EF3 TORNADO WITH A ½ TO 3/4 MILE-WIDE PATH DEVASTATES MAPLETON, IOWA ADVANCE WARNING SAVED LIVES 7:20 PM, SATURDAY APRIL 9, 2011 Walter Hays, Global Alliance for Disaster Reduction, University of North Carolina, USA

TEN TORNADOES The National Weather Service confirmed that 10 tornadoes hit Iowa over the weekend of April 9-10.

ADVANCE WARNING The advance warning system sirens sounded in Mapleton at 7:05 p.m., allowing residents 15 minutes to take cover.

THE PEOPLE RESPONDED They huddled in basements and bathrooms and hid beneath furniture or stairwells as winds up to 140 mph sucked, twisted and damaged buildings, homes and vehicles across Mapleton.

60 % OF MAPLETON, IA DESTROYED, BUT … One of the 10 twisters destroyed or damaged more than 100 homes and injured more than a dozen people.

BUT NO DEATHS, DUE IN PART TO 15 MINUTES OF ADVANCE WARNING

DAMAGE IN MAPLETON

BACKGROUND

TORNADOE SEASON The peak tornado season is late winter through midsummer,…. But tornadoes can happen any time of the year when the atmospheric conditions are right.

TORNADO ALLEY ALTHOUGH TORNADOES HAVE OCCURRED IN EVERY STATE, THEY OCCUR MOST FREQUENTLY IN “TORNADO ALLEY,” WHICH INCLUDES PARTS OF: TEXAS, OKLAHOMA, ARKANSAS, TENNESSEE, NEBRASKA, KANSAS, MISSOURI, ILLINOIS, AND IOWA.

PHYSICS OF A TORNADO

THE PHYSICS OF TORNADOES TORNADOES ARE CAUSED BY THE COLLISION OF DESCENDING COLD AIR MASSESS COMING FROM THE NORTH AND ASCENDING WARM AIR MASSES COMING FROM THE SOUTH.

THE PHYSICS OF TORNADOES THE COLLISION CREATES A FUNNEL OF HIGH-VELOCITY WIND THAT IS VERY DESTRUCTIVE AS IT “TOUCHES DOWN” ONE OR MORE TIMES ALONG A LONG, NARROW (TYPICALLY M) PATH.

HAZARDS OF A TORNADO (AKA POTENTIAL DISASTER AGENTS) WIND: SPEEDS CAN REACH 500 KM/HR (300 MI/HR) THUNDER, LIGHTNING, AND HEAVY PRECIPITATION E EXTREME HAIL CAN BE VERY DAMAGING

ILLUSTRATIVE PHOTOS OF SOME PAST TORNADOES

“TOUDHDOWN” –IOWA IN 2008

PHOTO OF A “TOUCHDOWN”

PJOTO OF A “TOUCHDOWN”