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 Earth's ocean-atmosphere system was technically in an ENSO-neutral state during June 2017. An upper-level ridge dominated the western half of the contiguous United States (CONUS) with attendant warmer- and drier-than-normal weather, which set the stage for an active wildfire season. An upper-level trough set up over the eastern CONUS and its associated short-wave troughs, fronts and low pressure systems brought cooler- and wetter-than-normal weather. Rain in the South and Southeast contracted drought and abnormal dryness, but persistent dry conditions in the West and, especially, northern Plains expanded and intensified drought. The fronts and lows generated rounds of severe weather, but fewer than the average number of tornadoes occurred. Without ENSO-related inhibitors shearing tropical systems, the Atlantic and East Pacific basins saw slightly more active tropical cyclone activity than average. The upper-level circulation, temperature, and precipitation anomaly patterns suggested that the atmospheric drivers originating in the North Pacific may have had an influence on the month's weather, but none of the drivers could be identified as strongly influential. 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 is at maximum with the high sun angle, arctic air masses are weakest, and the circumpolar vortex and jet stream have retreated far northward. Polar air masses influence the weather over the contiguous U.S. (CONUS) less, and the warm, dry subtropical high pressure belts dominate the weather.
During June 2017, the North Pacific High and an upper-level ridge with above-normal 500-mb height anomalies controlled the weather in the West. An upper-level trough and closed low moved across the western CONUS early in the month, but the upper-level ridge dominated the western CONUS during most of June. Upper-level troughs settled over the eastern CONUS most of the month, resulting in a mean long-wave trough manifested by below-normal 500-mb heights.
- Cold fronts and surface low pressure systems moving in the jet stream flow east of the Rockies were funneled into the eastern and southern CONUS by the long-wave trough where they tapped moisture from the Gulf of Mexico and Atlantic Ocean. The frontal lifting and convergence around the surface lows wrung out the moisture to produce areas of heavy rain, especially along the Gulf Coast. The fronts from the north and systems from the south diverged around the Mid-Atlantic region, resulting in an area around the DelMarVa that ended the month drier than normal. The system that moved across the West brought rain during the second and third weeks (weeks 1, 2, 3, 4, 5), but the ridge kept most of the West and Great Plains drier than normal. On a statewide basis, the circulation and precipitation pattern this month left Nebraska with the second driest June in the 1895-2017 record and Maryland with the seventh driest June. On the other end of the scale, the rain in the South and Northeast ranked five states in the top ten wettest category — Florida (second wettest), Alabama (fourth wettest), Mississippi (fifth wettest), Louisiana (eighth wettest), and Vermont (eighth wettest). The overall circulation and precipitation anomaly pattern gave the CONUS the 56th driest June on record.
- The cold fronts and low pressure systems combined with the upper-level troughs to create unstable air and dynamics favorable for severe weather. But based on preliminary data, there were 168 tornadoes in June 2017, which is well below the June average of 243. Most occurred in the northern Plains to Upper Midwest and Mid-Atlantic regions along the primary storm track.
- The above-normal precipitation helped shrink drought and abnormally dry areas across much of the Southeast to parts of the southern Plains. But the dry weather in the northern Plains during June was just a continuation of dryness that has lasted for much of the last three to six months. North Dakota, for example, had the seventh driest April-June and fourth driest January-June in the 1895-2017 record. Drought and abnormal dryness expanded in the northern Plains as well as parts of the central to southern Plains and West. As a result, the national moderate-to-exceptional drought footprint increased from 5.3 percent at the end of May to 8.0 percent at the end of June (from 4.5 percent to 7.8 percent for all of the U.S.). The heat and dry weather set the stage for several large wildfires in the West (wildfire maps for June 2, 22, 29).
- Without ENSO-related inhibitors shearing tropical systems, the Atlantic and East Pacific basins saw slightly more active tropical cyclone activity than average. The Atlantic hurricane season runs from June 1st through November 30th and two tropical systems developed in the North Atlantic during June. One (Tropical Storm Bret) dissipated in the Caribbean Sea and did not pose a threat to the CONUS. The second (Tropical Storm Cindy) brought above-normal precipitation to the Deep South as it approached and came ashore on the 21st-22nd and, when it later became entrained along a cold front, to parts of the Ohio Valley to Northeast (weeks 1, 2, 3, 4, 5). The Eastern North Pacific hurricane season runs from May 15th through November 30th, and three tropical systems developed during June. Tropical Storm Beatriz formed on the first of June and Tropical Storm Calvin formed at mid-month, but both lasted only a day or two before falling apart upon making landfall in Mexico. Hurricane Dora developed near the end of the month and dissipated a few days later south of Baja California. Three tropical disturbances developed in the western Tropical Pacific near the U.S.-Affiliated Pacific Islands in Micronesia, but none developed further while in Micronesian waters.
With the upper-level ridge dominating the weather in the West, June temperatures averaged warmer than normal across the West and much of the Great Plains. Four states in the Southwest ranked in the top ten warmest category for June — Arizona (second warmest June), Utah (fifth warmest), California (seventh warmest), and Nevada (ninth warmest) — along with Delaware (tenth warmest). The trough in the East funneled cold fronts into the Southeast. The cloudiness and rain which accompanied the fronts, along with the cooling effect of Cindy's rains, gave several states in the Southeast a cooler-than-normal June. Further north beneath the trough, the monthly temperature anomaly pattern was mixed with some parts of the Upper Mississippi Valley and Northeast cooler than normal and other parts of the region warmer than normal, no doubt a result of migratory cooler-than-normal troughs and warmer-than-normal short-wave ridges (temperature anomaly maps for weeks 1, 2, 3, 4, 5). When averaged across the CONUS, June ranked 20th warmest.
- Warm temperature records beneath the ridge dominated cold temperature records associated with the fronts. When integrated across the month, there were 4,343 record warm daily high (1,848) and low (2,495) temperature records. This was more than twice the 1,752 record cold daily high (974) and low (778) temperature records.
- The REDTI (Residential Energy Demand Temperature Index) for June 2017 ranked near the middle of the 123-year historical record for June at 42nd highest. This was due to the temperature anomaly pattern of near to below-normal temperatures in the high population centers of the South and East and above-normal temperatures in the less heavily populated West and Plains.
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. During June, the dominance of ridging resulted in warm temperature and dry weather extremes, frequent cold frontal passages resulted in cold temperature extremes, and frontal lifting, upper-level troughs and lows, and Tropical Storm Cindy brought wet weather extremes. These occurred in various parts of the country. However, while some components of the CEI ranked high, when the components are aggregated together the regional and national CEI values for June 2017 did not rank in the top ten category. Specifically, the East North Central region had the third most extreme wet spell component and the Northwest region had the seventh most extreme wet spell component. The Southeast region had the ninth most extreme days with precipitation component. The Southwest region had the sixth most extreme days without precipitation component, sixth most extreme warm maximum temperature component, and ninth most extreme warm minimum temperature component. The West region had the sixth most extreme warm minimum temperature component. But the other components were not extreme enough to give any of these regions a top ten regional CEI. When aggregated across the nation, none of the national components ranked in the top ten category.
The upper-level circulation pattern, averaged for the month, consisted of below-normal upper-level (500-mb) height anomalies centered over the Great Lakes and extending west to the northern Plains, south to the Ohio Valley, and east across New England. These resulted from troughs and closed lows frequently moving across the area. Above-normal height anomalies were over the West, centered over the Southwest. Below-normal height anomalies and mean trough were manifested over the western Gulf of Mexico into the Lower Mississippi Valley, reflecting the passage of Tropical Storm Cindy.
Weather systems during the first half of the month brought above-normal precipitation to parts of the Pacific Northwest and adjacent Great Basin, but the upper-level ridge kept monthly precipitation below normal across most of the West and Great Plains. Weather systems moving through the upper-level trough in the eastern CONUS brought above-normal precipitation to parts of the Midwest to Northeast but mostly missed other parts of the mid-Mississippi to Ohio Valleys. Moisture from the remnants of Tropical Storm Cindy contributed to above-normal precipitation in the South and, when it got entrained along a cold front, to areas further north. There was a mixed precipitation anomaly pattern across Alaska and Hawaii, although most of the Big Island was drier than normal.
The upper-level ridge over the western CONUS brought above-normal temperatures to the West and much of the Plains states, while cold fronts, clouds, and rain associated with upper-level troughing in the eastern CONUS brought near to below-normal temperatures to much of the South and East. Above-normal temperatures dominated much of Alaska.
Global Linkages: The upper-level (500-mb) circulation anomaly pattern over North America was part of a complex long-wave pattern that stretched across the Northern Hemisphere. The usual east-west trough/ridge pairs (or couplets) and anomaly couplets were evident across Asia and North America. But north-south coupling was also apparent in latitudinal bands across Europe, the North Atlantic, and North Pacific. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (over western North America, central Asia, and parts of Europe), below-normal snow cover (over parts of northern Canada), and above-normal surface temperatures over Europe, central Asia, and the western CONUS, and in the eastern North Atlantic and central North Pacific. The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over northwestern Russia and northern parts of the North Pacific and North Atlantic; above-normal precipitation over northwestern Russia and the North American Great Lakes; and above-normal snow cover over northern coastal Russia. Parts of Australia, North America, and Asia were near to cooler than normal. But with large parts of the continents still having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the June 2017 global temperature was 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: ENSO-neutral conditions were present during June 2017 with above-average SSTs across most of the equatorial Pacific Ocean.
- Teleconnections (influence on weather): The NWS CPC has no teleconnections for ENSO-neutral conditions. However, to the extent teleconnections are known, the typical temperature and precipitation patterns associated with El Niño during June include above-normal temperatures in the Pacific Northwest, near-normal temperatures in the Southeast, and below-normal temperatures in between from the Southwest and southern Plains, plus the Great Lakes and Northeast. La Niña temperature patterns during June include mostly below-normal temperatures in the West and Southeast to Mid-Atlantic.
- Comparison to Observed: The June 2017 temperature anomaly pattern does not match either of these teleconnection patterns.
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 Wheeler-Hendon index began the month in phase 4 and spent a few days at mid-month in phase 1, but was mostly incoherent (inside the unit circle). Other aspects of the tropical ocean-atmosphere system (referred to in the CPC reports as the background or low frequency state, Kelvin and Rossby waves) appeared to play a role in influencing the month's climate (especially Kelvin waves) (MJO updates for June 5, 12, 19, 26, and July 3).
- Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. But there are no teleconnections for an incoherent MJO.
- Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is usually more closely related to weekly weather patterns than monthly. In the case of June 2017, the monthly temperature anomaly pattern has a few similarities to the temperature teleconnection pattern for phase 1, but for the third week of the month, when the MJO was in phase 1, the weekly temperature anomaly pattern does not match. And when the MJO was in phase 4 (at the beginning of the month), the weekly temperature anomaly pattern does not match. Neither do the monthly precipitation anomaly pattern nor the precipitation patterns for weeks 1 and 3 match the precipitation teleconnection patterns for phases 1 or 4.
- 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 was weakly to strongly positive throughout the month, averaging positive for the month as a whole. The 3-month-averaged index was also positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive PNA for this time of year (July on the quarterly teleconnection maps) is associated with above-normal precipitation from Montana to Texas; below-normal precipitation across the Great Lakes; below-normal upper-level circulation anomalies over southeastern Canada and the northern North Pacific; and above-normal upper-level circulation anomalies across the central North Pacific, but all of the teleconnections are weak. The temperature teleconnections are weak to non-existent across North America.
- Comparison to Observed: The June 2017 precipitation anomaly pattern does not agree with the teleconnections. Notwithstanding that, the June upper-level circulation anomaly pattern has some similarity (although shifted) to the teleconnections for a positive PNA over the North Pacific and southeastern Canada.
- 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 slightly negative, then transitioned to positive for the last half, averaging 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 in the AO Composites table) is typically associated with dry conditions across the Great Plains, Northwest to northern Rockies, and Mid-Mississippi Valley to Northeast; wet conditions along the Gulf of Mexico Coast and Southeast; above-normal temperatures from the West into the northern and central Plains; near-normal temperatures elsewhere across the CONUS; below-normal upper-level circulation anomalies over the Arctic Ocean extending into the North Atlantic; and above-normal upper-level circulation anomalies spanning both the North Pacific and North Atlantic.
- Comparison to Observed: The June 2017 monthly upper-level circulation anomaly pattern does not agree with the teleconnections for a positive AO, especially over the Arctic. The precipitation anomaly pattern has some agreement, except from the Ohio Valley to Northeast. The temperature anomaly pattern agrees over the West and Plains where there is a warm signal.
- 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 began the month slightly negative then transitioned to slightly positive for the last half of the month, averaging zero for the month. The 3-month-averaged index was also near zero.
- Teleconnections (influence on weather): There are no teleconnections for a neutral (zero) NAO.
- 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, and the three-month average WP index was slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (July on the quarterly teleconnection maps) is typically associated with above-normal temperatures in the southwest CONUS and eastern North America; below-normal temperatures over northwestern Canada; above-normal circulation anomalies over the southwestern CONUS, eastern North America, and northeast Asia; and below-normal circulation anomalies over western Canada and the west-central Pacific. There are no discernible precipitation teleconnections over the CONUS.
- Comparison to Observed: The June 2017 monthly upper-level circulation anomaly pattern agrees with the teleconnections over the southwestern CONUS and adjacent Pacific, but not elsewhere. The temperature anomaly pattern agrees in the southwestern CONUS and a hint of agreement in northwestern Canada, but not in eastern North America. There are no teleconnections to compare against the precipitation anomaly pattern.
- 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: June North Pacific SSTs continued a stratified pattern of cooler-than-normal SST anomalies between 30 and 50 degrees North latitude that stretched from near Japan to the North American coast, and warmer-than-normal SSTs north and south of this cool area. But the warm SSTs anomalies between 20 to 30 degrees North latitude got warmer. The North Atlantic High (Bermuda High) and North Pacific High were evident on the average sea level pressure (SLP) field map. The SLP anomaly field indicated that the North Atlantic and North Pacific Highs were near normal strength, the Aleutian Low over the North Pacific and Icelandic Low over the North Atlantic were deeper and more active than normal, and low pressure systems were more active over the northeastern CONUS and southeastern Canada (coincident with the anomalous upper-level trough over eastern North America. These atmospheric pressure centers of action (Aleutian and Icelandic Lows) serve to both generate or intensify low pressure storm systems as well as reflect the migratory path of low pressure systems. The June 2017 monthly EP-NP index was slightly positive while the 3-month running mean was near zero.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index this time of year (July on the quarterly teleconnection maps) is typically associated with cooler-than-normal temperatures across the CONUS from the central Rockies to East Coast and across central to eastern Canada; warmer-than-normal temperatures along the immediate West Coast of North America; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada; and below-normal upper-level circulation anomalies across central to eastern North America (stronger upper-level trough) and over the central North Pacific. The precipitation teleconnections are weak but suggest wetter-than-normal conditions over Montana, Iowa, and the Great Lakes.
- Comparison to Observed: The June 2017 upper-level circulation anomaly pattern is a good match to the teleconnections associated with a positive EP-NP over the central to eastern CONUS and adjacent Canada, but does not agree over the western CONUS or central North Pacific. The temperature anomaly pattern shows little agreement. The precipitation anomaly pattern generally does not agree where the few teleconnections exist.
Examination of the available circulation indices and their teleconnection patterns, and comparison to observed June 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in June was difficult to trace to any specific atmospheric driver or drivers. The NAO index was near zero, indicating little influence; the MJO was weak to incoherent; and the equatorial Pacific was in an ENSO-neutral state. For the remaining indices, there was some agreement in the patterns for some variables while others did not agree. The strong negative circulation anomalies over central to eastern North America agree with the EP-NP and PNA teleconnections, and the North Pacific anomalies are consistent with the PNA teleconnections, suggesting the North Pacific drivers exerted some influence on the circulation pattern. But the temperature anomaly pattern isn't consistent with the teleconnections for these two indices. The temperature anomaly pattern is more in line with the teleconnections for the AO and WP drivers, but only in the western CONUS, not in the east. The below-normal temperature anomalies in the southeastern CONUS (and warmth in the West) are more consistent with a positive NAO, but the NAO this month was near zero.
This month illustrates how the weather and climate anomaly patterns might be weakly influenced by atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific, or the anomalies might reflect what you get when no driver is dominant (i.e., random variability).