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.
- Ridge over the Gulf of Alaska
- Ridge near eastern Canada
- Three landfalling tropical cyclones
- August 1-7
- Positive Arctic Oscillation (AO)
- Southwest Monsoon
- August 17-23
- Negative AO
- Negative NAO
- Hurricane Ida
The primary circulation drivers during August were a ridge over the Gulf of Alaska and another near the Canadian Maritimes. These ridges were associated with temperatures that were much above normal near the California Coast and record heat in portions of the Northeast. The Pacific ridge also inhibited precipitation along the West Coast. This combination of dry and hot conditions exacerbated the ongoing wildfires over the western U.S.
A series of troughs brought heavy precipitation to the Rockies—most notably over Utah during August 17-23. Farther east, a series of tropical cyclones brought record rainfall and flooding to the Tennessee Valley and the Northeast. These storms were compounded by a non-tropical flooding event in central Tennessee on August 21.
August began with a record strong positive PNA and a positive WPO that were both driven by a strong trough near the Bering Sea. As is typical for a positive PNA, the upper-level wave train featured a ridge along the West Coast and a trough over the Southeast. The trough was also associated with a positive NAO at the time. The ridge over the West brought record warmth to parts of the northern Rockies while the trough was associated with much below average temperatures for portions of the south-central U.S., the Tennessee Valley, and the East Coast.
The second week of August featured a ridge in the Gulf of Alaska that extended to the West Coast and another one over the North Atlantic that extended to the East Coast. Both ridges were part of a positive AO pattern. Temperatures were much above average on both sides of the country with the ridges, and record heat occurred near the Sierras. The only areas with below-normal temperatures in the Contiguous U.S. were associated with enhanced rainfall from the Southwest Monsoon.
During August 17-23, the positive AO weakened and a weak negative NAO developed. These played a role in developing a trough over the western U.S. while a ridge persisted near eastern Canada. The trough brought cooler temperatures to the western half of the country. It also produced above-normal rainfall along the Rockies, including record amounts over large portions of Utah.
Weather over the eastern U.S. was largely dominated by Tropical Storm Fred and Hurricane Henri. Fred brought record rainfall, flooding and landslides to the southern Appalachians, and Henri had similar effects on the Northeast. A stalled front on August 21 also led to extreme flooding in central Tennessee.
During the final week of August, the NAO remained negative and the AO became negative. The resulting circulation featured a trough near the Pacific Northwest and a broad ridge over the rest of the country that was most anomalous over the northeast. The trough was associated with cool temperatures for the Northwest and record precipitation for parts of the Northern Plains. The ridge also brought record warmth to portions of the Northeast. Hurricane Ida made landfall in Louisiana on August 29, and its northeastward motion was slowed by the broad ridge. This slow movement exacerbated the heavy rainfall and wind damage near the Gulf Coast. Ida went on to cause severe rainfall and flooding in the Northeast, but those effects happened in September.
- 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: ENSO conditions remained neutral in August. The most common metric for ENSO is the SST anomalies in the Central Pacific, the Niño 3.4 region. These remained weak at around -0.4° in August. The Southern Oscillation Index, which measures the atmospheric response, remained positive but weakened. Together, these indices continue to reflect ENSO-neutral conditions.
- Teleconnections (influence on weather): La Niña favors cooler than normal for the western U.S. and warmer for the Central Plains. It also favors drier conditions for the Central Plains. The temperature pattern only resembled La Niña during August 17-23, but the dry conditions over the Central Plains were present throughout the month.
- Description: The MJO is the biggest source of subseasonal (31-70 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 weak during August. The MJO index was centered on the Indian Ocean (phase 2) for most of the month, but this was more a reflection of low-frequency variability than the MJO.
- Teleconnections (influence on weather): The MJO's impacts on North American weather tend to be weaker during August. Combined with the disorganized MJO, there were no clear U.S. impacts.
- 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 set a new record high (since 1951) on August 5 with a value of +4.3, which exceeded the previous record of +3.4 from September 24, 2008. The positive PNA weakened during the second week of August. The PNA was generally weak during the remainder of the month. The monthly mean was positive.
- Teleconnections (influence on weather): The PNA's impact on North American temperatures is usually less during the summer. Even so, the positive PNA would have favored warmer conditions in the Pacific Northwest and cooler in the Southeast, which was apparent during the first week of August with the record strong positive PNA.
- 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 weak during most of August. It was positive near the middle 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 the opposite happening during the negative phase. The warmth in the Northern Plains was apparent during the positive AO August 8-16 and August 17-23. Similarly, the cool anomalies occurred there during the negative AO at the end of the month.
- 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 weak during most of August, but it was generally positive early in the month and then negative for the rest of the month. The monthly mean was near zero.
- Teleconnections (influence on weather): The positive NAO during the summer typically favors cooler than normal conditions for the southeast and warmer than normal for the Northern Rockies with the opposite occurring during the negative phase. The effects of the positive NAO were apparent August 1-7, and the negative NAO affected the temperature patterns August 17-23 and August 24-31.
- 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 during the first half of August, negative around August 21, and then weak at 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 Southwest and the Northeast with the opposite during the negative phase. The positive WPO early in the month may have been associated with the cool anomalies for the Northeast during August 1-7. The WPO did not appear to affect U.S. temperatures during the remainder 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 oscillated frequently between positive and negative values throughout the month. 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 and cooler along the West Coast with the opposite happening during the negative phase. The EPO's oscillations were generally too short to significantly contribute to the weekly means.