Synoptic Discussion - June 2016


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.


NCEI 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


The atmosphere was in an ENSO-neutral state during June 2016 — coming out of an El Niño but before entering an expected La Niña. Subtropical high pressure dominated the weather over the contiguous United States (CONUS), deflecting the jet stream and storm track to the Canadian border and blanketing much of the CONUS with unusually warm and dry weather. The high pressure ridge inhibited the development of severe weather, deflected tropical systems mostly away from the CONUS, aided the development of large wildfires in the Southwest, and contributed to the expansion or development of drought in many areas. The upper-level circulation, temperature, and precipitation anomaly patterns reflected the chaotic nature of an atmospheric-oceanic system in transition. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.
Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

In the Northern Hemisphere, June marks the beginning of climatological summer which is the time of year when solar heating forces the jet stream and circumpolar vortex to contract northward. Polar air masses can still influence the weather over the CONUS, 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 CONUS more frequently.

This was the case during June 2016. The long-wave circulation pattern in the upper atmosphere consisted of a high pressure ridge which dominated the CONUS, with its subsiding air and sunny skies bringing unusually hot and dry weather to much of the country. This kept the storm track to the north along the Canadian border, with upper-level troughs and lows occasionally dipping into the northern tier States. Surface fronts and lows associated with these systems brought areas of above-normal precipitation and, for the Great Lakes to Northeast, moderating temperatures. The fronts occasionally dipped further south, with one triggering mesoscale storms which dumped flooding rains over West Virginia. Summer monsoon showers brought above-normal precipitation to parts of the Southwest.

Tropical Storm Bonnie, which formed and dissipated in May, regenerated early in June in the Atlantic Ocean and contributed to rainfall along the East Coast. With El Niño over and the transition to ENSO-neutral conditions, wind shear in the Atlantic decreased and tropical activity increased. In addition to Bonnie, two other Tropical Storms — Colin and Danielle — formed in the Gulf of Mexico. The upper-level ridge deflected Tropical Storm Colin out into the Atlantic after crossing Florida, and deflected Tropical Storm Danielle into Mexico. The tropical Pacific Ocean, on the other hand, has been quiet. No tropical cyclones formed in the central equatorial Pacific and only one — Tropical Depression 01E — formed in the eastern equatorial Pacific — off the southern coast of Mexico early in June.

The upper-level dynamics associated with the CONUS ridge — especially descending air — inhibited the development of severe weather in addition to inhibiting precipitation. The troughs and frontal low pressure systems that did get through triggered severe weather east of the Rockies. Tornado outbreaks did occur, mainly in the High Plains and in the Midwest along the storm track. But the preliminary count of 90 tornadoes was well below the June average of 243 tornadoes.

In addition to inhibiting convection, which is important to the development of thunderstorms, the upper-level ridge deflected Pacific weather systems away from the West and blocked Gulf of Mexico moisture from reaching the Plains and Southeast. An upper-level trough with northwesterly flow dominated in the northeastern CONUS, which helped keep temperatures near normal but also blocked Atlantic moisture from reaching the region. This led to the expansion of drought and abnormal dryness in the Northwest, Great Plains, Southeast, and Northeast. The hot and dry weather associated with the upper-level ridge contributed to the development of wildfires across the West, especially during the last half of June (wildfire maps for June 22, 26, 29, 30).

With the upper-level ridge dominating the weather across much of the CONUS, June ended up with 5,768 record warm daily high (2,383) and low (3,385) temperature records, which is seven times the 819 record cold daily high (408) and low (411) temperature records. Nationally, June 2016 ranked as the warmest June in the 1895-2016 record. The REDTI (Residential Energy Demand Temperature Index) for June 2016 ranked ninth highest in the 122-year record for June. The unusual warmth across most of the CONUS increased cooling demands, but the REDTI would have been higher if not for the moderating temperatures in the heavily-populated Northeast.

The ridge also contributed to a rank of 14th driest June, nationally. The national moderate-to-exceptional drought footprint increased across the CONUS from 12.7 percent at the end of May to 16.2 percent at the end of June (from 10.7 percent to 13.6 percent for all of the U.S.). Drought contracted in the Hawaiian Islands and in the U.S.-Affiliated Pacific Islands.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Although temperature and precipitation extremes occurred during the month in many areas, the duration and extent of the extremes were not large enough to significantly affect the CEI components for most regions. Only one region ranked in the top ten category. The Southwest region had the second most extreme warm maximum and warm minimum temperature components, contributing to the sixth most extreme June regional CEI. When aggregated across the nation, June 2016 had the third most extreme warm minimum temperature component and sixth most extreme warm maximum temperature component which contributed to the 17th most extreme national June CEI on record.

North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern, averaged for the month, consisted of a dominant ridge with above-normal height anomalies over most of the CONUS and a trough with below-normal height anomalies over the Northeast. Troughs and ridges migrating through the jet stream flow varied the pattern throughout the month, but the anomalous ridge remained dominant most days.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

June 2016 was drier than normal across much of the CONUS, central to eastern Puerto Rico, and parts of Alaska. Western Puerto Rico, much of Hawaii, parts of Alaska, and a few parts of the CONUS were wetter than normal.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

June 2016 temperatures were above normal across most of the CONUS and much of Alaska. Central Alaska and parts of the Southern Plains, Great Lakes, and Northeast were cooler than normal.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.
Global Linkages: The upper-level (500-mb) circulation anomaly pattern over North America was part of a long-wave pattern that stretched across the Northern Hemisphere. Except for below-normal heights over the Arctic Ocean and four minor trough areas at the mid-latitudes, the hemispheric circulation pattern consisted mostly of upper-level ridges with above-normal heights. Beyond the Northern Hemisphere, above-normal heights dominated across the Tropics to Southern Hemisphere mid-latitudes, with below-normal heights dominating over Antarctica. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation and snow cover over North America and Eurasia; above-normal surface temperatures over the continents; and drying soils over parts of Asia. The few areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over North America and Eurasia; and above-normal precipitation over Asia. With most of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean, Indian Ocean, Atlantic Ocean, and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures, the June 2016 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 the available circulation indices and their teleconnection patterns, and comparison to observed June 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in June could not be definitely traced to any specific one or combination of atmospheric drivers. The atmosphere-ocean system was in a state of transition. El Niño has dissipated and the tropical Pacific was in an ENSO-neutral state potentially heading towards a La Niña. The MJO transitioned between phases, was incoherent part of the month, and had its influence affected by Kelvin waves. The daily values for the AO and NAO indices transitioned from negative during part of the month to positive during the remaining part. The PNA daily index values oscillated between positive and negative. The North Pacific SSTs, which are an important factor for the EP-NP index, and the Aleutian Low, which influences the jet stream storm track downstream over North America, underwent major transitions during June. As a result, the June 2016 temperature, precipitation, and circulation anomaly patterns were not consistently explained by any of the indices. Precipitation during the summer is not well correlated with most of the atmospheric drivers' teleconnections, the exceptions being the AO and La Niña. The atmosphere-ocean system is in an ENSO-neutral state, so this rules out La Niña. The June 2016 precipitation anomaly pattern was a very good match with the AO teleconnection pattern, and the temperature anomaly pattern matched the AO pattern where teleconnections exist (western CONUS), but the circulation anomaly pattern did not match. Elements of the June 2016 circulation anomaly pattern could be seen in the NAO teleconnections (western CONUS) and WP teleconnections (northeastern CONUS and southeastern Canada), but the teleconnections over other regions of the CONUS or Northern Hemisphere don't match and their temperature and precipitation teleconnections are poor matches.

This month illustrates how the weather and climate anomaly patterns can reflect the chaos of transition between atmospheric-oceanic states and not be strongly influenced by any single atmospheric driver (or mode of atmospheric variability).


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for June 2016, published online July 2016, retrieved on September 30, 2016 from http://www.ncdc.noaa.gov/sotc/synoptic/201606.