NOAA's National Centers for Environmental Information
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
| 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
- 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 for most of the month, particularly near the middle. It was negative when averaged over the month. The negative PNA was associated with a ridge in the Gulf of Alaska, a trough in the Midwest, and a ridge near Cuba, all of which were most prominent during the middle of the month. The pattern was shifted somewhat eastward of the canonical negative PNA but was strong enough to create a large signal.
- Teleconnections (influence on weather): A positive PNA favors cooler temperatures along the West Coast and warmer temperatures over the Southeast. That pattern was not observed in April 2020, probably because of the eastward shift of the PNA wavetrain.
AO: Arctic Oscillation
- Description: The AO teleconnection pattern generally measures the pressure difference between the low pressure over the North Pole and the higher pressures in the subtropical ridges. This pressure difference is larger during a positive AO, resulting in a stronger midlatitude jet. When the AO is negative, the jet is weaker and will have larger troughs and ridges.
- Status: The daily AO was positive for most of the month with two weak negative episodes around April 3 and 21. The monthly average was positive. The positive AO was primarily driven by the ridges in the North Pacific and western Europe combined with the anomalous trough near the North Pole.
- Teleconnections (influence on weather): The positive AO is associated with warmer than average temperatures east of the Rockies, which were only apparent April 7-11.
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
- Description: The WPO 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 daily WPO index was negative for most of April, particularly in the middle of the month. However, it ended the month strongly positive. Averaged over April, the WPO was negative. The negative WPO was primarily driven by a ridge over central Russia combined with a trough near Japan. They were also consistent with the ridge in the Gulf of Alaska.
- Teleconnections (influence on weather): The negative WPO is typically associated with cooler than normal temperatures over the Northern Plains, which were present in the middle of the month when the WPO was most negative.
EPO: East Pacific Oscillation
- Description: The EPO pattern identifies variations in the strength and location of the eastern Pacific jet stream. During the positive phase, the jet is stronger and shifted southward. The negative phase is associated with an Alaskan ridge that weakens the jet. The EPO is closely related to the East Pacific–North Pacific (EP–NP) teleconnection pattern, although the two are defined with opposite signs.
- Status: The daily EPO index was generally negative for the first two-thirds of April. However, it made a rapid transition from strongly negative around April 16 to strongly positive around April 27. Averaged over the month, the EPO was negative. The negative EPO was primarily associated with the ridge in the Gulf of Alaska, which was strongest in the middle of the month and was replaced by a trough at the end of the month.
- Teleconnections (influence on weather): The negative phase of the EPO favors cooler than normal temperatures across the Upper Midwest and Northeast. That pattern was most clear in the middle of April when the ridge was strongest.
