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

Indices and their agreement with the temperature, precipitation, and upper-level circulation anomaly patterns, by time period (month, week, or other sub-monthly period).
Time Period Key Driver Other Drivers
Month EPO-

April 1-6 NAO-

April 7-11 AO+
MJO
April 12-20 EPO-
WPO-, ENSO+
April 21-30 EPO+

The temperature and precipitation patterns in April 2020 were driven largely by the period April 12-20. A big ridge associated with a negative East Pacific Oscillation (EPO) brought above-normal temperatures to Alaska that drove that state's monthly mean anomalies. A trough downstream of that ridge was associated with a significant cold air event for the eastern U.S. That cold air had been preceded by warm anomalies in the southeast, which strengthened the cold front at the front edge of the air mass. That cold front led to more than 140 tornadoes from Texas to Maryland on April 12-13. The associated precipitation also led to the above-normal monthly totals for much of the Southeast.

The cold anomalies east of the Rockies from April 12-20 were moderated somewhat in the monthly average by the preceding warm anomalies during April 7-11. Another key feature in April was a trough near California early in the month that brought significant precipitation to southern California. Meanwhile, a persistent ridge near Cuba continued to bring hot temperatures to southern Florida.

Monthly Mean

Submonthly Evolution

April 1-6

April began with a ridge in the Gulf of Alaska, a trough in the Pacific Northwest, a ridge in the Midwest, and another trough off the East Coast. The eastern portions of this circulation was associated with a negative NAO. The trough in the Northwest brought moisture to California and the northern Rockies. It was also associated with cool temperatures in the northern Plains.

April 7-11

During April 7-11, the trough from the Northwest moved eastward to the Canadian Maritimes. The ridge to its south brought warm temperatures that would prime the atmosphere for a severe weather outbreak in the South a few days later. Meanwhile, a remnant cutoff trough near California brought cold temperatures and moisture to the Southwest.

April 12-20

On April 12-13, a strong cold front swept across the eastern part of the country that spawned 140 tornadoes from Texas to Maryland. It also brought significant precipitation to much of the Southeast. The cold air mass behind this front was associated with a strong trough downstream of a ridge in the Gulf of Alaska. That ridge also played a major role in the above-normal temperatures that Alaska experienced in April.

April 21-30

The Alaskan ridge quickly dissipated around April 21. Its remnants moved southwestward towards California where it brought warm and dry conditions to the western half of the country. A trough to its east was associated with cool and wet conditions east of the Mississippi.

Atmospheric Drivers

ENSO: El Niño Southern Oscillation

  • 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. These variations in SST change the locations of the Pacific's largest thunderstorms, which can in turn change circulation patterns around the globe.
  • Status: The ocean and atmosphere system continued to be in ENSO-neutral during April 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. The three-month averages have now been at least 0.5°C for five overlapping seasons going back to October-December, which is one of NOAA's criteria for an El Niño. However, the highest value has been only 0.6°C and the atmospheric coupling is still lacking.
  • Teleconnections (influence on weather): The teleconnections with ENSO are muted during near-neutral conditions. An active El Niño would have favored cooler conditions in the Southwest, which did not transpire. It would also favor wetter than normal conditions in the Southeastern U.S., which primarily occurred in the middle of the month.

MJO: Madden-Julian Oscillation

  • Description: The MJO is the biggest source of subseasonal (31-60 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms that develops over the Indian Ocean that then moves eastward. Like ENSO, the MJO's effects on tropical rainfall is so strong that it can alter the atmospheric circulation around the globe. The thunderstorms decay when they cross the Pacific, but the associated winds can often continue across the Western Hemisphere to initiate the next MJO in the Indian Ocean. The MJO is episodic, meaning that it is not always active. Most indices for tracking the MJO identify both the MJO's amplitude and the longitude of its strongest rainfall, usually described as one of eight phases.
  • Status: The MJO was active during April. The most commonly used MJO index began and ended the month near the Maritime Continent (phase 4). It moved quickly in the beginning of the month, reaching the Western Hemisphere (phase 8) in just 10 days. Its speed slowed as it coupled with anomalously warm SSTs over the Indian Ocean, and it took twice as long to complete the second half of its circumnavigation.
  • Teleconnections (influence on weather): The MJO's projection onto phase 5 (Maritime Continent) early in the month favored the warmer temperatures in the Eastern U.S., which occurred the following week.

PNA: Pacific/North American pattern

AO: Arctic Oscillation

NAO: North Atlantic Oscillation

  • Description: The NAO teleconnection pattern relates the pressure over the sub-polar low near Greenland and Iceland with the subtropical high over the Central Atlantic. It significantly affects the weather on both sides of the Atlantic.
  • Status: The daily NAO was negative at the beginning of April, weak but positive in the middle, and neutral towards the end. The monthly average was weak negative. The negative NAO was primarily driven by an anomalous trough in the North Atlantic, which was strongest early in the month. The ridge near Hudson Bay also contributed, but that feature would be shifted eastward to Greenland in a canonical negative NAO.
  • Teleconnections (influence on weather): The cool temperatures over the Northern Plains early in the month were consistent with the negative NAO.

WPO: West Pacific Oscillation

EPO: East Pacific Oscillation


Citing This Report

NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for April 2020, published online May 2020, retrieved on April 22, 2024 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/synoptic/202004.