How rare was that early blizzard in South Dakota?

Posted on October 15, 2013 by Weather Guys Editor

Our autumnal weather in southern Wisconsin last week was tame in comparison to the devastating blizzard that hit the western portions of South Dakota earlier this month. On Oct. 4, Rapid City, S.D., approached its all-time single-day snowfall record when it received 19 inches, ranking it the third-snowiest day in the city’s history. Interestingly, the two record-setting days ahead of it occurred in late April (in 1970 and 2001).

The recent storm dropped 23.1 inches of snow over three days from Oct. 3-5, ranking it second all-time in that category. Once again, the first and third all-time were in April (1927 and 2013).

Not surprisingly, the 19 inches of snow that fell in Rapid City on Oct. 4 shattered the all-time single October day snowfall by more than 9 inches as the prior record occurred on Oct. 19, 1919, with 9.9 inches of snow. And even though we are only one-third of the way through October, Rapid City has already established its all-time October monthly snowfall record at 23.1 inches — a whopping 8 inches more than any other year.

At the time we were preparing this report, the Rapid City area was under the threat of a substantial rain event this past Friday. The combination of record amounts of snow, little time for it to melt in the intervening week and heavy rains just seven days later prompted flood warnings in the area with the possibility of serious damage to property and agriculture.

As we enjoy the beautiful fall days that this time of year often brings to southern Wisconsin, these weather phenomena close to home should remind us all that the coming of winter is inevitable.

Category: Meteorology

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Were those the Northern Lights last Tuesday?

Posted on October 7, 2013 by Weather Guys Editor

Yes! If you were out late on Tuesday night, you might have seen the Northern Lights. The Northern Lights, also called aurora borealis, appear as a diffuse glow or as overlapping curtains of greenish-white and sometimes red light in the night sky.

Auroras are triggered when the sun ejects a cloud of gas, called a coronal mass ejection. It takes two to three days for the charged particles in this gas to reach Earth. Earth’s magnetic field deflects these particles toward the North and South poles. When these charged particles collide with a molecule or an atom, they can excite the molecule, increasing its energy state. When these molecules or atoms shift back down to their normal energy states, they emit light.

Auroras form between 60 and 250 miles above the Earth’s surface when these charged solar particles collide with two abundant constituents of our atmosphere: nitrogen and oxygen. The interaction of the charged particles with nitrogen molecules results in pinkish or magenta light, while oxygen atoms emit greenish light. A majority of the collisions occur near the poles, so the Northern Lights are usually seen at the higher latitudes of Canada and Alaska. When a large number of particles are emitted by the sun, which usually happens after a solar flare, the lights from the collisions can be seen throughout Wisconsin and other northern states.

Our sun goes through active and quiet energy cycles. The time between 2008 and 2010 was a very quiet time for the sun, and now the sun is in an active stage, providing opportunities to see more of these great natural light shows during cloud-free nights.

Category: Phenomena

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Was Superstorm Sandy a direct result of climate change?

Posted on October 1, 2013 by Weather Guys Editor

As the one-year anniversary of Superstorm Sandy approaches, we have news to report regarding current understanding of that tremendous storm.

One of the major questions confronting atmospheric scientists in the face of that unusual event was whether Sandy was a direct result of a warmer climate. This is a difficult question because there are so many elements that conspire to produce major storms.

One condition that influenced the impact of Sandy was the sea level. It is true that the sea level has risen as the planet warms, and so it is likely that at least some of the flooding associated with Sandy would have been less severe in the absence of climate change.

On the other hand, recent research, conducted using numerical forecast models, has suggested that the intensification of Sandy from the Caribbean to the New Jersey coast most likely was not influenced by climate change.

However, the development of conditions that set up this period of intensification may be linked to climate change — though there is not yet experimental or theoretical evidence that clearly makes this case.

The atmosphere-ocean system is complicated, so questions such as whether events like Sandy are direct consequences of the changing climate are also complicated. The interested citizen, like the interested scientist, may be well advised to be skeptical of such a connection while remaining open to being convinced as the evidence and analysis continues to be considered.

Category: Climate

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What’s a 100-year flood?

Posted on September 24, 2013 by Weather Guys Editor

It is wrong to think that a “100-year flood” happens only once every 100 years. The phrase “100-year flood” describes the estimated probability of a flood event happening in any given year.

A 100-year event has a 1 percent chance (or 1-in-100) of occurring in any given year.

While not likely, 200-year flooding events can occur within a month of each other.

Scientists collect data on how frequently different sizes of floods occur and the time between these floods.

They use the data to calculate the probability that a flood of a particular size will be equaled or exceeded during any year.

The term “100-year flood” is a statistical designation of an unlikely event.

Statistically, a 100-year flood has roughly a 63 percent chance of occurring in any 100-year period, not a 100 percent chance of occurring.

Climate can be defined as the average state of the atmosphere for a given place over a specified interval of time.

Extreme events, such as 100-year events, are part of a region’s climate. These extreme values are important for assessing the risk of unusual events and are used in determining flood insurance rates.

Recently, the front range of Colorado experienced a flooding event. Flooding is part of this region’s climate.

For example, Boulder, Colo., lies outside a canyon where rapid runoff from precipitation in the mountains can lead to flash flooding.

A flash flood is a sudden local flood characterized by a great volume of water and a short duration.

The area had major floods in 1919 and 1995. In 2013, between September 10-15, 17 inches of rain fell over Boulder causing major flooding that destroyed many bridges in the area. Many people had to be airlifted to safety.

Category: Climate

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How do weather radars work?

Posted on September 23, 2013 by Weather Guys Editor

Radar, an acronym for Radio Detection and Ranging, was invented during World War II to detect aircraft, but precipitation frequently got in the way. The military’s noise is meteorology’s signal.

A radar consists of a transmitter and a receiver. The transmitter emits pulses of radio waves outward in a circular pattern. Precipitation scatters these radio waves, sending some energy back to the transmitting point where it is detected by the radar’s receiver.

The intensity of this received signal, called the radar echo, indicates the intensity of the precipitation. Measuring the time it takes for the radio wave to leave the radar and return tells us how far away the storm is. The direction the radar is pointing locates the storm.

Uniquely, Doppler radar can measure the velocity of precipitation particles (and thus, the wind) in precipitating regions. A Doppler radar receiver “hears” waves of a higher frequency if precipitation particles are moving toward the radar and a lower frequency if particles are moving away. This allows Doppler radars to identify the detailed wind structure within severe thunderstorms. For example, if particles switch from moving toward and then away from the Doppler radar over a small distance, then a tornado is possible.

The National Weather Service is currently replacing its older radars with dual-polarization radars to improve observations of the interior of storm systems. A radio wave is an electromagnetic wave and therefore has electric and magnetic fields that are oriented perpendicular to one another. The orientation of these oscillations is referred to as polarization.

A polarizing filter for a camera, or polarizing sunglasses, can be used to observe the effects of polarization of light in a cloud-free sky. Rotate the filter, or glasses, while looking through them at a portion of the sky away from the sun — at a certain orientation, the intensity of the sky-light will be reduced. The filter is removing polarized light that is not aligned with the filter.

The additional information on polarization improves the precipitation rate measurement as well as the determining of the type of precipitation (snow, rain, freezing rain and possibly hail). The polarization radars can also measure information about both the horizontal and vertical dimensions of precipitation sized particles.

Category: Meteorology

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