May Tornadoes

May is the peak US tornado month by count - averaging 270 tornadoes in an average year. May 3, 1999 in Oklahoma. May 20, 2013 in Moore. May 22, 2011 in Joplin. May 4, 2007 in Greensburg. Some of the most consequential US tornadoes have occurred in May, all in a corridor that stretches from Texas through Iowa.

Why May Is the Peak Month

By May, the US atmosphere reaches maximum tornado potential in the Great Plains:

Gulf moisture is at peak levels - dew points routinely 65-70°F
Jet stream still strong - producing extreme wind shear
Surface heating maximizes - producing extreme atmospheric instability
Dryline positioned over Texas/Oklahoma - providing daily triggers for supercells

These four factors combine to produce the most consistently favorable tornado atmosphere anywhere on Earth. May 2011 saw 542 tornadoes in a single month - one of the highest monthly totals ever recorded.

Notable May Tornado Events

May 3, 1999 Bridge Creek-Moore F5 - 36 killed, 301 mph world record wind
May 22, 2011 Joplin EF5 - 158 killed, deadliest modern US tornado
May 20, 2013 Moore EF5 - 24 killed, Plaza Towers Elementary
May 31, 2013 El Reno - widest ever recorded (2.6 mi), TWISTEX deaths
May 4, 2007 Greensburg EF5 - first-ever EF5
May 11, 1953 Waco F5 - 114 killed, created the warning system
May 25, 2008 Parkersburg EF5 - second-ever EF5
May 4, 1991 Andover F5 (day after outbreak) - Golden Spur
May 27, 1997 Jarrell F5 - most extreme damage ever
May 25, 1955 Udall F5 - 80 killed at night

May vs. April - Which Is Deadlier?

Historically, April kills more people per year despite May having more tornadoes. Why: April concentrates outbreaks in the Deep South where population density and nighttime tornado rates are higher. May outbreaks tend to spread across the more sparsely populated Great Plains.

Exception: May 2011 killed 178+ Americans due to Joplin (158) plus other events. May 2013 killed 47 with Moore and El Reno. When May outbreaks strike major cities, the death toll rivals April.

The Great Plains May Corridor

Peak May tornado activity concentrates in a corridor running from north Texas through Oklahoma, Kansas, Nebraska, and into Iowa. This corridor - sometimes called the "spring supercell alley" - produces roughly 40% of all US May tornadoes.

Individual state peaks in May:

Oklahoma: May is by far the peak month
Kansas: May-June split as peak
Texas: May is peak in the Panhandle; April in the north
Nebraska: May-June split
Iowa: June is actually the peak, with May close behind

Late May Extremes

The last week of May has produced some of the most historically significant US tornado events:

May 25, 1955 - Udall, KS F5 (80 killed)
May 27, 1896 - St. Louis F4 (255 killed)
May 27, 1997 - Jarrell F5
May 22, 2011 - Joplin EF5
May 24, 2011 - El Reno-Piedmont EF5
May 25, 2008 - Parkersburg EF5
May 31, 2013 - El Reno (widest ever)

Meteorologists have noted this "late May peak" for years - a period of consistently extreme atmospheric setups over the central US.

Preparing for May

Household tornado drill in April, review results in early May
NOAA weather radio tested and batteries fresh
Insurance policy reviewed for wind/hail deductible
Shelter plan communicated to everyone in the household
Pay attention to SPC forecasts - especially "Enhanced" or "Moderate" risk days

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