Powerful wind

(MORE: Live Updates)

The National Weather Service in Monterey, California, said Monday that this storm is "expected to be one of the strongest storms in terms of wind and rain intensity" since storms in October 2009 and January 2008.

On Thursday afternoon in Oregon, the Portland International Airport recorded a 67-mph wind gust, its highest wind gust in 33 years.

(MORE: West Coast Storm Impacts)

Powerful wind gusts hit the San Francisco Bay Area Thursday morning, knocking out power to some 150,000 customers. Winds in the Sierra Nevada topped out at 147 mph on the summit of Mount Lincoln, near Truckee.

The storm is also slamming the Pacific Northwest, where hurricane force wind warnings have been posted for the waters along the coast of northwest Oregon and southwest Washington. Over 100,000 customers had lost power in the Northwest by Thursday evening as wind gusts as high as 95 mph ripped northward through Oregon and into Washington.

Numerous flash flood warnings have been issued Thursday, including virtually every county in the San Francisco Bay Area. Multiple freeways have been closed due to flooding, and parts of Sonoma County have seen over 9 inches of rain.

Cause of tornado

Many people always ask us what causes tornadoes? And while there are many different theories on tornadogenesis, the basic accepted idea is  that thunderstorms develop in warm, moist air in advance of eastward-moving cold fronts. These thunderstorms often produce large hail, strong winds, and tornadoes. Tornadoes in the winter and early spring are often associated with strong, frontal systems that form in the Central States and move east. Occasionally, large outbreaks of tornadoes occur with this type of weather pattern. Several states may be affected by numerous severe thunderstorms and tornadoes.

During the spring in the Central Plains, thunderstorms frequently develop along a "dryline," which separates very warm, moist air to the east from hot, dry air to the west. Tornado-producing thunderstorms may form as the dryline moves east during the afternoon hours.

Along the front range of the Rocky Mountains, in the Texas panhandle, and in the southern High Plains, thunderstorms frequently form as air near the ground flows "upslope" toward higher terrain. If other favorable conditions exist, these thunderstorms can produce tornadoes as well.

Storm Chaser

As you can see from the graphic above,"Chase Alley" and "Tornado Alley" certainly overlap. We usually start our early tours chasing from Texas to Oklahoma but as the season really kicks off we could be anywhere from Texas to Nebraska. As the season progresses, storm systems tend to remain to the Northern Plains, this usually happens from mid June to the latter part of July into August and is the reason why we have our Northern Plains tours. These tours are ideal if you are a photographer or interested in photography, as we encounter not only majestic landscapes but also severe weather stormscapes including massive supercell structures!

power outing

A tornado watch is in effect until 2 p.m. today for West Mississippi, Northeast Louisiana and Southeast Arkansas.
"Everyone needs to be on guard tomorrow," said Mississippi Emergency Management Agency Executive Director Robert Latham on Sunday evening. "The key is to have a plan and not panic."
National Weather Service forecasters updated MEMA Sunday about a strong cold front will begin moving into north and west Mississippi Monday afternoon and continue south and east through the night.
All kinds of severe weather are possible with this system, including damaging winds, heavy rains, hail and even tornadoes, a release from MEMA said.
A large portion of the middle part of the state will have an elevated risk for damaging winds, possibly upwards of 60 miles per hour, as well as possible tornadoes and small hail through Monday afternoon and into Monday night, according to the NWS.
"We are entering our fall severe weather period as big temperature swings occur with these systems" said Latham. "Please don't rely on any one way of getting weather alerts, make sure you have several."
The Mississippi Red Cross is on alert and has volunteers ready to respond, said regional disaster officer Bob Devaney.
"Our emergency response vehicles are ready to roll," Devaney said. "We urge everyone to take a few simple steps that can help save lives during severe weather. Be on your guard and have a plan."

California Storm

A monster storm is battering California, Oregon and Washington with extremely high winds and flooding rainfall, and it will eventually bring blizzard conditions to the Sierra Nevada. The National Weather Service in Monterey, California, said earlier this week it’s "expected to be one of the strongest storms in terms of wind and rain intensity" since storms in October 2009 and January 2008. Below are the latest updates on the storm from The Weather Channel, National Weather Service forecast offices and news sources from around the region.

A big storm will bring heavy snow, heavy rain, high winds and thunderstorms.

The development of an intense storm along the coast means heavy rains and gales along the coast and heavy wet snow interior locations. I think the higher elevations of the Northeast from northeastern Pennsylvania on north will see over a foot of snow. Snow mixed with rain at times for valley areas will keep amounts down. Flooding may occur along the coast and parts of southeastern New England may even have some heavy thunderstorms with strong winds.

Wettest Strom

The wettest storm to affect the U.S. West Coast since 2009 is gathering strength over the Pacific Ocean, and promises to bring much-needed drought relief to thirsty California Wednesday through Friday. Rainfall amounts of 3 - 8 inches are expected over most of Northern California, with snowfall amounts of 1 - 3 feet predicted in the Sierra Mountains. As noted by Wunderground weather historian Christopher C. Burt in his Monday post, California Drought Situation Improves, this week’s storm may be the strongest and wettest storm to hit the region since October 2009, when the last major ‘pineapple express’ soaked the state. California is already benefiting from widespread heavy rains that fell November 29th through December 6th, and most of California is now running a seasonal precipitation surplus—the first time they’ve seen such since December 2012.

Tropical Storm

Light to moderate rains are falling in Manila, the capital of the Philippines, as a much-weakened Tropical Storm Hagupit plows west-northwest at 7 mph. Hagupit made landfall in Dolores, Eastern Samar Island, at 9:15 pm local time on Saturday as a Category 3 storm with 125 mph winds, and will finally exit the Philippines on Tuesday morning after dumping widespread rains of 10 - 15" across the Central Philippines. Some 2-day rainfall totals from the storm include 17.06" (433 mm) at Catbalogan and 15.55" (395 mm) at Borongan on Samar Island, and 9.14" (232 mm) on Masbate Island. At least 21 deaths are being blamed on the typhoon so far, but rescue workers have not yet reached remote areas that received heavy damage where the typhoon initially made landfall, on northern Samar Island. Satellite loops show a large but weakening tropical storm, with much reduced heavy thunderstorm activity. Nevertheless, Hagupit is still a very serious heavy rainfall threat. The storm's slow forward speed of 5 - 10 mph through the western Philippines will potentially bring heavy rains of 5 - 10" to Manila, with a population of about 12 million, causing serious flooding. However, it appears that the Philippines have avoided a major catastrophe on the scale of last year's Super Typhoon Haiyan, which left over 7,300 people dead or missing.

Southern Blogs

We are going into a pattern where the southern jet will become active with storms over the next two weeks. The question becomes, how much cold air can these storms tap to produce snow. The map below is the ECMWF control run showing the snow accumulation through 360 hours. While the snowfall tends to be overblown on the ECMWF model, it does indicate that the storms can tap some of the cold air that will be around. This is not a cold weather pattern because the arctic air will be confined to northern Canada through much of December. It would appear that after the holidays the arctic air returns.

In the near term...

Light to moderate snow will continue into tonight but should wind down by Thursday morning. The heaviest snows will be across western New York and northwestern Pennsylvania.

Most areas will get 1-3 inches of snow.

The big story will be the heavy rains, flooding and mudslides across California through Friday. As you can see by the rainfall map, 6 inches of rain will fall which is a lot given the dry conditions. There will be serious flooding in some locations of central California.

California Flooding

Rain has finally moved into California, and it is just pouring now in the Bay Area. It's a warm rain, too, with temperatures in the 60s at the onset of the precipitation. Not only that, but it is a sideways rain, with sustained winds of 15 to 25 mph, and gusts well over 40 mph. In a season that has been unremarkable in terms of rain for a state desperately in need of rain, this is far too much in too short of a period of time and is already causing flooding.

Any rain prior to 12z today is obviously left off the storm totals on the 12z December NAM forecast. Still, even taking that into consideration, rainfall totals going forward through tomorrow are nothing short of astounding:

Earth Quake predictions

October 15, 2013 – LOS ANGELES, CA – More than 1,000 old concrete buildings in Los Angeles and hundreds more throughout the county may be at risk of collapsing in a major earthquake, according to a Times analysis. By the most conservative estimate, as many as 50 of these buildings in the city alone would be destroyed, exposing thousands to injury or death. A cross-section of the city lives and works in them: seamstresses in downtown factories, white-collar workers in Ventura Boulevard high-rises and condo dwellers on Millionaires’ Mile in Westwood. Despite their sturdy appearance, many older concrete buildings are vulnerable to the sideways movement of a major earthquake because they don’t have enough steel reinforcing bars to hold columns in place. Los Angeles officials have known about the dangers for more than 40 years but have failed to force owners to make their properties safer. The city has even rejected calls to make a list of concrete buildings. In the absence of city action, university scientists compiled the first comprehensive inventory of potentially dangerous concrete buildings in Los Angeles. The scientists, however, have declined to make the information public. They said they are willing to share it with L.A. officials, but only if the city requests a copy. The city has not done so, the scientists said. Recent earthquakes have spotlighted the deadly potential of buildings held up by concrete. A 2011 quake in Christchurch, New Zealand, more than two years ago toppled two concrete office towers, killing 133 people. Many of the 6,000 people killed in a 1995 earthquake in Kobe, Japan, were in concrete buildings. In 1971, the Sylmar earthquake brought down several concrete structures, killing 52.

Alaskas weather

While much of the central and eastern U.S. was shivering, Alaskans experienced their third warmest January in 96 years of record, according to NOAA's National Climatic Data Center.
Homer, Talkeetna, King Salmon, and Cold Bay (no pun intended) all chalked up their record warmest January. On Jan. 27, Port Alsworth tied the all-time January record high for the state, topping out at an incredible 62 degrees, according to Rick Thoman of the National Weather Service in Fairbanks.

Among the impacts of this warm spell:

• A closure of the Alyeska Ski Resort for two days.
• A closure of Fairbanks International Airport on Jan. 23 due to freezing rain. 
• Schools closed due to rain, not snow. (MORE: Alaska Dispatch column)

Warm air and a series of storms with heavy, wet snow, or even rain, triggered several large avalanches.

Interestingly, a wind chill of 97 degrees below zero was observed at Howard Pass on Valentine's Day, setting a new record cold wind chill for the state, previously held at Prudhoe Bay on Jan. 28, 1989 (-96F). The air temperature at that time was -42 degrees with a sustained wind of 71 mph.



http://www.weather.com/news/science/2014-world-extreme-weather-events-20140226?pageno=5http://www.weather.com/news/science/2014-world-extreme-weather-events-20140226?pageno=5

Tropical Cyclones

The intensity of Atlantic hurricanes has increased in recent years, a pattern that is consistent with climate change. Warming oceans store increasing amounts of heat energy and fuel stronger storms.3

Globally, the strongest tropical cyclones have increased in intensity, with the most significant increases occurring in the Atlantic. The available evidence is that this has been due to a combination of natural variability and climate change. Gaps in records from the last century and other issues make it difficult to be precise about the relative influences at this stage, but climate change is expected to dominate in the future.4

The Atlantic hurricane season is also getting longer, at a rate of 5 to 10 days per decade. The broadening of the season correlates with rising sea surface temperatures; each degree Celsius temperature change corresponds to a 20-day shift in both the beginning and end of hurricane season. Here too, however, due to the uneven state of record keeping, we cannot rule out the possibility of a role for natural variation in this trend.5

- See more at: http://www.climatecommunication.org/new/features/extreme-weather/summer-storms/#sthash.ztYwJCZ2.dpuf

Circulation Changes

Local weather, particularly extreme local weather, is often determined by fluctuations in large patterns of regional atmospheric pressure and sea surface temperatures, such as the Arctic Oscillation (and its close relative, the North Atlantic Oscillation) and other patterns associated with El Niño-Southern Oscillation (ENSO). These recognizable patterns come and go over a period of months to years. And these patterns may be altered by global warming.

Circulation variations, such as the ones associated with ENSO, largely determine where stormy, wet weather is favored and where dry weather prevails. In regions where the effects of these circulation variations are similar to global warming effects, new extremes are observed. While ENSO and other sources of natural variability can determine the location of extremes,1 the intensity and duration of the associated extremes such as droughts, and the associated heat waves, have increased with climate change. Similarly, in the wet areas, the intensity of rains and risk of flooding is greater.

Regional circulation patterns have significantly changed in recent years.2 For example, changes in the Arctic Oscillation cannot be explained by natural variation and it has been suggested that they are broadly consistent with the expected influence of human-induced climate change.3 The signature of global warming has also been identified in recent changes in the Pacific Decadal Oscillation, a pattern of variability in sea surface temperatures in the northern Pacific Ocean.4

El Niños have become more common and their intensity has nearly doubled in recent years, a statistically rare event. This recent shift towards more intense and frequent El Niños is related to the recent increase in dry areas around the world.5 However, past observations and reconstructions of El Niño events from non-instrumental records such as corals show that El Niño events naturally fluctuate in magnitude and frequency over time, and this has been demonstrated in long climate model simulations of past and future climate as well.6

El Niño’s center of action appears to be shifting from the eastern to the central Pacific, which in turn is affecting the distribution and frequency of weather events.7 However, due to the wide natural fluctuations within circulation patterns, it is difficult to attribute recent changes solely to human activity. Further research is needed to determine how global warming impacts these circulation patterns.

- See more at: http://www.climatecommunication.org/new/features/extreme-weather/circulation-changes/#sthash.3g79taYp.dpuf

Heat Waves

For many heat waves, there are also important feedbacks that come into play that amplify drought and heat and set the stage for wildfires. There is a direct local contribution to the drying and high temperatures in the absence of evaporative cooling.1 While heat waves with high humidity are oppressive and give no relief at night, heat waves often form in association with drought. In these cases, the prevailing dry conditions set the stage for the heat since the land is dried out, the vegetation is wilted, and all of the heat from the sun goes into raising temperatures, whereas ordinarily, in the process of evaporative cooling, surface water or wetness acts as an evaporative cooler (swamp cooler) of sorts.

Some extreme examples have occurred in recent years in south Australia in January 2009, in Russia in summer 2010, and in Texas and Oklahoma in summer 2011. The record high temperatures in each case, along with the tinder dry conditions, led to extensive wildfires that were extremely costly in terms of lives, structures, human dislocations, and costs.2 3 The exceptionally warm March in the U. S. is but part of record warmth for the first five months of the year (see Figure 8 below) and, along with an absence of snow, the rapid snow melt has left the Rocky Mountains almost without snow by 1 June 2012. The very hot and dry conditions throughout the Southwest and Rocky Mountain Region have led to exceptionally high risk of wildfire. Multiple wildfires have already occurred, and several in Colorado and Utah have expanded into huge burn areas, resulting in loss of life and structures.

As emissions of heat-trapping gases continue to rise, and global average temperatures continue to increase, we can expect even more of the of extreme heat and related impacts we’ve been witnessing in recent years.

- See more at: http://www.climatecommunication.org/new/features/heat-waves-and-climate-change/heat-waves-and-wildfires/#sthash.mvAzRKCw.dpuf

Cold

Increasing hot weather has been accompanied by a decrease in cold weather. Across the globe in recent decades, there has been a reduction in the number of cold extremes, such as very cold nights, and a widespread reduction in the number of frost days in mid-latitude regions.1 In the United States, there have been fewer unusually cold days. The 10-year period ending in 2007 witnessed fewer severe cold snaps than any other 10-year period since record keeping began in 1895.2 These changes cannot be explained by natural variation, and correspond very well with computer simulations that include human influences on climate.3 Snow cover has decreased in most regions, especially in the spring, and mountain snowpack has also decreased in several regions.4 - See more at: http://www.climatecommunication.org/new/features/extreme-weather/reduced-cold/#sthash.QC1sWc8U.dpuf

Severe Drought

While climate change has increased precipitation in some areas, in other regions it has contributed to drought.1 Though there are a number of factors that drive drought, such conditions are apt to develop in regions that lack rain; drought is also greatly intensified by increased evaporation from soil and vegetation associated with warming.2 Very dry areas across the globe have doubled in extent since the 1970s.3 In particular, a long-term drying trend (from 1900 to 2008) persists in Africa, East and South Asia, eastern Australia, southern Europe, northern South America, most of Alaska, and western Canada.4

The global increase in drier, hotter areas and the trend in which dry areas are becoming drier can both be traced to the human influence.5 Drying trends have been observed in both the Northern and Southern hemispheres since the 1950s.6 These trends cannot be explained by natural variations but do fit well with climate model expectations for global warming.7 In particular, greenhouse gas emissions have contributed significantly to recent drying by driving warming over land and ocean.8

The increase in drought is caused by many factors: shortfalls in precipitation; earlier snow melt; a shift away from light and moderate rains towards short, heavy precipitation events; and increased evaporation from soil and vegetation due to higher atmospheric temperatures, all of which have been driven at least in part by climate change. Increased heating leads to greater evaporation of moisture from land, thereby increasing the intensity and duration of drought.9

Individual droughts have been linked to climate change, such as the drought that hit central India in 2008 when the north-south pattern of precipitation was disrupted by unusual weather driven by abnormally high sea surface temperatures due in part to global warming

Climate Change

Recent weather events such as deadly heat waves and devastating floods have sparked popular interest in understanding the role of global warming in driving extreme weather. These events are part of a new pattern of more extreme weather across the globe, shaped in part by human-induced climate change.

As the climate has warmed, some types of extreme weather have become more frequent and severe in recent decades, with increases in extreme heat, intense precipitation, and drought. Heat waves are longer and hotter. Heavy rains and flooding are more frequent. In a wide swing between extremes, drought, too, is more intense and more widespread.

All weather events are now influenced by climate change because all weather now develops in a different environment than before. While natural variability continues to play a key role in extreme weather, climate change has shifted the odds and changed the natural limits, making certain types of extreme weather more frequent and more intense. The kinds of extreme weather events that would be expected to occur more often in a warming world are indeed increasing.

For example, 60 years ago in the continental United States, the number of new record high temperatures recorded around the country each year was roughly equal to the number of new record lows. Now, the number of new record highs recorded each year is twice the number of new record lows, a signature of a warming climate, and a clear example of its impact on extreme weather.1

The increase in record highs extends outside the U.S. as well. A similar two to one ratio of record highs to record lows recently has been observed in Australia.2 Over the past decade, 75 counties set all-time record highs but only 15 countries set all-time record lows. In 2010, 19 countries set new all-time record high temperatures, but not a single country set a new all-time record low (among those countries keeping these statistics). 3

- See more at: http://www.climatecommunication.org/new/features/extreme-weather/overview/#sthash.oVb0bEPt.d

Extreme Weather- Dust Storms

Huge dust storm engulfs Queensland town of Bedourie – in pictures

Enormous dark clouds of sand and dirt turn day into night in the small outback town on the edge of the Simpson Desert

A huge dust storm has rolled into the small Queensland town of Bedourie, engulfing the residents in dust.

Maggie den Ronden, a local council employee, took these pictures on Thursday afternoon as the clouds rolled in, plunging Bedourie into darkness for an hour-and-a-half.

Located in arid far-western Queensland on the edge of the Simpson desert, the town is no stranger to the storms.

“We do get them from time to time but that was a big one,” den Ronden told Guardian Australia.

As soon as it had begun, the dust moved on.

“We could see the stars by 7:30,” den Ronden explained.

http://www.theguardian.com/australia-news/2014/dec/05/huge-dust-storm-engulfs-queensland-town-of-bedourie-in-pictures

Sydney, AU- Severe Weather

Wild weather to continue to batter eastern Australia in lead-up to Christmas

Animated map of lightning strikes on each day from December 1 to December 7 from lightningmaps.org. Map points correspond to lightning strikes, coloured by age over the 24 hour period (more red is older, more yellow is newer). Map coverage depends on a network of contributors so coverage quality varies.

Tropfest in Centennial Park in Sydney was delayed due to severe weather. Photograph: Britta Campion/AAP Image

An afternoon storm moves across Sydney on Friday 5 December. Photograph: Nikki Short/AAP Image

Unusually tumultuous weather has swept parts of eastern Australia over the past week, with meteorologists warning that further storms, hail and rain are likely to return in the lead-up to Christmas.

A severe thunderstorm battered Sydney on Sunday, leaving more than 10,000 homes without power as large hailstones rained down on parts of the city.

The storms come as Brisbane residents clean up from a “supercell” storm that caused damage that could cost up to $1bn. Hail damaged nearly 2,000 homes, while fallen trees have caused a problem for many households.

A broad area of low pressure, stretching from central Australia to the south-east of the country, has been supplemented by a surge in humidity from tropical areas.

“That combination has led to an unstable environment in the atmosphere,” Craig Burke, a BoM meteorologist, told Guardian Australia.

http://www.theguardian.com/weather/2014/dec/08/wild-weather-continue-batter-eastern-australia

California Drought gets worst and Temperature still rising

California just had its worst drought in over 1200 years, as temperatures and risks rise

This photo shows the dried up lake bed of Huntington Lake which at only 30% capacity as California is gripped by its worst drought in over a millennium. Photograph: Mark Ralston/AFP/Getty Images

A new paper published in the journal Geophysical Research Letters by Griffin & Anchukaitis concludes that the 2012–2014 drought in California was its most intense in at least 1,200 years.

The study used drought reconstructions from tree-ring cores, from the North American Drought Atlas (NADA) and from cores Griffin & Anchukaitis collectedfrom blue oak trees in southern and central California. Blue oak tree ring widths are particularly sensitive to moisture changes.

Griffin states:

"California’s old blue oaks are as close to nature’s rain gauges as we get"

Pencil-like tree-ring cores are collected non-destructively using a Swedish increment borer. May 2014, image by Daniel Griffin.

The drought alone is anticipated to cost California over $2 billion this year. On top of that, heat and drought also create conditions ripe for wildfires; another major problem for California. As expected, California also saw an intense wildfire seasonin 2014, blowing well through its firefighting budget. Now that the state is finally seeing some significant rainfall in December, those wildfires created conditions conducive to flooding and mudslides.

http://www.theguardian.com/environment/climate-consensus-97-per-cent/2014/dec/08/california-just-had-its-worst-drought-in-over-1200-years

Cali Drought

California drought 'not caused by global warming', official study finds...

A dried-up irrigation ditch is a sign of the drought that has gripped California for three years, although climate change is not to blame, says the National Oceanic and Atmospheric Administration. Photograph: Jae C Hong/AP

Don’t blame man-made global warming for the devastating California drought, a new federal report says.

A report issued on Monday by the National Oceanic and Atmospheric Administration said natural variations – mostly a La Niña weather oscillation – were the primary drivers behind the drought that has now stretched to three years.

The study’s lead author, Richard Seager of Columbia University, said the paper has not yet been published in a peer-reviewed scientific journal. --

He and NOAA’s Martin Hoerling said 160 runs of computer models show heat-trapping gases should slightly increase winter rain in parts of California, not decrease.

Hoerling said La Niña, which is the cooler flip side of the warming of the central Pacific ocean, can only be blamed for about one-third of the drought. The rest of the causes can be from just random variation, he said.

Peer-reviewed studies are divided on whether the drought can be blamed on climate change. Others published earlier this year point more directly to changes in pressure of the Pacific that blocked rain from coming into California, but Hoerling and Seager dismissed them as not adequate.

http://www.theguardian.com/us-news/2014/dec/08/california-drought-not-caused-by-global-warming-official-study-finds

Gambling isn't the only risky business--Weather is too

Extreme weather is risky business

Thomas M. Kostigen, Special for USA TODAY

Nearly two-thirds of small businesses do not have an emergency plan in place for their businesses, according to an Ad Council survey. Moreover, 40% of businesses affected by natural or man-made disasters never re-open, an Insurance Information Institute study found.

Disaster disruption to businesses has become so chronic that the U.S. Small Business Administration has even helped create a web site, preparemybusiness.org, to help businesses protect employees, lessen the financial impact of disasters, and re-open more quickly after a disaster.

Natural hazards pose a $1.2 trillion loss to the economy through 2050, according to a Department of Homeland Security official's testimony before congress earlier this year.

Whether freak storms in Phoenix, or a longstanding drought in California, or damage from hailstorms in the Midwest (whose annual economic damage now regularly tops $1 billion), extreme weather is risky business.

There are five areas FEMA recommends businesses focus on:

Program management. Different ways to organize and administer a preparedness program.
Planning. Assessing risks and analyzing methods to prevent hazards.
Implementation. Setting tactics for emergency response, crisis communications, and business continuity.
Training. Testing and evaluating plans -- and learning new ones
Improvement. Reviewing plans frequently to make changes and/or make them better.

http://www.usatoday.com/story/news/nation/2014/10/04/extreme-weather-business-impact/16701599/

Extreme Weather- Nashville, TN

Weather Gone Wild

By Peter Miller

Photograph by Sean R. Heavey, Barcroft Media/Landov

The weekend forecast for Nashville, Tennessee, called for two to four inches of rain. But by the afternoon of Saturday, May 1, 2010, parts of the city had seen more than six inches, and the rain was still coming down in sheets.

Police, fire, and rescue teams were dispatched in boats. One crew in a skiff headed out to I-24 to pluck the driver of an 18-wheeler from the chest-high water. Other teams pulled families off rooftops and workers from flooded warehouses. Still, 11 people died in the city that weekend.

Over at NewsChannel 5, the local CBS station, meteorologist Charlie Neese could see where the weather was coming from. The jet stream had gotten stuck over the city, and one thunderstorm after another was sucking up warm, humid air from the Gulf of Mexico, rumbling hundreds of miles northeast, and dumping the water on Nashville. 
When the sun came out on Monday morning, parts of Nashville had seen more than 13 inches of rain—about twice the previous record of 6.6 inches set during Hurricane Frederic in 1979. 

What’s going on? Are these extreme events signals of a dangerous, human-made shift in Earth’s climate? Or are we just going through a natural stretch of bad luck?

The short answer is: probably both. The primary forces driving recent disasters have been natural climate cycles, especially El Niño and La Niña. Scientists have learned a lot during the past few decades about how that strange seesaw in the equatorial Pacific affects weather worldwide. During an El Niño a giant pool of warm water that normally sits in the central Pacific surges east all the way to South America; during a La Niña it shrinks and retreats into the western Pacific. Heat and water vapor coming off the warm pool generate thunderstorms so powerful and towering that their influence extends out of the tropics to the jet streams that blow across the middle latitudes. As the warm pool shifts back and forth along the Equator, the wavy paths of the jet streams shift north and south—which changes the tracks that storms follow across the continents. An El Niño tends to push drenching storms over the southern U.S. and Peru while visiting drought and fire on Australia. In a La Niña the rains flood Australia and fail in the American Southwest and Texas—and in even more distant places like East Africa.

http://ngm.nationalgeographic.com/2012/09/extreme-weather/miller-text