Archive for the 'Meterology' Category

Even though predicting weather a mere 24 hours in advance can be very difficult, every month the kind folks at the National Weather Service’s Climate Prediction Center take on the challenge of long term forecasting. While most of us have little practice use for this report, they can be encouraging for those of us still stuck in the dead of winter. I’m definitely getting tired of scraping Wisconsin ice and snow off of my car, so I’m happy to hear that the 3 month outlook forecast calls for warmer temperatures in the western third of country with a pocket of warmer climate that stretches along the northern border states to the Mississippi River Valley. Predictions for the rest of the country are up in the air.

The precipitation forecast calls for below average rainfall in the western and central Gulf Coast Regions along with the Far Southwest. Above average rainfall is predicted for the southern Rocky Mountains and the high plains state. It’s a toss up for the rest of the country.

Can’t forgot about Hawaii – long-term prediction: big surprise….paradise (If you must know, they are predicting average temperature with below average rainfall amounts).

How do they predict the long-term weather?
There are many factors that go into these predictions. An interesting factor is the so-called “memory” effect that ice and snow cover has on climate. For example, if there is a lot of snow on the sun doesn’t heat the ground very well because the energy from the sun is reflected by the snow so the climate “remembers” the recent colder weather. One of the most influential factors is the temperatures of the Atlantic and Pacific Ocean. While El Nino is the most powerful and well-known trend, there are other ocean trends like the Northern Atlantic Oscillation.

Speaking of El Nino, the current El Nino event is winding down and the Pacific Ocean will be ENSO-neutral (El Nino – Southern Oscillation is the full name of the ocean temperature pattern) sometime in the next month or two. In fact, by the summer we may be heading towards La Nina conditions but it’s way too soon to know for sure.

What are El Nino and La Nina?
El Nino and La Nina refers to changes in the average temperatures in the Pacific Ocean that have impacts on weather worldwide. An average increase of 0.5 degrees C in the central topical Pacific is the official definition of El Nino and a decrease of 0.5 degrees C is the definition of La Nina. This doesn’t seem like much, but .5 degrees C of warmer water multiplied by a huge area of ocean is a lot of energy and has a very noticeable effect on global weather.

If you are interested in learning the effects of El Nino on the weather in your location, check out this good website from the National Oceanic and Atomospheric Administartion (NOAA) Here’s a website discussing La Nina effects.

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Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Sources:
1. “Long-Lead Seasonal Outlook – February 15th 2007″, Climate Prediction Center, National Weather Service.

2. “El Nino-Southern Oscillation”, Wikipedia entry

The past two articles talked about the recent storm system that caused a blizzard in Wisconsin and heavy freezing rain in Illinois. The third and final article about this storm is about the southern end of the recent storm and a powerful tornado that devastated the town of Dumas, Arkansas.

What caused these tornadoes?
The cold front associated with low pressure system that causes a blizzard in Wisconsin and freezing rain in Illinois generated the formation of thunderstorms. The cold front brought in cold winter air from the northern US. This cold air combined with moist warm air to create updraft of air that formed the thunderstorm clouds. Once again, the currently strong jet stream over Arkansas played a big role. These upper levels winds enhanced spinning columns of air that are found in all thunderstorms. The updraft winds rotated these columns of air to a vertical position and this results in a rotating wall cloud.

As the storm becomes more powerful, heavy rainfall causes an area of air called the rear flank downdraft (RFD). The RFD’s rapid descent drags the rotating wall cloud close to the surface. At this point, a funnel cloud drops out of the funnel cloud and causes death and destruction on the ground.

Why does the funnel cloud form?
At this point, it remains unclear but many scientists are working on figuring this out. Stay tuned for more on this.

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Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Sources:
1. “Neccessary Ingredients for Thunderstorms”, NOAA’s JetStream – Online School for Weather
2. “Thunderstorm Hazards – Tornadoes”, NOAA’s JetStream – Online School for Weather
3. “Cyclogenesis”, Wikipedia entry

In addition, to the heavy snow that fell in Wisconsin last night and today there was a significant freezing rain event in Illinois. In Mercer County, there were reports of a 2 inch coating of ice on power lines. Add strong winds to the equation and it’s easy to see why 83,000 people lost their power.

What is freezing rain?
Freezing rain forms when a layer of warmer air aloft traps a shallow layer of colder air at the surface. As the rain or snow falls to the surface it melts in the warmer air, reaches the cold air and starts to supercool (cooling a liquid to temperature below it’s freezing point without the liquid becoming solid), then the precipitation freezes on contact with power lines, trees, and roads. The key is the depth of the cold air at the surface, if the layer is deep enough the rain will have time to freeze into relatively harmless sleet. Sleet is separate particles of moisture and can be moved around with a shovel or plow. Freezing rain on the other hand, forms a solid layer of ice because just before the liquid water freezes it molds to the power lines, pores in the road, and already formed freezing rain. One of the more notable freezing rain events took in place in 1998 when an area that included Montreal, Quebec and Northern New York was coated with a devastating 60 to 120 millimeters (3-4.5 inches) of freezing rain. Some people went without power for weeks and close to 30 people died due to the ice.

I couldn’t find a solid source for exact mechanism behind this particular freezing rain event, but my semi-educated guess is the counter-clockwise rotation of this storm system swept up warm air (via a warm front) from the south and this warm air was undercut by a strong and cold east wind that maintained freezing temperatures. This strong wind was due to the high pressure air mass of freezing temperatures that a particular strong jet stream rammed this storm system into.

For more about this storm system and the blizzard further north, click here.

The next article and stop on the tour of this storm will be the tornadoes that occurred to the south. Stay tuned for that.

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Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Sources:
1. “Midwest suffers through brutal snow system”, CNN.com article
2. “Freezing rain”, Wikipedia entry
3. “North American ice storm of 1998″, Wikipedia entry

Current weather bulletin for my corner of the world from from the U.S. National Weather Service:

“A blizzard warning is in effect tonight for all of South Central and Southeast Wisconsin. Snowfall tonight will reach 8 to 14 inches (20 to 36 cm) with some locally heavier amounts possible across much of the area. Strong easterly winds gusting up to 40 mph (64 km/h) at times will result in blizzard conditions at times with visibilities severely restricted to one-quarter mile (400 m) or less at time tonight into Sunday…..The heavy snow may be accompanied by lightning and thunder.”

Sure enough, about half an hour ago as I was doing the research for this article I could hear the rumble of thunder along with the howl of old man winter. As the above weather bulletin reads, the Upper Midwest especially Wisconsin is being slammed with a blizzard.

Here’s the current weather map for the Upper Midwest from the Weather Channel’s website.

As I’m writing this, there is very powerful low pressure system (the red “L” is the center) that is packed full of moisture. The light purple front line is called an occluded front and represents the merging of cold and warm fronts found in strong systems. (This will be explained in future artices). The faint white lines represent the same atmospheric pressure along the line. As you can see the lines are packed together tightly over the upper midwest. This means lots of wind because air is rapidly moving to stabilize the pressure gradient.

The driving force behind all the moisture and wind is the jet stream that is currently very dynamic. Below is the path of the current jet stream:

The very sharp upward swing of the jet stream helped to steer the system rapidly from the moisture-rich southern US and rammed the storm into the cold winter air of Wisconsin.

More importantly, the current jet stream is very fast-moving and created a stronger cyclonic effect on this low pressure area. This is known of extratropical cyclogenesis. Another way to describe this is that the jet stream helped to form a tighter ball of low pressure energy. Although the process of formations are vastly different, this is the same cyclonic effect that makes hurricanes so powerful. The low pressure center is essentially the “eye” of this storm.

There you have it. I’m just glad that I don’t have to go out tomorrow.

Update: The next two articles will be a north to south tour of this nasty storm system. The article about the freezing rain in Illinois can be found here and stay tuned for a 3rd article on the tornadoes to the south.

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Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Sources:
1. National Weather Service Bulletin from the Milwaukee, WI office – February 24, 2007 (no permalinks available)
2. Weather Channel TV broadcast February 24, 2007
3. Weather Channel website – Current Midwest Weather and Jet Stream (no permalinks for these constantly changing maps)
4. “Blizzard”, Wikipedia entry
5. “Cyclogenesis”, Wikipedia entry

What is the Jet Stream?

When you hear this term used by forecaster, they are referring to fast moving air currents found around 36,000 feet up (roughly 11,000 meters). These currents represent the boundaries between the cold and warm air. [1] Since the clash of warm and cold air is essentially the root of all weather, meteorologists use the jet stream in many ways. One of key ways is that the jet stream steers storm systems across continents. For example, my state of Wisconsin will get a lot of snow if the jet stream dips south over the central states and then steers the storm north towards Wisconsin. When the storm system is in warmer climates it will collect moisture (cold air can’t hold as much moisture) and then this moisture will hit the colder Wisconsin air resulting in a lot of snow…the rough appoximation is 1 inch of rain equals 10 inches of snow. That’s 1 cm rain to 10 cm snow for readers who live in countries smart enough to adopt the metric system…we Americans are a stubborn people!

Also, you might here about jet streams from the captain on your next flight because the winds can either shorten or lengthen transit time for flights. In the summer, the jet stream moves an average of 55 km/h (~35 mph) and in the winter an average of 120 km/h (~55 mph). Jet streams have been measured as high as 400 kh/h (250 mph)….that’s a mighty wind that could make you arrive early (or miss your connecting flight!) [1]

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Posted by Tim Roth, author of the political blog Think Anew and Act Anew

Source:
1. “Jet Stream”, Wikipedia entry