NOAA's National Centers for Environmental Information
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.
Overview
Key Drivers
- Month
- Negative Pacific/North American (PNA) pattern
- Positive West Pacific Oscillation (WPO)
- Positive East Pacific Oscillation (EPO)
-
May 1-10
- Negative Arctic Oscillation (AO)
- Negative PNA
- Positive WPO
- Positive EPO
-
May 11-17
- Positive WPO
- Positive EPO
- Negative PNA
- Positive AO
-
May 18-31
- Positive WPO
- Positive EPO
- Negative PNA
Summary
The North American circulation at the beginning of May was characterized by troughing across the western U.S., with ridging across the eastern half of the country. Numerous systems during the early part of the month brought enhanced rainfall over large parts of the U.S. During the middle of May, the troughing moved to the eastern and southern U.S. while a ridge centered over the North Pacific brought warmer conditions to the Pacific Northwest. At the end of the month, the synoptic pattern was similar to the beginning of the month, with cooler conditions across the Northwest and warmer conditions over the eastern and southern U.S. Overall, the month of May saw warmer than normal conditions across most of the eastern and southern U.S., with the largest departures from average occurring over south Texas and south Florida. During May, wetter than normal conditions were seen across portions of the north-central U.S. and in parts of the South and Southeast. Dry conditions were seen over much of the Southwest.
Submonthly Evolution
May 1-10
During the beginning of May, there was a fairly dynamic synoptic pattern, which resulted in troughing over the northwestern U.S. and ridging over the eastern U.S. This resulted in warmer than normal conditions across much of the eastern and southern U.S., with cooler than normal conditions over the Pacific Northwest and northern Rockies. There was enhanced precipitation over much of the U.S. during the early part of May due to numerous storm systems moving across the country. During this period the WPO and EPO were both positive, and both a positive WPO and EPO are typically associated with warmer than normal conditions in the eastern U.S. during May, which was observed. The negative AO and the negative PNA may have contributed to the enhanced precipitation over the northern and western U.S. during early May.
May 11-17
During the middle part of May, the synoptic pattern was characterized by a strong ridge centered over the North Pacific bringing warmer and drier conditions to the western U.S. Additionally troughing occurred over much of the rest of the U.S. This brought generally more mild conditions across most of the eastern and southern U.S. Some enhanced rainfall also occurred along the Gulf Coast. The WPO and EPO were both positive during the middle of May, but the troughing pattern generally overcame the typical temperature teleconnections. The PNA was negative during the middle of May, however once again the dynamic pattern largely countered the typical corresponding teleconnection pattern.
May 18-31
During the end of May, the overall synoptic pattern returned to a pattern that resembled the one seen during the early part of the month. Generally, there were warmer conditions across the eastern and southern U.S. with cooler conditions seen over the Pacific Northwest. The end of May saw wetter than normal conditions across the northern Great Plains and parts of the South and Southeast. There were also drier than normal conditions across the Southwest. Once again, as was the case at the beginning of the month, during this period the WPO and EPO were both positive. A positive WPO and EPO are typically associated with warmer than normal conditions in the eastern U.S. during May, which was observed.
Atmospheric Drivers
ENSO: El Niño Southern Oscillation
- 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: Neutral to weak El Niño conditions were observed during May. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. The SST anomalies in the Niño3.4 region decreased to around 0.3°C during May. The Southern Oscillation Index, which measures the atmospheric response, was around 0.5. Together, the coupled ocean-atmosphere system was consistent with a weakening El Niño.
- Teleconnections (influence on weather): El Niño favors cooler temperatures for most of the contiguous U.S. Cooler than normal conditions were seen during the beginning and end of May over the northwestern U.S. when there was troughing in the same region. El Niño is also associated with wetter conditions for the Great Plains and southern U.S. along with drier conditions for the northern U.S. Wetter than normal conditions were seen across the Southeast and northern U.S., matching the typical pattern in the Southeast.
MJO: Madden-Julian Oscillation
- Description: The MJO is the biggest source of subseasonal (20-60 day) tropical variability. It typically develops as a large envelope of tropical thunderstorms 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. The MJO index identifies both the MJO's amplitude and the longitude of its strongest rainfall. MJO amplitude is represented by the distance from the center of the figure with values outside the circle are considered active. The eight phases around the circle indicate the MJO's location.
- Status: The MJO index was weak during most of May, but did progress through phases 4 and 5 during the beginning and end of the month.
- Teleconnections (influence on weather): During May, the weak MJO meant that teleconnections were weak throughout most of the month. During the beginning and end of the month phase 4 was observed which typically is associated with warmer than normal conditions over the Rockies. During the end of the month MJO phase 5 was observed, which corresponds to the drier than normal conditions seen over the Southwest during the same period. It is likely that the MJO influenced the dry conditions across the Southwest.
PNA: Pacific/North American Pattern
- 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 negative throughout May. The monthly mean was -2.1.
- Teleconnections (influence on weather): The negative PNA generally favors cooler than normal conditions for the Pacific Northwest and warmer for the Southeast. These patterns were observed during the early and late parts of May. The negative PNA also favors drier than normal conditions for the southern Rockies and eastern U.S. These conditions were also partially observed during the month.
AO: Arctic Oscillation
- 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 began the month strongly negative, and transitioned to positive during the middle of the month. The AO index remained weak during the remainder of the month. The monthly mean was +0.0.
- Teleconnections (influence on weather): The negative AO is typically associated with cooler than normal conditions across the northern U.S., which was not strongly observed during the early part of May. During the middle of the month, positive AO conditions were observed. The positive AO is associated with warmer than normal conditions across the central part of the U.S. which was only partially observed. The negative AO observed during the beginning of the month corresponded to the wetter conditions that occurred over several regions of the country.
NAO: North Atlantic Oscillation
- 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 was near neutral throughout most of May, but alternated between periods of being weakly negative and weakly positive. The monthly mean was -0.3.
- Teleconnections (influence on weather): The NAO is positively correlated with temperatures over most of the U.S. during May. The corresponding patterns were generally not observed during May, likely due to the weak NAO. The NAO is correlated with below-normal precipitation in the Southeast, which again was not strongly observed during May.
WPO: West Pacific Oscillation
- 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 throughout the month of May. The monthly mean was +0.9.
- Teleconnections (influence on weather): The positive WPO is typically associated with warmer temperatures over the northern and central U.S., and with cooler conditions over the southwestern U.S. The cooler conditions over the Southwest were somewhat observed during the middle of May. The positive WPO may have contributed to some of the warmth over the central U.S. during May. The WPO does not have a strong influence on precipitation patterns during the month of May.
EPO: East Pacific Oscillation
- 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 throughout the month of May. The monthly mean was +0.9.
- Teleconnections (influence on weather): The positive phase of the EPO favors warmer than normal temperatures across the northern and eastern U.S. The observed patterns partially matched the positive EPO patterns across the northern and eastern U.S. during the beginning and end of May. During the middle of the month, troughs largely countered the effects of the positive EPO over the Southeast.
