Synoptic Discussion - February 2016


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 strong El Niño continued during February 2016 and contributed to a stronger-than-normal Aleutian Low which energized the upper-level circulation pattern over the contiguous United States (CONUS). Upper-level ridging dominated in the West, which kept precipitation drier than normal for much of the western and central CONUS and temperatures warmer than normal for most of the country. But troughs migrating through the jet stream flow intensified over the East, bringing above-normal precipitation and outbreaks of severe weather. A southerly flow of warmer-than-normal air helped contract the snow cover, especially during the last half of the month. The precipitation in the Northeast and improving conditions in the West helped to shrink the drought area slightly. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of February 2016 were largely the result of atmospheric drivers originating over the Pacific Ocean. 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, February marks the end of climatological winter which is the time of year when solar heating reaches a minimum with the low sun angle, arctic air masses are much colder, and an expanded circumpolar vortex forces the jet stream 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.

February 2016 was a little different from the norm. The strong El Niño energized the atmosphere and produced a stronger-than-normal Aleutian Low. The Aleutian Low fed an atmospheric circulation that was very active with several upper-level troughs and low pressure systems which moved across the CONUS. At the surface, this circulation pattern pushed a stream of Pacific weather systems into western North America. However, the North Pacific subtropical high pressure center was also active this month and reinforced an upper-level ridge over the western CONUS which weakened the Pacific weather systems as they migrated through it. The ridge inhibited precipitation over the West and contributed a northwesterly component to the circulation over the central CONUS which generally blocked Gulf of Mexico moisture. As a result, much of the western and central U.S. was drier than normal for the month. The strong Aleutian Low contributed to a warm southerly flow over Alaska which resulted in another warmer-than-normal month, wetter-than-normal conditions for some southern coastal locations, and drier-than-normal conditions for much of the rest of the state.

An upper-level trough dominated the eastern CONUS. Weather systems were re-energized when they reached the East, pulling in North Atlantic and some Gulf of Mexico moisture to give much of the East a wetter-than-normal month, which helped to contract drought in the Northeast. Cold Canadian air masses were pulled in behind many of the low pressure systems, giving the Southeast a monthly average temperature that was near to colder than normal. The clash of cold Canadian air and warmer and moist Gulf of Mexico and North Atlantic air, combined with a southerly flow of air aloft and the violent dynamics associated with these weather systems, resulted in outbreaks of severe weather over the Southeast and Mid-Atlantic States. There were 138 tornadoes nationwide during February (according to preliminary reports), which is well above the February average of 29.

A strong trough moved across the West at the beginning of the month, bringing below-normal temperatures across the Southwest and above-normal weekly precipitation to parts of the Southwest. Pacific weather systems plowed into the western ridge at mid-month, contributing to above-normal weekly precipitation in the Northwest and helping to contract drought in the area. But for the most part, the upper-level short-wave troughs and Pacific weather systems were no match for the western ridge, leaving the region drier than normal for the month.

The ridge also allowed warmer-than-normal air to dominate most of the country. The above-normal temperatures helped to contract the snow cover, especially during the last half of the month. About 40 percent of the CONUS was snow-covered for the first half of the month. As warmer air masses dominated during the second half, the snow cover extent contracted to 15-25 percent. By month's end, there were 6,805 record warm daily high (3,882) and low (2,923) temperature records, which is about six times the 1,109 record cold daily high (777) and low (332) temperature records, with the CONUS ranking seventh warmest nationally for February. The REDTI (Residential Energy Demand Temperature Index) for February 2016 ranked ninth lowest in the 1895-2016 record for February, illustrating how the unusually warm temperatures in the major metropolitan areas of the Northeast, Midwest, and West reduced heating demands.

Nationally, February 2016 ranked as the 46th driest February on record. This resulted from the unusually low precipitation in the usually wet west coast and south central regions. But above-normal precipitation in the Northeast, and improving hydrologic conditions in the Northwest, helped to contract drought and abnormal dryness in those regions. Areas in Wyoming, North Dakota, the Southwest, and Southern Plains missed out on the precipitation, so drought and abnormal dryness expanded there. Overall, the national moderate-to-exceptional drought footprint shrank slightly to 12.1 percent of the U.S. as a whole, and 14.3 percent of the CONUS.

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

As the troughs and ridges migrated through the jet stream flow across North America, the storm track favored troughing over the eastern CONUS and ridging over the West. A below-normal 500-mb height anomaly pattern prevailed across much of the North Pacific. Below-normal 500-mb height anomalies over the North Pacific and southeastern CONUS and above-normal anomalies over western North America are characteristic of a positive PNA index pattern.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

February 2016 was drier than normal over Hawaii, most of Alaska, and much of the West to Mississippi Valley. It was wetter than normal across most of Puerto Rico, parts of the Ohio Valley and Central to Northern Plains, and much of the eastern CONUS.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

February 2016 was warmer than normal across Alaska and most of the CONUS, and cooler than normal across parts of the intermountain basin and Southeast.

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. The circulation pattern was complicated this month. At the higher latitudes, above-normal 500-mb height anomalies stretched across the Arctic Ocean from Canada to central Russia. Between these areas of anomalous ridging, troughs with below-normal 500-mb height anomalies occurred over the north central portions of the North Pacific into eastern Siberia, and over northern Europe. At the lower portions of the Mid-Latitudes, ridge/trough couplets appear with a ridge and above-normal 500-mb heights over western North America with a trough and below-normal 500-mb heights over eastern North America; above-normal heights over the North Atlantic coupled with the below-normal heights over Northern Europe; and the above-normal heights over central Russia into northern Africa coupled with the below-normal heights over eastern Siberia and the North Pacific. The above-normal 500-mb heights were associated with upper-level ridging and above-normal surface temperatures over western North America and much of Eurasia, and with below-normal snow cover over Europe to southern Asia. The below-normal 500-mb heights were associated with upper-level troughing, near to below-normal surface temperatures over parts of eastern Siberia, and below-normal sea surface temperatures over the north central North Pacific. With large portions of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean (due to El Niño), Indian Ocean, Atlantic Ocean, and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures, the February 2016 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 February 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in February reflected influences from Pacific atmospheric drivers with some possible influence from the Arctic drivers. The El Niño was pumping energy and moisture from the tropical Pacific into the mid-latitudes, priming the atmosphere for action. This energy was reflected in an intense Aleutian Low over the north central North Pacific, as well as in positive values for the PNA and WP indices. The February 2016 temperature anomaly pattern from western Canada to much of the CONUS matched the temperature teleconnections associated with the PNA, modified by the WP, and agreed with the EP-NP teleconnections over Alaska. The upper-level circulation anomaly pattern is in good agreement with the PNA teleconnections. The February precipitation anomaly pattern seems to show some agreement with the teleconnections of several of the atmospheric drivers in various parts of the country. El Niño, PNA, AO, NAO, and even TNH seem to be partial players, although precipitation patterns are highly variable and some of the agreement may be due to random chance, especially since the temperature and upper-level circulation indicators suggested some of these drivers had little influence.

This month illustrates how the weather and climate anomaly patterns can reflect the influence of atmospheric drivers (or modes of atmospheric variability) from one general region (the Pacific).


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for February 2016, published online March 2016, retrieved on June 16, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201602.

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