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.
- Positive PNA
- La Niña
- La Niña
- November 20-30
The North American climate in November was driven by a Rossby wave-train that featured a trough near Alaska, a ridge over the western U.S., and a trough in the east. It resembled a positive PNA pattern, but the Alaskan trough was farther north and the ridge was farther south than the canonical pattern. The pattern was persistent throughout November but was strongest later in the month.
The Alaskan trough was associated with a series of atmospheric rivers that brought heavy rainfall to the Pacific Northwest. The East Coast trough led to a strong extratropical cyclone early in the month that brought heavy rain to Florida. The ridge suppressed precipitation across the rest of the contiguous U.S. The ridge was also associated with temperatures much above normal across the western half of the U.S., and the trough was associated with cooler anomalies for the East Coast.
November began with a trough in the Gulf of Alaska, a ridge over the Great Plains, and a trough along the East Coast. The trough in the Gulf of Alaska brought a series of atmospheric rivers to the Pacific Northwest. The ridge was associated with record warmth across portions of the southern Rockies and suppressed precipitation across most of the country. Meanwhile, the East Coast trough was associated with cooler than normal temperatures across the Southeast. A related extratropical cyclone brought heavy rain to Florida and coastal Georgia. The cyclone continued on to cause significant tidal flooding along the coast from Georgia to Virginia.
During the middle of November, the Pacific trough shifted northward over Alaska, the ridge moved towards the West Coast, and the trough stayed near the East Coast. This pattern remain in place for the rest of the month. The Alaskan trough continued bringing atmospheric rivers to the Pacific Northwest. A few of the related extratropical cyclones continued along the Canadian border to bring wintry precipitation to the Northern Plains.
The Rossby wave-train intensified in the last third of November. The ridge in particular became stronger and more expansive. The northwest extension of the ridge reduced the moisture supply to the Pacific Northwest. It also suppressed precipitation across the rest of the country. Most of the western half of the U.S. experienced temperatures much above normal, and the invigorated trough over the East Coast was associated with cooler temperatures there.
- 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 continued during November. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These persisted around -1.0°C in November. The positive Southern Oscillation Index, which measures the atmospheric response, strengthened in November. Together, these indices reflected coupled atmosphere and ocean La Niña conditions.
- Teleconnections (influence on weather): La Niña favors cooler than normal temperatures for the western U.S.. It also favors wetter conditions for the Pacific Northwest and drier for the Southeast. The temperature pattern only resembled La Niña early in the month when some cool anomalies were observed along the West Coast. Meanwhile, the wet conditions over the Pacific Northwest were more prevalent near the middle of the month.
- Description: The MJO is the biggest source of subseasonal (30-50 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 inactive until the final days of November. The MJO index was in phases 4/5 (Maritime Continent) for most of November, but that was a reflection of the La Niña conditions.
- Teleconnections (influence on weather): The MJO phase 5 favors warmer temperatures near the Great Plains, which were observed at the end of the month when the MJO became active.
- 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 most of November with positive values near the beginning and ends of the month. The monthly mean was positive.
- Teleconnections (influence on weather): The positive PNA favors warmer conditions in the Pacific Northwest and cooler in the Southeast. These were particularly prominent near the beginning and end of the month when the PNA was its most positive.
- 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 first two-thirds of the month and negative near the end of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive AO is typically associated with warmer than normal temperatures for the Northern Plains with cool anomalies from the Northern Plains to the East Coast during the negative phase. The Northern Plains were warmer than normal throughout the month. The warmest anomalies actually happened near the end of November when the AO was negative, contrary to the typical relationship. However, the negative AO may have contributed to the cool anomalies for the East Coast during that period.
- 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 near the beginning and end of the month with positive values near the middle of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The NAO has limited impacts on North America during the autumn. During winter, a positive NAO would have favored warmer than normal conditions for the Southeast with the opposite happening during the negative phase. These patterns were generally reversed in November.
- 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 negative during most of November except for positive values near the beginning and end of the month. The monthly mean was negative.
- Teleconnections (influence on weather): The negative WPO during the autumn is typically associated with cooler temperatures over the Midwest. This pattern was most apparent near the end of the month.
- 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 positive for most of November particularly towards the end of the month. The monthly mean was the fifth most positive November since 1948.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest and cooler along the West Coast. Despite the strongly positive EPO in November, this temperature pattern was not apparent. The anomalous ridge was shifted eastward over the Southwest instead of its typical location over the subtropical Pacific, and this difference may have limited the EPO's impacts.