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.
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The weather in December 2019 was largely driven by a circulation pattern that alternated between troughs along each coast with a ridge in the middle and the opposite. These fluctuations were closely tied to variations in the West Pacific Oscillation (WPO) and the Arctic Oscillation (AO). Averaged over the month, the pattern looks more like a trough along the West Coast and a ridge centered near the Great Lakes. This pattern reflects the stronger positive WPO from later in the month. The ridge was associated with warmer than normal temperatures across much of the east in the monthly mean, even though those features were less prominent earlier in the month.
The December total precipitation pattern was largely driven by major systems at the beginning and end of the month. The above normal precipitation for California and Nevada mostly came in association with a trough near the West Coast at the beginning of the month. Similarly, the above normal precipitation over the Great Plains came almost entirely during the final days of December.
December began with troughs on both coasts and a ridge near the Rockies. The West Coast trough was consistent with the negative EPO. It brought heavy rainfall to California and Nevada, bringing a resounding end to the 2019 California fire season. The ridge was consistent with the positive AO and brought warm and dry conditions throughout the Plains. The only precipitation in the region fell on December 1 in the Northern Plains (December 2-8 were dry there). Meanwhile, the trough brought cooler temperatures to New York and New England.
The North American circulation anomalies largely reversed in the middle of the month as the AO transitioned from positive to negative. A ridge along the West Coast extended northward through Alaska to the North Pole. The resulting northward surge of warm temperatures helped displace cold arctic air southward into the Northern Plains. That pattern is consistent with a negative AO, even though the index only became negative at the end of the period. The associated trough also enhanced precipitation along the East Coast, while a downstream ridge brought warm temperatures to that area.
The circulation flipped again December 19-26 as a positive WPO brought a trough back to the West Coast, although the precipitation was more concentrated in the Pacific Northwest this time. A strong ridge centered near the Great Lakes brought much above normal temperatures to the Plains and the Mississippi Valley. Aside from the South Atlantic, almost no precipitation fell east of the Rockies under the influence of that ridge.
The final days of 2019 saw a return of the positive AO along with a stronger positive EPO pattern. The positively tilted trough in the western US fostered a blizzard for the Northern Plains with heavy precipitation across most of the country.
- 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 December 2019. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These have been moderately warm since October after being near-zero in August and September. However, a much longer period of warming along with more substantial atmospheric coupling would be required for a return to El Niño conditions.
- Teleconnections (influence on weather): The teleconnections with ENSO are muted during near-neutral conditions. An active El Niño would have favored cooler than normal temperatures across the southern half of the country, which were never apparent this month. It would have also favored wetter conditions along the East Coast, which were most prominent December 9-18.
- 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 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 moderately active for the first half of December and very active by the end of the month as the Indian Ocean Dipole (IOD) weakened. The MJO was propagating faster than normal and nearly completely a circumnavigation during December. It began the month over the western Indian Ocean (Phase 2) end it in the Western Hemisphere (Phase 8).
- Teleconnections (influence on weather): The MJO's movement across the Indian Ocean during the middle of December favored warmer temperatures in the Northern Plains and the Northeast a week or so later, and they were observed later in 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 began fluctuated around neutral during December 2019. It was its most positive around December 9 and its most negative December 21-25. On average, the PNA was neutral during December 2019.
- Teleconnections (influence on weather): The positive PNA around December 9 would favor above-normal temperatures along the West Coast and cooler temperatures over the Southeast. The opposite may have been expected later in the month when the PNA was moderately negative. However, neither pattern was observed.
- 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 the first half of December and then negative for the second half until the final days of December when it returned to positive. Averaged over the month, it was moderately positive, primarily associated with the persistent ridge over the Northwest Pacific and the trough near Nunavut Canada.
- Teleconnections (influence on weather): The positive AO is associated with warmer than average temperatures over the Northern Plains, and the negative AO is associated with cold anomalies in that region extending to the East Coast. The temperature patterns during December 2019 largely followed the fluctuations of the AO, even when the AO was relatively weak.
- 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 positive for the first half of the month and then neutral for the second half. The monthly average was strongly positive. The positive values early in the month were largely due to the trough near Greenland.
- Teleconnections (influence on weather): The positive NAO would typically be associated with warmer temperatures over the Southeast, which were moderately present early in the month.
- 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 generally weak but positive except for December 19-26 when it was strongly positive. The monthly average was positive, due primarily to the persistent ridge over the northwestern Pacific. The WPO intensified when that ridge was coupled with a trough over eastern Russia during December 19-26.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer than normal temperatures over the Northern Plains, which occurred through much of the month and especially December 19-26.
- 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 half of the month and positive for the second. Averaged over December, the EPO was moderately positive. The circulation anomalies were generally north of the canonical EPO pattern, which might explain why its influence may have been reduced.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest and Northeast with the opposite occurring during the negative EPO. The negative EPO may have contributed to the cool anomalies in the northeast during the beginning of the month. However, the warm anomalies near the end of the month around the Ohio Valley were very consistent with the positive EPO in the latter half of December.