Synoptic Discussion - November 2014


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.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Summary


November 2014 was characterized by an active jet stream with a strong meridional flow over the contiguous United States (CONUS). A long-wave trough brought below-normal temperatures east of the Rockies for much of the month. Weather systems moving in the circulation generated areas of precipitation in parts of the country, which shrank the national drought footprint slightly, and a widespread snow cover at mid-month. But drier-than-normal weather accompanied a western ridge and a dominant northwesterly flow east of the Rockies, resulting in the 46th driest November in the 1895-2014 record. The upper-level circulation pattern, and temperature and precipitation patterns, suggest that the weather and climate of November 2014 were influenced very strongly by conditions in the North Pacific, with the remnants of tropical cyclone Nuri playing a crucial role. 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, November marks the end of climatological fall (autumn), which is the time of year when solar heating decreases as the sun angle decreases and an expanding circumpolar vortex forces the jet stream to migrate southward. In November 2014, the remnants of super typhoon Nuri moved into the North Pacific, generating record low pressure as an extra-tropical cyclone in the Bering Strait. This created a strong meridional circulation downstream, causing the jet stream to surge far south across the contiguous U.S. (CONUS) and funneling cold arctic air masses east of the Rockies. Short-wave troughs frequently moved through the strong meridional flow, bringing areas of precipitation and a rapidly expanding snow cover. The jet stream trough provided the below-freezing air, and the Gulf of Mexico provided the moisture, so that, by November 17th, snow covered about half of the CONUS, with the November North American and contiguous U.S. snow cover extent setting new highs in the 1966-2014 satellite record. Upper-level ridging over the western U.S. brought above-normal temperatures and below-normal precipitation to the Southwest, while the predominant northwesterly flow associated with the upper-level trough inhibited precipitation across the central CONUS.

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

The upper-level trough established itself by mid-month and persisted for a couple weeks, sending several cold fronts at the surface across the central and eastern CONUS. These reinforcing surges of cold air (weeks 1, 2, 3, 4) gradually sent the monthly temperature anomaly plummeting (November 9, 16, 23), with daily temperature records mounting. By the end of the month, about 9,197 record cold daily high (4,955) and low (4,242) temperature records had been set — 3.5 times as many (2,657) record warm daily high (1,337) and low (1,320) temperature records. This was reflected in the national monthly average temperature which ranked November 2014 as the 16th coldest November in the 1895-2014 record. It was also reflected in the 14th highest REDTI (Residential Energy Demand Temperature Index) for November, indicating that the widespread below-normal temperatures increased the energy needed to heat homes.

The remnants of Nuri forced a long-wave ridge in the upper-level circulation over western North America, Alaska, and the eastern North Pacific. A southwesterly flow, associated with the ridge, brought warmer-than-normal air to Alaska, while the ridge warmed the western CONUS. The ridge inhibited precipitation, giving Alaska a drier-than-normal month. The dry arctic air masses funneled by the northwesterly flow associated with the trough east of the Rockies resulted in fewer days with precipitation and a drier-than-normal month in the interior CONUS. This circulation pattern also inhibited the development of severe weather, with only 27 tornadoes developing compared to the average count of 58 tornadoes for November. Storms, which formed along the southern edge of the arctic air masses, tapped Gulf of Mexico moisture to bring above-normal precipitation to the drought areas of the Southern Plains and Southeast, improving drought conditions in those regions and shrinking the national drought footprint slightly compared to the end of October. The contrast in temperatures across the cold fronts, the presence of Gulf moisture, and dynamic localized circulation associated with some of these storms were responsible for the development of the tornadoes, all of which occurred in the Southeast.

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 western North America, with below-normal 500-mb heights (stronger-than-normal long-wave trough) over the eastern half of North America and also the North Pacific south of the Bering Strait. The anomaly couplet over the North Pacific strongly reflects the influence of the extra-tropical remnants of Nuri — Nuri's deep low pressure lowered the 500-mb heights near and south of the Bering Strait, while the strong southerly circulation east of the center of Nuri raised the heights over Alaska and the west coast of North America.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across parts of the Northern Rockies, Southern Plains, and Southeast. Precipitation was below normal across much of the country, including the West Coast, Southwest, Central Plains, Mississippi and Ohio Valleys, and interior Northeast. November was drier than normal over most of Alaska and the Hawaiian Islands.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

November temperatures were colder than normal across most of the CONUS and above-normal over the Southwest and Alaska.

Global Linkages: The upper-level ridge/trough circulation pattern over North America propagated downstream, with anomalous ridge/trough couplets occurring over the Atlantic Ocean and Eurasia. Above-normal 500-mb heights, associated with ridging in the upper atmosphere, were reflected by above-normal temperatures at the surface over Europe and eastern Asia and below-normal precipitation over eastern Europe to western Russia. Below-normal 500-mb heights, associated with troughs in the upper atmosphere, were reflected by near- to below-normal temperatures at the surface over eastern North America and northern Siberia, and above-normal snow cover across much of North America.


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 November 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in November was strongly influenced by activities in the North Pacific Ocean, although there were hints that the jet stream and ocean-atmosphere interactions over the Arctic (AO) and equatorial Pacific exerted some limited influence. The remnant cyclone that was Nuri had a major influence on the downstream circulation over North America when it moved into the North Pacific, an interaction that was consistent with a positive EP-NP. This Pacific influence appeared to overwhelm any influence that may have been exerted by the PNA, NAO, and MJO, although the AO could have been influential when it was in phase with the EP-NP (i.e., negative AO). The WP was neutral during the month. ENSO was still classified as neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state and there was some hint that precipitation anomaly patterns east of the Rockies could have been influenced by the developing El Niño.

This month illustrates how an extremely powerful weather system (the extratropical cyclone that was the remnants of Typhoon Nuri) can impact the monthly weather and climate anomaly patterns by reinforcing one atmospheric driver (or mode of atmospheric variability) (as reflected by the EP-NP North Pacific teleconnection index).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for November 2014, published online December 2014, retrieved on December 21, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/.