Synoptic Discussion - June 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


June marks the beginning of climatological summer in the Northern Hemisphere, which is the season of maximum solar heating and warmest temperatures. This season is normally characterized by a contraction of the circumpolar vortex and dominance of the subtropical high pressure centers, but the numerous large short-wave troughs and lows traversing the contiguous U.S. in a fast westerly flow indicated that the jet stream was reluctant to migrate north. The troughs and associated surface fronts generated areas of heavy precipitation in the Plains and Midwest, along with severe weather. The circulation pattern inhibited precipitation across much of the West, where drought intensified, but overall the national drought footprint contracted due to the beneficial rains in the Plains. 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 warmer-than-normal temperatures in the Southwest and northeastern United States. Signs of the influence of other modes of circulation were seen in the temperature and precipitation anomaly patterns, especially the Arctic (AO) and North Atlantic (NAO) modes of variability, as evidenced by colder-than-normal temperatures in the Northwest to Northern Plains and above-normal precipitation in the Plains to Midwest. 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, June is at the beginning of climatological summer which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract poleward. 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 normally dominates the weather as it shifts northward, with warm southerly air masses moving into the contiguous United States (CONUS) more frequently. In June 2014, the North Pacific and North Atlantic subtropical high pressure centers exerted influence over the southern tier states, but the jet stream lingered across the northern half of the U.S., producing a strong westerly flow in the upper-level circulation which sent several weather systems rippling across the CONUS. 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 cooler air surging southward behind them. There were more record cool high temperature records (676) than record warm high temperature records (391), but warm nighttime temperatures dominated with more record warm low temperatures (1257) than record cold low temperatures (344). When aggregated together, there were more than one and a half times as many record warm daily highs and lows (1648) as record cold daily highs and lows (1020), and this was reflected in the national monthly average temperature which ranked June 2014 as the 33rd warmest June in the 1895-2014 record. With the above-normal temperature anomalies centered over high population areas, the energy needed to cool the nation was up with the June 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 25th highest June REDTI in the 120-year record.

Severe weather developed along the Front Range and Plains, and points eastward, in association with these weather systems, as the upper-level lows over the Northwest and Northern Plains pulled warm and moist Gulf of Mexico surface air northward. Tornadoes and large hail occurred almost every day throughout the month in this unstable air ahead of the slow-moving upper-level troughs, with the preliminary count of 326 tornadoes ranking above the average count of 243 tornadoes for June. The warm, moist Gulf of Mexico air tended to keep nighttime temperatures warm along and east of the Mississippi River and dampen daytime highs in the Deep South.

The repeated occurrence of the short-wave weather systems traversing the upper-level flow, and the slow-moving nature of some of them, brought heavy precipitation to parts of the country, especially the Central Plains to Midwest, where widespread flooding occurred. Seven states in the Plains to Midwest had the tenth wettest, or wetter, June with Minnesota ranking wettest on record for June. The abundant precipitation helped reduce the area of drought in the Plains and Midwest. A westerly flow in the upper atmosphere inhibited precipitation in the West, where drought intensified and Arizona had the third driest June on record. The net change in drought area was a decrease in the moderate to exceptional national drought footprint compared to the end of May.

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 (weaker-than-normal long-wave trough) over eastern North America, especially eastern Canada, and below-normal 500-mb heights (weaker-than-normal long-wave ridge) over Alaska and the Pacific Northwest to Northern Plains. The below-normal anomalies in the Northwest to Northern Plains reflected the lingering dominance of troughs at the jet stream level as the circumpolar vortex resisted its seasonal retreat northward.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across much of the Central to Northern Plains, Midwest, Mississippi Valley, and eastern Great Lakes. This wet area was ringed on the east and south by areas of below-normal precipitation, especially in southern New England and parts of the Southeast and Southern Plains. On its western side lay a vast area of below-normal precipitation from the Rockies to the West Coast, especially across the Southwest. Frequent upper-level lows and troughs associated with below-normal 500 mb heights brought wetter-than-normal weather to Alaska, giving the state its second wettest June in the 1918-2014 record. The precipitation pattern across Hawaii was mixed.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The storm track, with its frequent upper-level lows and troughs, kept temperatures below normal in Alaska and funneled polar air masses into the Northern Rockies and Northern Plains, resulting in monthly temperature anomalies that were colder than normal in that region. But the warm southerly flow at the surface and above-normal heights in the upper atmosphere contributed to above-normal temperatures east of the Mississippi River, while warmer-than-normal temperatures were associated with above-normal heights in the Southwest.

The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The circulation during June reflected an interconnected meridional pattern of paired above-normal and below-normal 500-mb height anomalies over the Northern Hemisphere continents and oceans. Compared to earlier months, the pattern of anomalies in June reflects the seasonal shift toward the pole of the jet stream. There were more positive height anomalies over land than negative height anomalies, along with large areas of warmer-than-normal surface temperatures across the Northern Hemisphere associated with the above-normal 500 mb heights.


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 June 2014 temperature, precipitation, and circulation patterns, suggest that, in the absence of strong forcing from the equatorial Pacific, the weather over the CONUS in June was related to the jet stream and ocean-atmosphere interactions over the North Pacific Ocean, but the North Atlantic and Arctic oceanic drivers were also exerting some influence. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO. Even though the MJO was incoherent during most of the month, it appeared to have exerted some influence on temperature and precipitation during the third week. The PNA index showed very little correlation to the June weather patterns. The teleconnections for the NAO match June's precipitation, temperature, and upper-level circulation patterns fairly consistently from the Northwest to Midwest, indicating the match is likely more than coincidence. Likewise, the AO teleconnections are fairly consistent in the interior part of the country. The WP showed the strongest correlations, indicating the North Pacific still exerted influence, but EP-NP's influence was waning. The precipitation teleconnections with most of the indices are weak during the summer, but there was some indication that the MJO, PNA, AO, and NAO affected the precipitation patterns in some areas at various times during the month. This month illustrates how the weather and climate anomaly patterns can be the manifestation of influences from several atmospheric drivers (or modes of atmospheric variability).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for June 2014, published online July 2014, retrieved on October 23, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2014/6.