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
- Positive Pacific/North American (PNA) pattern
- Positive North Atlantic Oscillation (NAO)
- Active Southwest Monsoon
- La Niña
-
August 1-9
- Broad ridge
- Southwest Monsoon
- Hurricane Frank
-
August 10-20
- Positive PNA
- Southwest Monsoon and Hurricane Howard
-
August 21-31
- Madden-Julian Oscillation (MJO) over the Indian Ocean
- Strong cold front
- Ridges along the East and West Coasts
Monthly Mean
A pair of ridges anchored near the West Coast and the Northeast dominated the circulation pattern during August. The West Coast ridge was associated with a positive PNA pattern that persisted for the second half of August, while the ridge near the Northeast was associated with a positive NAO that lasted most of the month. These ridges were associated with record warmth for the Pacific Northwest and New England.
The Southwest Monsoon has been particularly active this summer, and that trend continued in August. The pressure gradient between the West Coast ridge and a series of East Pacific hurricanes brought repeated surges of moisture up the Gulf of California and into the Southwest. The moisture also moderated the temperature anomalies for the region. Farther east, a strong cold front brought heavy rain and flooding from Texas to Mississippi around August 20-25. It was particularly remarkable because Texas had been experiencing a prolonged heatwave and drought that began in April.
Submonthly Evolution
August 1-9
A broad ridge across the U.S. brought warmer than normal conditions to most of the country at the beginning of August. Record warmth was observed around New England. Temperatures were closer to normal in the Southwest where a surge of monsoonal moisture brought record rainfall and flooding to parts of California and Nevada. The precipitation and associated cloud cover contributed to the milder temperatures in the region. It was driven in part by the pressure gradient between the U.S. ridge and the remnants of Hurricane Frank in the East Pacific.
August 10-20
The middle of August exhibited a typical positive PNA pattern with a ridge near the Northern Rockies and a trough along the East Coast. The ridge was associated with record warmth around Idaho and western Montana, and the trough brought cool anomalies to the Eastern U.S. The gradient between the ridge and the remnants of Hurricane Howard caused another surge of monsoonal moisture with more rain and flooding in the Southwest.
August 21-31
Around August 21, a strong cold front swept across the Great Plains. The southwestern tail of that cold front tapped into moisture from the Gulf of Mexico and caused flooding around Dallas, TX. It brought an abrupt end to a heat wave that had dominated Texas since April and eased the drought in that region. The same system also caused flooding in Jackson, MI that breached the city's main water treatment facility. Meanwhile, a ridge along the West Coast brought record heat and dry conditions to the region, and another ridge along the East Coast was associated with record heat for the Northeast.
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: La Niña conditions continued during August. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. The cool anomalies associated with La Niña continued during August. The Southern Oscillation Index, which measures the atmospheric response, remained positive. Together, these indices suggested La Niña conditions.
- Teleconnections (influence on weather): La Niña favors cooler than normal temperatures for the Western U.S. It also favors warm and dry conditions for the Central Plains. The cool Western U.S. was not apparent during August, but the warm and dry conditions for the Central Plains were present throughout the month.
MJO: Madden-Julian Oscillation
- 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 index indicated that the MJO was moderately active in August. It circumnavigated the globe over the course of the month, beginning and ending near the Indian Ocean (phase 3). Its amplitude was weakest during the middle of the month as it crossed the Pacific Ocean (phase 7/8) and strongest at the end of the month.
- Teleconnections (influence on weather): During August, the most significant U.S. teleconnections occur during phase 2 when the Southern Plains are typically cooler and wetter than normal. This pattern was apparent towards the end of August.
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 alternated between positive and negative values during the first half of the month. It was positive throughout the second half of August. The monthly mean was positive.
- Teleconnections (influence on weather): The positive PNA favors warmer conditions in the Pacific Northwest and cooler in the Southeast with the opposite in the negative phase. The positive PNA temperature pattern was most apparent during August 10-20 even though the PNA was stronger later in 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 was positive during the first half of August and negative during the second half. The monthly mean was near zero.
- Teleconnections (influence on weather): The AO is typically associated with warm anomalies near the Northern Plains during the positive phase. Conditions tend to be drier near the Great Plains and wetter near the Southeast during the positive AO. These patterns were not observed during August.
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 positive during most of August except for some weak negative values near the end of the month. The monthly mean was positive.
- Teleconnections (influence on weather): During summer, the positive NAO favors warmer and drier than normal conditions for the Northern Plains with cooler and wetter conditions for the Southeast. These patterns were generally apparent throughout the month (temperature and precipitation).
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 negative at the beginning of August, positive in the middle, and oscillating between positive and negative towards the end of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive WPO during summer is typically associated with cooler temperatures over the Ohio Valley and the Southwest with the opposite during the negative phase. These patterns were not observed during August.
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 oscillated between strongly positive and negative values throughout August. The monthly mean was near zero.
- 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. These patterns were not observed during August.
