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.


Summary


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.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.

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.

Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

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.

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

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).

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

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.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

April 2015 temperatures averaged warmer than normal across most of Alaska and from the Southeast to Great Plains in the CONUS. Temperatures were colder than normal in the Northeast, with a mixed pattern in the West.

Global monthly upper-level circulation pattern and anomalies
Global monthly upper-level circulation pattern and anomalies.
Global Linkages: The upper-level circulation anomaly pattern over 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.


Atmospheric Drivers


Subtropical highs, and fronts and low pressure systems moving in the mid-latitude storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The Tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which largely drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:


Examination of 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).


Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for April 2015, published online May 2015, retrieved on May 25, 2015 from http://www.ncdc.noaa.gov/sotc/synoptic/201504.