What’s the Outlook for Hurricanes this Year?

Posted on May 22, 2011 by Weather Guys Editor

Hurricane season starts June 1 and runs through November. The National Oceanic and Atmospheric Administration is forecasting an above-average season.

We usually expect the hurricane season to have 11 named storms over the Atlantic Ocean; this year’s forecast is for 12 to 18. Of those, 6 to 10 might become hurricanes with winds of 74 mph or more. NOAA also is forecasting that as many as six of these storms could become major storms with wind of at least 111 mph.

There are a couple of conditions that contribute to this expected above-average season. One is the observation that the average ocean temperatures where hurricanes tend to form and travel is two degrees above normal. Sea-surface temperatures of at least 80 degrees is a required condition for hurricane formation. So, a warmer-than-average ocean means there is extra energy to help form and intensify hurricanes.

A second factor is that the La Nina conditions in the Pacific Ocean, which helped to contribute to our cold winter, are weakening. These changing La Nina conditions result in more favorable conditions for hurricane development over the Atlantic Ocean by promoting environments in which the difference between the surface and upper level winds is smaller than usual.

Hurricanes are named to better communicate forecasts to the general public as names can reduce confusion about which storm is being described. A list of names is generated by the National Hurricane Center. The first four Atlantic hurricanes this year will be named Arlene, Bret, Cindy and Don.

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What is with the Humidity?

Posted on May 15, 2011 by Weather Guys Editor

As the spring finally comes after a cool second half of April, it is not only the temperature that rises but also the humidity.  Humidity is a measure of the amount of invisible water vapor that is in the air.  This amount can be measured in a variety of ways and is easily the most highly variable part of the air throughout the year.  One measure of the water vapor content is known as the mixing ratio.  The mixing ratio is, as its name suggests, a ratio of the mass of water vapor in the air (measured in grams) to the mass of the dry air sample that contains the water vapor (measured in kilograms. The dry air is the mixture of every other chemical in the sample except water vapor!).  In the dead of winter, the mixing ratio may be as low as 0.5 grams/kilogram while during the middle of summer it can be as large as 20 grams/kilogram.  Naturally, the larger the amount of water vapor, the “muggier” the air feels and the more likely it is that thunderstorms with torrential rains may develop.

Another, perhaps more popular, measure of the humidity is the dewpoint temperature, often simply referred to as the dewpoint.  The dewpoint temperature is the temperature to which the air must be cooled (at constant pressure) before the water vapor contained within the air condenses into liquid water.  As you might guess, the higher the dewpoint temperature, the higher the water vapor content of the air and the “muggier” the air feels.  For this reason, the dewpoint temperature is often a reliable gauge of human comfort with the usual high dewpoints of summer leaving most of us feeling rather uncomfortable.

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How do We Stay Safe from Tornadoes?

Posted on May 8, 2011 by Weather Guys Editor

The U.S. has a wonderful tornado warning systems. The national system of severe weather watches and warnings has saved untold numbers of lives. In addition to the sirens, the National Oceanic and Atmospheric Administration’s (NOAA) Weather Radio network puts nearly everyone within range of government weather broadcasts. These radios can sound an alarm whenever severe weather warnings are issued for your area. Even with this excellent warning system, there are fatalities as we saw in these recent storms. Many deaths result directly from the force of the tornadoes.

In the event of a tornado, go into a tornado shelter, or the basement, or into a small interior room on the lowest floor of a building, such as a bathroom or closet. Protect yourself from flying debris and stay away from windows. If you are away from a sturdy home, you must seek adequate shelter. Avoid auditoriums, gymnasiums, and eating areas; their large, high roofs can blow off and the walls can collapse. If you are in a mobile home or car, leave it and go to a strong building. Many people are killed when cars and mobile homes are overturned in high winds. If there are no shelters nearby, get into the nearest ditch or depression and protect yourself from flying debris.

Videos have wrongly popularized the notion that it is safe to hide under a highway overpass as the tornado passes overhead. Don’t do it. It is far safer to take shelter in a sturdy building instead. A highway overpass creates a “wind tunnel” effect underneath it and can increase the amount of damage from a tornado.

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What Causes Tornadoes?

Posted on May 1, 2011 by Weather Guys Editor

We don’t know if a particular storm will produce a tornado so the truth is we really don’t know what causes a tornado. We do know the necessary conditions needed for tornado formation.

A tornado is a powerful column of winds that rotate around a center of low pressure. The winds inside a tornado spiral inward and upward, often exceeding speeds of 300 mph. For a thunderstorm to produce a tornado requires warm humid air near the surface with cold dry air above. These conditions make the atmosphere very unstable, in the sense that once air near the ground is forced upward, it moves upward quickly and forms a storm. Severe thunderstorm conditions also include a layer of hot dry air between the warm humid air near the ground and the cool dry air aloft. This hot layer acts as a lid that allows the sun to further heat the warm humid air — making the atmosphere even more unstable.

To form a tornado, the host thunderstorm must also rotate. From below, a rotating cloud base looks like someone is stirring the storm from above. This happens in a storm when wind at the ground is moving in a different direction and speed than the air above. The change in wind speed and direction with height is known as wind shear. This wind shear develops the rotation in the thunderstorm needed for tornado formation.

At this time of year, warm moist winds from the Gulf of Mexico move northward while above the jet stream warm dry winds from the Great Plains move eastward, providing the necessary conditions for severe thunderstorms.

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When is the Severe Weather Season?

Posted on April 23, 2011 by Weather Guys Editor

As the threat of winter snows recedes across the country, it is replaced by the threat of severe weather (i.e. thunderstorms with hail, damaging winds and tornadoes).  The severe weather season, though broadly spanning March – August across the United States, is actually quite regional.  It begins in March in the southern states, moves to the southern Plains during April and May, and then further north toward the Great Lakes states during the summer.  One of the basic underlying reasons for this northward migration of the severe weather threat during the spring and summer is the fact that the jet stream follows a similar seasonal cycle.

The jet stream is a ribbon of high wind speeds located near the top of the troposphere (~ 6 miles above the surface of the Earth).  The jet stream position is strongly tied to the southern edge of the dome of cold air that is centered on the North Pole.  During the depths of winter, that cold dome expands considerably, extending nearly to the Gulf of Mexico.  As the winter ends and spring approaches, the hemisphere begins to warm up and the cold dome shrinks dramatically.  Its southern edge moves to central Canada by early summer.  The jet stream is associated with vigorous upward and downward vertical motions.  The upward vertical motions are instrumental in producing thunderstorms.  Thus, when the jet stream migrates northward as the weather warms in spring/summer, so does the greatest concentration of severe weather outbreaks.

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