Report: ----- National ----- National Overview Drought Wildfires Hurricanes & Tropical Storms National Snow & Ice Tornadoes Synoptic Discussion ----- Global ----- Global Analysis Upper Air Global Snow & Ice Global Hazards El Niño/Southern Oscillation ----- Entire Report ----- Year: 2013 2012 2011 2010 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 Month: January February Winter March April May Spring June July August Summer September October November Autumn December Annual

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National Overview

• National Temperatures
• National Precipitation
• Severe Storms
• Atlantic Hurricanes
• Snow Season

This report was updated on May 1, 2007 to reflect revised statistics for the 2006 annual average temperature for the contiguous U.S. based on updates of preliminary data available in January 2007 as well as changes resulting from the switch from Version 1 to Version 2 of the U.S. Historical Climatology Network (USHCN) data set.

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National Temperature

The 2006 average annual temperature for the contiguous U.S. was the 2^nd warmest on record and within 0.1°F of the record set in 1998. Using final quality controlled data from the recently released USHCN Version 2 data set (see details below), the 2006 annual average temperature was 54.9°F, 2.1°F (1.2°C) above the 20^th Century mean and 0.08°F (0.04°C) cooler than 1998.

In a mid-January 2007 climate report and NOAA press release, NCDC indicated the 2006 annual average temperature for the contiguous United States was the warmest on record based on USHCN Version 1 preliminary data and that USHCN Version 2 was expected to show 2006 to be the 2^nd warmest year on record once operational testing was completed. Now that the USHCN Version 2 data set has replaced Version 1 as NCDC's official data set for calculations of U.S. national averages, this report has been revised to reflect a ranking of 2^nd warmest for 2006.

The USHCN Version 2 data set, now operational at NCDC, exploits recent scientific advances that better address uncertainties in the instrumental record. Because different algorithms were used in making adjustments to the station data which comprise both data sets, there are small differences in annual average temperatures between the Version1 and Version 2 data sets. These small differences in average temperatures result in minor differences in annual rankings for some years. The USHCN version 2 (V2) data set, shows 2006 and 1998 to be the two warmest years on record for the contiguous U.S.

All months, seasons, and years from 1895 to present are available on the U.S. Climate At A Glance website.

Reflecting the long-term warming trend in the Earth's climate, U.S. and global annual temperatures are now approximately 1.0°F warmer than at the start of the 20^th century, and the rate of warming has accelerated over the past 30 years, increasing globally since the mid-1970's at a rate approximately three times faster than the entury-scale trend. For additional information on global emperature trends, please see the 2006 annual temperature trends discussion.

The last eight 5-year periods (2002-2006, 2001-2005, 2000-2004, 1999-2003, 1998-2002, 1997-2001, 1996-2000, 1995-1999), were the warmest 5-year periods (i.e. pentads) in the last 112 years of national records, illustrating the anomalous warmth of the last decade. The 9^th warmest pentad was in the 1930s (1930-34), when the western U.S. was suffering from an extended drought coupled with anomalous warmth. The three warmest years on record are 1998, 2006 and 1934. In 1998, the record warmth was concentrated in the Northeast as compared with the Northwest during 1934. In 2006, much above average temperatures were present across most of the U.S. The West Coast and parts of the Ohio Valley and Southeast were above average. No state was near or below average for 2006.

Seasonal Analysis:
The temperature for the 2005-2006 winter season (Dec-Feb) was the 11^th warmest such period on record (1896-2006), with much warmer than average temperatures in parts of the central and northern Plains, Great Lakes, parts of the Northeast and across California. This was due in part to a record warm January for the U.S. Thirteen states in the central and northern Plains and Great Lakes ranked record warmest during this period.

Spring (March-May) temperatures were 8^th warmest for the nation with much above average to record warmest temperatures across the central third of the nation. Both Texas and Oklahoma were record warmest during this period, which exacerbated existing drought conditions in this region.

Warm temperatures persisted into the Summer (June-August) months, as the nation ranked 2^nd warmest in the last 112 years. Much above average temperatures were spread across the West, northern Plains and parts of the Mid-Atlantic states. The West region had its warmest summer on record. Many locations from the West Coast to the Central Plains broke records for the most days above 90°F and 100°F during a heat wave the last half of July. All-time record high temperatures were also set, breaking records which had stood since the 1930's Dust Bowl era in some places.

The year was the record warmest year-to-date period through September. During September and October, temperatures were unseasonably cold across a large portion of the country. November temperatures rebounded and were 18^th warmest on record. The resulting 2006 fall season (September-November) ranked 39^th warmest on record, or just above the 20^th century mean. Even with the November warmth, below average temperatures remained across much of the central Plains, Ohio Valley and the Southeast.

For the year, four states (New Jersey, Oklahoma, Texas, and Vermont.) were 2^nd warmest on record. All 48 of the contiguous states were either warmer or much warmer than average. No state ranked near to or below average during 2006.

Annual temperatures for 2006 averaged across the state of Alaska ranked 33^rd warmest since 1918: the coolest annual period since 1999. The previous six years had annual anomalies averaging from just under 1.8°F (1°C) to nearly 3.6°F (2°C) above the mean, which is unprecedented in the historical record. Winter temperatures in 2006 were above average for the 7^th consecutive year. Both spring and summer were slightly cooler than average and fall was slightly warmer. Wildfires across Alaska were not as active as in recent years. For additional information on the U.S. wildfire season, please see the Wildfire Season Summary.

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The adjacent figure shows the percentage of the contiguous U.S. that was very warm and the percentage that was very cold during each of the past 48 months. During 2006, only one month (September) averaged very cold over 10% or more of the country, with March and October near 5%. Over 20% of the U.S. was very warm for seven months in 2006. January was record warmest and April was 2^nd warmest across the U.S. during 2006 with over 60% of the U.S. very warm. Very warm and very cold conditions are defined as the warmest and coldest ten percent of recorded temperatures, respectively.

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Data collected by NOAA's TIROS-N polar-orbiting satellites and adjusted for time-dependent biases by NASA and the Global Hydrology and Climate Center at the University of Alabama in Huntsville, indicate that temperatures in the lower half of the atmosphere (lowest 8 km of the atmosphere) over the U.S. were warmer than the 20-year(1979-1998) average for the 9^th consecutive year. 2006 ranks as the 3^rd warmest year since this satellite record began in 1979.

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National Precipitation

Precipitation in the United States during 2006 was variable throughout much of the country with periods of excessive rainfall, especially across the Northwest, Great Lakes, and the Northeast, and persistent and developing drought in other areas. Winter storms in the Northwest contributed to the seasonal ranking of 10^th wettest for the region. Precipitation across the Southwest region ranked 2^nd driest during this same period. In spring, national precipitation was below average. Regionally, the Southeast ranked 3^rd driest and the West ranked 6^th wettest. The Northeast region had a record wettest summer exceeding the previous record by more than 1 inch (25.4 mm), while the U.S. as a whole was near average. Precipitation across the U.S. during the fall ranked 15^th wettest. Regionally, the Northwest and Northeast ranked 11^th and 4^th wettest such periods, respectively.

For the contiguous U.S. as a whole, five of the first seven months of the year were drier than average. Combined with unusually warm temperatures, this exacerbated drought across much of the country. By late July, half of the contiguous U.S. was in moderate to exceptional drought, as reported by the U.S. Drought Monitor. Nationally, annual precipitation was near the long-term mean, ranking 55^th driest year on record.

 

Below average precipitation occurred across portions of the Southwest, South and central Plains during the winter. Both Arizona and New Mexico had their driest such period on record. In contrast, a stormy period persisted in the Pacific Northwest, ranking Washington, Oregon and Idaho among their top ten wettest such periods on record. A dry winter exacerbated the drought conditions across the South and Southwest.

The Southeast and Mid-Atlantic states were exceptionally dry during the spring, whereas California ranked 6^th wettest. In May, river flooding across New England caused the evacuation of thousands of people from their homes. Massachusetts precipitation was record wettest for the month of May. Dry conditions across central Florida exacerbated wildfires, which forced a temporary closure of Interstate Highway 95 around Daytona Beach.

Summer precipitation across much of the Northeast and Mid-Atlantic states was much above average, with New York and Rhode Island record wettest. Rainfall during an event in June exceeded 10 inches (254 mm) in some areas with numerous daily and monthly rainfall records set. Flooding was widespread across the greater Washington D.C. area northward through parts of Pennsylvania and New York. Precipitation deficits across the Gulf Coast and Florida were evident during the summer due in part to a lack of tropical activity in this region.

Precipitation received during the fall was above average across the nation. Precipitation surpluses across the Northeast were evident for the second consecutive season. New Jersey, Maryland and Virginia each had its record wettest fall. Illustrating the persistent and ongoing precipitation across the region, accumulations from May to October in Vermont exceeded the previous 6-month record by nearly 5 inches (126 mm). Heavy rainfall, in excess of 20 inches, across Washington and Oregon resulted in flooding, evacuations and at least 3 fatalities during the first half of November. Heavy snowfall during this period prompted the closure of two mountain passes until spring due to the threat of avalanches: the earliest such closure for one of the passes in 12 years. Seattle, WA had its wettest month on record with 15.63 inches (397 mm) of rainfall: the wettest month in 115 years of record-keeping. Extreme drought continued to affect parts of Texas, Oklahoma, the northern Plains and northern Minnesota by the end of November, however drought across the Southeast and parts of the West alleviated as the year progressed.

Annual precipitation ranked Florida 3^rd driest, Georgia 6^th driest and New Hampshire and Indiana 2^nd and 3^rd wettest, respectively. The Northeast region had another wet year ranking 6^th wettest in 2006. In fact, the top two wettest annual periods in New Hampshire occurred during the past two years, making this two year period the wettest such period on record for the state.

 

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The adjacent figure shows the percent of the contiguous U.S. that was very wet and the percent that was very dry during each of the past 48 months. During 2006, more than a tenth of the country was very dry during March, May, June, July, August and November. More than 20% of the country was very dry during February. Conversely, nearly 20% of the contiguous U.S. was very wet in October. December was also very wet across more than 15% of the country. Over 10% of the U.S. was very wet during March, April, July and August. Very wet and very dry conditions are defined as the wettest and driest ten percent of recorded precipitation values, respectively.

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Severe Storms

Preliminary estimates indicate there were 23 very strong to violent tornadoes (wind speeds in excess of 158 mph, category F3-F5 on the Fujita Scale) during the 2006 official tornado season (March-August). This is below the 1971-2000 mean of 37. A slight negative trend in very strong to violent tornadoes has been observed since 1950. There was one F4 tornado and twenty-two F3 tornadoes during the main tornado season. No tornadoes were of F5 intensity.

Spring in the southern Plains and Tennessee and Ohio Valleys was punctuated by several severe weather outbreaks producing approximately 500 tornadoes and leading to nearly 50 deaths during March and April 2006. In mid-March, over 100 tornadoes were reported across 5 states from Oklahoma to Illinois. A major outbreak of severe weather in early April was responsible for at least 86 tornadoes spawned across Iowa, Illinois, Missouri, Arkansas, Kentucky, Indiana and Tennessee. Northwestern Tennessee was the hardest-hit by these storms with 19 confirmed deaths. Later that same week, another severe weather outbreak in the Nashville, TN region produced nearly 100 tornadoes. An additional 9 people were killed by this series of storms. Nearly 50 people in all were killed across the central U.S. in the March and April storms, many of them in Tennessee from the 2 most severe outbreaks (April 2^nd and 7th). Most people were killed from falling debris associated with damage to houses and buildings from straight line winds and tornadoes.

There were additional severe storm events in 2006. Severe weather affected parts of the southern Plains and Midwest in mid-March with over 100 tornadoes reported across 5 states from Oklahoma to Illinois. At least 10 fatalities were associated with these storms. Two episodes of severe thunderstorms caused massive power outages in the greater St. Louis, MO area. The severe weather caused the largest power outage in the city's history. Exacerbating the power disruptions was a heat wave which affected much of the region.

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Atlantic Hurricanes

The 2006 Atlantic basin hurricane season was near the 1950-2000 average with 9 named storms, of which 5 were hurricanes, including 2 major hurricanes. The ACE index of hurricane activity indicates a below-average season, with a preliminary value of approximately 46 x10⁴ knots². An average season is anywhere from 66 x 10⁴ knots² to 103 x 10⁴ knots² When compared to other seasons during the current active phase of the Atlantic Multidecadal Oscillation, which began in 1995, only 1997 had fewer named storms than the nine that formed during the official 2006 season. The relatively inactive season in 2006 was attributed in large part to the rapid onset of El Niño in the equatorial pacific, which acted to suppress conditions conducive to hurricane formation in the Atlantic.

While the 2006 season did not officially begin until June ^1st, Tropical Storm Zeta, which formed on December 30th, 2005 persisted until January 5^th of this year, making it the first storm of 2006 in the Atlantic basin. However, the first storm of the official 2006 season was Tropical Storm Alberto, which developed off the coast of Cuba in mid-June. Hurricane Gordon became the first major hurricane of the Atlantic season on September 13th, but did not make landfall. Isaac was the last named storm during the 2006 season. Prior to its dissipation in early October, Isaac brushed the Bermuda coast as a category 1 storm.

Only 2 storms made landfall with the mainland U.S. during 2006, Tropical Storm Alberto in Florida and Hurricane Ernesto as a tropical storm in Florida and North Carolina. More details about these and all the 2006 Atlantic tropical systems can be found on NCDC's hurricane page.

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Snow Season

The 2005/2006 snow season was generally above average across the parts of the Northwest including the Sierra and Cascade Mountains and below average across the Southwest. Snow cover was below average for the North American continent as a whole over the winter and spring, consistent with a trend towards reduced spring snow cover for North America as shown in the adjacent image.

Notable snow storms in 2006 include a blizzard in February that affected areas of the Mid-Atlantic and the Northeast from February 11-12, 2006. This storm produced 26.9 inches (68.3 cm) of snow in New York City's Central Park. This amount broke the all-time storm total record of 26.4 inches (67.1 cm) set during the December 26-27, 1947 storm. The Northeast Snowfall Impact Scale (NESIS) classified this as a Category 3 (Major) storm and ranked it as the 20^th most intense on record for the Northeast. In Hartford, CT, a snowfall total of 21.9 inches (55.6 cm) broke the old storm total record of 21 inches (53.3 cm) set in 1983. Many locations in the region reported between 10-20 inches (25-51 cm) during this event. In December, two major winter snow storms impacted Colorado and parts of the High Plains. Denver, Colorado had its 3^rd snowiest December on record and endured a major blizzard which brought the city to a standstill during the holiday travel season. More details of these and other snow and ice events are available in the annual summary of significant events.

By the end of Spring, snowpack in the West was above average across much of Oregon and parts of California, Washington, Nevada, Idaho and Utah. Many of the western states rely on melting winter snow to replenish reservoirs. In fact, precipitation received throughout the winter and spring helped to alleviate drought conditions in the West. An active Pacific storm track during the winter and spring brought a steady supply of rainfall and snow to the West, Northwest and Intermountain West. Excessive flooding occurred along the north California coastline and in the Sierra-Nevada Mountains in early April. Snowfall across Arizona and New Mexico was exceptionally low throughout the season with most regions reporting less than 40% of normal snowpack at the end of spring. The 2006-2007 snow season has begun with above average snowfall across parts of the West. Parts of the central Rockies and along the Cascades in Washington and Oregon have snow water equivalent percentages in excess of 160 percent of normal. Snowfall accumulations of up to two feet (60 cm) fell across the Cascades in late-November. Snowfall in excess of three feet (90 cm) fell across portions of the Colorado front-range and western High Plains during the two major snowstorms near the end of December.

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NOAA's National Climatic Data Center is the world's largest active archive of weather data. The preliminary temperature and precipitation rankings are available from the center by calling: 828-271-4800. NOAA works closely with the academic and science communities onclimate-related research projects to increase the understanding of El Niño and improve forecasting techniques. NOAA's Climate Prediction Center monitors, analyzes and predicts climate events ranging from weeks to seasons for the nation. NOAA also operates the network of data buoys and satellites that provide vital information about the ocean waters, and initiates research projects to improve future climate forecasts.

Global Analysis

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Global Highlights: Based on preliminary data, globally averaged combined land and sea surface temperature was the warmest on record for December 2006 and fifth warmest for January-December. December temperatures were above average in the United States, Europe, southern Asia, central Russia, and most of South America. Cooler-than-average conditions occurred in the Middle East Region. Precipitation during December was above average in Scandinavia, England, Japan, central U.S., southeastern Africa, and most of South America. Drier than average conditions were observed in the eastern U.S., central Europe, eastern Australia and southern India. ENSO conditions persisted in a warm phase (El Niño) during December.

Contents of this Section: Introduction Global Temperatures Global Precipitation ENSO SST Analysis References

The data presented in this report are preliminary. Ranks and anomalies may change as more complete data are received and processed. The most current data may be accessed via the Global Surface Temperature Anomalies page.

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Introduction Temperature anomalies for December 2006 are shown on the dot maps below. The dot map, below left, provide a spatial representation of anomalies calculated from the Global Historical Climatology Network (GHCN) data set of land surface stations using a 1961-1990 base period. The dot map, below right, is a product of a merged land surface and sea surface temperature anomaly analysis developed by Smith and Reynolds (2005). Temperature anomalies with respect to the 1961-1990 mean for land and ocean are analyzed separately and then merged to form the global analysis. Additional information on this product is available. Anomalously warm temperatures have covered much of the globe throughout the year. The January-December 2006 map of temperature anomalies show the presence of warmer than average temperatures across all land areas except central Russia. Warmer than average SSTs occurred in all oceans with the expection of portions of the South Pacific and South Indian Oceans where cooler than average conditions were observed.

During December, there were above average temperatures across the U.S., Europe, southern Asia, central Russia, eastern South America, and western coast of Canada. Cooler than average temperatures were observed in the Middle East Region. Warmer than average SSTs occurred in the North Atlantic and the Niño regions. Temperatures in parts of these Niño regions were more than 1°C (1.80°F) above average, and the average temperature anomaly in the Niño 3.4 region increased in December to approximately 1.37°C (2.47°F). Please see the latest ENSO discussion for additional information on the developing El Niño event.

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The mean position of upper level ridges of high pressure and troughs of low pressure (depicted by positive and negative 500-millibar height anomalies on the December map) are generally reflected by areas of positive and negative temperature anomalies at the surface, respectively. For other Global products see the Climate Monitoring Global Products page.

Images of sea surface temperature conditions are available for all weeks during 2006 at the weekly SST page.

Temperature Rankings and Graphics

Effective with the January, 2006 report, NCDC transitioned from the use of the Operational Global Surface Temperature Index (Quayle et al. 1999) to the blended land and ocean dataset developed by Smith and Reynolds (2005). The differences between the two methods are discussed in Smith et al. 2005. December 2006 was the warmest December since global surface records began in 1880 for combined global land and ocean surface temperatures. December land surface temperatures were 4th warmest, while ocean surface temperatures were 2nd warmest in the 127-year record, behind 1997 during which the very strong 1997/1998 El Niño event was developing. The January - December 2006 land and ocean combined temperature is tied for 5th warmest on record.

Current Month / Year-to-date

December	Anomaly	Rank	Warmest Year on Record
Global Land Ocean Land and Ocean	+1.29°C (+2.32°F) +0.51°C (+0.92°F) +0.72°C (+1.30°F)	4th warmest 2nd warmest 1st warmest	1939 (+1.34°C/2.41°F) 1997 (+0.56°C/1.01°F) 2003 (+0.70°C/1.26°F)
Northern Hemisphere Land Ocean Land and Ocean	+1.51°C (+2.72°F) +0.55°C (+0.99°F) +0.91°C (+1.64°F)	4th warmest 1st warmest 2nd warmest	1939 (+1.82°C/3.28°F) 2004 (+0.52°C/0.94°F) 2003 (+0.92°C/1.66°F)
Southern Hemisphere Land Ocean Land and Ocean	+0.65°C (+1.17°F) +0.49°C (+0.88°F) +0.51°C (+0.92°F)	7th warmest 3rd warmest 3rd warmest	1997 (+0.80°C/1.44°F) 1997 (+0.60°C/1.08°F) 1997 (+0.63°C/1.13°F)

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January-December	Anomaly	Rank	Warmest Year on Record
Global Land Ocean Land and Ocean	+0.78°C (+1.40°F) +0.45°C (+0.81°F) +0.54°C (+0.97°F)	4th warmest 5th warmest 5th warmest	2005 (+0.97°C/1.75°F) 2003 (+0.48°C/0.86°F) 2005 (+0.61°C/1.10°F)
Northern Hemisphere Land Ocean Land and Ocean	+0.87°C (+1.57°F) +0.49°C (+0.88°F) +0.63°C (+1.13°F)	3rd warmest 4th warmest 2nd warmest	2005 (+1.02°C/1.84°F) 2005 (+0.54°C/0.97°F) 2005 (+0.72°C/1.30°F)
Southern Hemisphere Land Ocean Land and Ocean	+0.54°C (+0.97°F) +0.43°C (+0.77°F) +0.44°C (+0.79°F)	6th warmest 5th warmest 6th warmest	2005 (+0.83°C/1.49°F) 1998 (+0.50°C/0.90°F) 1998 (+0.54°C/0.97°F)

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The most current data may be accessed via the Global Surface Temperature Anomalies page.

Precipitation The maps below represent anomaly values based on the GHCN data set of land surface stations using a base period of 1961-1990. During December 2006, above average precipitation fell over areas that include Scandinavia, Japan, central U.S., southeastern Africa, and most of South America. Below average precipitation was observed in eastern Australia, southcentral Europe, eastern U.S., eastern Brazil, and southern India. Additional details on flooding and drought can also be found on the December Global Hazards page.

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ENSO SST Analysis Click here for animated loop

Sea Surface Temperatures continued to increase across the equatorial Pacific Ocean during December, indicative of a moderate El Niño episode (shown in the adjacent animation of weekly sea surface temperature anomalies). A comprehensive summary of December 2006 ENSO conditions can be found on the ENSO monitoring page. For the latest advisory on ENSO conditions go to NOAA's Climate Prediction Center (CPC) and the CPC ENSO Diagnostic Discussion Images of sea surface temperature conditions are available for all weeks since 2003 at the weekly SST page

References Peterson, T.C. and R.S. Vose, 1997: An Overview of the Global Historical Climatology Network Database. Bull. Amer. Meteorol. Soc., 78, 2837-2849. Quayle, R. G., T. C. Peterson, A. N. Basist, and C. S. Godfrey, 1999: An operational near-real-time global temperature index. Geophys. Res. Lett., 26, 333-335. Smith, T. M., and R. W. Reynolds (2005), A global merged land air and sea surface temperature reconstruction based on historical observations (1880-1997), J. Clim., 18, 2021-2036.

Global Hazards

Please note: Material provided in this report is chosen subjectively and included at the discretion of the National Climatic Data Center (NCDC). The ability to report on a given event is limited by the amount of information available to NCDC at the time of publication. Inclusion of a particular event does not constitute a greater importance in comparison with an event that has not been incorporated into the discussion. Data included in this report are preliminary unless otherwise stated. Links to supporting information are valid at the time of publication, but they are not maintained or changed after publication.

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Indonesia Rainfall Anomalies

Global Hazards And Significant Events December 2006 Severe flooding in the Aceh province of Indonesia during late-December produced significant loss of life. Additional information can be found below.

Drought & Heat | Flooding | Storms | Tropical Cyclones | Extratropical Cyclones | Severe Winter Weather

Across the United States, significant drought affected areas of the Plains and Rocky Mountains. Extreme drought was also concentrated in northern areas of Minnesota, parts of Wyoming and Nebraska, as well as sections of Texas and Oklahoma.

U.S. Drought Monitor

National Snow & Ice

The two satellite-derived animations above show the daily snow cover across the Northern Hemisphere (left map) and North America (right map) throughout December 2006. By clicking on the images, the advance of sea-ice across Hudson Bay and parts of the Arctic can be seen through the month as well as the snowstorm across the Plains and Great Lakes in early December, the Rockies blizzard on December 20-21 and another snowstorm in the Rockies and High Plains on December 29-31.

More information on significant winter weather and other hazards can be found on NCDC's Hazards page.

The map to the left depicts the snowfall totals for the blizzard which affected primarily the front-range of the Colorado Rockies on December 20-21, 2006. Some of the largest snowfall totals during this event ranged from 21 inches (53.3 cm) in Fort Collins to 42 inches (106.7 cm) at Conifer, southwest of Denver. Meteorologists at the National Weather Service office in Boulder measured 19 inches (48.3 cm) of snowfall. This blizzard forced the closure of interstates, businesses, schools and airports stranding thousands of holiday travelers. Denver International Airport closed on the 19th, leaving about 5000 air travelers stranded in the terminals until the weather cleared. Blowing and drifting snows, creating drifts up to 5 feet (152 cm) deep, prevented the airport from reopening until the 22nd. For the month, Denver received 29.4 inches (74.7 cm) of snowfall: the 3rd snowiest December on record (1882-2006). The holiday 2006 blizzard was also the 7th largest for Denver since 1946.

Other notable snowfall accumulations in other states, as seen in the map to the left, include: Cheyenne, WY with 13.6 inches (34.5 cm), Scottsbluff, NE with 10.0 inches (25.4 cm) and Bridgeport, NE with 17.5 inches (44.5 cm). More information on December severe winter weather can also be found on NCDC's Hazards page.

Drought

Contents Of This Report: National Drought Overview Detailed Drought Discussion State/Regional/National Moisture Status Pre-Instrumental Perspective

National Overview

Based on the Palmer Drought Index, severe to extreme drought affected about 10 percent of the contiguous United States as of the end of December 2006, a decrease of about 3 percent compared to last month. By contrast, about 10 percent of the contiguous U.S. fell in the severely to extremely wet categories. About 19 percent of the contiguous U.S. fell in the moderate to extreme drought categories (based on the Palmer Drought Index) at the end of December. On a broad scale, the previous two decades (1980s and 1990s) were characterized by unusual wetness with short periods of extensive droughts, whereas the 1930s and 1950s were characterized by prolonged periods of extensive droughts with little wetness (moderate to extreme drought, severe to extreme drought).

A file containing the national monthly percent area severely dry and wet from 1900 to present is available for the severe to extreme and moderate to extreme categories. Historical temperature, precipitation, and Palmer drought data from 1895 to present for climate divisions, states, and regions in the contiguous U.S. are available at the Climate Division: Temperature-Precipitation-Drought Data page in files having names that start with "drd964x" and ending with "txt" (without the quotes).

Detailed Drought Discussion

At the end of December drought was concentrated in Texas and Oklahoma. Hydrologic drought continued in the central and northern Plains, Wyoming, Arizona, and northern Minnesota. Two snowstorms at the end of the month improved conditions in eastern Colorado and the central Plains. Abnormal dryness was experienced in the central Appalachians. Except in Florida, conditions improved in the Southeast. In Florida, dryness continued in the eastern, and especially the northeastern, part of the state (December 26 Drought Monitor). In the drought areas, soil moisture was low, vegetative health was fair to poor, and streamflow was low.

In Texas, hay prices have almost doubled since the drought began, and this year's cotton yield was about half of last year's record yield. Also in Texas, low water levels on lakes and closed boat ramps reduced income for businesses dependent on recreational uses of the lakes, christmas tree production was stunted, and burning bans were common. The shipping season on the Missouri River was the worst since 1951 because of low water levels. In northwest Nebraska wildfires caused significant damage to pasture lands and trees. Mandatory or voluntary water restrictions were in effect in parts of Florida, Alabama, Texas and Oklahoma as lake and reservoir levels dropped and other municipal water supplies were reduced. Increased salinity in the Chesapeake Bay led to a much higher than normal oyster death rate. Agricultural impacts of drought were noted throughout the Plains and in many parts of the Southeast. Some of the impacts were farmers being driven out of business, hay shortages, low crop yields, and lower cattle production. Impacts in drought-stricken areas have been collected and summarized by county at the National Drought Mitigation Center's Drought Impact Reporter.

State/Regional/National Moisture Status

A detailed review of drought and moisture conditions is available for all contiguous U.S. states, the nine standard regions, and the nation (contiguous U.S.): STATES: Alabama Arizona Arkansas California Colorado Connecticut Delaware Florida Georgia Idaho Illinois Indiana Iowa Kansas Kentucky Louisiana Maine Maryland Massachusetts Michigan Minnesota Mississippi Missouri Montana Nebraska Nevada New Hampshire New Jersey New Mexico New York North Carolina North Dakota Ohio Oklahoma Oregon Pennsylvania Rhode Island South Carolina South Dakota Tennessee Texas Utah Vermont Virginia Washington West Virginia Wisconsin Wyoming REGIONS: Northeast Region East North Central Region Central Region Southeast Region West North Central Region South Region Southwest Region Northwest Region West Region NATIONAL: Contiguous U.S.

Pre-Instrumental Perspective

There is no December 2006 Paleoclimatic Perspective

Upper Air

Troposphere Current Month / Year-to-date Temperatures above the Earth's surface are measured using in-situ balloon-borne instruments (radiosondes) and polar-orbiting satellites (NOAA's TIROS-N). The radiosonde and the satellite records have been adjusted to remove time-dependent biases (artificialities caused by changes in radiosonde instruments and measurement practices as well as changes in satellite instruments and orbital features through time). The radiosonde data used in this global analysis were developed using the Lanzante, Klein, Seidel (2003) ("LKS") bias-adjusted dataset and the First Difference Method (Free et al. 2004). Additional details are available. Satellite data have been adjusted by the Global Hydrology and Climate Center at the University of Alabama in Huntsville (UAH). An independent analysis is also performed by Remote Sensing Systems (RSS) and a third analysis has been performed by Dr. Qiang Fu of the the University of Washington (UW) (Fu et al. 2004)** to remove the influence of the stratosphere on the mid-troposphere value. Global averages from radiosonde data are available from 1958 to present, while satellite measurements began in 1979.

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Radiosonde measurements indicate that for the January-December period, temperatures in the mid-troposphere (approximately 2 to 6 miles above the Earth's surface) were 0.56°C (1.01°F) above average; the 3rd warmest January-December since global measurements began in 1958. However, satellite measurements of the January-December 2006 period varied from 5th and 7th warmest on record, depending on the analysis method.

Although the rankings from satellite measurements are lower than the in-situ radiosonde measurements for 2006, the 1979-2006 trends from the radiosonde measurements (0.16°C/decade) are similar to those calculated from satellite observations, with the exception of the UAH version, as shown in the table below. The 1958-2006 mid-troposphere trend from radiosonde observations is 0.15°C/decade. Note: These temperatures are for the atmospheric layer centered in the mid-troposphere (approximately 2-6 miles above the Earth's surface) which also includes a portion of the lower stratosphere. (The MSU channel used to measure mid-tropospheric temperatures receives about 25 percent of its signal above 6 miles). Because the stratosphere has cooled due to increasing greenhouse gases in the troposphere and losses of ozone in the stratosphere, the stratospheric contribution to the tropospheric average, as measured from satellites, may create an artificial component of cooling to the mid-troposphere temperatures. The University of Washington (UW) versions of the UAH and RSS analyses attempt to remove the stratospheric influence from the mid-troposphere measurements, and as a result the UW versions tend to have a larger warming trend than either the UAH or RSS versions. For additional information, please see NCDC's Microwave Sounding Unit page. January- December Anomaly Rank Warmest Year on Record Trend UAH mid-trop +0.11°C/+0.20°F 7th warmest 1998 (+0.44°C/0.79°F) +0.05°C/decade *RSS mid-trop +0.18°C/0.32°F 7th warmest 1998 (+0.52°C/0.94°F) +0.13°C/decade **UW-UAH mid-trop +0.21°C/0.38°F 5th warmest 1998 (+0.57°C/1.03°F) +0.12°C/decade **UW-*RSS mid-trop +0.28°C/0.50°F 7th warmest 1998 (+0.63°C/1.13°F) +0.19°C/decade RATPAC +0.56°C/1.01°F 3rd warmest 1998 (+0.74°C/1.33°F) +0.16°C/decade *Version 02_1 The December 2006 mid-troposphere rankings were also warmer than average as shown in the table below.

December	Anomaly	Rank	Warmest Year on Record	Trend
UAH mid-trop	+0.07°C/+0.13°F	12th warmest	1987 (+0.37°C/0.67°F)	+0.02°C/decade
*RSS mid-trop	+0.12°C/0.22°F	9th warmest	2003 (+0.38°C/0.64°F)	+0.09°C/decade
**UW-UAH mid-trop	+0.17°C/+0.31°F	8th warmest	1987 (+0.42°C/0.76°F)	+0.08°C/decade
**UW-*RSS mid-trop	+0.20°C/+0.36°F	8th warmest	2003 (+0.48°C/0.86°F)	+0.15°C/decade

*Version 02_1

Stratosphere Current Month The table below summarizes stratospheric conditions for December 2006. On average, the stratosphere is located approximately between 10-14 miles above the Earth's surface. Over the last decade, stratospheric temperatures have been below average in part due to the depletion of ozone. The large positive anomaly in 1982 was caused by the volcanic eruption of El Chichon in Mexico, and the sharp jump in temperature in 1991 was a result of the eruption of Mt. Pinatubo in the Philippines. In both cases the temperatures returned to pre-eruption levels within two years.

December	Anomaly	Rank	Coolest Year on Record
UAH stratosphere	-0.68°C (-1.22°F)	4th coolest	2000 (-0.80°C/-1.44°F)
*RSS stratosphere	-0.58°C (-1.04°F)	4th coolest	2000 (-0.71°C/-1.28°F)

*Version 02_1

For additional details on precipitation and temperatures in December, see the Global Hazards page.

References Christy, John R., R.W. Spencer, and W.D. Braswell, 2000: MSU Tropospheric Temperatures: Dataset Construction and Radiosonde Comparisons. J. of Atmos. and Oceanic Technology 17 1153-1170. Free M., D.J. Seidel, J.K. Angell, J. Lanzante, I. Durre and T.C. Peterson (2005) Radiosonde Atmospheric Temperature Products for Assessing Climate (RATPAC): A new dataset of large-area anomaly time series, J. Geophys. Res., 10.1029/2005JD006169. Free, M., J.K. Angell, I. Durre, J. Lanzante, T.C. Peterson and D.J. Seidel(2004), Using first differences to reduce inhomogeneity in radiosonde temperature datasets, J. Climate, 21, 4171-4179. Fu, Q., C.M. Johanson, S.G. Warren, and D.J. Seidel, 2004: Contribution of stratospheric cooling to satellite-inferred tropospheric temperature trends. Nature, 429, 55-58. Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003a), Temporal homogenization of monthly radiosonde temperature data. Part I: Methodology, J. Climate, 16, 224 240. Lanzante, J.R., S.A. Klein, and D.J. Seidel (2003b), Temporal homogenization of monthly radiosonde temperature data. Part II: Trends, sensitivities, and MSU comparison, J. Climate, 16, 241 262. Mears, Carl A., M.C. Schabel, F.J. Wentz, 2003: A Reanalysis of the MSU Channel 2 Tropospheric Temperature Record. J. Clim 16, 3650-3664.

El Niño/Southern Oscillation

Larger image of December SSTs Loop of December SSTs Larger image of December Sub-Surface Temperatures Loop of December Sub-Surface Temperatures Larger image of December SST Anomalies Larger image of 3-month (Oct-Dec) averaged SST Anomalies

Sea-Surface Temperatures (SSTs) and Mixed-Layer Conditions: A large area of Sea-Surface Temperature (SST) anomalies greater than +1.0°C (+1.8°F) stretched from the Date Line in the west-central Pacific to the South American coast in December, with anomalies greater than +1.5°C (+2.7°F) near the Dateline between 170°E and 150°W. Water temperatures in the mixed-layer also remained warm over the past month, with a large area of +3.0°C (+5.4°F) and greater temperature anomalies as deep as 150 meters in the eastern equatorial Pacific. In addition, a layer of cooler water below 100 m depth developed in mid-November in the western Pacific and moved eastward in December. For the month of December, the SST anomaly in the Niño 3.4 Index region was +1.37°C (+2.47°F), which was an increase of +0.29°C (+0.52°F) compared to the November anomaly. The SSTs in the Niño 4 Index region of the western equatorial Pacific also remained warm during December, although they decreased slightly from the previous month to an anomaly of +1.06°C (+1.91°F) above the mean (map of Niño regions). For the most recent global ocean surface temperatures, please see the loop of satellite-derived weekly SST anomalies for December 2006. With warmer SSTs in the Niño 3.4 index region in December, the 3-month running mean remained above the +0.5°C (+0.9°F) threshold that indicates the presence of an El Niño episode (NOTE: For NOAA's official ENSO classification scheme, please see NOAA's El Niño/La Niña Index Definition). The Climate Prediction Center's most recent ENSO Diagnostic Discussion indicated that the warm event (El Niño) had reached maturity and has now shown signs of weakening slightly over the past month. The ENSO forecast from the Australian Bureau of Meteorology (BoM) also continues to reflect the persistence of warm event conditions in the tropical Pacific basin into the first part of 2007 (see the Australian BoM ENSO Wrap-Up).

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Larger image of December Zonal Winds Loop of December Zonal Winds Larger image of 17 December 2006 Pacific Sea Level Anomalsies Loop of Pacific Sea-Level Altimetry Loop of Global Sea-Level Altimetry

Equatorial Zonal Winds (U-Component Winds) and Sea-Level Topography: Anomalous westerly winds were observed across a portion of the far western equatorial Pacific during December, while the easterly Trade winds were slightly above normal across the central and eastern equatorial Pacific. Significant week-to-week variability in the near-surface winds has been observed along the equatorial region of the Pacific over the past month, as shown in the loop of December zonal winds. A period of anomalous westerly flow occurred in the equatorial Pacific region during early December, followed by a return to easterly Trade winds across the Pacific basin later in the month. Pacific sea levels measured by the NASA/JPL Jason-1 satellite were above average across the eastern equatorial Pacific in early and mid-December, reflecting the warmer-than-average ocean temperatures and the maturing El Niño event (see the most recent image of 17 December 2006 sea level anomalies).

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Larger image of December OLR Anomalies Larger image of October-December OLR Anomalies Larger image of December OLR Index

Outgoing Longwave Radiation (OLR): The map to the left shows the spatial pattern of global OLR (in W m-2) measured by satellite during December. A region of negative OLR anomalies was observed in the western equatorial Pacific near the Date Line, which suggests that enhanced tropical convection has developed in this region. The monthly OLR index for December was -0.3 W m-2 averaged across an area in the western Pacific between 160° E and 160° W. This was the fifth consecutive month that the OLR index was below the long-term mean, although an OLR Index value of -0.3 is considered near-neutral. Persistently negative OLR indices are typical of the mature phase of a warm event. At present, NOAA's Climate Prediction Center (CPC) has forecasted the current El Niño episode to persist into 2007. Therefore, it is expected that the OLR Index will decrease further as the Walker Circulation shifts to the east and tropical convection in the central equatorial Pacific intensifies. Note that high frequency variability in OLR is typically associated with the Madden-Julian Oscillation (MJO, which is convective activity that propagates west to east in the near-equatorial region from the Indian Ocean into the Pacific Ocean approximately every 30-60 days). The latest MJO activity can be seen in CPC's graphs of Daily MJO Indices.

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Larger image of December SOI

Southern Oscillation Index (SOI): The standardized SOI was -0.5 in December, switching signs from a positive value in November. The near-neutral SOI in November followed six consecutive months with negative index values. Note that consistently negative (positive) values of the SOI are typical of El Niño (La Niña) conditions. However, negative SOI values are expected to redevelop by the end of the year, as NOAA's Climate Prediction Center (CPC) continues to forecast the persistence of the current El Niño episode into the first few months of 2007.

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Additional El Niño/Southern Oscillation Links

• ENSO Monitoring
  • Sea Level Anomalies
  • Sea Temperatures
  • SST Anomalies
  • Zonal Winds
• NOAA El Niño Observations Page
• NOAA El Niño / La Niña Index Definition
• NOAA's Pacific Marine Environmental Laboratory (PMEL):
  • PMEL ENSO Pages
  • Tropical Atmosphere Ocean (TAO) Project Buoy Array
  • Latest ENSO Measurements on the Web
• NOAA's Climate Prediction Center (CPC):
  • El Niño/La Niña Home Page
  • Current ENSO Diagnostics Discussion
  • Archive of past ENSO Diagnostics Discussions
  • Weekly ENSO Update
  • Daily MJO Indices
  • Glossary of Terms
• NOAA's Climate Diagnostics Center (CDC)
  • ENSO Information Page
• NASA/JPL Ocean Surface Topography from Space
• Australian Bureau of Meteorology (BoM) ENSO Wrap-Up
• IRI - International Research Institute
  • IRI ENSO Web
  • IRI's Current ENSO Discussion
  • IRI's Technical ENSO Discussion