Synoptic Discussion - September 2016

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.


The atmosphere was in an ENSO-neutral state during September 2016. Like the previous month, the weather over the contiguous United States (CONUS) this month was dominated by a complex interplay between subtropical high pressure systems, jet stream weather systems, and tropical cyclone rainfall. Troughs and lows in the jet stream flow funneled cold fronts and surface lows into the western U.S., giving the West a colder-than-normal month, while the Bermuda High and upper-level ridges kept the U.S. east of the Rockies warmer than normal. The fronts and lows brought precipitation to parts of the western and central CONUS, while tropical systems drenched parts of the East Coast, but the North Pacific High and Bermuda High kept much of the rest of the country drier than normal. The fronts and upper-level troughs provided the dynamics that triggered severe weather, but, again, the Bermuda High helped keep the number of tornadoes below average for the month. Drought expanded in the South and New England and contracted in parts of the West and Midwest, with the overall national drought footprint changing very little, shrinking slightly to about 19.4 percent of the CONUS. The upper-level circulation and temperature anomaly patterns suggested the atmospheric drivers originating in the North Pacific and Arctic Oceans had the greatest 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, September marks the beginning of climatological fall (autumn) which is the time of year when solar heating decreases as the sun angle decreases, and an expanding circumpolar vortex forces the jet stream to migrate southward. Polar air masses begin to influence the weather over the contiguous U.S. (CONUS) more, and the warm, dry subtropical high pressure belts influence the weather less.

The mid-latitude circulation was very active during September 2016, with numerous troughs and ridges moving in the jet stream flow. The circulation became extremely meridional over Canada near the end of the month, with large areas of north-flowing and south-flowing air. This pattern resulted in the storm track staying mostly along the U.S.-Canadian border, with deep troughs of low pressure frequently plunging deep into the CONUS. The North Pacific and Bermuda subtropical highs struggled to maintain their dominance over the U.S. in spite of the onslaught of jet stream troughs and upper-level lows. The troughs and upper lows dragged cooler Canadian air behind cold fronts along with them at the surface. Under the dominance of the Bermuda High, temperatures averaged warmer than normal for the month from the Plains eastward, while the dominance of upper-level troughs gave much of the West cooler-than-normal temperatures.

The North Pacific High kept much of the Far West drier than normal for September, while descending air associated with the Bermuda High inhibited precipitation across the central and eastern CONUS. The fronts and upper-level troughs parried the dry trend by bringing above-normal precipitation to parts of the Interior West to Northern Plains and Southern Plains to Upper Midwest. Moisture from the remnants of East Pacific tropical cyclones gave parts of the Southwest above-normal precipitation, and moisture from North Atlantic tropical cyclones resulted in a wetter-than-normal month along the coast from the Southeast to Mid-Atlantic states.

The combination of lifting along the frontal boundaries and dynamical support from the upper-level troughs and lows triggered severe weather, mostly east of the Rockies, but the overall dominance of the Bermuda High kept the tornado count below average. According to preliminary data, 36 tornadoes occurred during September 2016, which is about half of the September average of 74. Most occurred in the Southwest to Central Plains and Midwest in association with fronts and low pressure systems along the storm track.

Normally, on the southern side of the North Pacific and North Atlantic Highs, the winds blow out of the east to northeast and toward the equator to produce the northeasterly Trade Winds. The Trade Winds steer tropical weather systems that are close to the equator, thus making the subtropical highs important drivers of tropical cyclone motion. ENSO conditions along the equatorial Pacific Ocean (El Niño or La Niña) can influence the development of tropical cyclones in both the Pacific and Atlantic. The tropical Pacific was in an ENSO-neutral state during September, so it's influence on tropical cyclones was muted this month.

Five named tropical cyclones (Tropical Storms Ian, Julia, Karl, and Lisa, and Hurricane Matthew) formed in the Atlantic Basin in September. Three tropical cyclones which formed in August (Hurricanes Gaston and Hermine and Tropical Depression 8) were also active to some degree into early September. The Bermuda High steered Gaston, Ian, Karl, and Lisa into the central North Atlantic, keeping them away from the CONUS. Tropical Depression 8 brushed the North Carolina coast before being deflected out to sea. Hermine tracked across northern Florida on September 2 after developing in the Gulf of Mexico in late August. It joined with a cold front along the Southeast coast and transformed into a frontal low, then tracked just offshore along the Eastern Seaboard during the next several days, dropping tropical moisture along the coast from Florida to the Mid-Atlantic States. Julia developed along the Atlantic Coast near Jacksonville, Florida in mid-September and slowly meandered just off the Florida to North Carolina coast for the next several days, occasionally interacting with a passing cold front. The languid motion of Hermine and Julia was influenced by a complex interaction between the Bermuda High, which wanted to swing them out into the North Atlantic, and troughs in the mid-latitude jet stream, which kept pulling them back to the coast. Matthew developed near the end of the month in the vicinity of the Lesser Antilles and was steered westward into the Caribbean by easterly Trade Winds, until being drawn north by an upper-level trough dipping southward from the westerlies.

Five named tropical cyclones (Tropical Storm Roslyn and Hurricanes Newton, Orlene, Paine, and Ulika) formed in the central and eastern equatorial Pacific Basin in September. Two tropical cyclones which formed in August (Hurricanes Lester and Madeline) were also active to some degree in early September. Most of these cyclones moved westward in the easterly Trade Winds, with Lester and Madeline sweeping by Hawaii. Madeline passed by close enough to drop heavy rains on the Big Island. Deep troughs in the jet stream over western North America helped draw Newton and Paine northward along Baja California, with their tropical moisture eventually making it into southern California and the Southwest U.S., enhancing summer monsoon rainfall.

The Intertropical Convergence Zone was active across the U.S.-Affiliated Pacific Islands (USAPI) in the western equatorial Pacific. Several tropical disturbances moved westward across Micronesia in the easterly Trade Winds and developed into named storms as they moved west of the Mariana Islands and into the Philippine Sea. These included Typhoons Meranti, Malakas, Megi, and Chaba. Other than heavy rainfall, the USAPI were largely spared damaging effects from these cyclones.

The subtropical highs allowed warmer tropical-type air masses to cover the CONUS more often than cooler polar air masses. As a result, September ended up with 3,715 record warm daily high (1,100) and low (2,615) temperature records. This was about five and a half times the 669 record cold daily high (422) and low (247) temperature records that were associated with the air masses behind the cold fronts. Nationally, September 2016 ranked as the ninth warmest September in the 1895-2016 record. The last eight Septembers have been consistently much warmer than average. The REDTI (Residential Energy Demand Temperature Index) for September 2016 ranked seventh highest in the 122-year record for September, due to much-warmer-than-normal temperatures which increased cooling demand in the heavily-populated East Coast, Midwest, and Southeast regions.

Above-normal precipitation from fronts and tropical systems helped offset the dryness associated with the North Pacific and Bermuda Highs, with the result being the 40th wettest September on record for the CONUS. Drought and abnormal dryness shrank where heavy rains fell, but it expanded and intensified in the South and Northeast where it was dry and much warmer-than-normal temperatures enhanced evapotranspiration. Drought contraction and expansion roughly balanced each other out, so that the national moderate-to-exceptional drought footprint decreased only slightly from 19.5 percent at the end of August to 19.4 percent at the end of September (from 16.3 percent to 16.2 percent for all of the U.S.).

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Although temperature and precipitation extremes occurred during the month in many areas, only two regions had a CEI that ranked in the top ten category. The Northeast region had the fifth most extreme September CEI in the 1910-2016 record due to the second most extreme warm maximum and seventh most extreme warm minimum temperature components, and 13th most extreme Palmer drought component. The East North Central region had the tenth most extreme September CEI in the 1910-2016 record due to the second most extreme warm minimum temperature component and fourth most extreme Palmer wet spell component. When aggregated across the nation, September 2016 had the seventh most extreme warm minimum temperature component, but the other national components ranked lower, with the September 2016 national CEI ranking as only the 39th most extreme September CEI in the 107-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 near to below-normal upper-level (500-mb) height anomalies over the western CONUS extending into western Canada, resulting from troughs frequently moving across and deepening over the West. Above-normal heights dominated over the eastern CONUS and extending into eastern Canada. Of the circulation indices usually discussed on this page, the teleconnections aggregated for a negative EP-NP and a positive AO most closely match the September 2016 500-mb circulation anomalies.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

September was drier than normal across most of the Far West, from the Central Rockies to parts of the Central High Plains, from the Lower Mississippi Valley to Southern Appalachians, and across most of the Northeast. It was wetter than normal from the Interior West to Northern Plains, along a strip from the Southern High Plains to western Great Lakes, and along the coast from Florida to the Mid-Atlantic states. Alaska was wetter than normal except in the southeast. Hawaii was generally wetter than normal except on the Big Island and Kauai. Puerto Rico was wetter than normal in the northwest and drier than normal along the southern coast and in the east.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

September temperatures averaged below normal roughly from the Rockies to the West Coast, and above normal from the Plains to the East Coast, with Ohio having a record-warm September. Temperatures were above normal in Alaska except in the interior where they were near to below normal.

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. There were several regional ridge (above-normal 500-mb height anomalies) / trough (below-normal heights) "couplets" in the monthly average pattern, but positive height anomalies (ridges) seemed to dominate. The most notable below-normal height departures (troughs) occurred over western North America, the northeastern North Atlantic (near the Icelandic Low), southwestern Russia, and the northwestern North Pacific. The most notable above-normal height departures (ridges) occurred over eastern North America, Europe, north central Eurasia, and the eastern North Pacific. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation; and above-normal surface temperatures over the continents and oceans. The few areas of below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures over the continents and oceans, and above-normal precipitation over southwestern Russia. With most of the continents having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the September 2016 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 September 2016 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in September was traced mostly to atmospheric drivers originating over the North Pacific and Arctic Oceans. The PNA index was near zero (neutral) and there is little agreement between the September 2016 anomaly patterns and those expected with the PT, NAO, and WP drivers. The tropical Pacific Ocean was in an ENSO-neutral state. Rossby and Kelvin waves, the IOD, tropical cyclones, and the monsoon trough in the western North Pacific may have masked the influence of the MJO. Yet, despite what the ENSO and MJO indices are indicating, the observed temperature and precipitation anomaly patterns show some similarities to those expected with a phase 4 MJO and with a La Niña. This may be a coincidence. There is some similarity in the circulation and precipitation anomaly patterns expected with a positive AO and those observed for September 2016, which suggests the Arctic Ocean drivers may have had some influence. But the greatest agreement was with the EP-NP pattern. The temperature and circulation patterns for a negative EP-NP index are an excellent match for those observed during this September. The North Pacific High was stronger than normal, and upper-level troughs and low pressure systems in the jet stream flow originating over the Pacific strongly influenced the weather over North America. The combination of the EP-NP pattern and AO pattern accounts for the shape of the September 2016 upper-level circulation anomaly pattern over the North American sector. Most of the indices have poor teleconnections for precipitation; this reflects the random nature of warm season convection.

This month illustrates how the weather and climate anomaly patterns can be strongly influenced by atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific and Arctic Oceans, with the Bermuda and North Pacific subtropical highs playing an important role.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for September 2016, published online October 2016, retrieved on July 21, 2019 from