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2015 Rochelle–Fairdale tornado

EF4 tornado in Illinois in 2015

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Meteorological synopsis

On April 4, 2015, 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 Ark-La-Tex 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 Northeastern 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 Convective Available Potential Energy (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 850 mb (4,781 ft) strengthened at or above 45 mph (72 km/h) below winds of 70 mph (110 km/h) at 700 m (2,300 ft), 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.

At 1:50 p.m. CDT (18:50 UTC), the SPC issued a tornado watch for northern and central Illinois, far northwestern Indiana, far southern Wisconsin, and portions of Lake Michigan. A 60% chance of two or more tornadoes was assessed across the watch box, with a 40% chance of at least one strong tornado. Isolated shower activity began forming across the region just prior to the issuance of watch, eventually growing into a line of strong to severe thunderstorms, including supercells. One such high-precipitation supercell was first severe warned at 6:11 p.m. CDT (23:11 UTC); the National Weather Service office in Chicago, Illinois, warned of winds up to 70 mph (110 km/h) and quarter- to golf ball-sized hail. The weather forecast office upgraded the severe thunderstorm warning to a tornado warning at 6:35 p.m. CDT (23:35 UTC), and four minutes later, trained storm spotters reported a damaging tornado north of Ashton. This was one of several tornadoes observed across the Central United States on April 8 and April 9.

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