Monday, July 3, 2017

Risk of Tornadoes Begins to Decline in July, But Is Fourth Most Active Month

Linda Lam
Published: July 1,2017

July is past the peak months for tornado activity, but the threat of tornadoes increases in some areas of the northern U.S.
Severe weather activity can develop throughout much of the country in July due to the availability of moisture and warmth. However, the overall weather pattern in July is not as conducive for tornado formation in many areas, as in the peak months of April through June.
(MORE: Tornado Central)
As the jet stream continues to migrate northward, the greater risk for tornadoes also shifts northward compared to the spring. A higher risk of tornadoes is found in areas from the central and northern Plains eastward into parts of the Midwest and Northeast in July.
Areas shaded in darker red have a greater risk of tornadoes.
(Storm Prediction Center/NOAA)
The threat of tornadoes also remains elevated in portions of the Florida Peninsula through July, as the wet season ramps up. Waterspouts coming ashore here is one of the reasons for the higher tornado risk.
The states with the highest average number of July tornadoes, based on data from the Storm Prediction Center for 1989-2013, are Minnesota (11 tornadoes), Colorado (10 tornadoes), North Dakota (10 tornadoes), Kansas (7 tornadoes) and Florida (7 tornadoes).
(Virtual Reality: Experience the Formation of a Tornado)
An important point to remember is that tornadoes can develop in areas not shaded in red. It is always a good idea to be aware of weather conditions, especially when participating in outdoor activities.
Based on data from 1996-2015, July sees the fourth most tornadoes on average. However, the drop in tornado count from June to July is noticeable. July typically experiences almost half as many tornadoes as June, with 112 tornadoes in July compared to 217 in June.
Average monthly U.S. tornadoes from 1996-2015.
Over the past three years, 102 tornadoes have been confirmed in July. This count still puts the month in the top fourth spot, behind April, May and June, which is the time of year climatologically when the ingredients for tornadoes are most likely to come together.
One limiting factor in the number of tornadoes in the heart of summer is there is typically lower wind shear across most of the U.S. Damaging wind gusts are a bigger threat in July, especially from squall lines. This includes the risk of derechoes, or large clusters of thunderstorms that produce widespread wind damage.
(MORE: What Is Typical Summer Weather)
Although wind shear is generally limited, if the jet stream does dig southward, the higher dew points (moisture) and very warm temperatures can help to spawn tornadoes.
Farther south, upper-level high pressure is also usually in place over much of the South, which results in pop-up thunderstorms in the afternoon across the region with a lower risk of tornadoes compared to other times of the year.

July's Tornado History

A few metro areas that you might not normally think of regarding tornadoes have experienced the most tornadoes in July, compared to the rest of the year, according to severe weather expert Dr. Greg Forbes.
Minneapolis/St. Paul has seen 66 tornadoes in the three counties of Ramsey, Hennepin and Dakota from 1950-2013.
(Dr. Greg Forbes, The Weather Channel)
Minneapolis is one of those metro areas, where 20 tornadoes were recorded from 1950 through 2013, accounting for about a third of all tornadoes here during this period. This is no surprise given the more northern location of the jet stream. In addition, July sees the warmest temperatures for the Twin Cities, with an average high for the month in the mid-80s.
Strong tornadoes have caused significant destruction in July. One example took place in southwestern Minnesota when a F4 tornado struck the city of Granite Falls on the evening of July 25, 2000. One person was killed, more than a dozen were injured and millions of dollars in property damage occurred in the area.
(MORE: Tornado Strength Has Larger Effect Than Population on Casualties)
Another city that has seen the most tornadoes recorded in July from 1950 through 2013 is Boston. Although tornadoes are not a regular occurrence in the region, eight tornadoes have been confirmed in the Boston metro area in the month, which is more than a quarter of the tornadoes that occurred during this time period. This also has to do with the more northern storm track in July, along with the warm and humid conditions that can be in place.
Boston has seen 29 tornadoes in the three counties of Suffolk, Middlesex and Norfolk from 1950-2013.
(Dr. Greg Forbes/The Weather Channel)
Other parts of the Northeast have been impacted by tornadoes in July. On the afternoon of July 12, 2006, a F2 tornado tracked through part of Westchester county, New York. Six people were injured and there was over $10 million in property damages.
A southern city that sees its peak tornado activity in July is the Tampa-St. Petersburg-Clearwater area. Data from 1950 through 2013 indicates that July sees slightly more tornadoes, with 51, than other months. June is a close second, with 48. The higher number of confirmed tornadoes in the area is partly due to waterspouts coming onshore.
Tropical activity also contributes to tornado counts in coastal areas. One July example is Hurricane Cindy in 2005. More than 40 tornadoes were confirmed from July 6-8, 2005, across portions of the South into the mid-Atlantic. Three of the tornadoes spawned were rated F2, including one in Hampton, Georgia which significantly damaged the Atlanta Motor Speedway.
MORE: Tornadoes Around The World

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