low 50s.

A bounce back to near average temperatures is expected through the
middle of the week as an upper level ridge builds over the Plains
states. The warm up will be short lived as a deep upper level low
digs southeast and dislodges a large dome of cold air to our north.
The GFS and EC remain out of sync with their timing and evolution of
the trough, although the slower EC has trended toward the faster GFS
with the latest run. Regardless of any model discrepancies, there is
high confidence that the coldest air we have seen so far this season
will be ushered in by a strong cold frontal passage in the late
Thursday or Friday time frame. Latest guidance suggests highs will
struggle to get out of the 40s on Friday and Saturday with below
freezing temperatures possible overnight. Light rain and/or a few
snow flakes are also possible Friday night into Saturday as moisture
wraps around the back side of the upper level low. At this time it
looks like the better lift/moisture will slide north of the area.

&&

.AVIATION.../For the 00Z TAFS through 00Z Sunday evening/
Issued at 627 PM CDT Sat Oct 21 2017

Broken line of convection with occasional MVFR cigs moving across
area now...and will exit the far east by 10-12z Sunday. Activity
may increase southeast overnight near KOTM as low level jet
increases aft 03z. Some wind and mainly MVFR VSBY reduction as
storms roll through. Aft 06-10z nearly all areas will be VFR with
either high clouds or skc aft 12z. Winds increase aft 15z with
gusts near 16-18kt until 21z. /rev

&&

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

SHORT TERM...Small
LONG TERM...Martin
AVIATION...REV

Hazardous Weather Outlook
National Weather Service Des Moines IA
328 PM CDT Sat Oct 21 2017

IAZ004>007-015>017-023>028-033>039-044>050-057>062-070>075-
081>086-092>097-221100-
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-
328 PM CDT Sat Oct 21 2017

This Hazardous Weather Outlook is for much central Iowa.

.DAY ONE...This afternoon and tonight

Thunderstorms are expected into the evening, lingering into
eastern Iowa overnight. There is a marginal to slight risk of
severe weather with large hail, damaging wind and a brief tornado
possible. Moderate to locally heavy rains may occur as well,
especially central and east.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Freezing temperatures are possible Thursday night far north and
Friday night over all but far southeast Iowa.

.SPOTTER INFORMATION STATEMENT...

Spotter activation may be needed briefly into the evening hours.

Small

Special Weather Statement
National Weather Service Des Moines IA
737 PM CDT SAT OCT 21 2017

IAZ094>096-220100-
Appanoose IA-Wayne IA-Decatur IA-
737 PM CDT SAT OCT 21 2017

...A line of strong storms may produce wind gusts to 50 mph and
small hail...

At 736 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from 9 miles east of Leon to 10 miles northeast of
Mercer. Movement was northeast at 60 mph.

Half inch hail and wind gusts up to 50 mph will be possible with
these storms.

Locations impacted include...
Centerville, Corydon, Rathbun Lake, Seymour, Allerton, Mystic,
Lineville, Promise City, Numa, Clio, Plano and Millerton.

LAT...LON 4067 9370 4090 9338 4090 9325 4075 9283
      4058 9325 4058 9349
TIME...MOT...LOC 0036Z 238DEG 52KT 4071 9358 4061 9338

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
805 PM CDT SAT OCT 21 2017

IAZ074-084-085-095>097-220145-
Appanoose IA-Marion IA-Wayne IA-Lucas IA-Davis IA-Monroe IA-
805 PM CDT SAT OCT 21 2017

...Line of storms may produce wind gusts to 50 mph...

At 805 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from Chariton to 11 miles northwest of Unionville.
Movement was northeast at 45 mph.

Wind gusts up to 50 mph will be possible with these storms.

Locations impacted include...
Centerville, Chariton, Albia, Lake Sundown, Rathbun Lake, Seymour,
Moravia, Russell, Lovilia, Mystic, Williamson, Hamilton, Melrose,
Promise City, Unionville, Numa, Rathbun, Plano, Marysville and Udell.

LAT...LON 4100 9339 4132 9301 4118 9287 4116 9287
      4116 9285 4095 9264 4090 9264 4090 9258
      4084 9252 4058 9309 4058 9317
TIME...MOT...LOC 0105Z 226DEG 37KT 4103 9332 4061 9313

Kotenberg

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