Synoptic Discussion - January 2015

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.

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January 2015 was characterized by an active jet stream with a strong meridional flow over the contiguous United States (CONUS). Vigorous short-wave troughs moving in this flow generated areas of precipitation in parts of the country. The dominant long-wave upper-level circulation pattern kept much of the CONUS drier than normal with a reduced snow cover, but the overall national drought footprint changed little during the month. Monthly temperatures averaged warmer than normal in the West beneath a long-wave ridge and cooler than normal in the East beneath a long-wave trough. The upper-level circulation pattern, and temperature and precipitation patterns, suggest that the weather and climate of January 2015 were influenced very strongly by conditions in the North Pacific. 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, January marks the mid-point of climatological winter, which is the time of year when the sun angle and solar heating reach their minimum and an expanded circumpolar vortex forces the jet stream to migrate southward. In January 2015, the long-wave jet stream pattern consisted of an upper-level ridge over the western CONUS and a trough in the East. The ridge kept temperatures warmer than normal and deflected Pacific weather systems away from the drought-stricken West, with several western states having the top ten warmest and top ten driest January in the 1895-2015 record. The northerly flow over central North America, associated with the trough, funneled cold Canadian and Arctic air masses into the central and eastern CONUS, especially during the first half of the month. The northerly flow also kept Gulf of Mexico moisture out of the central to eastern CONUS, resulting in below-normal precipitation for January and a decreasing snow cover throughout the month.

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

The long-wave circulation pattern shifted at mid-month, with the ridge/trough pattern being replaced by a more zonal/westerly flow. This blocked the cold Canadian air masses and allowed milder Pacific air masses to move across the central and eastern CONUS, which warmed the national temperature as seen in the month-to-date temperature anomaly progression through the month (January 1st through the 5th, 11th, 17th, 25th, and 29th). By the end of the month, there were 3,499 record warm daily high (1,906) and low (1,593) temperature records, which is more than four times as many (775) record cold daily high (441) and low (334) temperature records. This was reflected in the national monthly average temperature which ranked January 2015 as the 24th warmest January in the 1895-2015 record. It was also reflected in the REDTI (Residential Energy Demand Temperature Index) for January 2015, which ranked as the 53rd lowest January value in the 121-year record. This near-average rank indicated that energy was still needed to heat homes in the heavily-populated East which remained cold while the central and western parts of the country warmed.

Parts of the CONUS received above-normal precipitation thanks to energetic short-wave troughs and lows moving in the upper-level flow. Some of these systems had enough moisture and below-freezing air to generate snow storms, but they weren't enough to bring the monthly snow cover area above average, with January 2015 ranking as the 18th least snowy January for the CONUS in the 1967-2015 satellite snow cover record. A potent storm system triggered an outbreak of tornadoes across the Southeast during the 3rd-4th, but the overall circulation pattern generally inhibited severe weather with the preliminary monthly total of 26 tornadoes below the January average of 35. The January precipitation pattern had little effect on the overall national drought footprint, with 28.4 percent of the CONUS experiencing moderate to exceptional drought at the end of January, slightly less than at the end of December.

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 (stronger-than-normal long-wave ridge) over the eastern North Pacific and western North America extending into the central CONUS, with below-normal 500-mb heights over eastern Canada extending across the far northern North Atlantic.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Parts of the Northern Rockies and High Plains, Southwest and Southern Plains, and coastal Mid-Atlantic to Northeast were wetter than normal during January 2015. Precipitation was below normal across much of the rest of the CONUS, especially in the West and Central Plains to Midwest. January was drier than normal across much of Alaska and the Hawaiian Islands.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

January 2015 temperatures averaged warmer than normal across the western and north central CONUS, southern Florida, and Alaska. Temperatures were colder than normal from the Southern Plains to Northeast.

Global Linkages: The upper-level circulation anomaly pattern over North America was part of a long-wave pattern that stretched across the Northern Hemisphere. This pattern consisted of ridges (with positive 500-mb height anomalies) over the central North Atlantic and western North America, with the North American ridge extending across the Bering Strait into Siberia. Troughs (with negative 500-mb height anomalies) were centered over the central North Pacific, north central Siberia, and northeastern North America stretching across the far northern North Atlantic into northern Europe. The above-normal 500-mb heights were reflected by above-normal temperatures at the surface over western North America to eastern Asia, while the below-normal 500-mb heights were reflected by near- to below-normal temperatures over northeast North America and north central Siberia.

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 January 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in January reflected influences from the Pacific. The EP-NP, WP, and PNA teleconnection indices had the strongest agreement with the January observations. Elements of the circulation anomalies matched the teleconnections for the PNA and WP, but had the best agreement with the EP-NP. The temperature anomaly pattern appeared to be a combination of those expected from the PNA, WP, and EP-NP. Elements of the NAO and TNH could be seen in the circulation anomaly pattern, so they may have had some influence but it would have been minor. Precipitation correlations are weak, but the WP and PNA appeared to have the greatest influence, with possible influence from the TNH and NAO and even the MJO on a weekly time scale. ENSO was officially neutral because of the disconnect between the oceanic and atmospheric indicators. There was some agreement between the January temperature and precipitation anomaly patterns and those expected with an El Niño, but the similarity could simply be coincidence and due to other factors.

This month illustrates how the weather and climate anomaly patterns can reflect the influence of one primary atmospheric driver (or mode of atmospheric variability), as seen in the temperature and circulation patterns, with other drivers exerting a limited influence.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for January 2015, published online February 2015, retrieved on February 28, 2015 from