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


December 2014 was characterized by an active jet stream with a strong zonal flow over the contiguous United States (CONUS). Pacific weather systems moving in the circulation generated areas of precipitation in parts of the country, especially in the West and Southeast, which shrank the national drought footprint slightly. The mild Pacific air masses, and relative absence of cold polar air masses, resulted in widespread warmer-than-normal monthly temperatures. A shift in the jet stream at the end of the month brought a return of very cold weather to the central and western CONUS. The upper-level circulation pattern, and temperature and precipitation patterns, suggest that the weather and climate of December 2014 were influenced very strongly by conditions in the North Pacific and Arctic. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.

In the Northern Hemisphere, December marks the beginning of climatological winter, which is the time of year when the sun angle and solar heating reach their minimum and an expanding circumpolar vortex forces the jet stream to migrate southward. In December 2014, a zonal flow dominated the jet stream pattern, keeping cold arctic air masses bottled up in Canada and allowing milder Pacific air masses to move across the CONUS. Short-wave troughs and ridges frequently moved through the upper-level flow, with the troughs bringing areas of precipitation. The circulation pattern fed Pacific moisture into the West and Gulf of Mexico moisture into the Southeast, but blocked these moisture sources from most of the central part of the country for most of the month. Upper-level troughs and closed lows generated low pressure systems at the surface which were able to tap Gulf of Mexico moisture. These low pressure systems had enough below-freezing air with them that they were able to lay down an expanding snow cover at mid-month and the end of the month — over 42 percent of the CONUS was snow-covered by December 31st, the largest extent at any point during the month. A particularly energetic low pressure and frontal system brought a severe weather outbreak on the 23rd-24th. A (preliminary) total of 26 tornadoes occurred during December 2014, most in the Southeast, which is close to the December average of 24.

Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

The relatively mild air associated with the Pacific air masses kept low temperatures from getting too cold, while frequent short-wave ridges moving in the upper-level flow brought warmer-than-normal air on southerly winds (weeks 1, 2, 3, 4). In fact, the nationally-averaged minimum temperature for the month ranked December 2014 as the warmest December in the 1895-2014 record. A shift in the jet stream occurred near the end of the month, with a long-wave ridge developing over the eastern North Pacific which induced a northerly flow in the jet stream over western Canada. This funneled cold arctic air into the CONUS during the last few days of the month. By the end of the month, there were 5,060 record warm daily high (1,371) and low (3,689) temperature records, which is more than eight times as many (580) record cold daily high (370) and low (210) temperature records. This was reflected in the national monthly average temperature which ranked December 2014 as the second warmest December in the 1895-2014 record. It was also reflected in the sixth lowest REDTI (Residential Energy Demand Temperature Index) for December, indicating that the widespread above-normal temperatures decreased the energy needed to heat homes.

The Pacific weather systems brought much-needed precipitation to the western U.S., which has been suffering from drought for much of the last three years. The precipitation was not enough to end the drought, but it did help erase some soil moisture deficits and replenish small northern California reservoirs. These were "warm" storm systems, which dropped much of their moisture in the form of rain instead of snow, resulting in a mountain snow pack which was still well below seasonal norms. Drought contracted in parts of the West — and parts of the East where above-normal precipitation fell — but expanded in the Southern Plains, with the national drought footprint shrinking slightly compared to the end of November.

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

When integrated across the month, the atmospheric circulation indicated a pattern of above-normal 500-mb heights (weaker-than-normal long-wave trough) over much of eastern and northern North America, with below-normal 500-mb heights over part of the eastern North Pacific. While cold troughs and weather systems did occur in December, this pattern of monthly upper-level height anomalies reflects the more frequent occurrence of warm ridges over North America.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across much of the West and Central Plains, and parts of the Northeast and Southeast. Precipitation was below normal across much of the Southern and Northern Plains, Mississippi Valley, and southern Great Lakes. December was drier than normal across southeastern Alaska and much of the Hawaiian Islands.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

December temperatures were warmer than normal across virtually all of the CONUS and Alaska.

Global Linkages: The upper-level circulation anomaly pattern over North America was part of a contracted circumpolar vortex across much of the Northern Hemisphere, with positive 500-mb height anomalies extending into the North Atlantic and across much of Asia. Above-normal 500-mb heights, associated with ridging in the upper atmosphere, were reflected by above-normal temperatures at the surface over much of North America and parts of Eurasia.


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:


Global monthly upper-level circulation pattern and anomalies
Global monthly upper-level circulation pattern and anomalies.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed December 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in December reflected influences from the Pacific and Arctic. Even though the TNH index was only weakly negative (i.e., near zero), the TNH pattern reflects large-scale changes in both the location and eastward extent of the Pacific jet stream, and also in the strength and position of the climatological mean Hudson Bay Low. The December upper-level circulation pattern reflected a significant shift (weakening) in the Hudson Bay Low. The TNH pattern significantly modulates the flow of marine air into North America, as well as the southward transport of cold Canadian air into the north-central United States, and the December 2014 daily upper-level atmospheric circulation features reflected the dominance of Pacific marine air masses and absence of cold Canadian air masses over the CONUS. The December temperature pattern suggests the possible influence of the equatorial Pacific (MJO) and higher latitudes (AO and NAO) as well. There was some hint that the precipitation pattern may have been influenced by the drivers behind the MJO, TNH, AO, and even El Niño (even though ENSO was officially neutral). The drivers behind the PNA and WP appeared to have little influence on December's weather.

This month illustrates how the weather and climate anomaly patterns can be the manifestation of normal (random) atmospheric variability, as seen in the precipitation pattern, but also reflect influences from several atmospheric drivers (or modes of atmospheric variability), as seen in the temperature and circulation patterns.

Citing This Report

NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for December 2014, published online January 2015, retrieved on April 19, 2024 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/synoptic/201412.