Synoptic Discussion - February 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


February 2015 was characterized by an upper-level circulation pattern that 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. The ridge produced record warm monthly temperatures in the West and the trough generated record to near-record cold monthly temperatures in the East. The upper-level circulation and temperature anomaly patterns suggest that the weather and climate of February 2015 were influenced very strongly by jet stream conditions over 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, February marks the end 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 February 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 over the West, with extensive areas experiencing the warmest February in the 1895-2015 record. It also deflected most Pacific weather systems away from the drought-stricken West, resulting in many areas having a drier-than-normal month. Short-wave troughs and low pressure centers moving in the upper-level flow would ride across the top of the ridge and occasionally produce a storm system that brought precipitation, generally to the Pacific Northwest and Rocky Mountains. But the precipitation was mostly rain instead of snow, due to the well-above-normal temperatures, and contributed little toward helping the much-below-normal mountain snowpack recover, especially in the coastal ranges.

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 last half of the month. The Northeast was under seige from cold air masses throughout the month (weeks 1, 2, 3, 4). By the end of the month, there were 8,281 record cold daily high (4,778) and low (3,503) temperature records, which is one and a half times as many (5,448) record warm daily high (2,866) and low (2,582) temperature records. This was reflected in the REDTI (Residential Energy Demand Temperature Index) for February 2015, which ranked as the tenth highest February value in the 121-year record, indicating that extra energy was needed to heat homes in the heavily-populated East. The monthly average temperature ranked February 2015 as the 53rd coolest February in the 1895-2015 record, showing how the warm West mostly canceled the cold East on the national scale.

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

The polar air masses that were pushed into the central and eastern CONUS by the upper-level northerly flow over central North America, in addition to being cold, were also dry. The circulation pattern also kept Gulf of Mexico moisture out of the Northern Plains to Upper Midwest, which were drier than normal for the month. The cold and dry circulation pattern inhibited the occurrence of severe weather, with only two tornadoes reported during February 2015, compared to a February average of 29. Low pressure systems formed along the cold fronts associated with the polar air masses, bringing a wintry mix of precipitation to the Southern and Central Plains, Midwest, South, and Northeast, expanding the CONUS snow cover area during the last half of the month. While these regions were affected by numerous winter storms, the amount of precipitation associated with them was frequently low, so only a few of these areas had normal to above-normal total precipitation for the month. The precipitation was enough to reduce the number of large wildfires, which were widespread from Oklahoma to Florida at mid-month (large wildfires on February 13, 20, and 27). The dryness in parts of the West, Plains, and Southeast resulted in an expansion of the overall national drought footprint, with 31.9 percent of the CONUS experiencing moderate to exceptional drought at the end of February, compared to 28.4 percent at the end of January.

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 February CEI for the U.S. ranked as the fifth highest for the month, due largely to the eighth highest cold maximum temperature component, ninth highest warm maximum temperature component, ninth highest cold minimum temperature component, and 16th highest warm minimum temperature component. The cold temperature extremes drove the regional CEI's to top ten status for two regions in the east — the Central region had the fifth most extreme February CEI in the 1910-2015 record and the Northeast region had the ninth most extreme February CEI. The West region had the most extreme February CEI on record due to extremely warm and dry conditions, while the Southwest region had the fourth most extreme February CEI due to extremely warm temperatures and an unusually high frequency of heavy one-day precipitation events.

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, extending into the North Pacific, with below-normal 500-mb heights (stronger-than-normal trough) over eastern North America extending from the eastern CONUS to Greenland.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Parts of the West, Plains, and Southeast were wetter than normal during February 2015. Precipitation was below normal across much of the Southwest, Southern Plains to Central Gulf Coast, Northeast to Upper Midwest, and parts of the Northern Plains. February was drier than normal across much of Alaska and the Hawaiian Islands.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

February 2015 temperatures averaged warmer than normal across the western CONUS and Alaska. Temperatures were colder than normal from the Plains to the East Coast.

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 the central North Pacific/eastern North Pacific, western North America/eastern North America, eastern North America/North Atlantic, and western Europe/eastern Europe to western Asia. Positive 500-mb height anomalies dominated most of Asia. The below-normal 500-mb heights were associated with near-normal to slightly below-normal temperatures at the surface over Western Europe and northwest Africa. Eastern North America was the only region where the upper-level trough brought much below-normal temperatures. The above-normal 500-mb heights were reflected by above-normal temperatures at the surface over western North America and most of Asia. With most of the continents having warmer-than-normal temperatures, the February 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 February 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in February reflected influences from the Pacific. The EP-NP and WP teleconnection indices had the strongest agreement with the February observations, with elements of the circulation anomalies matching the teleconnections for the PNA. The temperature anomaly pattern was a good match for those expected from the EP-NP and WP, although there was some agreement with the PNA and, for the first half of the month, possibly with the MJO. Precipitation correlations are weak, but for isolated areas there was some agreement between the precipitation anomalies and teleconnections for almost all of the indices.

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 February 2015, published online March 2015, retrieved on March 30, 2015 from http://www.ncdc.noaa.gov/sotc/synoptic.