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|
|November 15-21||AO+||La Niña, NAO+|
The dominant pattern during November 2020 was the positive phase of the Arctic Oscillation (AO). It was associated with an enhanced trough in northern Canada and anomalous ridges throughout the midlatitudes. A series of broad ridges over the conterminous U.S. brought warm and dry anomalies to much of the country during most of November.
The most notable departure from this positive AO pattern came during November 8-14. A weak negative phase of the Pacific/North American pattern (PNA) brought a trough to the western U.S. This trough was associated with the only notable cool anomalies during the entire month. Farther east, moisture from Hurricane Eta brought heavy rain from Florida to the Mid-Atlantic Coast, which drove the above normal November precipitation in that region.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for November 1-7 2020
An anomalously strong upper-level ridge has dominated the weather of the Southwest U.S. since mid-April. That ridge was still in place in early November. It was centered near the Northern Plains and was associated with warmer than normal temperatures across the western two-thirds of the country. It also suppressed precipitation over the vast majority of the conterminous U.S.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for November 8-14 2020
The ridge moved to the eastern U.S. during the second week of November, as a trough developed over the western U.S. in association with a weak negative PNA. The trough brought cool anomalies to the western half of the country, and these would be the only notable cool anomalies anywhere during the entire month. It also brought enhanced precipitation to much of the Northern Rockies and Northern Plains.
On November 8, Hurricane Eta made landfall in Lower Matecumbe Key, FL as a tropical storm. This landfall brought the total number of U.S. landfalling named storms to 12 -- a record. Florida usually receives more landfalling storms than any other state, so it is particularly remarkable that Eta was the first named storm to make landfall within the state in 2020. After passing through the Florida Keys, Eta briefly regained hurricane status over the eastern Gulf of Mexico before making another landfall in Central Florida. Moisture from Eta collided with a cold front to bring heavy rain from Florida to the Mid-Atlantic Coast, which was the primary driver of above-normal November precipitation for that region.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for November 15-21 2020
Beginning on November 15, the trough from the previous week moved eastward towards the North Atlantic. The associated powerful cold front brought widespread wind damage from Indiana to New England. In its wake, another broad ridge anchored over the Southwest dominated the U.S. circulation. It brought a return to warm and dry conditions for most of the country. Notably, Phoenix, AZ reached 92°F on November 17, breaking its record for the latest day in the year when the high temperature was at least 90°F.
500-mb height mean (contours) and anomalies (shading) for the Northern Hemisphere for November 22-30 2020
The final week November had a more meridional and propagating weather pattern. A series of disturbances in the subtropical jet brought precipitation to the South-Central U.S. Temperatures were generally above normal, particularly for the Northern Plains and the Gulf Coast.
- 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 November 2020. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These continued cooling to -1.3°C in November, which is indicative of a moderately strong La Niña. The Southern Oscillation Index, which measures the atmospheric response, was more La Niña-like in November after weakening during October in association with the Madden-Julian Oscillation (MJO).
- Teleconnections (influence on weather): La Niña favors warmer than normal temperatures for the Central Plains, drier than normal conditions for the Lower Mississippi, and wetter than normal in the Pacific Northwest. These patterns were apparent throughout the month, particularly around November 15-21.
- Description: The MJO is the biggest source of subseasonal (31-70 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 active during November. It began the month over the Western Pacific and the Americas (Phases 7/8) and propagated eastward to the Maritime Continent (Phase 4).
- Teleconnections (influence on weather): The MJO in Phase 8 early in the month may have favored the cool anomalies over the western U.S. that were observed near the middle of the month. The MJO also contributed to the unusually strong Atlantic hurricane activity in November and two tropical cyclones in the North Indian Ocean.
- 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 weak during November with weak negative values in the first half of the month and weak positive values in the second half. The monthly mean was near zero. The negative values early in the month were associated with a ridge in the North Pacific, a trough in the western U.S., and a ridge near the East Coast. This pattern was more zonal than a typical negative PNA, which limited its amplitude. Similarly, the positive values later in the month were associated with a trough near the Bering Sea and a weak ridge over the western U.S.
- Teleconnections (influence on weather): The negative PNA generally favors warmer temperatures along Pacific Northwest and cooler in the Southeast with the opposite happening in the negative PNA. The temperatures during November 8-14 were consistent with the negative PNA around that time. However, the positive PNA at the end of the month did not manifest its typical response.
- 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 strongly positive for nearly the entire month, and the November mean AO was strongly positive. The strongly positive signal was associated with the canonical enhanced trough near Northern Canada and ridges over the North Atlantic and North Pacific. These features were prominent throughout November.
- Teleconnections (influence on weather): Temperatures over the Northern Plains are typically warmer than normal during a positive AO. That pattern was observed throughout November except for November 8-14.
- 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: Similar to the AO, the daily NAO strongly positive throughout November, and the November mean NAO was also strongly positive. The strong signal was associated with the persistent trough in Northern Canada and a ridge from the eastern U.S. to the North Atlantic. This pattern was present for most of November. The primary exception was November 8-14 when the ridge broke down over the North Atlantic.
- Teleconnections (influence on weather): The NAO's teleconnections with the U.S. are generally weak during the Fall. During Winter, the positive NAO would have favored warmer than normal conditions over the eastern U.S., as was observed during most of November.