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.
Summary
The El Niño continued during April 2016. Although it was weakening, it still contributed to a stronger-than-normal Aleutian Low which energized the upper-level circulation pattern over the contiguous United States (CONUS). An upper-level ridge across western North America gave the Pacific Northwest much warmer-than-normal temperatures, while upper-level troughs spun off from the Aleutian Low and undercut the ridge, bringing cooler and wetter-than-normal weather to the Southwest. The upper-level troughs also funneled Gulf of Mexico moisture into the Great Plains, contracting drought and abnormal dryness there. Meanwhile, drought and abnormal dryness expanded in the East where a drier-than-normal April across much of the area compounded dryness which has been building over the last four months. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of April 2016 were the result of a combination of atmospheric drivers originating over the Atlantic, Pacific, and Arctic Oceans. See below for details.
Synoptic Discussion
Animation of daily upper-level circulation for the month.
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Animation of daily surface fronts and pressure systems for the month.
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In the Northern Hemisphere, April is in the middle of climatological spring which is the time of year when solar heating increases with the rising sun angle, arctic air masses are not as cold, and a contracting circumpolar vortex forces the jet stream northward. Polar air masses influence the weather over the contiguous U.S. (CONUS) less, and the warm, dry subtropical high pressure belts influence the weather more.
During April 2016, the upper-level circulation reflected this battle between the subtropical high pressure systems and the circumpolar vortex. High pressure ridging dominated parts of western North America and the eastern CONUS, resulting in above-normal 500-mb height anomalies. But the Aleutian Low was stronger than normal, contributing to below-normal 500-mb heights over the north central North Pacific. Low pressure troughs and energy centers spun off the Aleutian Low and penetrated the western North America ridge, creating large upper-level low pressure systems which moved across the western CONUS. These frequently stalled or got cut off from the jet stream flow, resulting in near- to below-normal 500-mb heights, above-normal precipitation, and near- to below-normal temperatures over the southwestern CONUS.
As the lows spun slowly over the Southwest, the circulation around their eastern edge pulled in moisture from the Gulf of Mexico and funneled it into the Southern Plains. The upper-level dynamics on their eastern edge also supported the development of strong storms which tapped into the Gulf moisture and created widespread heavy rains and flooding, especially over Texas. The dynamics supported the development of severe weather as well, with 109 tornadoes (according to preliminary reports) occurring from the Southern Plains to Ohio Valley as well as in the Southeast. But with ridging from the Bermuda High (which is also known as the North Atlantic High) competing with the upper-level lows, the tornado count was below the April average of 155 tornadoes.
The upper-level ridging over western North America deflected many Pacific storm systems away from the Pacific Northwest. Those that penetrated the ridge were weakened. With ridging dominating, the month was warmer than normal across much of the West and drier than normal over the Pacific Northwest. The warmer- and drier-than-normal conditions extended into western Canada, with one consequence being a major wildfire in northern Alberta in early May.
Upper-level troughs dominated the jet stream flow over eastern North America and funneled cooler and drier Canadian air masses into the eastern CONUS, especially during the first half of the month (temperature anomaly maps for weeks 1, 2, 3, 4) (precipitation anomaly maps for weeks 1, 2, 3, 4). This resulted in cooler-than-normal monthly temperatures from the Great Lakes to Northeast. Upper-level ridging, reinforced by the Bermuda High, brought warmer-than-normal temperatures to the eastern CONUS during the last half of the month. The combination of dry Canadian air masses, followed by upper-level ridges, contributed to a drier-than-normal month across parts of the CONUS east of the Mississippi.
The storm track was mostly limited to the northern CONUS and southern Canada during April 2016, with most surface low pressure systems migrating near the border or moving up from the Southwest to the Great Lakes. Most of the precipitation fell as rain, but some of the lows were accompanied by below-freezing air masses which generated areas of snow. The month began with about 13 percent of the CONUS snow-covered. The snow cover steadily decreased during the month with occasional blips in coverage occurring as storm systems drew in below-freezing air to lay down new snow, mainly in the northern Plains or New England. Snow coverage reached a low of about 4 percent by the 23rd, but a storm system near the end of the month brought the snow coverage back up to 13 percent on the 28th. Based on the NOAA/Rutgers 1966-2016 satellite record, April 2016 had the 19th smallest April monthly snow cover area for the CONUS.
Dry weather during the first three months of 2016 contributed to the development of numerous large wildfires across the Southern Plains (especially Oklahoma) and the Southern Appalachians. Heavy rains beginning in mid-April quenched the wildfires in the Plains and rains near the end of the month reduced their number in the Appalachians (wildfire maps for April 1, 8, 15, 22, 29).
With ridges dominating in the Northwest, troughs in the Northeast early in the month, and frequent passage of frontal systems, April ended up with 3,078 record warm daily high (1,382) and low (1,696) temperature records, which is about three times the 1,044 record cold daily high (592) and low (452) temperature records. Nationally, April 2016 ranked as the 18th warmest April in the 1895-2016 record. The REDTI (Residential Energy Demand Temperature Index) for April 2016 ranked 27th lowest in the 122-year record for April, illustrating how the varying pattern of temperature anomalies across the country and throughout the month muted their impact on heating and cooling demands during this transition season.
Nationally, the mix of abnormally wet and abnormally dry areas ranked April 2016 as the 21st wettest April on record. The wet conditions in the Great Plains helped contract drought and abnormally dry areas. But the drier-than-normal weather expanded drought and abnormally dry areas in parts of the East. Drought contracted slightly in the Hawaiian Islands, but El Niño-induced severe to extreme drought continued to affect much of the U.S.-Affiliated Pacific Islands. Overall, the national moderate-to-exceptional drought footprint shrank slightly to 12.3 percent of the U.S. as a whole, and 14.6 percent of the CONUS.
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Only one region (Northwest region) ranked in the top ten most extreme category for the April regional CEI. With the most extreme warm minimum temperature component and second highest warm maximum temperature component, the Northwest region had the tenth highest regional CEI. While other regions had extreme ranks for some components — for example, third most extreme 1-day precipitation component for the West North Central Region and fourth most extreme 1-day precipitation component for the South Region — none had a combination of components extreme enough to break into the top ten category for the regional CEI, nor did the national April CEI rank in the top ten category.
The upper-level circulation pattern, averaged for the month, consisted of a dominant ridge over western North America extending across most of the western CONUS. Troughs and ridges migrating through the jet stream flow varied the pattern throughout the month, with frequent troughs and cutoff lows resulting in a near- to below-normal 500-mb height anomaly pattern for the month in the Southwest. Slightly above-normal 500-mb heights dominated across the eastern CONUS, largely due to frequent ridging during the last half of the month. |
April 2016 was drier than normal across much of Hawaii, parts of Alaska and Puerto Rico, much of the Pacific Northwest and Northeast, and parts of the Mississippi Valley to Southeast. It was wetter than normal across much of Puerto Rico, parts of Alaska, much of the Great Plains and Southwest, and parts of the Southeast. |
April 2016 was warmer than normal in Alaska, much of the West, and parts of the Central Plains to Southeast. It was colder than normal in parts of the Southwest to Southern Plains and Southeast, and most of the Great Lakes to Northeast. |
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. The circulation pattern was complicated this month. At the mid- and higher latitudes, the circulation pattern consisted of a series of longwave troughs and ridges. These usually form pairs or couplets in an east-west orientation, but this month their orientation was both east-west and north-south and scattered. Troughs with below-normal 500-mb heights occurred over the North Pacific, eastern Canada, Northern Europe, northeast North Atlantic, and eastern Asia where Mongolia, Russia, and China intersect. Ridges with above-normal 500-mb heights occurred over western North America (coupled with the North Pacific trough), eastern Mediterranean (coupled with the Northern Europe trough), north central Asia (coupled with the East Asian and North Pacific troughs), and Arctic Ocean. Multiple couplings beyond those pairings described were also evident. Beyond the Northern Hemisphere, above-normal heights extended across the Tropics to Southern Hemisphere mid-latitudes where ridge/trough couplets were evident. The below-normal 500-mb heights were associated with upper-level troughing and near- to below-normal surface temperatures over eastern Canada, Northern and Western Europe, and eastern Asia; below-normal sea surface temperatures over the north central North Pacific and North Atlantic; above-normal snow cover over eastern Canada and eastern Asia; and above-normal precipitation over Northern and Western Europe. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal snow cover over western North America, above-normal sea surface temperatures in most oceans, and above-normal surface temperatures over most continents. With most of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean (due to El Niño), Indian Ocean, Atlantic Ocean, and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures, the April 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:
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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 April 2016, El Niño conditions were observed as above-average SSTs continued across the western and central equatorial Pacific and continued to be coupled to the tropical atmosphere. Collectively, the atmospheric and oceanic features reflected a weakening El Niño.
- Teleconnections (influence on weather): To the extent teleconnections are known, the typical temperature and precipitation patterns associated with El Niño during April include above-normal precipitation in California and along the Oregon coast, the Southern Plains to Southeast, and along the East Coast; below-normal precipitation in parts of the interior Northwest, Northern Rockies, Upper Mississippi Valley, and Ohio Valley; above-normal temperatures in Washington, northern Minnesota, and the Northeast; and below-normal temperatures across the Southwest to Southern and Central Plains, as well as parts of the Southeast.
- Comparison to Observed: The April 2016 precipitation anomaly pattern agrees in most parts of the country where teleconnections exist with an El Niño, except the Northern Plains and Northeast. The temperature anomaly pattern shows some hint of agreement in the West but is opposite to that expected historically with an El Niño in the Northern Plains to Northeast.
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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 transitioned weakly through phases 1-3 during the first week of April, then became incoherent. Other modes of coherent subseasonal tropical variability, including the background El Niño state and Kelvin and Rossby wave activity, influenced the tropical Pacific and global climate (MJO updates for April 4, 11, 18, 25, and May 2).
- 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 March-May 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 April 2016, the temperature anomalies for the first week matched the teleconnections for phase 1 reasonably well. With the MJO incoherent for the rest of the month, none of the teleconnection patterns apply. The monthly temperature temperature anomaly pattern did not match the teleconnections for MJO phases 1-3. There was no agreement between the teleconnection pattern for phase 1 and the precipitation anomaly pattern for week 1. The monthly precipitation anomaly pattern generally matched the teleconnection pattern for phase 1, but that may have been coincidence since most of the month the MJO was incoherent.
- 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 positive for most of 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 (April on the teleconnection maps) is correlated with below-normal temperatures over the Southern Plains to East Coast; above-normal temperature anomalies along the West Coast and across western Canada and Alaska; below-normal precipitation in the western Great Lakes; above-normal precipitation in Colorado and Kansas; near-normal (low correlation) for the rest of the CONUS and Alaska; above-normal upper-level circulation anomalies over most of Canada and the northwestern CONUS; and below-normal circulation anomalies over the Southern Plains to East Coast and over the North Pacific.
- Comparison to Observed: The April 2016 temperature anomaly pattern matches the teleconnections expected with a positive PNA over Alaska and much of the West, but not east of the Rockies. The April upper-level circulation anomaly pattern agrees over the North Pacific, western Canada, and Pacific Northwest, but not over the rest of North America. The April precipitation anomaly pattern agrees over Colorado-Kansas, but there are few teleconnections.
- 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 positive but quickly went negative where it stayed for the rest of the month, averaging negative for the month. The 3-month-averaged index was slightly negative to near zero.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (March-May) is typically associated with dry conditions over western Washington and over the CONUS east of the Mississippi River; below-normal temperatures across most of the CONUS; and upper-level circulation anomalies which are above normal in the North Central Pacific and eastern CONUS and below normal over the Arctic Ocean and into northern Canada.
- Comparison to Observed: The April 2016 monthly temperature and precipitation anomaly patterns match those expected with a positive AO in the CONUS east of the Rockies, but not across much of the West. The March 2016 monthly upper-level circulation anomaly pattern agrees over the eastern CONUS, but generally not across most of the rest of the 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 began the month positive, oscillated between negative and positive during the month, ending negative but averaging positive for the month. The 3-month-averaged index was positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (April on the teleconnection maps) is associated with above-normal temperature and upper-level circulation anomalies over most of the CONUS. The precipitation teleconnections are very weak, with drier-than-normal anomalies in northern California to Nevada, the Central Plains, and Southeast.
- Comparison to Observed: The April 2016 temperature anomaly pattern agrees with that expected with a positive NAO across most of the CONUS except in the Southwest and Northeast. The upper-level circulation anomaly pattern agrees across most of the CONUS except in the Southwest. The precipitation anomaly pattern has few teleconnections, but they are opposite the anomalies expected in the Central Plains and Southeast.
- 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 and three-month average WP index were both slightly negative.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (April on the maps) is typically associated with above-normal temperatures in the Southwest; below-normal temperatures from the Mid-Mississippi Valley to northwest Canada; above-normal precipitation in Washington (although there are few other teleconnections); above-normal circulation anomalies over the Southwest; and below-normal circulation anomalies over the Northern Plains into southwest Canada.
- Comparison to Observed: The April 2016 monthly temperature anomaly pattern little agreement with that expected with a negative WP over the CONUS. The precipitation anomaly pattern has few teleconnections, but both the precipitation and upper-level circulation anomaly patterns are the opposite of the expected patterns over the CONUS where teleconnections exist.
- 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 in the north central portion of the North Pacific, with the warmer-than-normal ring along its southern, eastern, and northeastern edges, continued during April 2016, with little change along the North American coastline since the end of March. The cooler-than-normal SST anomalies in the north central North Pacific were roughly located beneath the stronger-than-normal Aleutian Low, and the warmer-than-normal SST anomalies along the North American coast roughly coincided with the upper-level ridging across the Pacific Northwest to western Canada and adjacent North Pacific. The April monthly EP-NP index and 3-month running mean were both positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index during this time of year (April on the maps) is typically associated with cooler-than-normal temperatures across central to eastern Canada and across the CONUS east of the Rockies, and warmer-than-normal temperatures across Alaska and the extreme West Coast of the CONUS; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska to the western CONUS, and below-normal upper-level circulation anomalies (stronger upper-level trough) over eastern Canada and the adjacent CONUS. The precipitation teleconnections are weak but show evidence of wetter-than-normal conditions over Wyoming and the Ohio Valley.
- Comparison to Observed: The April 2016 temperature and upper-level circulation anomaly patterns generally agree with those expected for a positive EP-NP, except they appear shifted. The below-normal temperatures and 500-mb heights over the eastern CONUS seem shifted a bit to the north, while the above-normal temperatures over the western CONUS appear shifted a little further east than expected. The precipitation anomaly pattern shows some agreement to the few teleconnections to compare to.
Examination of the available circulation indices and their teleconnection patterns, and comparison to observed April 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in April reflected influences from Atlantic, Pacific, and Arctic atmospheric drivers. Although it was weakening, the El Niño was still pumping energy and moisture from the tropical Pacific into the mid-latitudes, priming the atmosphere for action. This energy was reflected in an intense Aleutian Low over the north central North Pacific. This contributed to an active jet stream and numerous Pacific weather systems. These weather systems and their storm tracks gave above-normal precipitation to some parts of the CONUS that normally expect it during an El Niño. The MJO was mostly incoherent but during the first week may have had an influence on the circulation and, hence, temperature anomalies. But for the month as a whole, the temperature and upper-level circulation anomalies seem to reflect the combined influence of El Niño and the NAO and EP-NP drivers — NAO (with its ridging) warmed most of the CONUS while EP-NP cooled the Northeast and El Niño (with its wet upper-level lows) cooled the Southwest. The WP index was weak and the PNA teleconnections showed little similarity to the observed patterns, so these Pacific drivers likely had little influence on April's weather, or their influence was overwhelmed by the other drivers.
This month illustrates how the weather and climate anomaly patterns can reflect the combined influence of atmospheric drivers (or modes of atmospheric variability) from several regions — the Atlantic, Arctic, eastern North Pacific, and equatorial Pacific.