Synoptic Discussion - August 2017


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.


NCEI added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.


Summary


The Earth's ocean-atmosphere system was technically in an ENSO-neutral state during August 2017. An upper-level ridge dominated the western half of the contiguous United States (CONUS) with attendant warmer- and drier-than-normal weather, which continued conditions for an active wildfire season. An upper-level trough set up over the eastern CONUS and its associated short-wave troughs, fronts and low pressure systems brought cooler-than-normal weather. Parts of the Plains, South, and Mid-Atlantic regions had wetter-than-normal weather due to rain associated with the fronts and troughs, and Hurricane Harvey inundated eastern Texas and the Lower Mississippi Valley at the end of the month. Drought and abnormal dryness expanded and intensified in the Pacific Northwest and High Plains of Montana under the relentless heat and dryness, but storms in the Plains contracted drought areas, especially in the hard-hit Dakotas. The fronts and lows, and especially Hurricane Harvey, generated rounds of severe weather, with more than the average number of tornadoes occurring. Without ENSO-related inhibitors shearing tropical systems, the Atlantic and East Pacific basins saw more active tropical cyclone activity than average. The upper-level circulation, temperature, and precipitation anomaly patterns suggested that the atmospheric drivers originating in the subtropical North Pacific may have had an influence on the month's weather. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.
Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

In the Northern Hemisphere, August marks the end of climatological summer which is the time of year when solar heating is at maximum with the high sun angle, arctic air masses are weakest, and the circumpolar vortex and jet stream have retreated far northward. Polar air masses influence the weather over the contiguous U.S. (CONUS) less, and the warm, dry subtropical high pressure belts dominate the weather.

During August 2017, however, the atmospheric circulation was highly meridional. Strong short-wave troughs dragged Canadian air masses southward across the eastern and central CONUS, resulting in well-below-normal temperatures, while ridging dominated in the West. The resulting long-wave circulation pattern, averaged over the month, consisted of a trough over the eastern CONUS and a ridge over the western CONUS. And at the surface, the North Atlantic High (Bermuda High) and North Pacific High were near normal strength.

With the upper-level ridge dominating the weather in the West, August temperatures averaged warmer than normal across the West. Four states ranked in the top ten warmest category for August — California (warmest August on record), Oregon (warmest), Washington (warmest), and Nevada (fourth warmest). The persistent passage of cold fronts east of the Rockies beneath the upper-level trough in the East resulted in six states in the central Plains to Midwest having a top ten coldest August. These included Missouri (seventh coldest), Iowa (eighth coldest), Kansas (eighth coldest), Oklahoma (ninth coldest), Illinois (tenth coldest), and Nebraska (tenth coldest). These opposite extremes nearly balanced each other out. When averaged across the CONUS, August ranked near the middle of the historical distribution at 53rd coolest.

  • The extreme warmth in the West was a little more extreme than the extreme cold in the central part of the country. Warm temperature records beneath the ridge dominated cold temperature records associated with the fronts. When integrated across the month, there were 3,596 record warm daily high (1,363) and low (2,233) temperature records. This was about one and a half times the 2,277 record cold daily high (1,667) and low (610) temperature records.
  • The REDTI (Residential Energy Demand Temperature Index) for August 2017 ranked near the middle of the 123-year historical record for August at 61st highest. This was due to the temperature anomaly pattern of below-normal temperatures in the high population centers of the central and eastern CONUS and the smaller area of above-normal temperatures in the less heavily populated West which reduced the national cooling demand.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. During August, the dominance of ridging resulted in warm temperature extremes, frequent cold frontal passages resulted in cold temperature extremes, and frontal lifting, upper-level troughs and lows, and Hurricane Harvey brought wet weather extremes. These occurred in various parts of the country. However, while some components of the CEI ranked high, when the components are aggregated together the only regional CEI that ranked in the top ten category was for the Northwest region, and that only barely. The South and West regions ranked in the top 20 most extreme category. Even the national CEI for August 2017 didn't make the top ten cut — with the tropical cyclone component, it ranked twelfth most extreme, but without it the rank was 33rd most extreme.

Specifically, the Northwest region had its tenth most extreme August CEI in the 1910-2017 record due to the most extreme warm minimum temperature component and third most extreme warm maximum temperature component. The East North Central region had the second most extreme wet spell component, the South region had the fourth most extreme days with precipitation component and seventh most extreme cold maximum temperature component, and the West region had the most extreme warm minimum temperature component, but none of these regions had a regional CEI in the top ten category. When aggregated across the nation, August 2017 had the tenth most extreme cold maximum temperature component and eleventh most extreme days with precipitation component. For the other national components, the regional extremes balanced each other out, nationally. As noted earlier, with these extreme components and the tropical cyclone component, the CONUS had only the twelfth most extreme national August CEI in the 108-year record.

North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern, averaged for the month, consisted of below-normal upper-level (500-mb) height anomalies centered over the Great Lakes and extending west to the central Plains and northeast into southeastern Canada. These resulted from troughs and closed lows frequently moving across the area. Above-normal height anomalies were over the West, centered over the Northwest and extending into northern Canada. Of the circulation indices usually discussed on this page, the teleconnections for a positive PT come closest to the August 2017 500-mb circulation anomalies over North America and the North Pacific.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Below-normal precipitation dominated much of the West as the upper-level ridge deflected most weather systems into Canada. It was also drier than normal across the Ohio Valley to lower Great Lakes and parts of the Southeast and Northeast. Showers and thunderstorms that developed along cold fronts gave parts of the Plains above-normal precipitation, and rain from fronts and Hurricane Harvey resulted in above-normal precipitation in the southern Plains to Lower Mississippi Valley. Scattered showers gave parts of California and Nevada above-normal precipitation, but this is their dry season and normals are very low. Alaska and Hawaii had a mixed precipitation anomaly pattern, although it was mostly dry across the Big Island in Hawaii. Puerto Rico was mostly near to wetter than normal.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The upper-level ridge over the western CONUS brought above-normal temperatures to the West, while cold fronts associated with upper-level troughing in the eastern CONUS brought below-normal temperatures to most of the country east of the Rockies, but especially in the central Plains to Midwest. Alaska had a mixed temperature anomaly pattern with generally below-normal temperatures in the northwest and above-normal temperatures dominating in the south and interior.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.

Global Linkages: The upper-level (500-mb) circulation anomaly pattern over North America was part of a complex long-wave pattern that stretched across the Northern Hemisphere. The usual east-west trough/ridge pairs (or couplets) and anomaly couplets were evident across Asia and North America (some examples of anomaly couplets include eastern and western North America, western North America and Alaska/Gulf of Alaska, Alaska/Gulf of Alaska and eastern Siberia). But north-south coupling was also apparent in latitudinal bands across Eurasia, North America, and the Arctic (examples include eastern CONUS and northern Canada, northern Canada and Arctic Ocean, Arctic Ocean to eastern Siberia or eastern Eurasia). The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation (over western North America, western Asia, and parts of Siberia); above-normal surface temperatures over western Eurasia, eastern Siberia, and western to northern North America; and warming SSTs in parts of the North Atlantic (near Greenland). The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over central North America, Alaska, and north central Siberia; cooling SSTs in parts of the North Pacific (Gulf of Alaska); and above-normal precipitation over Alaska, north central Siberia, and the north central CONUS. Small parts of southeast Australia, central North America, northwest South America, and north central Asia were near to cooler than normal. But with large parts of the continents still having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the August 2017 global temperature was well above normal.


Atmospheric Drivers


Subtropical highs, and fronts and low pressure systems moving in the mid-latitude storm track flow, are influenced by the broadscale atmospheric circulation. The circulation of the atmosphere can be analyzed and categorized into specific patterns. The Tropics, especially the equatorial Pacific Ocean, provides abundant heat energy which largely drives the world's atmospheric and oceanic circulation. The following describes several of these modes or patterns of the atmospheric circulation, their drivers, the temperature and precipitation patterns (or teleconnections) associated with them, and their index values this month:


Examination of the available circulation indices and their teleconnection patterns, and comparison to observed August 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in August was traced mostly to atmospheric drivers originating over the subtropical North Pacific. The PNA and AO indices were close to zero (weakly positive), indicating little influence; the MJO was weak to incoherent; and the equatorial Pacific was in an ENSO-neutral state. For the NAO and WP, there was some agreement in some anomaly patterns, but nothing conclusive. The observed anomaly patterns were opposite those expected with a negative EP-NP. There was some agreement in patterns for the MJO, but the MJO was weak to incoherent so the apparent similarities may have been a coincidence. Only one teleconnection pattern showed strong similarities. The observed anomaly patterns were shifted a little to the west, but otherwise they had good agreement with the teleconnections of the PT index. As noted by the NOAA Climate Prediction Center, "The positive phase of the PT pattern features above-average heights west of Hawaii and across western North America, and below-average heights in the Gulf of Alaska and over the southeastern United States. The PT pattern is associated with above-average surface temperatures in the western subtropical North Pacific, the subtropical North Atlantic, and throughout western North America, and with below-average temperatures over the eastern half of the United States." These upper-level circulation and oceanic and CONUS temperature conditions were strongly satisfied.

This month illustrates how the weather and climate anomaly patterns are influenced by atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for August 2017, published online September 2017, retrieved on September 19, 2017 from https://www.ncdc.noaa.gov/sotc/synoptic/201708.

Metadata