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 East Pacific Oscillation (EPO)
- Repeated upper-level troughs and extratropical cyclones near the Mississippi Valley
- La Niña
- Negative PNA
- Positive Arctic Oscillation (AO)
- Positive EPO
- Negative West Pacific Oscillation (WPO)
- Strong upper-level trough and cold front in the eastern U.S. followed by another in the West
May featured an active weather pattern with quickly moving systems. They spread enhanced precipitation across many parts of the country with the notable exception of the Southwest, which remains in persistent drought. Overall, the temperature patterns were consistent with the ongoing La Niña event and its associated negative phase of the PNA. In this pattern, a trough near the Pacific Northwest is associated with cooler anomalies there, while a ridge along the East Coast brings warm anomalies to the Southeast. The moist conditions for the Pacific Northwest are less common with La Niña, but they were driven by the active weather pattern bringing numerous extratropical cyclones to the region.
May began with a strongly positive phase of the EPO. This event did not manifest in the typical warm temperatures for the Ohio Valley. However, the positive EPO is defined by a trough near Alaska and a ridge to its south, which strengthens the Pacific jet. This enhanced jet led to a more active weather pattern for the U.S. with a series of upper-level troughs and their associated surface extratropical cyclones moving across the country. They brought repeated cold temperatures to the Ohio Valley and enhanced precipitation from coast to coast.
The second week of May featured a circulation pattern consistent with the ongoing La Niña event. A ridge extending from the Southwest to the Northeast created anomalous heat across the Great Plains and the Mississippi Valley. It also suppressed precipitation across most of the country. A trough in the Pacific Northwest brought cool and moist conditions there. Another trough near the Southeast was associated with cool temperatures for that region.
The circulation during the third week of May was most consistent with the negative phase of the WPO. A broad trough anchored near Hudson Bay was associated with cooler temperatures from the Pacific Northwest to the Great Lakes. A persistent frontal boundary on the east side of the trough enhanced precipitation from the Gulf Coast to the Northeast. Meanwhile, a ridge off the California coast was associated with hot and dry conditions for the Southwest. Another ridge along the East Coast led to warmer temperatures there.
The weather pattern near the end of May was dominated by a pair of strong upper-level troughs and their associated cold fronts. The first one was near the Mississippi River on May 25 and moved to the East Coast by May 28. The second reached the Pacific Northwest on May 28 and moved eastward to the Great Plains by the end of the month. These two systems enhanced precipitation across the country with the notable exception of the Southwest. They were also associated with cool temperatures for the Pacific Northwest and the Lower Mississippi Valley.
- 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 May. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These remained steady around -1.0°C during May. The Southern Oscillation Index, which measures the atmospheric response, also remained strongly positive. Together, these indices suggested a persistent La Niña.
- Teleconnections (influence on weather): La Niña favors cooler than normal temperatures for the western U.S. and the Northeast. It also favors drier conditions for the Pacific Northwest, the Mississippi Valley, and the Gulf Coast. These patterns were most apparent May 9-16 although the Pacific Northwest was anomalously wet during that period.
- Description: The MJO is the biggest source of subseasonal (31-50 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 active during May, but its fast eastward movement was more consistent with an atmospheric Kelvin wave. The MJO index became active around May 5 in Phase 2 (Indian Ocean), circumnavigated the globe by around May 23, and then rapidly moved back to the western Pacific (phase 7) by the end of the month.
- Teleconnections (influence on weather): The most significant teleconnections for the U.S. occur during phase 6 (West Pacific) when the Ohio Valley experiences warm anomalies. This phase is also associated with drier conditions for the western U.S. and the East Coast. These patterns occurred during May 9-16. The MJO was in phase 6 on May 13-14, but its unusually fast phase speed meant that it was also 3, 4, 5, and 7 during May 9-16.
- 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 or negative during most of May. The monthly mean was negative.
- Teleconnections (influence on weather): The negative PNA favors cooler conditions in the Pacific Northwest and warmer in the Southeast. This pattern was most apparent during May 9-16 although the warmth was shifted westward.
- 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 index was strongly positive during most of May. It peaked on May 12 with its fourth highest daily May value on record since 1951. It became strongly negative in the final days of the month. The monthly mean was the third most positive May on record.
- Teleconnections (influence on weather): The positive AO is typically associated with warmer and drier conditions around the Central Plains. These were only observed during May 9-16 (temperature and precipitation) near the peak of the positive AO.
- 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 index positive during most of May. The monthly mean was positive.
- Teleconnections (influence on weather): The positive NAO favors warmer than normal conditions for most of the U.S. These patterns were not clear during May.
- 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 positive at the beginning of May and then negative for the rest of the month. The monthly mean was negative.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer temperatures over the Ohio Valley and cooler for the Southwest with the opposite during the negative phase. These patterns were generally out of phase with the WPO except for May 17-24, which best matched the negative WPO then.
- 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 positive for most of May except for a negative EPO event around May 20. The EPO also had its sixth highest May value since 1951 on May 4. The monthly mean was positive.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the Upper Midwest. The related negative phase of the EP-NP is also associated with cooler temperatures along the West Coast. The effects of the positive EPO were only apparent during May 9-16.