Review of winter 2009 in North America

Temperature

Fig. 1. Winter temperature averaged over the contiguous US, 1895-2009.

According to NOAA, winter (December 2008 - February 2009) temperature across the contiguous United States was near average, based on records dating back to 1895 (Fig. 1). It is interesting to note, that the US experiences a decline in extreme events since the turn of the 21st century (click here for more details). Figure 1 also shows a noticeable cooling trend since the late 1990s.

Average Canadian temperature this winter was also close to the long-term mean, but, given a strong upward trend, it was much lower than the average temperature for the past 10 years. It was the coldest winter in Canada since 1996.

The distribution of winter (DJF) 2009 temperature anomalies over North America is presented in Fig. 2. Warmer than normal temperatures were observed in the southern United States and colder than normal temperatures in a band stretching from western Canada to the Midwestern United States.

Fig. 2. Temperature anomalies in the winter of 2009.

The observed temperature anomaly pattern is in general agreement with our forecast issued in August 2008 and its update issued in October 2008. This is reflected in high skill scores. For U.S. cities, the skills of heating degree day forecasts were 72 (relative to climatology), 82 (relative to persistence), and 82 (relative to the OCN).  For Canadian cities, the scores were 45, 53, and 81 respectively.

The general setting for atmospheric circulation over the Pacific-North American sector was pretty much the same as during the previous winter, largely determined by a negative phase of the PDO and a weak Aleutian low. Therefore, both the winter of 2008 and 2009 were characterized by an increased frequency of upper atmospheric troughing along the western seaboard of North America.

Fig. 3. Winter 2009 temperature ranks.

An important difference between these two winters was an anomalously high pressure cell established over the south-central region of the United States during the winter of 2009.  This cell was responsible for anomalously high temperatures in this region . It did not allow Pacific storms to penetrate too far south over the western US, pushing them toward the Midwest instead, where winter temperatures were much below normal (Fig. 3). 

More frequent troughing over the western US and ridging over the its eastern part is reflected in a negative PNA index (Fig. 4). The mean winter index value of -0.58 was the lowest since 1979.

Atmospheric circulation in January 2009, however, was different from that in the two other winter months. The January PNA index was positive, resulting in more atmospheric troughing over the eastern US, and hence more frequent outbreaks of cold air.  January temperatures were below average across much of the eastern US, while the western half of the nation experienced warmer than average temperatures. California had its sixth warmest January on record. Maine and Michigan had their eighth and ninth coldest January on record, respectively.

Precipitation

Fig. 4. Winter PNA index, 1951-2009.

The United States experienced its fifth driest December-February period on record. Texas had its driest winter ever and the Southeast experienced its 10th driest winter. Only the East-North-Central region (Iowa, Michigan, Minnesota, and Wisconsin) had precipitation averages that were above normal (Fig. 5). January-February 2009 was the driest first-two-month-period in the 1895-2009 record for the contiguous United States. Winter precipitation in Canada was close to normal.

Fig. 5. Winter 2009 precipitation ranks.

Despite the overall dryness in the contiguous United States, there were two major flooding events during the winter season. On January 6-8, a 'Pineapple Express' brought mild temperatures and torrential rain to the Pacific Northwest U.S., melting snowpack from last month's record snowstorms and causing widespread flooding, mudslides, and avalanches across the state of Washington. More than 30,000 people were encouraged to evacuate their homes due to flooding. Roads and railways were severed as highway officials closed a 20-mile stretch of I-5 and Amtrak passenger service out of Seattle was suspended. Several cities, including Tacoma, declared a civil emergency. A number of precipitation records were set, including Seattle where a record 58.2 mm (2.29 inches) of rain fell at Sea-Tac Airport, and Olympia, which received a record 122.4 mm (4.82 inches) of rainfall. The Snoqualmie River at Carnation reached its highest levels since records began in 1932, measuring a height of 61.5 feet (7.5 feet above flood stage) on January 7. The National Weather Service considers the flood event to be among Washington State's most memorable. The cost of damage is estimated at $125 million (Associated Press).

The second major flooding occurred in Minnesota and North Dakota, as the Red River swelled to record and near-record levels during late March. Well-above-normal precipitation during March compounded the flooding potential brought on by extreme precipitation surpluses accumulated during autumn and winter. The six-month precipitation for the October-through-March period was the wettest on record (114 years) for five climate divisions that cover most of the Red's drainage basin in eastern North Dakota and western Minnesota. The combined area of these regions is 50,346 square miles, or roughly the size of Louisiana.