Synoptic Discussion - April 2015
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 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.
April 2015 was characterized by an active upper-level circulation pattern over the contiguous United States (CONUS), with several weather systems moving in the jet stream flow. These weather systems disrupted the long-wave pattern of ridge over the western CONUS and trough over the East. They also brought severe weather and areas of precipitation, mainly to the south central and southeastern regions, as well as a changing pattern of temperatures across the nation. With the storm track to the south, precipitation was below-normal in the western and northern states, which helped increase the national drought footprint. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of April 2015 were the result of influences from multiple atmospheric drivers originating over the North Pacific, Arctic, and Equatorial Pacific, with the El Niño influencing precipitation patterns east of the Rockies. See below for details.
In the Northern Hemisphere, April is in the middle of climatological spring, which is the transition period between winter and summer. The sun angle gets higher in the sky and solar heating increases, which contracts the circumpolar vortex and causes the jet stream to retreat northward. But it was not an orderly retreat during April 2015 as different air masses and weather systems competed for dominance. The long-wave circulation pattern in the upper atmosphere consisted of a ridge over the western CONUS and trough over the East. The ridge kept temperatures warmer-than-normal over Alaska. But short-wave troughs and low pressure centers migrating through the upper-level flow counteracted the effects of the long-wave pattern over the CONUS, resulting in a mixed monthly average temperature anomaly pattern over the West. A southerly circulation ahead of the short-wave troughs pushed short-wave ridges and warmer-than-normal air ahead of them over the Southeast. The upper-level circulation funneled air masses from different regions across the CONUS, alternating between cooler Canadian air masses and milder Pacific air masses.
By the end of the month, there were 2,238 record warm daily high (756) and low (1,482) temperature records, which is twice as many (1,071) record cold daily high (678) and low (393) temperature records. The monthly average temperature ranked April 2015 as the 17th warmest April in the 1895-2015 record, showing how the warmth dominated the national statistics. The REDTI (Residential Energy Demand Temperature Index) for April 2015 ranked 34th lowest for April.
The short-wave troughs and lows moving in the upper-level flow brought areas of precipitation to parts of the Northwest, Central and Southern Plains, and much of the Southeast to Ohio Valley. With a southerly flow, the systems tapped abundant Gulf of Mexico moisture to bring widespread above-normal rain to the Southern Plains, Southeast, and Ohio Valley. They also generated numerous reports of severe weather across the region as well as widespread flooding over parts of the Ohio Valley. There were 185 preliminary reports of tornadoes, which compares to an April average of 155.
While the upper-level troughs and lows brought cooler temperatures to the West, the long-wave ridge pattern inhibited precipitation, with most areas having a drier-than-normal April. And with the main storm track over the South, much of the Northern Plains and parts of the Upper Midwest and Northeast were drier-than-normal. The month began with several large wildfires burning across parts of the Plains (wildfires on April 3, 10, 17, 24, and May 1), but rain later in the month helped extinguish the fires in the Southern Plains (precipitation anomaly maps for weeks 1, 2, 3, 4, 5).
The above-normal precipitation improved drought conditions from New Mexico and the Southern Plains to the Southern Appalachians, but continued dryness expanded and intensified drought in the West. April 2015 continued a 7-month dry trend in the Northern Plains and Upper Midwest, which prompted the expansion of drought and abnormally dry conditions. The net effect was an expansion of the overall national drought footprint, with 37.4 percent of the CONUS experiencing moderate to exceptional drought at the end of April, compared to 36.8 percent at the end of March.
The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. With temperature and precipitation extremes reduced on a national scale by the numerous weather systems this month, the April 2015 CEI for the CONUS was muted, ranking as the 22nd lowest for the month. The regional CEI ranks were also muted, but still notable in the West and Southeast. The West region had the 20th most extreme April CEI in the 106-year record due largely to the most extreme Palmer drought component. The Southeast region also had the 20th most extreme April CEI on record due mostly to the second most extreme days with precipitation component and fourth most extreme warm minimum temperature component.
The migration of short-wave troughs and lows through the upper-level circulation dampened the long-wave anomaly pattern. So, when integrated across the month, the atmospheric circulation indicated a pattern of reduced 500-mb height anomalies. Above-normal 500-mb heights (weaker-than-normal long-wave trough) occurred over the southeastern U.S., while 500-mb heights were near normal (indicating a near-normal-strength long-wave ridge) over the West. The pattern reflected a slight weakening of the long-wave ridge over Alaska (below-normal 500-mb heights) and strengthening of the long-wave trough over eastern Canada (below-normal 500-mb heights).
Most of the West and Northern Plains, and parts of the Midwest and Northeast, were drier than normal during April 2015. Precipitation was above normal across the Southern Plains to Southeast and Ohio Valley. April was drier than normal across parts of the Hawaiian Islands and interior stations in Alaska, but wetter than normal along the southern and northern Alaska coasts.
North America was part of a long-wave pattern that stretched across the Northern Hemisphere. Unlike North America, strong anomalous ridge/trough couplets were evident over Western Europe/Western Asia and Central Asia/Eastern Siberia. The above-normal 500-mb heights were associated with upper-level ridging and above-normal surface temperatures over Eurasia, as well as near to below-normal precipitation. The below-normal 500-mb heights were associated with upper-level troughing and were reflected by near-normal temperatures at the surface and near to above-normal precipitation over Eurasia. With large portions of the continents having warmer-than-normal temperatures, the April 2015 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 April 2015, El Niño conditions were observed as the above-average SSTs across the western and central equatorial Pacific continued to be coupled to the tropical atmosphere. The positive SST anomalies have strengthened across the equatorial Pacific.
- 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, the Southwest, Southern Plains, Southeast, and Atlantic Coast States; below-normal precipitation in the Northwest, Northern Rockies, Upper Midwest, and parts of the Ohio Valley; below-normal temperatures across the southern half of the CONUS; and above-normal temperatures across parts of the Northern Tier States.
- Comparison to Observed: The April 2015 precipitation anomaly pattern agrees from the Southern Plains to Southeast, and from the Northwest to Upper Midwest, but not in California or the Ohio Valley. The April 2015 temperature anomaly pattern is opposite to that expected with an El Niño.
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 started the month in phase 3, then became incoherent for the rest of the month. The MJO indices appeared to be influenced by other modes of coherent subseasonal tropical variability, including the background El Niño state, tropical cyclone activity, and Kelvin and Rossby waves (MJO updates for April 6, 13, 20, and 27).
- 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. The April 2015 monthly temperature and weekly temperature (weeks 1, 2, 3, 4, 5) anomaly patterns do not match those expected with MJO phase 3. The monthly precipitation anomaly pattern generally matches that expected for phase 3 in some areas, as illustrated in the weekly correlations. The precipitation anomaly patterns for week 1 matches in the Ohio Valley and Upper Midwest, and for week 2 agrees in the Ohio Valley, Northern Plains, and Southwest.
- 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 negative then turned positive during the month, averaging slightly negative for the month as a whole. The 3-month-averaged index turned negative after being positive for the previous several months.
- Teleconnections (influence on weather): To the extent teleconnections are known, for a negative PNA, the temperature teleconnection map for this time of year (April on the teleconnection maps) shows cooler-than-normal temperatures in Alaska, Canada, and the Pacific Northwest of the U.S., and warmer-than-normal temperatures for the Southern and Central Plains to Atlantic Coast of the CONUS. The precipitation teleconnection map shows drier-than-normal weather from Utah to Kansas and wetter-than-normal anomalies over the western Great Lakes, but the precipitation teleconnections are weak. The upper-level circulation anomaly teleconnections show above-normal heights over the southeastern CONUS, and below-normal heights across Canada and the Pacific Northwest.
- Comparison to Observed: The April 2015 temperature anomaly pattern agrees with that expected with a negative PNA across most of the CONUS, but not Alaska. The upper-level circulation anomaly pattern agrees over the eastern half of North America, but not elsewhere. The precipitation anomaly pattern shows no agreement, but the teleconnections are weak.
- 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 was positive for the first half of the month, then turned neutral, with the index averaging positive for the month. The 3-month-averaged index was positive.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (March-May) is typically associated with dry conditions in the Southeast, Mid-Atlantic, Central Plains, Central to Northern Rockies, and northern California; wet conditions in eastern Texas; above-normal temperatures from the Rocky Mountains to the Appalachians, near normal temperatures elsewhere; and upper-level circulation anomalies which are below normal across the Arctic into northern North America, and above normal over the eastern CONUS, North Atlantic and North Pacific.
- Comparison to Observed: The April 2015 monthly temperature anomaly pattern is a good match for the central CONUS. The upper-level circulation anomaly pattern agrees in most areas, but seems slightly shifted over the Pacific. The precipitation anomaly pattern shows some agreement in the West and North, but not in the South and Southeast.
- 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 positive then turned negative, 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 drier-than-normal conditions in the Southeast, Central Plains, and parts of the West (although the teleconnections are weak for precipitation); warmer-than-normal temperatures everywhere in the CONUS except the Gulf of Mexico coast to Mid-Atlantic; positive upper-level circulation anomalies across the North Atlantic and most of the CONUS, and negative upper-level circulation anomalies over the Canadian Archipelago and Greenland.
- Comparison to Observed: The April 2015 precipitation anomaly pattern is opposite to that for a positive NAO where teleconnections exist. The monthly temperature and upper-level circulation anomaly patterns show little agreement.
- 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 positive for the month, while the three-month average WP index turned positive after many months in negative territory.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive WP during this time of year (April on the maps) is typically associated with below-normal temperatures in the Southwest, above-normal temperatures from the Ohio Valley to Northern Plains, drier-than-normal conditions in the Northwest, and below-normal circulation anomalies over the western CONUS and above-normal circulation anomalies over the Northern Plains (although the precipitation and circulation teleconnections are weak).
- Comparison to Observed: The April 2015 monthly temperature anomaly pattern shows some agreement in the Ohio Valley and Southwest, but not elsewhere. The precipitation and upper-level circulation anomaly patterns show little resemblance to those expected for a positive WP over the CONUS.
- 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 magnitude of the warmth of the SSTs in the northeastern North Pacific decreased this month, but the April SST pattern still showed above-normal SSTs along the North American coast and cooler-than-normal SSTs in the central North Pacific. The monthly EP-NP index was slightly negative during April, but the 3-month running mean was still in positive territory.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative EP-NP index during this time of year (April on the maps) is typically associated with warmer-than-normal temperatures east of the Rockies, cooler-than-normal temperatures across the extreme West Coast and in Alaska, drier-than-normal conditions in Wyoming and parts of the Ohio Valley (although the precipitation teleconnections are very weak), below-normal upper-level circulation anomalies (weaker upper-level ridge) over Alaska and western Canada and into the western CONUS, and above-normal upper-level circulation anomalies (weaker upper-level trough) over the northeastern CONUS, eastern Canada, and the central North Pacific.
- Comparison to Observed: The April 2015 upper-level circulation anomaly pattern agrees with a negative EP-NP pattern over Alaska and the North Pacific, but not elsewhere. The April temperature anomaly pattern matches over the Plains, but not elsewhere. The precipitation anomaly pattern shows no agreement but few teleconnections exist.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed April 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in April reflected influences from several atmospheric drivers. The El Niño funneled energy and moisture from the eastern equatorial Pacific across Mexico and into the U.S. Southern Plains and Southeast. The Gulf of Mexico also served as a moisture source. The MJO tapped into this pattern and likely enhanced precipitation across the Ohio Valley early in the month. Temperatures were likely influenced by the North Pacific and Arctic atmospheric drivers, as indicated by the PNA and AO teleconnections and, to a lesser extent, by the WP and EP-NP. The North Atlantic (NAO) did not appear to have much influence on April's weather in the CONUS.
This month illustrates how the weather and climate anomaly patterns can reflect the combined influence of several atmospheric drivers (or modes of atmospheric variability).