Synoptic Discussion - March 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

The long-wave circulation pattern for March 2014 over North America was a continuation of the overall pattern seen throughout the winter, but with some changes. It consisted of a dominant upper-level trough over eastern Canada which extended into western Canada and the northeastern U.S., thus bisecting the upper-level ridge over western North America. Short-wave troughs undercut the western ridge, allowing Pacific weather systems to penetrate into the Pacific Northwest. This long-wave pattern reflects the influence of atmospheric modes of variability (East Pacific-North Pacific and West Pacific) associated with the North Pacific driver, whose teleconnections include colder-than-normal temperatures east of the Rockies and warmer-than-normal temperatures in the Southwest, although signs of the influence of other modes of circulation were evident in the precipitation anomaly pattern. The combination of cold and moist air masses produced frequent winter storm systems east of the Rockies, while the Pacific weather systems that broached the western ridge brought mostly rain in the warmer air over the West. The March precipitation was not enough to eliminate seasonal deficits, and the crucial mountain snowpack remained deficient in many areas, so drought conditions continued across much of the West, although the precipitation brought improvement to some areas, especially the Northwest. The western ridge continued in force over the Southwest into the Southern Plains, where the lack of precipitation saw expansion of drought. 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 time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. Cold polar air masses can still influence the weather, but warm southerly air masses begin to move into the contiguous United States (CONUS) more frequently. In March 2014, however, a strong long-wave trough in the upper-level circulation remained entrenched over eastern Canada. The northerly flow over central North America, associated with the trough, directed cold northerly air masses into the CONUS.

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

This caused Canadian cold fronts to frequently charge into the U.S. east of the Rockies all the way to the Gulf of Mexico coast and beyond. This northerly flow of cold dry air masses kept much of the central and eastern U.S. colder than normal for the month, resulting in top ten coldest March state rankings concentrated in the Great Lakes and Northeast, and keeping nearly two-thirds of the Great Lakes frozen into early April. With colder-than-normal monthly temperatures affecting the high population centers of the Midwest and Northeast, the national REDTI (Residential Energy Demand Temperature Index) ranked as the 26th highest for March.

The circulation pattern also deflected much of the Gulf of Mexico and Atlantic moisture away from the interior CONUS, resulting in a drier-than-normal month from the Great Plains to Central Appalachians and some top ten driest March state rankings. A long-wave ridge in the upper atmosphere dominated the southwestern U.S., bringing warmer-than-normal temperatures and inhibiting widespread precipitation. This combination of northerly flow in the central U.S. and ridge over the Southwest caused drought to expand in the Plains, Midwest, South, Southeast, Southwest, and U.S. as a whole, and the intensity of drought to increase significantly in the Southern Plains.

Meanwhile, energetic short-wave troughs moving in the upper-level flow picked up Pacific moisture and slammed into the western CONUS, bringing precipitation to the Pacific Northwest, Northern Rockies, and northern California. The rain and snow contracted drought in the Northwest, but expansion and intensification of drought in the Southwest resulted in a net small increase of drought for the West. The short-wave troughs had enough energy and moisture left in them, after traversing the Rockies, to generate surface low pressure systems which, when combined with the below-freezing temperatures, blanketed the northern tier states with a persistent and heavy snow cover, continuing a pattern that has occurred for much of the winter.

The first week of March saw the greatest surge of colder-than-normal air and winter snow storms which created the most extensive snow cover of the month. The circulation at higher latitudes (Canada) relaxed and became more zonal at mid-month, with warmer temperatures (weeks 2 and 3) and less extensive snow cover becoming common over the CONUS. March went out like a lion during the last week as the upper-level circulation again became more northerly at high latitudes, sending cold air masses surging back into the CONUS, dropping temperatures, and triggering a round of severe weather, including several tornadoes. The northerly flow and relatively dry air masses kept the number of tornadoes low — the preliminary count of 25 tornadoes was well below the average count of 80 tornadoes for March. However, the dry and windy weather that accompanied the cold fronts set the stage for a surge of large wildfires near the end of the month in the central to southern Plains and southern Appalachians.

By the end of March, moderate to exceptional drought affected 38.4 percent of the CONUS (according to the U.S. Drought Monitor). There were five times as many record cold daily maximum and minimum temperatures (5822) as record warm daily maximum and minimum temperatures (1149). The unusually cold maximum and minimum temperatures caused by the persistent upper-level trough, combined with fewer-than-normal days with precipitation, but heavier precipitation events when it did rain, resulted in the the biggest CEI (Climate Extremes Index) for March in the Northeast. A different combination of factors generated the seventh largest March CEI for the West — unusually warm maximum and minimum temperatures and intense, widespread long-term drought associated with persistent upper-level ridging, and heavy 1-day precipitation events caused by extreme weather systems which penetrated the ridge. The combination of these diverse regional extremes, caused by opposing weather factors, gave the CONUS the ninth largest CEI for March — largely due to cold minimum temperatures but also to widespread drought and heavy 1-day precipitation events.

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

When integrated across the month, the circulation produced a pattern of below-normal 500-mb heights (stronger-than-normal long-wave trough) over eastern North America which extended into the north central and northeastern CONUS and the Canadian Prairies. Above-normal 500-mb heights extended over northern Alaska and northwest Canada, as well as the southwestern CONUS extending into the adjacent Pacific Ocean, reflecting a bisected western long-wave ridge, much like last month.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

The Pacific storm track brought above-normal precipitation to the Pacific Northwest and Northern Rockies, while short waves moving along southerly fronts moistened areas along the Gulf of Mexico and Atlantic coasts. Although precipitation did fall in the interior CONUS, monthly totals were below normal across the Plains, Midwest, and parts of the Great Lakes, Northeast, and Southeast, as well as in the Southwest beneath the upper-level ridge. With the Pacific storm track hammering the Pacific Northwest, most weather systems were directed south of Alaska, resulting in a drier-than-normal month for much of the state.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The pattern of monthly temperature anomalies over the CONUS followed the upper-level circulation pattern — warmer than normal beneath the southwest ridge and cooler than normal east of the Rockies beneath the upper-level trough. Temperatures in Alaska averaged warmer than normal in the north, under the influence of above-normal upper-level heights, and cooler than normal in the southeast.

The upper-level circulation over North America in March is an example of an interconnected and amplified meridional pattern. The atmospheric circulation is interconnected around the world, and this interconnectedness was manifested this month by a pattern of paired above-normal and below-normal 500-mb height anomalies over North America, the North Atlantic, and Eurasia. A dipole of below-normal heights can be traced from the high latitudes over north central Asia, extending across the Arctic into lower latitudes over eastern North America. Above-normal heights dominated much of Europe and Asia. This was reflected at the surface over Eurasia as widespread above-normal temperatures and below-normal snow cover, with areas of below-normal precipitation. The atmosphere compensated for the Eurasian warmth by extending the circumpolar vortex over North America with its accompanying cold and snowy weather.


Atmospheric Drivers

Subtropical highs, and cold fronts and low pressure systems moving in the 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 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:


Upper-level circulation pattern and anomalies averaged for the last three months
Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed March 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in March was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, and thus not a player. Slowly-evolving atmospheric and oceanic forcings in the western Pacific weakened the influence of the MJO, but the MJO may have exerted some influence on precipitation during the first half of the month. The teleconnections for the NAO match March's temperature and circulation patterns in the Southwest and precipitation patterns in the Central Plains, but not elsewhere, so the match could have been coincidence. The PNA and AO indices show essentially no significant correlation to the March weather patterns. It is the North Pacific indices which have the best correlations — EP-NP and WP with the March temperature and circulation anomaly patterns. The lack of agreement with the EP-NP upper-level circulation teleconnections in western Canada and the Pacific Northwest probably reflects the undercutting of the western North America ridge by moist short-wave troughs. The EP-NP and WP show some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, PNA, and NAO. It should be noted that precipitation relationships with these indices are generally weak. This month illustrates how the anomaly patterns can be strongly represented by the indices (EP-NP, WP) measuring one atmospheric driver (the North Pacific Ocean) for circulation and temperature but also have elements of several other drivers (or modes of atmospheric variability) for precipitation.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for March 2014, published online April 2014, retrieved on July 22, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2014/3.