Synoptic Discussion - November 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.



Summary


Like last month, November 2015 was characterized by a strong El Niño which contributed to an energized upper-level circulation pattern over the contiguous United States (CONUS). Several troughs and ridges migrated through the jet stream flow at northern latitudes. Strong upper-level troughs and cutoff lows competed with the subtropical high pressure systems at lower latitudes. El Niño-enhanced convection triggered tropical cyclones over the eastern equatorial Pacific, and moisture from some of these systems moved across the CONUS. A complex synoptic weather pattern near the end of the month funneled tropical moisture into the Plains, bringing flooding rains. Fronts and weather systems moving in the jet stream flow brought areas of above-normal precipitation and severe weather to parts of the country, especially across the Plains and Midwest to Southeast. The air behind some of these fronts was cold enough to produce extensive areas of snow cover. High pressure ridging kept the Northeast drier than normal and much of the country east of the Rockies warmer than normal. The widespread heavy precipitation contracted drought across the Plains, Midwest, Southeast, and coastal Northwest. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of November 2015 were the result largely of atmospheric drivers originating over the Pacific Ocean, with the El Niño playing an important 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.
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, November marks the end of climatological fall (autumn) which is the time of year when solar heating decreases as the sun angle decreases, arctic air masses become much colder, and an expanding circumpolar vortex forces the jet stream to migrate southward. Polar air masses influence the weather over the contiguous U.S. (CONUS) more, and the warm, dry subtropical high pressure belts influence the weather less.

That wasn't the case during November 2015. The North Pacific and North Atlantic subtropical high pressure centers continued to have a strong influence on the weather across the CONUS. An active upper-level circulation dominated at higher latitudes, with several short-wave troughs and ridges migrating through the jet stream flow. The troughs frequently plunged south to form cutoff lows which competed with the subtropical highs across the southern tier states. The jet stream circulation was energized by Pacific tropical cyclones spawned under the influence of a strong El Niño.

The oceanic and atmospheric conditions associated with El Niño enhanced tropical cyclone activity in the eastern and central equatorial Pacific Ocean, while increasing vertical wind shear over the North Atlantic Ocean, which inhibited the development of tropical cyclones in the Atlantic and Caribbean. Two tropical cyclones (Tropical Storm Rick and Hurricane Sandra) formed in the Eastern Tropical Pacific, which is highly unusual for this time of year, and one tropical cyclone (Typhoon In-Fa) formed in the Central to Western Tropical Pacific during November. Tropical moisture from the remnants of Hurricane Sandra moved across Mexico then became entrained in a complex synoptic circulation over the Southern Plains to Mississippi Valley at the end of November — the moisture was caught up in the southwesterly flow over Mexico along the southern edge of a large upper-level trough, then directed into the Southern Plains. Several days of heavy rains resulted as the moist air was lifted over a stationary front (November 27, 28, 29).

One tropical cyclone, Hurricane Kate, formed in the Atlantic basin. This is near the end of the Atlantic hurricane season and about average for November.

The number of major hurricanes observed in the Eastern Tropical Pacific for the 2015 season was the highest since reliable records began in 1971, and the seasonal Accumulated Cyclone Energy (ACE) was above average. Both of these results are expected during an El Niño. In terms of ACE, the 2015 Atlantic season was below average, which is consistent with an El Niño.

Typhoon In-Fa moved northward in the Pacific and added its energy to the jet stream flow. This energy propagated downstream over North America, contributing to the vigor of upper-level ridges and troughs over the CONUS.

The subtropical high pressure systems and strong El Niño helped keep the jet stream and storm track well to the north. Although cool fronts brought Pacific and Canadian air masses across the CONUS, the subtropical highs and El Niño, in combination with migrating jet stream ridges, kept temperatures warmer than normal across the central and eastern CONUS. The tendency for cutoff lows to form and slowly move across the western CONUS and into the central U.S. resulted in below-normal 500-mb heights and cooler-than-normal temperatures over the West. By the end of the month, there were 4,502 record warm daily high (1,642) and low (2,860) temperature records, which is five times the 866 record cold daily high (494) and low (372) temperature records. New Jersey had the warmest November in the 1895-2015 record and 28 other states east of the Rockies ranked in the top ten warmest category, with the CONUS ranking 13th warmest nationally. The REDTI (Residential Energy Demand Temperature Index) for November 2015 ranked fourth lowest for November, illustrating how the unusually warm temperatures reduced heating demands in the heavily-populated Plains to East Coast.

With strong cutoff lows moving slowly out of the West and into the Plains, a divergent southerly flow is set up over the Plains. This pattern contributes to the development of thunderstorms and severe weather. There were 95 tornadoes nationwide during November (according to preliminary reports), which is above the November average of 58. Most of the tornadoes occurred in the Plains to Midwest and Southeast with cutoff lows and strong troughs on the 11th and 16th-17th.

An unusual feature of this month's circulation was the highly elongated or meridional nature of the jet stream flow. As noted earlier, this resulted in the development of several strong troughs and cutoff lows which moved across the southern tier states, especially in the Southwest. The southerly circulation along the east end of these systems funneled Gulf of Mexico moisture into the Plains to Midwest and Southeast, resulting in much above-normal precipitation. Arkansas and Missouri had the wettest November in the 1895-2015 record, while eleven other states ranked in the top ten wettest category.

Cold fronts and surface lows were associated with the upper-level systems. The air behind some of these fronts was cold enough to produce snow instead of rain. A new storm system would lay down a new snow cover every five to six days, with nearly 40 percent of the CONUS snow covered at the peak on November 28th. The storms increased the snow cover across much of the West and from the Central Plains to Midwest, but mostly missed the Northern Plains to south central Canada and Northeast.

Another unusual feature of this month's atmospheric circulation was the persistent flow of Pacific weather systems into the Northwest. Locally heavy rainfall amounts led to above-normal monthly precipitation in coastal Washington and contributed to retreat of drought west of the Cascades in that state. It is unusual because during typical El Niño years storm systems are deflected away from the Pacific Northwest.

Nationally, November 2015 ranked as the fourth wettest November in the 1895-2015 record. The heavy rains reduced drought and abnormal dryness in the Plains to Midwest and Southeast. Drought also contracted slightly in parts of the West, and above-normal precipitation across parts of Alaska and Puerto Rico reduced drought and abnormal dryness in those areas. Overall, the national moderate-to-exceptional drought footprint shrank to 17.2 percent of the U.S. as a whole, and 20.6 percent of the CONUS.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Several parts of the U.S. experienced temperature and precipitation extremes during November. These included the East North Central and Southeast regions, which had the eighth most extreme November CEI in the 1910-2015 record, the Northeast region (12th most extreme), and South region (14th most extreme). The East North Central region's eighth most extreme November CEI was due to the third most extreme warm minimum and fourth most extreme warm maximum temperature components, 28th most extreme 1-day precipitation component, 31st most extreme days with precipitation component, and 33rd most extreme wet spell component. The Southeast region's eighth most extreme November CEI was due to the second most extreme warm minimum and 12thmost extreme warm maximum temperature components, tenth most extreme days with precipitation component, and 19th most extreme wet spell component. When aggregated across the nation, November 2015 had the 14th most extreme national November CEI on record.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Wetter-than-normal monthly precipitation anomalies dominated across the Southeast, most of the Midwest and Plains, and parts of the West. November was drier than normal across the Northeast and parts of the Ohio Valley, Northern Plains, and West. The precipitation anomaly pattern was mixed in Puerto Rico but mostly wetter than normal in Alaska and Hawaii.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

November 2015 averaged warmer than normal across Alaska and the CONUS east of the Rocky Mountains. Temperatures were cooler than normal across the West.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.
Global Linkages: The upper-level circulation anomaly pattern over North America was part of a long-wave pattern that stretched across the Northern Hemisphere. Above-normal 500-mb heights occurred across the eastern CONUS, Southern Europe, eastern Asia, and the northern North Pacific. Below-normal 500-mb heights occurred between these above-normal anomalies — over the southwestern CONUS, North Atlantic, southwestern Asia, and Arctic Ocean to Bering Strait. The above-normal 500-mb heights were associated with upper-level ridging, above-normal surface temperatures, below-normal snow cover (over eastern North America and Southern Europe), and near- to below-normal precipitation. The below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures, above-normal snow cover, and (over southwestern Asia) above-normal precipitation. With large portions of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean (due to El Niño) and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures, the November 2015 global temperature was well above normal.


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 November 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in November reflected influences from Arctic and Pacific atmospheric drivers. The NAO showed little correlation with the November anomaly patterns, indicating that the North Atlantic atmospheric driver had little influence on November's weather over the CONUS. For temperature, the PNA, WP, and EP-NP had strong agreement, especially east of the Rockies, suggesting that the Pacific drivers were mostly in control. The AO temperature teleconnections were also strong, suggesting that the Arctic drivers also influenced temperatures, but it may have been a coincidence since the AO teleconnection pattern falls within the EP-NP teleconnection pattern. The MJO temperature pattern also matched, but that may also have been a coincidence for similar reasons since the MJO was weak this month. For the upper-level circulation, the November monthly anomaly pattern matches that expected with the PNA, WP, and EP-NP indices, suggesting that the Pacific drivers exerted the greatest influence, but the pattern was shifted in the west, hinting at other forces exerting an influence. For precipitation, most of the indices have weak or few teleconnections. El Niño, MJO, and AO had the closest agreement with the November precipitation anomaly patterns, but not everywhere.

The El Niño created atmospheric and oceanic conditions that enhanced the development of tropical storms and hurricanes in the eastern and central tropical Pacific Ocean. Moisture and energy from the remnants of some of these tropical cyclones directly impacted the CONUS, especially through a southern branch of the jet stream. The northern branch of the jet stream is influenced more by the drivers behind the PNA, WP, and EP-NP, as well as the AO. The interaction between these two branches likely created a chaotic pattern of precipitation anomalies which makes it difficult to trace them to any particular indicator.

This month illustrates how the weather and climate anomaly patterns can reflect the influence of several atmospheric drivers (or mode of atmospheric variability), but also how random variability can influence the month's weather.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for November 2015, published online December 2015, retrieved on December 14, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201511.

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