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
- West Pacific Oscillation (WPO) variability
- Ridge anchored near Texas
-
July 1-9
- La Niña
- Positive WPO
- Tropical Storm Colin
-
July 10-24
- Negative WPO
- Broad ridge over the western U.S.
-
July 25-31
- Madden-Julian Oscillation (MJO)
- Positive Pacific/North American (PNA) pattern
- Positive WPO
- Negative East Pacific Oscillation (EPO)
- Active Southwest Monsoon
Monthly Mean
A strong ridge anchored near Texas continued to dominate the North American climate in July. It was associated with record heat and continuing drought conditions over Texas. During summer, these strong ridges are often associated with enhanced precipitation along their periphery, as was observed during July. The Southwest Monsoon was moderately active, particularly later in the month. Flooding also occurred near Kentucky and Missouri with a trough near the Great Lakes later in the month.
Submonthly Evolution
July 1-9
The key circulation features at the beginning of July were a trough along the West Coast and a ridge anchored near Texas. The trough was associated with cool and wet conditions for the Pacific Northwest, while the ridge continued the record hot and dry conditions for Texas. As often happens with strong ridges in summer, precipitation was enhanced along its periphery. The wet conditions included a slightly stronger than normal Southwest Monsoon. Precipitation was also enhanced from the Central Plains to the Southeast. Tropical Storm Colin also produced heavy rain near the Carolina Coast.
July 10-24
During the middle of July, the ridge strengthened and extended westward. It brought persistent hot and dry conditions to most areas west of the Mississippi. A trough near the Appalachians brought near-normal temperatures and slightly wetter than normal conditions to the eastern U.S. However, a ridge over the Atlantic was also associated with a heat wave over the Northern Atlantic Coast.
July 25-31
The ridge moved farther northwestward towards the end of July bringing record heat and dry conditions to the Pacific Northwest. The record heat also continued over Texas. Around Texas, a trough near the Great Lakes brought cool and wet conditions from the Southwest to the Great Lakes. This included another active period for the Southwest Monsoon and severe flooding in Kentucky and Missouri.
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 July. The most common metric for ENSO is the SST anomalies in the equatorial Pacific. The cool anomalies associated with La Niña continued during July. The Southern Oscillation Index, which measures the atmospheric response, weakened but remained 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 East Coast. It also favors wet conditions for the western U.S. with drier conditions to the east. These patterns were somewhat apparent July 1-9 (temperature and precipitation).
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 July. It began the month near the Maritime Continent (phase 4) and slowly moved eastward towards the western Pacific over the first half of the month. Later in July, the MJO moved more quickly across the Western Hemisphere and the Indian Ocean.
- Teleconnections (influence on weather): The most significant teleconnections for the U.S. in July occur during phase 8 when the Pacific Northwest is warmer than normal and the Ohio Valley is anomalously cool, which was apparent towards the end of July.
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 positive throughout July. The monthly mean was the third most positive July PNA since 1951.
- 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. These patterns are generally weaker during the summer, but the positive PNA may have contributed to the temperature pattern during July 25-31.
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 early in July and negative later in the month. 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 and cool anomalies there during the negative phase. Conditions tend to be drier near the Great Plains and wetter near the Southeast during the positive AO with the opposite occurring during the negative phase. These patterns were not observed during July.
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 weak during most of July with positive values near the end of the month. The monthly mean was near zero.
- 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 not apparent during July because of the NAO's weak amplitude.
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 during the beginning and end of July with negative values near the middle 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 generally observed during July with each phase of the WPO. It was particularly strong during the positive WPO at the end of the month.
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 strongly negative at the beginning and end of July with strong positive values in between. 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. Only the negative phase near the end of the month was apparent.
