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.
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.
Animation of daily upper-level circulation 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.
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.
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.
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:
El Niño Southern Oscillation (ENSO)
- Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
- Status: During June 2016, ENSO-neutral conditions continued as equatorial SSTs were near or below average in the east-central and eastern equatorial Pacific.
- Teleconnections (influence on weather): The NWS CPC has no teleconnections for ENSO-neutral conditions.
- Comparison to Observed: However, the June 2016 precipitation anomaly pattern was dry across much of the country. The precipitation anomaly pattern associated with La Niña during June is also dry across much of the CONUS, but the similarity could be a coincidence. The June 2016 observed temperature anomaly pattern does not agree with that for either an El Niño or a La Niña.
Madden-Julian Oscillation (MJO)
- Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. The MJO can enter periods of little or no activity, when it becomes neutral or incoherent and has little influence on the weather. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
- Status: The MJO index began the month incoherent, then moved through phases 2 to 4 and ended the month incoherent. Other modes of coherent subseasonal tropical variability, including Kelvin wave activity, influenced the tropical Pacific and global, and especially CONUS, climate (MJO updates for June 6, 13, 20, 27, and July 4).
- Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the June (May-July) teleconnections for temperature are shown here and for precipitation are shown here.
- Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. In the case of June 2016, the weekly temperature anomaly patterns generally don't match the teleconnections for the corresponding MJO phases, and the monthly temperature anomaly pattern does match the teleconnections. There is generally no agreement between the weekly precipitation anomaly patterns and the corresponding teleconnection patterns. The monthly precipitation anomaly pattern also shows little agreement.
- The Pacific/North American (PNA) pattern
- Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
- Status: The daily PNA index began the month positive, then turned negative, then ended the month positive, averaging negative for the month as a whole. The 3-month-averaged index was also negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative PNA for this time of year (July on the quarterly teleconnection maps) is correlated with below-normal precipitation from Montana to Texas; above-normal precipitation in the Great Lakes; and little correlation with precipitation for the rest of the CONUS and Alaska. There is very little correlation with temperatures or upper-level circulation anomalies.
- Comparison to Observed: The June 2016 precipitation anomaly pattern agrees with that expected with a negative PNA where teleconnections exist. The temperature and upper-level circulation anomaly patterns have nothing to compare to (no correlations over the CONUS).
- The Arctic Oscillation (AO) pattern
- Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
- Status: The daily AO index began the month negative and ended positive, averaging slightly positive for the month. The 3-month-averaged index was slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO in June (May-July) is typically associated with dry conditions along the immediate Northwest coast, from the Northern High Plains to the Southern Plains, and from the Mid- to Upper-Mississippi Valley to the Northeast; wet conditions in the Coastal Southeast; above-normal temperatures from the Central Plains to West Coast; and upper-level circulation anomalies which are above normal across much of the North Pacific into the Pacific Northwest, and above normal from the Northeast, across the North Atlantic, to Western Europe, and below normal across the north polar regions.
- Comparison to Observed: The June 2016 monthly temperature and precipitation anomaly patterns over the CONUS agree quite well where teleconnections exist. The upper-level circulation anomaly pattern over the CONUS does not match those expected with a positive AO. The upper-level circulation anomalies agree over the North Pacific but not elsewhere in the Northern Hemisphere.
- The North Atlantic Oscillation (NAO) pattern
- Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
- Status: The daily NAO index was negative for the first half of the month and positive for the second half, averaging negative for the month. The 3-month-averaged index was slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during June (July on the quarterly teleconnection maps) is associated with below-normal temperatures in the Northern Rockies to Northern Plains; above-normal temperatures from the Southern Plains to Southeast; below-normal upper-level circulation anomalies over much of the West to Great Lakes, and from the North Atlantic to Western Europe; and above-normal upper-level circulation anomalies along the coastal Southeast, in the northeastern North Pacific, and over Greenland. The precipitation teleconnections are very weak, with wetter-than-normal anomalies in the Northern High Plains and Mid-Mississippi Valley, and drier-than-normal conditions in parts of the Southern Plains.
- Comparison to Observed: The June 2016 temperature and precipitation anomaly patterns do not agree with those expected with a negative NAO across most of the CONUS. The upper-level circulation anomaly pattern does not agree over the CONUS or North Pacific, but does agree over the western North Atlantic and Greenland.
- The West Pacific (WP) pattern
- Description: The WP teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the (East Asian) jet stream in the western Pacific.
- Status: The monthly WP index was negative while the three-month average WP index was near zero.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during June (July on the quarterly teleconnection maps) is typically associated with above-normal temperatures in the Southwest, Mid- and Upper-Mississippi Valley to East Coast, and eastern Canada; below-normal temperatures in northwest Canada; below-normal circulation anomalies over western Canada; and above-normal circulation anomalies over eastern Canada, most of the eastern CONUS, eastern Siberia, and over the North Pacific from Hawaii to California-Nevada. There are no precipitation teleconnections in the CONUS.
- Comparison to Observed: The June 2016 monthly temperature and upper-level circulation anomaly patterns generally do not agree with those expected with a negative WP where teleconnections exist. The precipitation anomaly pattern has no teleconnections over the CONUS to compare to.
- The East Pacific-North Pacific (EP-NP) pattern
- Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns (geopotential height anomalies) over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
- Status: The colder-than-normal SST anomaly pattern, which existed in the north central portion of the North Pacific during May 2016 and previous months, warmed considerably during June. Remnant pools of colder-than-normal SSTs remained in the central North Pacific, but they were surrounded by warmer-than-normal SSTs at lower latitudes and along the Asian and North American coasts. The area of greatest warming of SST anomalies in the north central North Pacific was roughly located beneath the area of strongest positive height anomalies in the upper-level circulation. This was associated with the weakening of the Aleutian Low and dominance of the subtropical North Pacific High. The June 2016 monthly EP-NP index and the 3-month running mean were both positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during June (July on the quarterly teleconnection maps) is typically associated with cooler-than-normal temperatures across central to eastern Canada and across the CONUS from the Central Rockies to Great Lakes; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada; and below-normal upper-level circulation anomalies (stronger upper-level trough) from the Central Rockies (in the CONUS) to Hudson Bay in eastern Canada, and over the central North Pacific. The precipitation teleconnections are weak but show above-normal precipitation in Montana and Iowa.
- Comparison to Observed: The June 2016 upper-level circulation, temperature, and precipitation anomaly patterns shows little agreement to the teleconnections.
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).