Tornado outbreak of April 8–9, 2015
High-end EF4 damage to a farmstead in Rochelle, Illinois
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| Type | Tornado outbreak |
|---|---|
| Duration | April 8, 2015 – April 9, 2015 |
| Tornadoes confirmed | 27 |
| Max rating1 | EF4 tornado |
| Duration of tornado outbreak2 | about 36 hours |
| Highest winds |
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| Casualties | 2 deaths |
| Areas affected | High Plains, parts of the Midwest and Ohio Valley |
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1Most severe tornado damage; see Enhanced Fujita scale 2Time from first tornado to last tornado Part of the Tornadoes of 2015 |
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1Most severe tornado damage; see Enhanced Fujita scale 2Time from first tornado to last tornado
The tornado outbreak of April 8–9, 2015 was a relatively small but damaging outbreak of tornadoes that occurred in parts of the Great Plains and in the Midwestern United States, As much as 27 tornadoes were confirmed during the two days, most of them weak, however a select few of them were powerful and damaging.
By far the most significant tornado of the outbreak was a very high-end EF4 wedge tornado that struck the towns of Rochelle and Fairdale, Illinois, causing catastrophic damage. It was also the strongest tornado recorded in 2015, as well as the strongest to occur in Illinois since the F5 tornado that struck Plainfield in 1990.
On April 4, the Storm Prediction Center (SPC) issued their Day 6 severe weather outlook, highlighting a substantial and widespread risk for severe weather from southeastern Oklahoma to northeastern Illinois valid for April 9. This threat area was maintained in the Day 5 outlook and narrowed to a corridor from southern Missouri into northern Illinois in the Day 4 outlook. On April 7, a Day 3 Enhanced risk was issued across most of Illinois, eastern Missouri, and small portions of adjacent states. No further changes to the threat level were made, although the Enhanced risk was significantly expanded late on April 8 to include portions of the southern Great Lakes, lower Ohio Valley, Ozark Plateau, and Arklatex region. Around midday on April 9, the SPC issued a 10% hatched tornado threat area across much of northern Illinois and small portions of nearby states, signifying the potential for several tornadoes, of which one or two had the potential to be strong (EF2 or stronger on the Enhanced Fujita scale).
The catalyst for the severe weather outbreak came as a positively-tilted shortwave trough progressed across the central High Plains into the Great Plains and eventually through the western Great Lakes region. A weaker disturbance pushed from the Ozarks into the Northeast United States, acting to strengthen southwesterly winds aloft across the risk area. At the surface, a weak area of low pressure initially centered over northeastern Kansas early on April 9 progressed steadily northeast while intensifying, reaching the trisection of Iowa, Wisconsin, and Illinois by late that evening. A cold front stemming from the low progressed eastward across the Mid-South, whereas an arching warm front slowly pushed northward across eastern Iowa and northern Illinois. Modest surface heating ahead of the cold front allowed mid-level CAPE values to reach 1,000–1,500 J/kg, and a mass of rich moisture transported northward from the Gulf of Mexico pushed dewpoints into the lower 60s °F across the Enhanced risk area. Winds at 850mb strengthened at or above 45 mph (75 km/h) atop winds of 70 mph (110 km/h) at 700mb, creating a favorable setup for sustained supercells. Although the overall directional component of low-level winds was expected to be less than ideal as a whole, a small area of southeasterly surface winds developed near the surface low in northern Illinois.
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