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.
|Time Period||Key Driver||Other Drivers|
|December 17-23||EPO+||PNA+, La Niña|
The dominant pattern during December 2020 was the positive phase of the Pacific/North American pattern (PNA). The associated ridge over northwestern North America extended particularly far east and brought above normal temperatures to the Northern Plains. It also suppressed precipitation across the contiguous U.S. except for two nor'easters that brought heavy snow to the Northeast. The downstream trough also brought cooler than normal temperatures to the Southeast.
The Arctic Oscillation (AO) was also negative for most of December. It did not lead to the significant cold-air outbreaks that are typical for the negative AO. However, it did contribute to a more stationary and meridional circulation. It was also associated with a strong ridge that extended from the North Atlantic to Greenland and the North Pole. The southern periphery of this ridge contributed to the warm anomalies for New England.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for December 1-10 2020
Early December was dominated by a strongly positive PNA event that produced the typical trough near the Aleutians, ridge near the Pacific Northwest, and trough along the East Coast. The ridge over the western U.S. has been a persistent feature bringing dry conditions to the Southwest since mid-April. The ridge also brought much above normal temperatures to the Northern Plains, while the trough was associated with cooler temperatures for the Southeast. The only significant precipitation during this period was associated with a nor'easter that affected New England December 5-6.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for December 11-16 2020
The positive PNA weakened towards the middle of December. Meanwhile, the AO became strongly negative. This combination brought a trough and cold-air outbreak to the eastern Rockies and Great Plains. It also brought a more active weather pattern to the eastern U.S. with a series of low pressure systems bringing snow and rain to the lower Midwest and the Southeast.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for December 17-23 2020
The trough moved eastward from the Great Plains to the East Coast December 16-17. It spawned a powerful nor'easter that brought one to two feet of snow from central Pennsylvania to southern Maine. A ridge developed east of the Rockies in its wake and brought unusually warm temperatures to the Northern Plains.
Farther west, the trough with the positive PNA extended from the Aleutians to the Hudson Bay. Combined with an enhanced ridge over the subtropical Pacific enhanced the Pacific jet, consistent with a positive East Pacific Oscillation (EPO). The onshore flow brought enhanced moisture to the Pacific Northwest. The precipitation extended as far south as Las Vegas, which ended a record 240 consecutive dry days there.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for December 24-31 2020
The final week of December had a more meridional and propagating weather pattern. A ridge stretched along the Rockies from Colorado to Alaska and brought anomalous warmth to the Great Plains. A series of low pressure systems brought rain and snow across the eastern U.S., while a trough brought much needed rain to southern California.
- 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: La Niña conditions persisted during December 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These warmed to -1.2°C in December from its peak of -1.5°C in November, but they are still indicative of a moderately strong La Niña. The Southern Oscillation Index, which measures the atmospheric response, became more positive in December, which indicates a stronger atmospheric response to the La Niña ocean conditions.
- Teleconnections (influence on weather): La Niña favors warmer than normal temperatures for the Southern Plains, drier than normal conditions across the Southeast, and wetter than normal for the West Coast. The warm anomalies were generally concentrated near the Northern Plains this December. The West Coast was also generally dry except for December 17-23 when the Pacific Northwest was anomalously wet.
- Description: The MJO is the biggest source of subseasonal (31-100 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 index was generally weak during December with most of the tropical convection concentrated near the Maritime Continent (phases 4/5) in association with the La Niña event.
- Teleconnections (influence on weather): The lack of a strong propagating MJO limited its potential teleconnections in December. However, the projection onto phases 4 and 5 of the La Niña signal would have favored the warm anomalies over the Northern Plains that were observed most of the month.
- 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 strongly positive in early December, near-zero in the middle of December, and positive again near the end of the month. The monthly mean was strongly positive. The positive PNA was associated with a strong trough near the Aleutians and a ridge near the Pacific Northwest. The trough was present throughout the month, but the ridge was strongest early in the month when the PNA index was strongest.
- Teleconnections (influence on weather): The positive PNA generally favors warmer and drier conditions in the Pacific Northwest along with cool and dry conditions in the Southeast. Both patterns were present through most of the month.
- 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 negative for most of December with strongly negative values in mid-December and the end of the month. The monthly mean was strongly negative. The negative AO was associated with ridges near Greenland and the Barents Sea along with a trough near the Aleutians. The trough was most prominent early in the month when the PNA was strongly positive, while the ridges were strongest at the end of the month.
- Teleconnections (influence on weather): Temperatures over the Northern Plains are typically cooler than normal during a negative AO. That pattern was only observed December 11-16.
- 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 weak but negative during most of December, and the monthly mean NAO was slightly negative. The negative NAO was primarily associated with the ridge near Greenland, but the NAO's amplitude was reduced because that ridge extended southward into the North Atlantic. During a typical negative NAO, a trough would have been to the south of the Greenland ridge.
- Teleconnections (influence on weather): The negative NAO typically favors warmer than normal conditions over the eastern U.S.. However the lack of a trough over the North Atlantic probably limited its impacts in December 2020.