Special Weather Statement, NWS Des Moines, IA

Area Forecast Discussion
National Weather Service Des Moines IA
645 AM CDT Sat May 27 2017

...Updated for 12z Aviation Discussion...

.SHORT TERM.../Today through Tonight/
Issued at 340 AM CDT Sat May 27 2017

Main focus as well as questions lies with the possibility of severe
weather today, but also in the near term have had areas of dense fog
over eastern parts of the CWA. Areas around Waterloo have had
visibilities drop briefly to 1/4 mile at times early this morning.
Will monitor near term trends, but have issued an SPS for patchy
dense fog.

Rest of attention then is on later today and questions around
moisture return, the amount of sunshine, or perhaps lack thereof,
and how convection south of the state may play into our potential.
While winds are light at 8z, winds generally have had some sort of
northerly component early this morning. Winds should start to shift
and blow from the east after sunrise, but do not obtain a southerly
component until mid to late morning. This will limit the amount of
moisture return as convection and showers currently over Nebraska
and northern Kansas will be moving into Iowa. Secondly, the
aforementioned convection cluster out west is already spreading mid-
level clouds into western and southern portions of the state per
latest GOES-13/16 imagery. These clouds, which may allow for some
filtered sunshine initially, will limit our overall surface heating.
Still, there will be instability with MUCAPE values between 1500 and
2500 J/kg with bulk shear between 40 and 55 knots. Finally, a strong
line of thunderstorms is forecasted to move across areas south of
Iowa along a warm front later today. This may be too late in the day
to have a large impact on our storms, but something that will have
to be watched.

Bottom line, strongest storms that are able to develop will be
focused over areas along and south of Interstate 80, where a
damaging wind threat will be the main concern. Secondary concerns
will be brief tornadoes on the leading edge of the line of storms
along with perhaps isolated large hail.

.LONG TERM.../Sunday through Friday/
Issued at 340 AM CDT Sat May 27 2017

This period will be relatively inactive with little in the way of
sensible weather. The long wave trough axis and embedded short
waves currently moving through the Rockies should reach eastern
Iowa at onset, while the core of the upper low should move from
Manitoba into the northern Great Lakes by Monday. This pattern
will place Iowa in broad, deep cyclonic flow for an extended
period keeping us mainly dry and temperatures seasonal at best.
Some weak instability is evident north and east during peak
heating Monday, but any showers that develop will likely be sparse
and unorganized with little in the way of convergence. Thus PoPs
are no better than low chance category.

Our cyclonic flow should finally weaken into the middle of next
week as surface high pressure very slowly builds from the high
Plains into the MS Valley. This will gradually increase warm
advection and thus finally re-introduce low PoPs back into the
forecast south and west by Thursday and just beyond the valid
period to end the week.

&&

.AVIATION.../For the 12Z TAFS through 12Z Sunday morning/
Issued at 645 AM CDT Sat May 27 2017

LIFR and IFR visibilities are impacting many eastern Iowa
terminals early this morning, but restrictions should improve into
VFR no later than mid-morning. A line of thunderstorms over
southeastern Nebraska and northeastern Kansas will move through
the state today. Impacts are expected to arrive to the western
central Iowa terminals late this morning and the eastern central
Iowa terminals this afternoon. Recent observations suggest if the
storms maintain a steady state, wind speeds will be 20 knots with
gusts to around 30 knots as they arrive. However, confidence in
the thunderstorms maintaining a steady state not high enough to
put that in at this time so have VCTS wording. After the storms
clear the area, should see a return to mainly VFR conditions.

&&

.DMX WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 8 AM CDT this morning for IAZ027-028-
037>039-048>050-060>062-073>075-084>086.

&&

SHORT TERM...Ansorge
LONG TERM...Small
AVIATION...Ansorge

Hazardous Weather Outlook
National Weather Service Des Moines IA
502 AM CDT Sat May 27 2017

IAZ070>075-081>086-092>097-281100-
Cass-Adair-Madison-Warren-Marion-Mahaska-Adams-Union-Clarke-Lucas-
Monroe-Wapello-Taylor-Ringgold-Decatur-Wayne-Appanoose-Davis-
502 AM CDT Sat May 27 2017

This Hazardous Weather Outlook is for portions of southern Iowa.

.DAY ONE...Today and tonight

There will be a chance of thunderstorms from the late morning
through the evening hours. There will be a Marginal to Slight
Risk of severe weather with damaging wind and large hail the
primary threats.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Slight chances for thunderstorms return to the forecast late
Thursday into Friday with low severe weather potential.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed for a brief period later this afternoon or
this evening.

IAZ037>039-044>050-057>062-281100-
Hardin-Grundy-Black Hawk-Crawford-Carroll-Greene-Boone-Story-
Marshall-Tama-Audubon-Guthrie-Dallas-Polk-Jasper-Poweshiek-
502 AM CDT Sat May 27 2017

This Hazardous Weather Outlook is for portions of central Iowa.

.DAY ONE...Today and tonight

There will be a chance of thunderstorms from the late morning
through the evening hours. There will be a Marginal Risk of
severe weather with damaging wind and hail the primary threats.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

Slight chances for thunderstorms return to the forecast around
Friday with low severe weather potential.

.SPOTTER INFORMATION STATEMENT...

Spotters may be needed for a brief period later this afternoon or
this evening.

IAZ004>007-015>017-023>028-033>036-281100-
Emmet-Kossuth-Winnebago-Worth-Palo Alto-Hancock-Cerro Gordo-
Pocahontas-Humboldt-Wright-Franklin-Butler-Bremer-Sac-Calhoun-
Webster-Hamilton-
502 AM CDT Sat May 27 2017

This Hazardous Weather Outlook is for portions of northern Iowa.

.DAY ONE...Today and tonight

No hazardous weather is expected at this time.

.DAYS TWO THROUGH SEVEN...Sunday through Friday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotters will likely not be needed through next week.

Small

Special Weather Statement
National Weather Service Des Moines IA
859 AM CDT SAT MAY 27 2017

IAZ057-070-271500-
Audubon IA-Cass IA-
859 AM CDT SAT MAY 27 2017

...RAPIDLY INTENSIFYING STORM MAY PRODUCE DIME TO NICKEL SIZE
HAIL...

At 859 AM CDT, Doppler radar was tracking a strong thunderstorm near
Griswold, or 9 miles southwest of Atlantic, moving north at 30 mph.

Nickel size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Atlantic, Griswold, Exira, Elk Horn, Lewis, Kimballton, Cumberland,
Brayton, Marne, Wiota, Cold Springs State Park, Griswold Park,
Atlantic Municipal Airport and Lake Anita State Park.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

This storm may intensify, so be certain to monitor local radio
stations and available television stations for additional information
and possible warnings from the National Weather Service.

To report severe weather, contact your nearest law enforcement
agency. They will relay your report to the National Weather Service
office in Des Moines .

&&

LAT...LON 4150 9515 4151 9504 4160 9504 4160 9509
      4163 9509 4162 9476 4116 9482 4116 9516
TIME...MOT...LOC 1359Z 190DEG 24KT 4128 9510

Kotenberg

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Special Weather Statement
National Weather Service Des Moines IA
958 AM CDT SAT MAY 27 2017

IAZ070-071-271545-
Adair IA-Cass IA-
958 AM CDT SAT MAY 27 2017

...LINE OF STORMS MAY PRODUCE DIME SIZE HAIL...

At 957 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Griswold to 9 miles north of Lake Icaria.
Movement was north at 20 mph.

Dime size hail will be possible with these storms.

Locations impacted include...
Atlantic, Griswold, Lewis, Massena, Cumberland, Bridgewater, Wiota,
Cold Springs State Park, Griswold Park, Atlantic Municipal Airport
and Lake Anita State Park.

LAT...LON 4143 9515 4141 9456 4116 9460 4116 9516
TIME...MOT...LOC 1457Z 184DEG 18KT 4122 9506 4120 9470

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
1000 AM CDT SAT MAY 27 2017

IAZ045-046-057-058-271600-
Guthrie IA-Audubon IA-Greene IA-Carroll IA-
1000 AM CDT SAT MAY 27 2017

...CLUSTER OF STORMS MAY PRODUCE DIME SIZE HAIL...

At 1000 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from 9 miles southeast of Irwin to 6 miles southeast
of Audubon. Movement was north at 30 mph.

Dime size hail and winds in excess of 30 mph will be possible with
these storms.

Locations impacted include...
Audubon, Manning, Coon Rapids, Exira, Elk Horn, Templeton,
Kimballton, Dedham, Halbur, Brayton, Willey, Gray, Manning Municipal
Airport and Audubon County Airport.

LAT...LON 4200 9509 4199 9454 4150 9475 4151 9504
      4160 9504 4160 9508
TIME...MOT...LOC 1500Z 198DEG 24KT 4170 9507 4168 9483

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
1031 AM CDT SAT MAY 27 2017

IAZ072-073-082>084-271615-
Clarke IA-Lucas IA-Union IA-Madison IA-Warren IA-
1031 AM CDT SAT MAY 27 2017

...LINE OF STORMS CAPABLE OF PRODUCING DIME SIZE HAIL...

At 1030 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from 10 miles northwest of Lakeside Casino to 12
miles west of Chariton. Movement was north at 25 mph.

Dime size hail and winds in excess of 30 mph will be possible with
these storms.

Locations impacted include...
Winterset, Osceola, Lakeside Casino, Avenue Of The Saints Event
Center, Milo, Murray, St. Charles, New Virginia, Truro, Lacona,
Lorimor, Lucas, St. Marys, East Peru, Lake Ahquabi State Park and
Otter Creek County Park.

LAT...LON 4104 9411 4134 9409 4130 9333 4119 9333
      4102 9343
TIME...MOT...LOC 1530Z 195DEG 23KT 4110 9398 4108 9353

Kotenberg

Special Weather Statement
National Weather Service Des Moines IA
1139 AM CDT SAT MAY 27 2017

IAZ047-059>061-271715-
Boone IA-Jasper IA-Polk IA-Dallas IA-
1139 AM CDT SAT MAY 27 2017

...LINE OF STRONG STORMS MAY PRODUCE WIND GUSTS TO 40 MPH AND
BRIEF HEAVY DOWNPOURS...

At 1139 AM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Granger to near Pleasant Hill. Movement
was northeast at 30 mph.

Winds in excess of 40 mph will be possible with these storms.
visibility may also be reduced to one mile during periods of heavy
rain... potentially impacting travel.

Locations impacted include...
Des Moines, Ankeny, Johnston, Altoona, Pleasant Hill, Grimes,
Bondurant, Polk City, Saylorville, Madrid, Mitchellville, Colfax,
Prairie City, Granger, Saylorville Lake, Adventureland Amusement
Park, Big Creek Lake, Prairie Meadows, Iowa State Fairgrounds and
Elkhart.

LAT...LON 4151 9337 4176 9392 4196 9378 4192 9370
      4186 9370 4186 9356 4166 9310
TIME...MOT...LOC 1639Z 207DEG 26KT 4178 9389 4154 9338

Kotenberg