Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.


Summary


The ocean-atmosphere system was in a La Niña state during December 2016, but a transition to ENSO-neutral conditions was expected in early 2017. The weather over the contiguous United States (CONUS) this month was dominated by strong troughs and lows moving in the fast jet stream flow. Cold fronts and surface lows dragged along by the upper-level troughs brought areas of precipitation to much of the country. Precipitation was above normal across parts of the West and Southeast, improving drought conditions. But a large part of the country, from Texas to Indiana, was in between the storm tracks and experienced a drier-than-normal month, with drought or abnormal dryness expanding. The troughs tended to congregate in the northwestern CONUS and spread their associated cold fronts east across the country. The result was a colder-than-normal month from the Pacific Northwest to Ohio Valley, with warmer-than-normal temperatures hanging on along the southern tier states and parts of the East Coast. The upper-level circulation, temperature, and precipitation anomaly patterns suggested the atmospheric drivers originating in the Pacific Ocean had the greatest influence on the month's weather. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.
 Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

In the Northern Hemisphere, December is the first month of climatological winter which is the time of year when solar heating is at its minimum due to the low sun angle, and an expanded circumpolar vortex results in the furthest southern extent of the jet stream. Polar air masses dominate the weather over the contiguous U.S. (CONUS), and the warm, dry subtropical high pressure belts have their least influence.

A strong westerly flow dominated the upper-level circulation across the CONUS during December 2016. Troughs and cutoff lows moving in the jet stream flow dragged cold fronts and surface low pressure systems with them which brought areas of precipitation and below-normal temperatures during the first and last couple weeks of the month, while a fairly zonal flow with fewer cutoff lows dominated during the second week (precipitation anomaly maps for weeks 1, 2, 3, 4, 5) (temperature anomaly maps for weeks 1, 2, 3, 4, 5). The storm systems typically took a southwest to northeast track, dumping precipitation along the way. This resulted in a monthly precipitation pattern that consisted of southwest-to-northeast bands of anomalies.

Above-normal precipitation fell across much of the West and parts of the Southeast and Northeast, improving drought conditions in those regions. But in between the storm tracks, a large area from the Southern Plains to southern Great Lakes was drier than normal, with drought and abnormal dryness expanding there. The net effect was a decrease in the national moderate-to-exceptional drought footprint from 31.5 percent at the end of November to 22.5 percent at the end of December (from 26.3 percent to 18.8 percent for all of the U.S.). December began with several large wildfires still burning in the Southeast. Heavy rains during the last week of November and first weeks of December quenched most of the Southeast fires, but the prolonged dry conditions contributed to the development of several large wildfires the last half of the month in the Southern to Central Plains (wildfire maps for December 1, 22, 29, 31).

The lows that were especially strong and deep drew in very cold arctic air and generated intense snow storms that expanded snow cover across the CONUS. The national snow cover area grew from a low of about 25.8 percent of the CONUS on December 3rd to a peak of 54.8 percent on the 18th when a low pressure system blanketed much of the Central to Southern Plains with snow. Warmer temperatures during the last week of the month contracted the snow cover area to about 37 percent. When integrated across the month, December 2016 had the 13th largest December snow cover area for the CONUS, based on the 1966-2016 satellite record, with the storm track keeping snow cover above-normal for the month across the northern tier states.

Some of the upper-level lows where characterized by upper-level dynamics which enhanced the upward twisting of air. These dynamics, coupled with frontal lifting, set the stage for the development of severe weather, especially during the 17th and 25th. According to preliminary reports, 14 tornadoes occurred in December 2016, which is less than the December average of 24. Most occurred in the Central Plains and Lower Mississippi Valley in association with the strong fronts and upper-level low pressure systems.

The upper-level troughs tended to intensify and congregate over the northwestern and north central U.S., while the subtropical high pressure ridges dominated the Southeast and southern tier states. The resulting monthly temperature anomaly pattern consisted of colder-than-normal temperatures beneath the troughs and above-normal temperatures beneath the ridges, with state rank extremes ranging from eleventh coldest December in Oregon to fourth warmest in Florida. But cold fronts and surface lows associated with the upper-level troughs swept across the entire CONUS at times. There were 2,891 record warm daily high (1,607) and low (1,284) temperature records. This was about one and a half times the 2,003 record cold daily high (928) and low (1,075) temperature records that were associated with the air masses behind the cold fronts. The combination of warm and cold extremes resulted in the 54th coolest December, nationally, in the 1895-2016 record, or about average. The REDTI (Residential Energy Demand Temperature Index) for December 2016 ranked 40th lowest in the 122-year record for December. The above-normal temperatures along much of the East Coast and South reduced heating demand, but this was counterbalanced by below-normal temperatures from the Pacific Northwest to Central Plains which increased heating demand, resulting in a national REDTI value for December which was near the middle of the historical distribution.

La Niña conditions existed during December and La Niña conditions are typically associated with drier-than-normal weather over much of the CONUS. La Niña also typically suppresses hurricane activity in the central and eastern Pacific basins, and enhances it in the Atlantic basin. The tropical cyclone season in the North Atlantic and eastern tropical Pacific oceans normally ends in November, and no tropical systems developed in these two basins during December 2016. But in the western tropical Pacific, four tropical disturbances developed, with one becoming a typhoon, and some of them affected U.S.-Affiliated Pacific Islands. Super Typhoon Nock-Ten developed as a tropical depression in western Micronesia. Nock-Ten became a Tropical Storm as it moved over Yap State in the Federated States of Micronesia, then intensified to typhoon strength as it exited the Republic of Palau, moved over warmer waters, and experienced more favorable upper-level dynamics. Two tropical disturbances developed in the South Pacific, one of which influenced the weather over American Samoa. Neither of these developed into tropical storms.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Temperature and precipitation extremes occurred during the month in many areas, but only a few regions had component extremes in the top ten category, and none of the regions had a top ten integrated regional CEI. The Northeast region had the fifth most extreme days with precipitation component. The East North Central region had the ninth most extreme wet spell component. The Southwest region had the fifth most extreme one-day heavy precipitation component. The West region had the second most extreme one-day heavy precipitation component. When aggregated across the nation, December 2016 had only the 74th most extreme December CEI in the 107-year record.

North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern, averaged for the month, consisted of below-normal upper-level (500-mb) height anomalies over the northwestern and north central CONUS and much of Canada, resulting from troughs frequently moving across the area, and above-normal height anomalies over the south central to southeast CONUS. The below-normal heights reflected a weakening of the Aleutian Low and an increase in cyclonic activity over western North America. Of the circulation indices usually discussed on this page, the teleconnections aggregated for a negative PNA, positive TNH, and positive AO most closely match the December 2016 500-mb circulation anomalies.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

There were several bands of alternating precipitation anomalies, stretching from the southwest to northeast across the CONUS, during December 2016. The month was wetter than normal from the Southwest to western Great Lakes, from the Tennessee Valley to New England, and from coastal Texas to the coastal Carolinas. It was drier than normal across much of the Central Plains to Ohio Valley, northern Mississippi to Virginia coast, and central Florida. The December precipitation anomaly pattern across Alaska was mixed, but generally drier than normal except wetter than normal at some interior and panhandle locations. In Hawaii, December was generally drier than normal in the north from Kauai to Oahu and wetter than normal in the south from Maui to The Big Island. The anomaly pattern across Puerto Rico was mixed, but generally wetter than normal in the west and extreme southeast.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

December 2016 was cooler than normal from the Pacific Northwest to Ohio Valley, and warmer than normal across the southern tier states and much of the East Coast. Most of Alaska was cooler than normal, except the northern and western coasts which were warmer than normal.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.

Global Linkages: The upper-level (500-mb) circulation anomaly pattern over North America was part of a complex long-wave pattern that stretched across the Northern Hemisphere. It was difficult to discern trough/ridge couplets. East-west couplets could be seen (trough or below-normal heights over central North America and central Eurasia, coupled with ridge or above-normal heights over the North Pacific and Western Europe) as well as north-south couplets (one over North America and one over Eurasia). These couplets illustrate the wave nature of the atmosphere. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (in Europe), below-normal snow cover (over Europe and the Himalayas), and above-normal surface temperatures over the southern CONUS, Europe, much of southern Asia, and the North Atlantic, and a warming trend over the central North Pacific. The areas of below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures over north central Asia, western North America, and the extreme northern North Atlantic, and above-normal precipitation and snow cover over parts of Asia and North America. With Central and South America, Africa, and much of Asia having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the December 2016 global temperature was above normal, but the cooler-than-normal temperatures over parts of North America and Asia and parts of the oceans had a moderating effect.


Atmospheric Drivers


Subtropical highs, and fronts and low pressure systems moving in the mid-latitude storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The Tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which largely drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:


Examination of the available circulation indices and their teleconnection patterns, and comparison to observed December 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in December was traced mostly to atmospheric drivers originating over the North Pacific and equatorial Pacific Oceans, with possibly some influence from the Arctic. The MJO was incoherent and had little influence on the weather over North America. The precipitation teleconnections for all of the circulation indices were either weak or not strongly correlated with the observed precipitation anomaly pattern. The temperature and circulation anomaly patterns were not consistent with those expected with the WP driver. The circulation anomaly pattern fits very well as an integration of the patterns associated with the PNA, TNH, and AO drivers, while the temperature anomaly pattern appears to be an integration of the patterns associated with La Niña, the PNA, and the TNH drivers. There is some agreement between the December circulation and temperature anomaly patterns and the NAO teleconnections in the eastern CONUS, but not anywhere else, so that could be a coincidence.

This month illustrates how the weather and climate anomaly patterns can be strongly influenced by atmospheric drivers (or modes of atmospheric variability) originating in the Pacific Ocean, with Arctic drivers possibly playing a role.

Citing This Report

NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for December 2016, published online January 2017, retrieved on July 27, 2024 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/synoptic/201612.