Does a Warm Winter Lead to an Earlier Start to Severe Weather's Prime Season?
Published: February 16, 2017
This winter's warm temperatures and occasional tornado outbreaks beg the question: do warmer conditions early in the year lead to an earlier start to severe weather's prime season?
(RECAPS: Late January Outbreak | New Orleans EF3 Tornado | Southeast Texas Valentines Tornadoes)
The simple answer is yes, but to explain this answer, we must look at the basics of when and how tornadoes and other types of severe weather form.
When is Severe Weather Season?
This depends on where you are located. Severe weather can happen any day of the year virtually anywhere in the country, but damaging storms are most likely where conditions for the growth of severe storms are the most favorable on any given day. In general, severe weather is most likely in the U.S. from late winter in the southern U.S., through spring and early summer elsewhere.
Severe weather probabilities in March and June. (Storm Prediction Center)
In the South, some locations, such as Florida, can have two severe weather seasons – one in the winter and spring and another during the summer. The farther south in the country you are, the more likely it is to see severe weather earlier in the year. This is mainly because warm air arrives sooner in the South. Ingredients come together later in spring and summer closer to the Canadian border as warm air and an active jet stream push north.
(MORE: Tornado Central)
Why Might Tornadoes Be More Common Following a Warm Winter?
One of the ingredients needed for severe weather is warm, humid air near the surface, contributing to instability. If warm air is increasingly more common near the surface, as it has been for decades, instability should also be more common, assuming the air aloft remains somewhat relatively cool.
This is not the only variable that goes into the formation tornadoes and other severe weather, but it is one of the more fundamental factors.
(MORE: Spring Outlook)
Warm air earlier in the season, or lasting through the winter, could foster more severe weather in parts of the Midwest and central and northern Plains states earlier in the year. If temperatures in March, for example, are more typical of April, then we would expect April-like severe weather chances. Remember, the weather does not have a calendar.
This warm air could also power more severe weather and potentially stronger storms in the South during the cooler months.
Severe storms that do occur during the winter months in the south are typically in environments of high vertical wind shear and low instability. The high vertical wind shear part of that balance comes from the jet stream that causes wind to change in strength and direction with height, which allows storms to rotate. In the winter, this ingredient is usually common.
The low instability portion of that balance is almost always in question during the cooler months because without heat (instability), you get plain rain or clusters of non-severe thunderstorms. If we get a warmer winter or spring, the chance of severe weather increases.
Conditions become increasingly more dangerous with higher levels of shear and instability.
What is the Effect of El Niño and La Niña?
Although there is no direct cause-and-effect relationship between El Niño/La Niña and weather that occurs on the scale of individual thunderstorms, it is known that those patterns can foster conditions that lead to more severe weather.
El Niño and La Niña are patterns of warming or cooling in the equatorial Pacific Ocean that exhibit various forcings on the atmosphere (collectively ENSO) with various precipitation and temperature effects around the world.
Typical wintertime effects of La Niña.
(MORE: La Niña is Gone; Now What?)
Notice the warmer and wetter area in parts of the Mississippi Valley and Ohio Valley? Both of those conditions are favorable for wintertime tornadoes during La Niña.
According to The Weather Channel severe weather expert Dr. Greg Forbes, La Niña led to twice as many January-through-April outbreaks (6) compared to El Niño (3) in the 27 largest, most impactful tornado outbreaks in U.S. history.
MORE: Severe Weather Hits the South
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