Thunderstorms!

Thunderstorms occur all over the United States, and Alaska is no different. Even though the state does not receive as many thunderstorms as others, it is still affected.

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So what is a thunderstorm?

A thunderstorm is a tall, vertically developed cloud that produces lightning and thunder. These storms usually produce heavy precipitation, which includes heavy rain and sometimes hail.

In order for a thunderstorm to occur, they must require instability. This instability can be caused by warm or humid air near the surface and cold air aloft. The change is key, however, as without instability, the impending storm will either die out or not form in the first place.

Air must rise in order for instability to hit. This air can rise due to many different triggers. These triggers can be: fronts and dry lines, sea or lake breezes, gust fronts, or mountains.

Another ingredient of thunderstorms is that they require vertical wind shear, which is a change in wind direction or speed with height. This helps determine the storm’s severity and can directly influence the strength of the wind’s draft.

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So what is a SEVERE thunderstorm?

In order to be considered severe, a thunderstorm must contain any of the following ingredients: wind gusts at the surface greater than 50 knots (57.5 mph), hail with a diameter greater than 1.00 inch (2.54 cm), or a tornado. The National Weather Service follows these criteria and determines the potential of significant damage caused by the thunderstorm.

According to the National Weather Service, their motto is, “When Thunder Roars, Go Indoors…and stay there at least 30 minutes after the last clap of thunder.”

Regarding lightning, the NWS states that lightning “DOES NOT constitute a severe thunderstorm!  As stated above EVERY thunderstorm has lightning.”

What are the different types of thunderstorms?

There are three different types of thunderstorms, an Ordinary thunderstorm, Multicell thunderstorm, and a Supercell thunderstorm. The most important way we determine which type of thunderstorm is due to vertical wind shear.

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Ordinary Thunderstorms

These are the most common thunderstorms, and are typically not severe. They usually develop in areas where there is weak vertical wind shear, especially in the summer away from fronts. They last about an hour, but have different stages associated with them.

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The cumulus stage is when a cloud forms because warm air rises and cools, creating an updraft. The mature stage is when the updraft reaches the tropopause, which acts as a lid on the storm. The dissipation stage is when the updraft weakens and is replaced by the downdraft, which creates precipitation and causes dissipation of the thunderstorm.m9tnin2

Multicell Thunderstorms

These thunderstorms are also called Mesoscale Convective Systems, abbreviated as MCS. These thunderstorms produce a lot of rainfall, creating hazards such as flooding, straight-line winds, and occasional tornadoes. Sometimes covering an entire state, this complex thunderstorm begins as a cluster of scattered thunderstorms, each being at a different stage in their life cycle.

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Supercell Thunderstorms

These thunderstorms are long-lived, rotating thunderstorms that can last for many hours. Always rotating, they are often isolated, which is why they are almost always severe. Being the strongest thunderstorm, they are still rare, however, almost all of the violent tornados are produced by Supercell thunderstorms, and account for most of the hail larger than the size of golf balls. These thunderstorms have been known to produce winds and flash flooding.

According to the Western Regional Climate Center, Anchorage receives only 1 thunderstorm a year, on average. The total number, on average, for the entire state of Alaska producing thunderstorms is 40, according to the NWS.

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Thunderstorms in the Area

Around Anchorage, there was a freak thunderstorm on July 26, 2012. The residents of the city of Barrow, the nation’s coldest and northernmost city, experienced thunder and lightning for the first time in 8 years. This was a rare occurance, however, the timespan between sightings was even more uncommon, as the city sees a thunderstorm once every five years, on average.

This thunderstorm was due to the Arctic Ocean’s cold front meeting unusually warm, moist air. The city that averages only about 37 degrees Fahrenheit in the summer began to see temperatures 30 degrees higher, creating the instability the storm needed. In the city, lightning was masked by the heavy rainfall, however, 30 miles south recorded severe lightning strikes.

According to a member of the community, “rather than running in, we’re running outside to see it.” Even though this is an interesting and rare occurance, if she follows the NWS safety procedures, she would have headed inside after hearing the thunder.

Hazards

Lightning, Hailstorms, and Tornadoes are all hazards that can affect a US city. In Anchorage, there have been occurances or each, however, the frequency is not as high as one might think.

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Tornadoes

There have only been four recorded tornadoes since 1950 in the entire state of Alaska. This is major, because with Alaska being the largest state in terms of land area, this means that Alaska averages one tornado every 142,595 square miles. In comparision, Oklahoma, the state known for tornadoes, sees a twister every 20 square miles.

The impact may not be as great in terms of frequency, but since they are such a rare occurance, the severity of the situation is increased during the storm.

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Hailstorms

In 2015, the first NWS severe thunderstorm warning was issued for Anchorage in 6 years. The severe thunderstorm warning was issued in southern Alaska, and had hail one inch in diameter. Anchorage was not hit, however, the sightings were in extreme proximity to the city.

According to the Weather Channel, hail has only been reported 13 separate days since 2004, making Alaska an unlikely place for the phenomenon to occur. The website stated that the “severe thunderstorms were triggered in the unstable, cold air aloft on the back edge of a vigorous upper-level trough swinging through the state. This same feature triggered heavy rain in other parts of coastal Alaska, leading to a deadly landslide in the town of Sitka.” This is in congruence with what we have covered in class and makes the description of the storm make more sense.

According to the article, summer thunderstorms are not uncommon in Alaska, particularly in the interior. However, the combination of hot, humid air, and wind shear, or changing wind speed and direction with height supportive of severe thunderstorms, is rarely a norm in the state.

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Lightning

There has been 48 fatalities related to lighting, on average per year, in the last 30 years.. With the cold temperatures, Alaska does not get many or if any lighting fatalities, however, the last one was in 2006. The hazards associated to lightning in Alaska are not as severe as in other states, however, weather related fatalities in Alaska in 2015 was 7, while there were 8 others injured.

Due to weather related events, the property damage Alaska suffered in 2015 was upwards of 25 million dollars, ranking in the top 40% of the country in terms of dollars lost. For a state that doesn’t see much severe weather, when it does hit, it makes a major impact.

Sources are hyperlinked, however, here are more sources that we used:

http://www.srh.noaa.gov/jetstream/tstorms/tstorms_intro.html

http://www.nssl.noaa.gov/education/svrwx101/thunderstorms/

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