What is a nor’easter?

Posted on November 15, 2012 by Weather Guys Editor

A nor’easter is an extratropical cyclone that affects the northeastern United States and extreme eastern Canada. An extratropical cyclone is a low-pressure system that forms outside of the tropics and is usually associated with fronts, unlike a tropical cyclone. A nor’easter is named for the strong northeasterly winds that blow across this region as the path of the low pressure moves northeastward, slightly to the east of the North American coastline.

The counterclockwise winds that flow around the low pressure system draw moist maritime air from over the Atlantic Ocean. At the same time, cold polar air from Canada air moves southward toward the East Coast and encounters northward-moving warmer and more humid air originally from the Gulf of Mexico. The dynamics of the resulting warm and cold fronts produces some of the heaviest snowfalls that occur along the East Coast.

Nor’easters often occur between the months of October and April, but are particularly dangerous during winter months, when cold weather-related deaths occur due to exposure to cold air and loss of power due to high winds, ice, and snow. Nor’easters also can cause coastal flooding, coastal erosion and strong winds.

The nor’easter of Halloween 2011 was accompanied by record-breaking snowfall across New England and killed 39 people. The Presidents’ Day Weekend, February 15-17, 2003, was a nor’easter that dumped record snowfall in many areas along the East Coast. It claimed the lives of 42 people nationwide and stranded thousands of travelers. Perhaps the most notorious nor’easter is the Great Blizzard of 1888 which dropped 4 feet of snow in New York and killed 400 people.

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What can we learn from Hurricane Sandy?

Posted on November 15, 2012 by Weather Guys Editor

Nearly a week after Hurricane Sandy struck the Mid-Atlantic coast of the United States, the affected region is still reeling from the shock. This really was an unprecedented storm in the truest sense of that word.

Among the amazing aspects of the event was the extraordinarily accurate and early forecasting of the storm. Numerical forecast models were latching on to the correct scenario, including the unusual and rapid leftward turn off the Mid-Atlantic coast, as early as five to seven days before the event (depending on the particular model in question).

This kind of forecast accuracy was a pipe dream, even for run-of-the-mill storms, just 20 years ago. We are all living through a quiet revolution in weather forecasting and high-profile events such as Sandy make it very clear. The advance warning offered by these accurate medium-range forecasts undoubtedly saved dozens, perhaps hundreds of lives, and literally billions of dollars in damage in New York City alone.

From what we know in its immediate aftermath, the storm itself was the result of an unusually strong interaction between a late-season tropical cyclone (the original Hurricane Sandy) and a mid-level extratropical disturbance that grew in intensity as it approached the coast. Such interaction is a common feature of the transition seasons — fall or spring.

There are reasons to believe that in a warming climate the frequency of these kinds of interactions may increase potentially leading to an increase in strong examples, like Sandy, of this kind of severe weather.

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Are wind turbines detected by weather radars?

Posted on November 1, 2012 by Weather Guys Editor

While a single wind turbine is unlikely to confuse a radar return signal, a wind farm, particularly one 20 square miles or larger, will pose a problem. For example, the radar returns from the weather radar in Sullivan continually measures what looks to be a rain cloud to the north. This signal is always there and is the location of a wind farm.

A radar consists of a transmitter and a receiver. The transmitter emits pulses of radiowaves outward in a circular pattern. Precipitation scatters these radiowaves, sending some energy back to the transmitting point where it is detected by the radar’s receiver as a radar return signal. The wind turbine towers are made of metal and have a strong radar return signal and will appear as a fixed object in a radar image. Many of the rotating blades are made of materials transparent to radar, but they often have metal lightning ground wires running through each blade — increasing the radar signature. The various speeds of the different turbines can modulate the radar return, so the radar image will continually change.

There are ways to get around to this problem, ranging in expense. Perhaps the simplest is to know that the farm is there and thus recognize that when it exists in an isolated manner that it is not raining.

Wind farms can also show up on surveillance radar used for aviation airspace control, which perhaps poses a more serious problem. As the number of wind farms continues to increase, their impact on radar is important to understand.

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How were recent heavy rains so well-predicted so many days in advance?

Posted on October 22, 2012 by Weather Guys Editor

Though we have been relatively dry for much of the autumn, on the weekend of Oct. 13-14 we received a soaking rain of 0.86 inches on Saturday followed by 1.74 inches on Sunday. Two aspects of this heavy rain event are noteworthy to us. First, though Madison averages an inch of rain in a single day about six times each year, the 1.74 inches that fell on Sunday was the most in a single calendar day in Madison since 3.61 inches of rain fell on July 22, 2010. That long stretch includes two full summers (2011 and 2012) in which we never received such a rain.

Second, the rainy weekend was clearly in the forecast for almost seven days in advance. In other words, at the end of the prior weekend, it was clear that next weekend was going to be a washout.

That forecasting success is a particular example of a revolution that has quietly occurred in the science of numerical weather prediction (forecasting with the aid of high-speed computers) over the last 30 years. It used to be that the one- to two-day forecast was the apex of forecasting skill but the combination of more powerful computers and continued research into the underlying science of weather systems has led to our present ability to forecast the weather sometimes several days in advance.

It is exciting to imagine where the state of the science may be in another 30 years.

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What is an air mass?

Posted on October 15, 2012 by Weather Guys Editor

An air mass is a large body of air whose properties of temperature and humidity are similar in any horizontal direction. Air masses can cover hundreds of thousands of square miles. Air masses are formed when air stagnates for long periods of time over a uniform surface. The characteristic temperature and moisture of air masses are determined by the surface over which they form. An air mass acquires these attributes through heat and moisture exchanges with the surface.

Air masses are classified according to the temperature and moisture characteristics where they develop. Cold air masses originate in polar regions and are therefore called polar air masses. Warm air masses usually form in tropical or subtropical regions and are called tropical air masses. Moist air masses form over oceans and are referred to as maritime air masses. Dry air masses that form over land surfaces are called continental air masses.

In a typical year, weather changes resulting from the movement of air masses kill more people in the United States than all other weather phenomena combined. For instance, people from warmer winter climates (like Dallas, Texas) might be at greater risk from a cold air outbreak than people living in colder climates (like Madison). Conversely, Madisonians may be at greater risk to heat waves than residents of Dallas.

Socioeconomic status, cultural backgrounds and the size of the elderly population can also put a city’s residents at greater or lesser risk. For example, households with heaters in the north and air conditioners in the south may reflect socioeconomic status, whereas cultural backgrounds may influence the amount of time people spend outdoors.

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