Synoptic Discussion - May 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


May marks the end of climatological spring, which is the season of transition from the atmospheric forcings of winter to a new set of forcings behind summer. This change was reflected by the seasonal migration northward of the circumpolar vortex and increasing influence of the subtropical high pressure centers and high pressure anomalies, but the numerous large short-wave troughs and lows traversing the contiguous U.S. in a fast westerly flow indicated that the cold season was reluctant to depart. The upper-level circulation pattern still reflected the influence of the West Pacific (WP) mode of variability associated with conditions in the North Pacific. The WP's teleconnections include colder-than-normal temperatures in the north central U.S. and warmer-than-normal temperatures in the Southwest, although signs of the influence of other modes of circulation were evident in the precipitation anomaly patterns. The short-wave troughs generated areas of heavy precipitation in the south and east, and some produced severe weather when slowed by blocking patterns in the atmospheric circulation. The circulation pattern inhibited precipitation across much of the West and Plains to Appalachians, but overall the national drought footprint contracted slightly. Dry weather in the central U.S. contributed to the growth of several large wildfires. 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, May is at the end 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 they are not as cold as in previous months. The warm, dry subtropical high pressure belt begins to exert more influence as it shifts northward, with warm southerly air masses moving into the contiguous United States (CONUS) more frequently. In May 2014, a strong westerly flow in the upper-level circulation sent several weather systems rippling across the CONUS, but the North Pacific and North Atlantic subtropical high pressure centers helped to poke warm ridges into the jet stream flow. Occasionally the upper-level trough/ridge pattern would stall and amplify, with upper-level lows and troughs moving slowly across a region.

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

These upper-level weather systems generated low pressure systems with their associated warm and cold fronts at the surface — warm air flowed ahead of the lows, with cold air surging southward behind them. There were slightly more record warm daily highs and lows (3275) as record cold daily highs and lows (2937), but the roughly equal number of daily extremes associated with the parade of competing cold and warm air masses (weeks 1, 2, 3, 4, 5) resulted in a monthly temperature anomaly pattern where monthly extremes were mostly washed out. Averaged across the nation, May 2014 ranked as the 32nd warmest May in the 1895-2014 record. Since the cold was focused more on the less populated interior of the U.S., the May 2014 REDTI (Residential Energy Demand Temperature Index) ranked as the eleventh lowest May REDTI in the 120-year record.

Severe weather developed from the Plains eastward in association with these weather systems, with several major outbreaks occurring during the month. Tornadoes occurred across the country throughout the month in the unstable air ahead of the slow-moving upper-level troughs, but the preliminary count of 150 tornadoes was well below the average count of 276 tornadoes for May.

The slow-moving nature of some of the short-wave weather systems traversing the upper-level flow brought heavy precipitation to parts of the country, especially the Gulf of Mexico coast. The haphazard character of the slow-moving upper-level systems split the storm track east of the Rockies, resulting in drier-than-normal weather from the Southern Appalachians to Central Plains, where Kansas had its sixth driest May on record, and contributing to the occurrence of large wildfires in these regions early in the month. Ridging in the upper atmosphere dominated in the West, bringing mostly below-normal precipitation and above-normal temperatures, with May 2014 ranking as the ninth warmest May for California. Drought expanded in some areas, but beneficial rains from the upper-level lows and cold fronts contracted drought in others, with the net change being a decrease in the national drought footprint compared to the end of April.

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 above-normal 500-mb heights (stronger-than-normal long-wave ridge) over western North America, the southeastern U.S., and northeast Canada, and below-normal 500-mb heights (stronger-than-normal long-wave trough) over west-central Canada. The above-normal anomalies in the west and southeast reflected the dominance of long-wave ridging in the jet stream and the influence of the subtropical high pressure centers. The below-normal anomalies over west-central Canada reflected the lingering dominance of troughs at the jet stream level as the circumpolar vortex continued its seasonal retreat northward.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell in a ring along the Gulf of Mexico coast, central Atlantic coastal plains, Northeast, Great Lakes, and Central to Southern Rockies. This "above-normal ring" surrounded a vast area of below-normal precipitation from the Central Plains to Southern Appalachians. Conditions were mostly drier than normal across the West, but parts of the West had above-normal precipitation, although normals are low this time of year. Upper-level ridging associated with above-normal 500 mb heights brought dry weather to much of Alaska, while Hawaii was generally wetter than normal in the northern islands and drier than normal in the south.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

While the roller coaster pattern of warm fronts and cold fronts traversing the CONUS kept monthly temperatures from getting too extreme, a distinct monthly temperature anomaly pattern was evident. Near normal to below normal temperatures dominated the middle third of the country, reflecting frequent cold frontal passages, while above-normal temperatures along the west and east coasts are associated with the dominance of upper-level ridging. The western North America ridge brought above-normal temperatures to Alaska, resulting in the sixth warmest May in the state's 1918-2014 record.

The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The circulation anomalies during May form an interconnected meridional pattern with the anomalies over Eurasia and the North Pacific and North Atlantic oceans, consisting of paired above-normal and below-normal 500-mb height anomalies over the continents and oceans. Compared to April, the pattern of anomalies in May reflects a significant seasonal shift toward the pole of the positive anomalies and a weakening of the negative anomalies. Consequently, there were large areas of warmer-than-normal temperatures across the Northern Hemisphere associated with the above-normal 500 mb heights, and pockets of below-normal precipitation.


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:


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 May 2014 temperature, precipitation, and circulation patterns, suggest that, as has been the case for the last several months, the weather over the CONUS in May was most closely related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO. Even though ENSO was neutral, the May precipitation anomaly pattern was characteristic of that expected with an El Niño in some areas, and the MJO appeared to have exerted some influence on temperature and precipitation. The teleconnections for the NAO match May's precipitation patterns along the Gulf of Mexico coast, but not elsewhere, so the match could have been coincidence. Likewise, the AO teleconnections match for precipitation in parts of the country, but don't match for temperature, so the match could have been coincidence. The PNA index shows essentially no significant correlation to the May weather patterns. It is the North Pacific indices which have the best correlations — EP-NP and WP with the May temperature and circulation anomaly patterns. But the poor agreement in the east for the EP-NP suggests that the WP had the strongest influence. The WP shows some agreement with the precipitation anomaly pattern where teleconnections exist, but so do the MJO, AO, 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 an index (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), and how precipitation can be a reflection of several of the drivers.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for May 2014, published online June 2014, retrieved on November 27, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2014/5.