Synoptic Discussion - September 2014

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.

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Like most of the rest of the year, September 2014 was characterized by an active jet stream with many vigorous short-wave troughs and ridges migrating through the upper-level circulation over the contiguous United States (CONUS), with much variability throughout the month. The competition between cool fronts beneath the troughs and warmer air associated with the ridges produced a net result of warmer-than-normal temperature anomalies in the West and cooler-than-normal temperature anomalies in the central U.S., with a mixed pattern in the South and East. Tropical moisture and the jet stream circulation helped feed monsoon showers which dumped moisture over large parts of the West, helping to combat large wildfires and to reduce drought. Fronts and surface low pressure systems, associated with the jet stream troughs, brought above-normal rainfall to parts of the Plains, Midwest, and Southeast, along with areas of severe weather, but the month was largely drier than normal east of the Rockies beneath dominating ridges which inhibited severe weather outbreaks and spurred the growth of drought and abnormally dry areas. The net result was a contraction in the national drought footprint. The upper-level circulation pattern, and temperature and precipitation patterns, suggest that the weather and climate of September 2014 were influenced, in part, by modes of variability associated with conditions in the North Pacific, Arctic, and equatorial Pacific oceans, but that normal random variability of the atmosphere also played a role. See below for details.

Synoptic Discussion

Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.

In the Northern Hemisphere, September is 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 more, and the warm, dry subtropical high pressure belts influence the weather less. In September 2014, a long-wave pattern consisting of a ridge in the west and trough in the east dominated the circulation for most of the month, with an active jet stream sending many vigorous short-wave troughs and ridges through the upper-level circulation. The pattern changed (or "flipped") near the end of the month when a large upper-level trough/cutoff low moved slowly across the western CONUS, forcing a long-wave ridge over the central to eastern CONUS.

Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

These upper-level weather systems generated low pressure systems with their associated cold fronts at the surface — warm air flowed ahead of the lows, with cooler air surging southward behind them. The competition between cool fronts beneath the troughs and warmer air associated with the ridges, as well as the "flip" in the circulation pattern near the end of the month, caused the temperature pattern to change week to week (weeks 1, 2, 3, 4, 5). Cold anomalies in the central U.S. beneath the trough by mid-month were gradually eroded by warmer air beneath a ridge as the month progressed (September 1-17, September 1-25, September 1-27, September 1-29), resulting in a mixed pattern of monthly temperature anomalies. This seesaw of warm and cold fronts produced 4,285 record warm daily high (1,091) and low (3,194) temperature records and about two-thirds as many (2,869) record cold daily high (2,122) and low (747) temperature records. This was reflected in the national monthly average temperature which ranked September 2014 as the 26th warmest September in the 1895-2014 record.

The fronts and surface low pressure systems, associated with the jet stream troughs, brought frequent days with rain and above-normal monthly total rainfall to parts of the Southern Plains, Upper Midwest, and Coastal Southeast. Subsiding air beneath upper-level ridges, especially near the end of the month, dominated the rest of the country east of the Rockies, keeping precipitation below normal and contributing to expansion of drought in the South and East. Tropical moisture, and the cutoff low near the end of the month, powered monsoon showers which dumped precipitation over much of the Southwest and Intermountain Basin. This contracted drought where it rained, but the precipitation generally missed southern California and large parts of the Northwest. At mid-month, the remnants of Hurricane Odile deluged parts of the southern Arizona, southern New Mexico, and western Texas with flooding rains which helped refill low reservoirs. The upper-level trough/cutoff low helped reduce the number of large wildfires which were scorching the West. Parts of California received precipitation from the upper-level trough, but moisture deficits were so large that the rain had little effect on the drought. The net change in drought area was a contraction of the moderate to exceptional national drought footprint compared to the end of August.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. While the aggregated national CEI for September ranked slightly below average, the regional CEI reflected the extreme weather out west. The highest warm minimum temperature component, eighth most extreme drought component, and 13th highest warm maximum temperature component contributed to the seventh most extreme September CEI for the West region. The persistence of the unusual warmth and dryness gave the West region the most extreme July-September and January-September CEI in the 1910-2014 record, and second most extreme April-September CEI. At this time of year, energy is needed both to heat homes in the north as cold air masses begin their southward trek, and to cool homes in regions where warm ridges still dominate the weather, as in the West during this month. With a mixed temperature anomaly pattern across the country and throughout the month, the energy needs for the nation as a whole were slightly higher than average, with the September 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 37th highest September REDTI in the 120-year record.

Areas of severe weather developed ahead of the troughs, especially early in the month. But the migratory nature of the ridges and troughs in the upper-level circulation, as well as the large ridge near the end of the month, kept the total number of tornadoes below normal, with a preliminary count of 45 tornadoes comparing to the average count of 74 tornadoes for September. The circulation pattern also helped to deflect the two North Atlantic tropical systems (Tropical Storm Dolly and Hurricane Edouard) away from the CONUS.

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

When integrated across the month, the atmospheric circulation produced a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over southern Alaska, the Gulf of Alaska, western Canada, and the U.S. Rocky Mountains, and another area of above-normal heights (weaker-than-normal long-wave trough) over the northeast U.S. and into the North Atlantic. The circulation pattern consisted of below-normal 500-mb heights over the northern Canadian Arctic islands (Canadian Archipelago) to Greenland. This anomaly pattern masks the large variability in the circulation which resulted from the numerous short-wave ridges and troughs that migrated through the westerly flow.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across much of the West, parts of the Plains to Midwest, and parts of the coastal Southeast. Precipitation was below normal across parts of the Northwest, Southwest, and southern California; large parts of the Great Plains and Deep South; and the Northeast. The precipitation pattern over Alaska and Hawaii was mixed.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The frequent passage of cool short-wave troughs and warm short-wave ridges across the CONUS, and the "flip" in the long-wave pattern, resulted in a mixed temperature anomaly pattern. Temperatures generally averaged below normal in parts of the Central to Northern Plains and much of the Midwest. September temperatures were near to above normal across much of the West and parts of the Southern Plains to Southeast. Alaska averaged warmer than normal in the south and east, and cooler than normal in parts of the interior.

Global Linkages: The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The circulation anomaly pattern over North America matches up with the anomaly pattern over Eurasia. The September mid-latitude (jet stream) pattern across the Northern Hemisphere consisted of two large centers of negative anomalies — one over northeast Canada and Greenland, and the other over north central Siberia — with two large areas of positive anomalies in between — one over Europe and the other extending from northeast Siberia to western North America. Above-normal 500-mb heights, associated with ridging in the upper atmosphere, were reflected by above-normal temperatures at the surface over Europe and northwestern Siberia to western North America. Below-normal 500-mb heights, associated with troughs in the upper atmosphere, were reflected by below-normal temperatures and above-normal precipitation at the surface over central to north central Siberia.

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:

Upper-level circulation pattern and anomalies averaged for the last three months
Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed September 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in September was related to the normal chaotic nature of the transition between seasons, but there were hints that the jet stream and ocean-atmosphere interactions over the North Pacific and Arctic played a role, and the equatorial Pacific may have exerted some limited influence. The MJO was mostly weak or incoherent and the NAO and WP showed little correlation. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state. The jet stream circulation (ridge/trough pattern) underwent a significant "flip" during the month, which complicated and flattened the temperature and circulation anomalies. The anomaly patterns suggest that Arctic (AO) and Pacific (PNA, EP-NP, and PT) drivers had the strongest influence on the circulation, especially in the West. There was some indication that the equatorial Pacific may have influenced the temperature pattern early in the month (week 1), and the Arctic (AO) may have influenced it near the end of the month (week 4). The Pacific indices (PNA, EP-NP, and PT) had some agreement with the monthly temperature pattern, especially in the West. The precipitation pattern was also complicated by the variations in the circulation, with the AO having the best agreement monthly but the EP-NP and PT having some agreement in places. This month illustrates how weather and climate anomaly patterns can be the manifestation of normal (random) atmospheric variability, but also how they reflect influences from several atmospheric drivers (or modes of atmospheric variability).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for September 2014, published online October 2014, retrieved on October 21, 2014 from