Remarkable fall storm showed links to El Niño
Two images capture a remarkable storm that dumped record snow last week in Colorado, flooded parts of the South Central states, and spawned an EF-2 tornado that tore through downtown Shreveport, La. (population 200,000).
The first image (above) shows the absolutely phenomenal snowfall totals in Colorado, where accumulations of one to two feet were common in the Front Range metro areas, including Denver and Boulder. The jackpot was found in the mountains and foothills to the west and northwest of Denver, with 46 inches of snow recorded near Pinecliffe.
Keep in mind that this snowstorm occurred at the same time baseball's World Series kicked off -- a period most people typically call "fall." It was a bit early for 46 inches of snow, don't you think?
The other image (below) shows the wet side of the storm, with an unbroken band of heavy rain that fell Oct. 29-31, stretching from northeast Texas through southern Illinois. The widespread three to six inches of rain fell on already saturated ground, and flash flooding and river flooding ensued.
The 5.88 inches of rain that fell in Shreveport on Oct. 29 was the seventh wettest 24-hour rainfall total on record for that city, according to Victor Murphy, Climate Service Program Manager for the National Weather Service Southern Region in Fort Worth, Texas.
More significant than the single event, however, was how it aggravated preexisting flooding concerns. The storm pushed the water-logged Arkansas/Louisiana/Texas border area -- known as the "ArkLaTex" -- well into record territory for October, following an unusually wet September. October 2009 was the wettest on record in Shreveport and Monroe, La., as well as in Tyler, Texas, and El Dorado, Ark.
With 20.35 inches of rain, Shreveport broke its previous October record by more than six inches, and even recorded its third wettest month overall since records began in 1930.
"These periods would be the wettest 30- and 60-day periods for this time of year for these areas dating back to 1895," Murphy said in an email conversation.
Links to El Niño
While specific weather events cannot be definitively linked to large-scale climate factors, the storm, and the unusually wet fall season in parts of the nation's mid-section, may have had ties to a well-known source of natural climate variability -- our old friend El Niño, currently lurking in the tropical Pacific Ocean.
An El Niño event is characterized by warmer-than-normal sea-surface temperatures in the central and eastern equatorial Pacific. El Niño can lead to significant shifts in weather patterns worldwide.
As occasional CWG contributor Robert Henson of the University Corporation for Atmospheric Research (UCAR) explains in the current edition of UCAR Magazine, El Niño is associated with heavy early-season snowstorms in Colorado.
"...the presence of El Niño boosts the odds of a big dump considerably, even though our winters as a whole aren't substantially wetter during El Niño," Henson wrote. "It's a good example of nuance in the relationship between El Niño and climate, the kind of connections worth exploring in many parts of the world."
According to Henson, research shows almost half of the major snowstorms (12 inches plus) in Boulder occurred in El Niño years, compared to only 22 percent during El Niño's opposite phase, La Niña. Even heavier snowstorms, on the order of 20 or more inches, are almost seven times more likely to occur in an El Niño year compared to non-El Niño years. In addition, El Niño winters tend to bring more snows in the early and late parts of the season, with a dry spell in between.
However, the Weather Service's Murphy said the storm that buried Colorado and flooded the ArkLaTex did not feature the hallmarks of a typical El Niño-related event, such as a subtropical jet stream feeding Pacific moisture into the southern U.S. He also noted that "the impacts of El Niño are most commonly felt deeper into the winter and spring (i.e. mainly from November into March or April)," rather than during the fall.
"On the other hand, this El Niño continues to strengthen. If the El Niño weren't in place, would this event have been less likely to occur? Probably," Murphy said.
Connections with climate change?
There is another elephant in the room that may have played a role in the numerous rounds of rainfall that have contributed to a historically wet fall in the ArkLaTex, and to the heavy snowfall in Colorado: manmade climate change. Like El Niño, climate change does not cause a particular weather event, but it can increase the odds that events will pass certain thresholds, kind of like how taking performance-enhancing drugs may increase the odds of a baseball player hitting a home run.
According to Murphy, this was Louisiana's wettest October on record, and Arkansas may have recorded its wettest as well. The monthly values for both September and October in these two states constitute roughly a one in 100-year event for this time of year, Murphy told me.
We know from numerous studies, including a 2008 federal report on climate extremes, that climate change increases the odds of heavy rain events. That report showed that the nature of rainfall in North America, how heavy it is and how often it occurs, has shifted during the past century to favor heavier downpours. The report tied this change to the increase of water vapor in the atmosphere that has occurred as a feedback to warming from greenhouse-gas emissions.
However, the evidence tying climate change to the recent historic wet spell, and to last week's storm in particular, is not as strong as the potential link to El Niño. El Niño has strengthened throughout the fall, potentially increasing its impacts on U.S. weather patterns.
As will be detailed in CWG's winter outlook later this week, El Niño may play a big part in determining whether our winter will be a particularly white one, or if D.C. area snow lovers are going to be disappointed yet again.
The views expressed here are the author's alone and do not represent any position of the Washington Post, its news staff or the Capital Weather Gang.
| November 3, 2009; 10:45 AM ET
Categories: Climate Change, Freedman, News & Notes, Science
Save & Share: Previous: Forecast: Very nice weather for voters
Next: PM Update: Mostly clear (and frosty?) tonight
Posted by: Josh-CapitalWeatherGang | November 3, 2009 11:27 AM | Report abuse
Posted by: Bombo47jea | November 3, 2009 1:56 PM | Report abuse
Posted by: Mr_Q | November 4, 2009 12:40 PM | Report abuse
Posted by: Mr_Q | November 4, 2009 12:55 PM | Report abuse
Posted by: CapitalWeatherGang | November 4, 2009 10:45 PM | Report abuse
Posted by: Mr_Q | November 4, 2009 10:47 PM | Report abuse
Posted by: Mr_Q | November 4, 2009 10:53 PM | Report abuse
Posted by: Mr_Q | November 4, 2009 11:11 PM | Report abuse
Posted by: Mr_Q | November 5, 2009 12:32 PM | Report abuse
The comments to this entry are closed.