Synoptic Discussion - March 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.


NCDC added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.


Summary


March 2015 was characterized by an upper-level circulation pattern that generally consisted of a long-wave ridge over the western contiguous United States (CONUS) and a long-wave trough over the East. Short-wave troughs and low pressure centers moving in this flow brought areas of rain and snow to parts of the country, but the dominant long-wave circulation pattern kept much of the CONUS drier than normal with a reduced occurrence of severe weather. Several of the short-wave troughs generated winter storm systems which tapped Gulf of Mexico moisture to lay down an extensive snow cover east of the Rockies early in the month, but a pattern change to warmer and drier weather rapidly shrank the national snow cover area. The ridge produced near-record warm monthly temperatures in the West and the trough generated much below-normal monthly temperatures in the Northeast. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of March 2015 were the result of influences from multiple atmospheric drivers originating over the North Pacific, North Atlantic, and Equatorial 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, March is the beginning of climatological spring, which is the transition period between winter and summer. The sun angle gets higher in the sky and solar heating increases, which contracts the circumpolar vortex and causes the jet stream to retreat northward. March 2015 began cold and snowy with the long-wave jet stream pattern consisting of an upper-level ridge over the western CONUS and a trough in the East. But, as the month wore on, the jet stream retreated, temperatures warmed (weeks 1, 2, 3, 4), and the snow cover melted away. The long-wave ridge/trough pattern persisted throughout the retreat, with short-wave troughs and low pressure centers migrating through the upper-level flow and distorting its shape. In between the ridge and trough, a northwesterly flow funneled cool and dry Canadian air masses into the CONUS east of the Rockies. This circulation pattern kept the Northeast mostly cold and snow covered. The western ridge was reinforced by the North Pacific High and kept temperatures warmer than normal over the West, with extensive areas experiencing a record to near-record warm March. The retreating jet stream allowed the western ridge to intrude into the Northern Great Plains, where it brought unseasonable warmth.

By the end of the month, there were 5,534 record warm daily high (3,105) and low (2,429) temperature records, which is twice as many (2,550) record cold daily high (1,365) and low (1,185) temperature records. The monthly average temperature ranked March 2015 as the 12th warmest March in the 1895-2015 record, showing how the warmth dominated the national statistics. The REDTI (Residential Energy Demand Temperature Index) for March 2015 ranked 50th lowest for March, reflecting the influence of the cold anomalies in the heavily-populated Northeast and indicating that extra energy was needed to heat homes there.

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

The short-wave troughs and lows moving in the upper-level flow brought areas of precipitation to parts of the Northwest, Southwest, and northern tier states, but the long-wave ridge pattern kept total precipitation amounts well below normal. The combination of drier- and warmer-than-normal weather intensified and expanded drought and abnormally dry conditions in the West, Central to Northern Plains, and western Great Lakes, with several large wildfires developing in the Central to Southern Plains at mid-month. The North Atlantic semi-permanent high pressure system, also known as the Bermuda High, occasionally approached the southeast coast of the CONUS. With the upper-level trough to the north and Bermuda High to the south, cold fronts and surface lows were directed into a storm track from eastern Texas to the Central Appalachians. This region experienced above-normal precipitation for the month, which helped keep temperatures cool, as well as outbreaks of severe weather. The circulation pattern generally inhibited the development of tornadoes, with a preliminary count of only 13 tornadoes developing across the nation compared to a March average of 80. The above-normal precipitation improved drought conditions from Texas and the Lower Mississippi Valley to the Ohio Valley, but the Bermuda High contributed to drier- and warmer-than-normal conditions, and expanding drought, in the Southeast. The dryness in the West, Plains, western Great Lakes, and Southeast resulted in an expansion of the overall national drought footprint, with 36.8 percent of the CONUS experiencing moderate to exceptional drought at the end of March, compared to 31.9 percent at the end of February.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The extremes in temperature created by the ridge/trough circulation pattern contributed to extreme climate as measured by the CEI. The March CEI for the U.S. ranked as the 19th highest for the month, due largely to unusually warm maximum and minimum temperatures. Unusual warmth and dryness drove the CEI ranks in the western regions. The West region had the most extreme March CEI on record due largely to the most extreme Palmer drought component, third most extreme warm maximum temperature component, and fourth most extreme warm minimum temperature component. The Northwest region had the eighth most extreme March CEI on record due mostly to the most extreme warm maximum temperature component and fourth most extreme warm minimum temperature component. The West North Central region had the tenth most extreme March CEI on record due largely to the second most extreme warm maximum temperature component and eighth most extreme warm minimum temperature component.

North America monthly upper-level circulation pattern and anomalies
North America 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 western CONUS and all along the North American west coast to Alaska, above-normal heights extending across the central to southeastern CONUS (weaker-than-normal long-wave trough in the Southeast), and below-normal 500-mb heights (stronger-than-normal trough) over eastern Canada.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Most of the West, Central to Northern Plains, Upper Midwest, Northeast, and Southeast were drier than normal during March 2015. Precipitation was above normal across southern and eastern Texas, the Lower Mississippi Valley, and Ohio Valley. March was drier than normal across much of the Hawaiian Islands and southern to interior Alaska.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

March 2015 temperatures averaged warmer than normal across Alaska and the western, central, and southeastern CONUS. Temperatures were colder than normal from Texas to the Northeast.

Global monthly upper-level circulation pattern and anomalies
Global 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. Anomalous ridge/trough or trough/ridge couplets are evident over Eurasia, the North Pacific, and North America/the North Atlantic. The below-normal 500-mb heights were associated with below-normal temperatures at the surface over eastern Canada and the northeastern CONUS. The above-normal 500-mb heights were reflected by warmer-than-normal temperatures at the surface over western North America and most of Eurasia. The below-normal heights over north-central Asia reduced the magnitude of the warm anomalies there. Drier-than-normal weather occurred beneath the above-normal heights and upper-level ridges over the western CONUS and western Russia. With most of the continents having warmer-than-normal temperatures, the March 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 these circulation indices and their teleconnection patterns, and comparison to observed March 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in March reflected influences from several atmospheric drivers. The EP-NP, PNA, and NAO teleconnection indices indicate that the North Pacific region drivers influenced the overall circulation pattern, but the North Atlantic region driver also exerted an influence. The EP-NP, NAO, and MJO teleconnection indices reflected a shared influence on CONUS temperature by the North Pacific, North Atlantic, and Equatorial Pacific region drivers. The AO, NAO, and MJO teleconnection indices had the best overall correlation with the observed precipitation pattern.

This month illustrates how the weather and climate anomaly patterns can reflect the combined influence of several atmospheric drivers (or modes of atmospheric variability).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for March 2015, published online April 2015, retrieved on April 18, 2015 from http://www.ncdc.noaa.gov/sotc/synoptic/.