Thursday, April 16, 2015

April 9th Tornadoes in Northern Illinois

This year's severe weather season got off to a slow start for much of the country. In fact, the first severe weather watches in March weren't issued until March 24, which was the latest first March watch in 45 years. In the three weeks following another 53 watches have been issued, a rate more typical of this time of year. Last year there were 1045 severe weather reports through April 15, and this year the count was at 923.

2015 severe weather reports (wind (blue), hail (green), and tornadoes (red)) through April 15.
One of the more spectacular, impressive, and certainly damaging events of the year to date occurred on April 9 in the Midwest.

By now you have probably seen some of the photos and video taken of the tornado in northern Illinois. Now that the dust has settled on this event it's a good time to look at some of the summaries and descriptions of the event that are available. It's not too unusual for there to be numerous videos and countless photographs of tornadoes these days, but this event caught my attention because it was, for lack of a better description, so photogenic and several people captured incredible images. Wide open spaces and clear views of storms are usually the calling card of the southern, central, and northern Plains, not northern Illinois.

Last Thursday's severe weather was anticipated and well-forecast by the NWS. The bulls-eye location for this severe weather event extended from eastern Iowa through northern Illinois, an area located southwest of an intensifying surface low. A warm, humid highly unstable air mass was in place over this area, strong winds aloft, converging winds at the surface, and a 30°F temperature difference from north to south of the warm front.

Annotated surface map for 6:00 p.m. April 9.
Source: National Weather Service Chicago

Most of Illinois and eastern Iowa was in an area of Enhanced Risk for severe weather in the 11:30 CDT convective outlook from the Storm Prediction Center.

The convective outlook issued at 11:30 a.m. CDT on April 9 showing and area of Enhanced Risk from Texas to Illinois and Indiana. On the left is a graphic showing the highest tornado probability from eastern Iowa across northern Illinois.

Storms began to develop during the late afternoon in eastern Iowa and northwestern Illinois. By 6:00 p.m. a strong thunderstorm was developing in northern Ogle County. At 6:09 p.m. the National Weather Service issued a tornado warning for this storm.

Radar image at 6:10 p.m.

At 6:29 p.m. a funnel cloud was reported south of the Rockford Airport, and eight minutes later a tornado was on the ground just south of Cherry Valley, IL. This was a short-lived EF0 tornado. The storm that produced the EF4 was was rapidly developing to the southwest of the first storm.

At 6:35 p.m. a tornado warning was issued on the big supercell. A tornado was reported on the ground near Franklin, IL, between Dixon and Rochell just north of Interstate 88.

Radar image at 6:35 p.m.showing warned storm that produced the Fairdale tornado.
The Fairdale tornado just after formation as it intensifies and strikes Crest Foods in Ashton, IL. Debris is clearly seen in the lower half of the funnel.  Photo by Walker Ashley, used with permission.

Over the next 40 minutes this tornado traveled 30 miles, destroying much of the village of Fairdale, IL and producing EF4 damage at several points along its path.this one thunderstorm eventually produced six tornadoes.

When the day was over there were a total of 11 tornadoes confirmed in Illinois, seven of those in northern Illinois, and one in Iowa. There were also three tornadoes reported in eastern Missouri and three in Texas that day.

The path of this tornado was through a lightly populated area of northern Illinois, well west of the Chicago metropolitan area. However, it passed just three miles north of Rochelle, a city of 9,500 people. Had the path of this tornado been located 13 miles to the northwest it would have plowed through the city of Rockford (pop. 150,250). If the track of this tornado were 15 miles to the southeast of the actual path the tornado would have passed through De Kalb (pop. 43,850), the home of Northern Illinois University, and Sycamore (pop. 17,500).

Here is a map of all severe reports received on April 9 superimposed on the outlook issued by the Storm Prediction Center at 11:30 CDT.

There are number of descriptions of the analysis and events of that day that are worth reading. The NWS Chicago office has an updated page, "April 9, 2015 Tornado Event, Including Rochelle/Fairdale EF-4 Tornado". At the bottom of that page are links to event descriptions from NWS Quad Cities, NWS Central Illinois, and NWS Paducah, KY where the other tornadoes occurred. The page has photos, detailed tornado  tracks, radar images, and an analysis of the weather of that day.

Walker Ashley, a professor of meteorology at Northern Illinois University, Certified Consulting Meteorologist,  and well-known storm chaser has written a blog post about his experiences that day chasing the Fairdale tornado, including remarkable photographs and videos he recorded.

The NWS Chicago has a Facebook photo album of storm photos submitted by the public of the storms that day. You do not need a Facebook account to view these.

Dennis Mersereau at The Vane also has a nice blog post with an in-depth look of the tornado.

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