Synoptic Discussion - August 2019

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.


Summary

Indices and their agreement with the temperature, precipitation, and upper-level circulation anomaly patterns, by time period (month, week, or other sub-monthly period).
Time Period Key Driver Other Drivers
Month NAO, AO WP
August 1-7 WP NAO, AO
August 8-14 AO, NAO EP–NP
August 15-21 EP–NP NAO, AO, WP
August 22-30 WP Dorian

The synoptic pattern in August 2019 was dominated by the ebb and flow of a strong subtropical ridge centered near New Mexico. That ridge contributed to August being the second warmest on record in Texas, New Mexico, and Arizona. The ridge also suppressed the North American Monsoon, leading to particularly dry conditions across the Southwestern U.S. Several times throughout the month, the ridge extended farther north, west, or east, bringing the above normal temperatures to other portions of the country.

The primary teleconnections for the month were a negative Arctic Oscillation (AO) and a negative North Atlantic Oscillation (NAO). Both of these are generally associated with the above normal temperatures observed near Texas during August 2019. A negative AO is also associated with above-normal precipitation over the Central Plains, which may explain why Nebraska and Kansas each had their wettest years on record.

In the tropics, the El Niño–Southern Oscillation (ENSO) was an unusual flavor of neutral with warm ocean anomalies in the western ENSO region and cool anomalies in the east canceling one another. The most significant tropical cyclone for the U.S. during August was Hurricane Dorian. Dorian’s impacts on the contiguous U.S. will be included in next month’s report. During August, it brought strong winds and rain as a tropical storm to the U.S. Virgin Islands and Puerto Rico. In the following days, Dorian strengthened into one of the strongest hurricanes on record as it made landfall over the Bahamas.

Monthly Mean

Submonthly Evolution

August 1-7

The beginning of August saw a return of the anomalous ridge over Alaska that has been a major player in North America for several months. Meanwhile, the contiguous U.S. was dominated by a ridge centered near New Mexico, which extended unusually far northwestward to the Pacific Northwest. This extension brought unusually warm conditions to the western U.S. It was also very dry, but not unusually so. The gray shading in the precipitation anomalies map shows areas where the climatological and observed precipitation were both zero. A trough over the eastern U.S., on the other hand, brought cooler than average conditions to the Mississippi Valley.

August 8-14

The ridge over the Southwest retreated during the second week of August as anomalous troughs developed along both coasts of the contiguous U.S. The West Coast trough was particularly impactful, bringing cooler than normal conditions there and enhanced rainfall to the Northern Rockies and the Northern Plains. Even though the Southwestern Ridge was more compact, it still contributed to warmer than normal temperatures across the South–Central U.S.

August 15-21

August 15–21 was dominated by a more zonal pattern across the contiguous U.S. The primary feature was a trough over Central Canada that brought below normal temperatures to the Northern Plains and wet conditions to Central and Northern Plains. The Southwestern ridge combined with an enhanced subtropical ridge over the Atlantic to bring warmer than normal temperatures from the Southwest to the Northeast. The western half of the contiguous U.S. to much of the Southeast were also drier than normal with this ridge.

August 22-30

The final week of August saw a shift westward in the atmospheric circulation over North America. The ridge that had been over Alaska moved across the Bering Sea to eastern Russia, leaving an enhanced trough over Alaska and northern Canada. The remnants of Tropical Storm Ivo in the eastern Pacific moved northward along the Baja coast, bringing warmer temperatures and a strengthening ridge on the West Coast. This also prolonged the dry conditions in the Southwest and the Central Rockies. A trough near the Great Lakes brought only slightly below normal temperatures to the Central U.S. but contributed wetter conditions from the Central Plains to the East Coast.

Atmospheric Drivers


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for August 2019, published online September 2019, retrieved on October 22, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201908.

Metadata