Power Outage in Denison, IA

Trees were blown down along with power lines.

High winds in excess of 60 MPH downed trees and power lines.

A cold front moved slowly into Iowa from the northwest as an upper level shortwave lifted northeast across the area. The atmosphere was marginally unstable with afternoon highs reaching the mid to upper 80s and dewpoint readings in the upper 50s to low 60s. The lifted index was just 0 to -3 C by early evening. CAPE was limited to around 1000 J/kg. With relatively dry air in place, downdraft CAPE was around 1200 J/kg. CAPE in the -10 to -30 C layer of the atmosphere was between 200 and 400 J/kg, but the freezing level was around 15,000 feet. There was sufficient shear available with 50 to 55 kts present. A narrow area of higher precipitable water values moved into the state by evening, with 1.25 to 1.5 inches available. A line of thunderstorms moved into west central and northwest Iowa during the early evening. One of the storms produced severe weather with quarter size hail in Sac County. High winds also occurred with damage from east of Odebolt to Early to Lake View. Numerous power poles were blown down, roofs were taken from outbuildings including turkey and hog confinements. Boats were thrown at a dealership. In the Lake View area, a tree fell and killed a dog. There were no other injuries. Prior to the storms moving into Sac County, wind damage was reported in Crawford County in the Ricketts area. Several trees were blown down.

A strong line of thunderstorms formed along a cold front to the west of Iowa. As it approached Iowa, the line weakened as there was a very strong cap in place over Iowa and low level winds became divergent. The atmosphere was quite unstable as the line approached with about 2000 J/kg of CAPE available and a lifted index of -5 C. The freezing level was quite high, around 15,000 feet. Downdraft CAPE was very high and was between 1400 and 1600 J/kg. In spite of a 65 to 70 kt low level jet, the storms weakened as they moved into Iowa. One of the storms bowed out in the Denison area of Crawford County and produced a 63 MPH wind gust at the Denison Airport. Tree and power line damage occurred in Dow City and the town of Denison. A shed was blown over near Dow City.

A very dynamic weather pattern set up over the central U.S. during the day on the 9th. A deep trough formed aloft over the southeast U.S. with a strong southwest flow setting up over Iowa at all levels. During the afternoon and evening, the low level jet increased steadily and reached 50 to 70 kts by midnight. Considerable moisture advection took place with surface dewpoints rising into the low to middle 60s by evening. A strong warm front lifted north into Iowa during the afternoon. By the early evening hours it extended from west central Iowa into the east central and southeast counties of the state. This front served as a focusing mechanism for thunderstorm development. A strong elevated mixed layer was in place over the state during the afternoon of the 9th. This kept convection from developing until the evening hours. The freezing level was quite high for mid April, around 13,000 feet. By early evening, at the initiation of storm development, the atmosphere had become very unstable. Lifted indices were in the -9 to -12 C range with elevated CAPE feeding into the state in the 3000 to 5000 J/kg range. The atmosphere was very sheared with effective shear of 60 to 65 kts. As the moisture increased, precipitable water values increased to between 0.8 and 1.1 inches. There was considerable downdraft CAPE available, with 1100 J/kg available. CAPE in the -10 to -30 C layer of the atmosphere was between 500 and 1000 J/kg. The effective helicity in the low layers of the atmosphere was around 800 J/Kg. The LCL was around 1250 meters. Thunderstorms developed along the warm frontal boundary during the very late afternoon and evening hours. The storms formed more as discrete supercells rather than forming into a line. The strongest supercells moved through Onawa County along the northern Crawford County line into Sac County. Another strong supercell moved through Kossuth County. In addition to the tornadoes that occurred, there were scattered reports of hail ranging from golf ball to baseball size with the storms. There were a few reports of wind damage, however most of the damage that occurred was tornadic in nature. On the north edge of the storms, what appeared to be a gravity wave lifted northeast along the back side of the line. It produced a wind gust of near 60 MPH for a short period of time in Emmet County. Several tornadoes touched down across western into northwest Iowa. In Sac County, an EF3 tornado touched down with peak winds of 140 MPH. The path length was about 13 miles long and reached a mile in width at its widest. The tornado began north of Odebolt and ended about 6 miles northwest of Early. Damage was done to a farmstead and one home was totally destroyed. Shortly before that, a tornado touched down in Crawford County from northwest of Ricketts to to about 4 miles northeast of Schleswig. Damage was done to farmsteads along its path. Another tornado touched down in Sac County northwest of Odeboldt and tracked to just northeast of Early. This tornado had winds of 132 MPH, EF2 strength, with a half mile wide peak width. It was on the ground for about 13 miles. In what was the longest tornado track of the day, about 30 miles in length, a tornado touched down just west of Nemaha and tracked to about 3 1/2 miles southwest of Havelock through Sac, Buena Vista, and Pocahontas Counties. The tornado was EF3 in strength and had a peak width of 2 to 3 miles. Major damage occurred along the route with hog buildings destroyed, several farmsteads damaged or destroyed, and residential damage reported. A turkey containment was destroyed, with the building completely removed. Numerous hogs died in the tornado was well. There were at least 4 satellite tornadoes associated with this long track tornado. Two of the satellite tornadoes were a cyclonic/anticyclonic pair as they pushed north into Pocahontas County. Another tornado, related to the same parent cell, tracked southeast and crossed paths with the long track tornado in Pocahontas County. This tornado had peak winds of 170 MPH and was rated EF4. Several farmsteads were destroyed by the tornado. The path width was nearly 600 yards wide. Yet another tornado touched down in Sac County northwest of Nemaha, and tracked to about 9 miles northeast of Nemaha. It was on the ground for about 12 miles and had a peak width of 1/2 mile. Damage was done to farms and a hog confinement building. In Kossuth County, a tornado touched down briefly south of Algona and was on the ground for about a mile. It was 200 yards wide and was EF1 strength with 90 MPH winds. A smaller tornado touched down briefly southwest of Algona in Kossuth County. Damage occurred to a hog confinement building and to power lines. Following the NWS storm survey, there were two other tracks of EF-0 strength tornadoes discovered. One occurred in farm fields of Sac County, north-northwest of Odebolt. The path was about a mile long and 200 yards wide at its widest, with peak winds estimated at 65 MPH. The tornado had an erratic track which actually looped back on itself. Another tornado touched down in farm fields in Pocahontas County. It was also an EF-0 tornado with a path length of about 2.5 miles and maximum width of 250 yards. Winds with the tornado were around 100 MPH. Governor Terry Branstad proclaimed a State Disaster Declaration for Buena Vista, Sac, Calhoun, and Pocahontas Counties.

A strong low pressure developed over Oklahoma and southern Kansas during the day on the 12th. The low lifted north-northeast, passing through central Iowa during the evening of the 12th, reaching southeast Minnesota by the early afternoon of the 13th. A strong push of moisture occurred with the low, bringing widespread precipitation to the state. For the most part, precipitation was in the form of rain during the day of the 12th. There was some wet snow mixed with the rain over western into north central Iowa. Rain changed to snow over west central into northwest Iowa during the evening of the 12th and continued through the overnight hours. There were two main areas of heavy snow. Initially an area of heavy snow formed over west central Iowa. As it lifted northeast, a stronger area of convergence formed over the northwest part of the CWA during the late night hours. This area produced snowfall rates of 1 to 2 inches per hour for several hours. Thunder was also reported with the snow event. An accumulation of 6 inches had already occurred in the Denison area by shortly after midnight. Overnight, a narrow band of very heavy snow developed with a swath of 6 to 12 inches of snow extending from Denison, north-northeast through the Emmetsburg area. By noon on the 13th, Emmetsburg reported the highest snowfall total with 14 inches. Roads became slick and snow packed as heavy snow occurred. Travel was not advised due to slushy roads and visibility being reduced below 1/4 mile in heavy snow. During the morning of the 13th, conditions improved with the rising sun and temperatures holding near to just above freezing. By mid day conditions had improved to where the roads were partly to mostly snow and slush covered. Temperatures were near freezing during the event. As a result, the snow was wet and heavy. This resulted in some tree limb and power line damage. Several power poles were snapped as well. Winds contributed to the power line damage with northwest to north winds of 20 to 35 MPH common. Power was knocked out to several thousand customers at some point during the storm.

A strong southerly flow developed over Iowa as a cold frontal boundary slid southeast into the state. Low level winds of 30 to 45 kts impinged on the front during the late afternoon and into the night. Dew points rose into the low 70s, with surface temperatures in the mid to upper 80s. CAPE values were around 1500 J/kg with lifted indices in the -4 to -5 C. range. Plenty of moisture was drawn into the state with precipitable water values in the 1.5 to 2 inch range. Thunderstorms formed during the late afternoon and evening hours and produced three quarter to one inch diameter hail in west central into parts of northwest Iowa. A few storms produced high winds that blew down trees and broke power lines as well. Heavy rainfall fell over northern Iowa. The rain fell in areas that had about 1.5 inches of rain the previous night. As the additional 3 to 4 inches of rain fell in Kossuth County, flash flooding was reported in the northern part of the county near the intersection of U.S. Highway 169 and Iowa Highway 9. Three to five inches of rain fell in southern Winnebago County. Flash flooding occurred north of Forest City with U.S. Highway 69 flooded and closed at 380th Street two and one half miles north of Forest City. Damage was also reported in the Thompson area and toward Lake Mills. Six to twelve inches of flowing water washed out gravel roads in the area. The same situation occurred in Worth County, where 8 inches of rain washed out roads in the Joice area. The heaviest rainfall occurred in the northwest part of the state with officials reporting 4 to 9 inches of rainfall overnight in Kossuth County, and 6 to 12 inches in Emmet County. In Emmet County, 5 to 6 feet of water was reported in several homes and there were numerous washouts of gravel roads. Highway 4 in Emmet County was closed with water running over the highway one quarter of a mile wide. Some of the areas were hit by two rounds of heavy precipitation. One flash flooding event occurred around midnight, the second from heavy rainfall shortly before dawn.

As had been the case for the past few days across Iowa, the airmass was very unstable over the state. A stationary outflow boundary was located east to west along the southern Iowa border. During the late afternoon heating, lifted indices reached -10 C. again with CAPE values in excess of 4000 J/kg. Strong convergence overcame the mid level cap and thunderstorms fired just north of the surface outflow boundary. The storms became severe rather quickly producing hail and high winds. Most of the hail was under an inch in diameter, however golf ball size hail occurred in Adair and Wayne Counties as stronger storms moved through. High winds caused damage in Wayne and Ringgold Counties with gusts as high as 70 MPH. The winds downed trees and power lines. A strong funnel cloud was spotted 1 mile northeast of Kent in Union County around 1758 CST, however it did not touch down.

Moisture streamed northeast into Iowa ahead of a cold front approaching from the north. Dew point temperatures rose to +15 degrees C. at 850 mb by early evening. At the same time, colder air was moving aloft. This destabilized the atmosphere with thunderstorms erupting in northwest Iowa by late afternoon. The small cluster of storms became severe and moved southeast into the DSM County Warning Area. For the most part, the storms produced hail due to the low freezing level. The freezing level was down to just over 12,000 feet at the time of the evening sounding at 0000 UTC. Numerous reports of one to two inch diameter hail were received from Crawford County southeast into Adams County. A tornado touched down in Crawford County near Buck Grove. The tornado tracked southeast a little over 6 miles, lifting just before crossing the Shelby County line southwest of Manilla. The tornado downed power lines, destroyed several out buildings, and caused extensive crop damage along its path.

Conditions remained relatively unchanged from the previous two days. Temperatures warmed in to the mid 80s to low 90s as dew point temperatures in the low to mid 70s. CAPE values soared to between 4000 and 5000 J/kg over western Iowa and areas west. Thunderstorms erupted during the early evening hours from South Dakota, south into Kansas. Wind shear profiles were favorable for tornadic development. A few tornadoes were reported to the west of the Des Moines CWA, however the main severe event for the Des Moines CWA came in the form of high winds as small bow shaped storms moved northeast along the line. One storm moved into Emmet County by mid evening. This storm produced winds of 75 MPH in the town of Estherville, resulting in tree and power line damage around town. Winds gusted to around 60 MPH as one of the stronger storms moved through Crawford County. A bow echo moved into Sac County late in the evening. High winds overturned a semi-tractortrailer truck east of Sac City just south of the intersection of U.S. Highway 20 and State Road 196. High winds damaged roofs in the town of Sac City. Winds in Sac City were measured by an observer in town at 92 MPH.