Summer forecast questions: How hot? How many thunderstorms?

Published 7:00 am Tuesday, June 9, 2015

Last week was the start of meteorological summer, which is comprised of June, July, and August. And, it certainly felt like summer, as Friday began our first stretch of multiple days with highs above 90 degrees.

The dominant weather system affecting both our summer temperatures and rainfall is the North Atlantic Subtropical High, sometimes called the Bermuda High. Usually the circulation around this high pumps warm, moist air over the northern Gulf Coast. The strength and position of this high is a dominant factor for our summer weather.

Often in summer our warmest temperatures are accompanied by mainly dry conditions with only a few isolated afternoon showers and thunderstorms across south Mississippi. This type of weather is usually associated with strong high pressure ridging in the upper parts of the atmosphere. This may be due to a strong extension of the Bermuda High into the Gulf South, or a strong upper ridge of high pressure building over the central USA. In both cases, sinking, warming air from above usually puts a lid on the development of thunderstorms. With this pattern, 30 percent or less of south Mississippi and southeast Louisiana gets hit with showers.

Sign up for our daily email newsletter

Get the latest news sent to your inbox

When the upper ridge and its capping temperature inversion weakens, blobs of hot air from near the Earth’s surface can rise high into the afternoon atmosphere and form the thunderheads, also known as cumulonimbus clouds, that we often see in summer.

This is what will happen as the upper ridge weakens today, and it will likely be the case for the rest of the week as 50-70 percent of the Gulf South is forecast to experience showers or thunderstorms each afternoon or evening.

When we are in this wetter mode, our temperatures, while not cool by any means, tend to top out near or below 90. The temperatures throughout the atmosphere are not as warm when the upper ridge is weaker, and the increased clouds and showers keep our afternoons less hot.

Regardless of whether we are in the hot and drier mode, or the warm and wetter pattern, everyone needs to have a healthy respect for any thunderstorms that do develop. While most of our summer thunderstorms do not reach severe levels, they still bring one of the most dangerous weather phenomena we encounter: lightning.

According to the National Weather Service lightning killed 322 people in the U.S. over the past decade. Eight died in Mississippi.

In an NWS study by John Jensenius, factors that reduce the likelihood of lightning fatalities are willingness to cancel or postpone outdoor activities, ability to be aware of an approaching or developing storm, vulnerability of the activity, and ability to get to a safe place quickly.

Additionally, lightning results in tens of millions of dollars of U.S. private property damage each year. The National Fire Protection Association reports that fire departments respond to over 20,000 lightning-caused fires annually.

Heat is an even more dangerous summer threat than lightning. The Centers for Disease Control reports that 7,233 heat-related deaths occurred in the U.S. from 1999 to 2009.

The best weather advice for south Mississippians during summer is (1) reduce your exposure to the heat, and (2) move inside when a thunderstorm is approaching.

By Skip Rigney