Thursday, April 23, 2015

Not Where You Would Expect a Tornado

With much cooler air over much of the country this week has been rather quiet with respect to severe weather. Wednesday has been the most active day with a number of severe storms in northeastern Texas, including several weak tornadoes and hail to three inches.

On Tuesday there was a somewhat rare occurrence in the desert of southern California. A landspout tornado developed out of an isolated thunderstorm and caused some damage to a solar panel array near Desert Center, CA, near the California-Arizona border. If you're wondering what a "landspout tornado" is, it's a tornado that is not associated with a mesocyclone or rotating thunderstorm. They are typically weaker and smaller than tornadoes associated with a supercell and tend to have a smooth appearance, similar to a waterspout (hence the name "landspout"). It is a tornado because the rotating column of air is in contact with the ground and with the parent cumulonimbus cloud - the definition of a tornado.

Landspout tornado near Desert Center, Ca on April 21.
Photo via NWS Phoenix Facebook

The tornado was first reported by a pilot flying in the area. The circulation associated with the tornado was not evident on radar since it was at low levels. The nearest radar was in Yuma, AZ, and at the distance the radar beam at the location of the storm is at about 10,000 feet above the ground. A news crew and a number of other people were able to capture photographs of the tornado.

Another photograph of the Desert Center landspout.
Photo by Russell Fischer via Facebook.

This is the radar image of the storm at 4:00 PDT, about 13 minutes after the tornado was first reported by the pilot. This was at the storm's peak strength. Fifteen minutes later the storm had weakened considerably. The tornado icon indicates where the tornado was reported.
Based on early reports of damage, mostly to the solar array, the National Weather Service has preliminarily given this a rating of EF0. A number of panels were bent and twisted and others were damaged by rocks and other debris flung about by the tornado.

Solar panels damaged by the landspout

The NWS will be sending out a team to survey the damage in a few days. A description of this tornado, photos, and maps can be found at the NWS Phoenix web site.

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.

Tuesday, April 7, 2015

Undertanding Severe Thunderstorm Risk Categories

In October the Storm Prediction Center implemented two additional severe storm risk categories in their outlooks. I blogged about these changes at the time (Changes to Severe Weather Outlooks), but now that we are in the heart of severe storm season I thought it would be a good idea to mention these changes again. You can read a lot more about the specifics in my October blog post. Recently the National Weather Service office in Kansas City (Pleasant Hill) and the Storm Prediction Center in produced a new graphic that should make understanding the differences between these categories easier. The SPC worked with NWS offices and forecasters, communications experts, social scientists, and the public to come up with the descriptions for the severe thunderstorm risk categories.

First, let's take a look at the Day 2 outlook (for Wednesday) issued earlier today, as it shows both the Marginal and Enhanced categories.

This is the SPC outlook for Wednesday, April 8 made on Tuesday, April 7. The Marginal Risk is in dark green, the Slight Risk in yellow, and the Enhanced Risk in Orange.

How should you interpret the risk for severe thunderstorms in each area? Here's the graphic produced by the NWS.

You can read more about the science and numbers behind the severe storm categories at the Storm Prediction Center web site.

One important thing to remember is that any thunderstorm, severe or not, is potentially dangerous. Lightning is the second leading cause of weather fatalities in the U.S., second only behind floods. Lightning itself is not a criteria for a severe thunderstorm. Don't be lulled into a false sense of security by a lower risk category.

You can read more about the science and numbers behind the severe storm risk categories at the Storm Prediction Center web site.

Also, on April 14 the SPC will add a new Summary section to Public Watch Notification Messages. Per the SPC, "The new Summary section is a general 1-2 sentence statement of the severe weather expected in and close to the watch area. This new section facilitates consistent, forecaster-driven, concise communication for public consumption. The Summary is intended to be useful for a variety of communication needs including web page headlines, social media and multimedia briefings."

Wednesday, April 1, 2015

Slow Start to Severe Weather Season

The first Severe Thunderstorm Watch of March 2015
The late but long winter segued into a very slow start to the convective severe weather season in the U.S. Severe weather during the month of March was practically non-existent until the last week of March. The first severe thunderstorm watch of March 2015, and only the fifth of the year was issued for the afternoon and evening of March 24th in eastern Oklahoma, northwest Arkansas, and the southwestern quarter of Missouri. This was the latest first March watch since 1970. Thunderstorms developed as forecast, and when all was said and done there were 70 reports of severe hail (1.00 inch or greater), with three reports of two-inch hail. There were no reports of high winds with these storms.


The atmosphere was a little more primed for severe weather on March 25th, and the Storm Prediction Center had a Moderate Risk of severe weather across central and northeastern Oklahoma into extreme southwestern Missouri and northwestern Arkansas.

Storm Prediction Center outlook issued at 11:30 a.m. CDT March 25, 2015

This time the storms were more widespread and spawned a more springlike menu of hail, damaging winds, and tornadoes. There were a number of reports of hail between two and three inches, and one report of 4.5 inch hail in Tulsa.

The bigger news from this event was the tornado that hit Moore, OK. Yes, this is the same Moore, OK that was devastated by a massive EF-5 tornado on May 20, 2013. This time Moore fared a lot better. There was some low-end EF-2 damage, but most was EF-0 to EF-1. A recap of this event, including video, photos, and maps, is available at the Norman, OK National Weather Service office web site.

Those two days were followed by a few days of relatively quiet weather. On March 31st an area of Slight Risk was painted from northern Texas and southern Oklahoma eastward across Arkansas into west central Alabama. There were many reports of hail and high wind but fortunately no tornadoes. Tornado warnings were issued yesterday afternoon for a storm moving through Arkansas but there were no touchdowns, although a funnel cloud was reported by a spotter.

There will continue to be a slight risk of severe thunderstorms through Friday as the cold front now moving into the central U.S. moves to the east coast. Showers and thunderstorms will return to the Midwest and lower Great Lakes next week as a frontal system stalls across the northern U.S.