Synoptic Discussion - August 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.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Summary


August 2014 was characterized by an active jet stream with many vigorous short-wave troughs and ridges migrating through the upper-level circulation. The competition between cool fronts beneath the troughs and warmer air associated with the ridges and subtropical highs resulted in a mixed pattern of temperature anomalies. Monsoon showers dumped moisture over parts of the southwestern U.S., while large wildfires ravaged areas farther west. Fronts and surface low pressure systems, associated with the jet stream troughs, brought above-normal rainfall to Northern Plains and Midwestern states, and areas of severe weather, mostly east of the Rockies. But the orientation of the upper-level circulation kept the number of tornadoes below normal and helped deflect tropical systems away from the continental United States (CONUS). Subsiding air from the subtropical Bermuda High kept precipitation below normal across much of the Southern Plains to Southeast. Drought contracted where it rained, and expanded where it didn't, with the net result being a contraction in the national drought footprint. The upper-level circulation pattern, and temperature and precipitation patterns, suggested that the weather and climate of August 2014 were influenced, in part, by modes of variability associated with conditions in the North Pacific, North Atlantic, Arctic, and equatorial Pacific oceans, but that normal random summer 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, August is at the end of climatological summer which is the time of year when solar heating brings the warmest temperatures and forces the jet stream and circumpolar vortex to contract poleward. Polar air masses can still influence the weather, but they are usually rare and not as cold as in winter months. The warm, dry subtropical high pressure belt normally dominates the weather as it shifts northward, with warm southerly air masses holding sway across the contiguous United States (CONUS). In August 2014, the North Atlantic subtropical high pressure center (Burmuda High) exerted influence over the southeastern states and occasionally the Southern Plains states, but an active jet stream dominated the north, with many vigorous short-wave troughs and ridges migrating through the upper-level circulation.

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 and subtropical highs resulted in a mixed pattern of temperature anomalies. This seesaw of warm and cold fronts produced 2,179 record warm daily high (431) and low (1,748) temperature records and about the same number (2,189) of record cold daily high (1,763) and low (426) temperature records. This was reflected in the national monthly average temperature which ranked August 2014 as the 53rd warmest August in the 1895-2014 record, or about average.

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 the Northern Plains, Northern Rockies, and Midwestern states. Subsiding air from the Bermuda High dominated much of the Southern Plains to Southeast, keeping precipitation below normal and contributing to expansion of drought. Monsoon showers dumped moisture over parts of the southwestern U.S., contracting drought, while upper-level ridges dominated the Pacific Northwest, bringing persistent warmer-than-normal weather (weeks 1, 2, 3, 4, 5). The warm and dry (weeks 1, 2, 3, 4, 5) weather with the ridges helped the development and spread of large wildfires in the Northwest and California, but rain from troughs and fronts at mid-month helped reduce the wildfire threat by the end of the month. The net change in drought area was a contraction of the moderate to exceptional national drought footprint compared to the end of July.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The fronts and low pressure systems from the stream of upper-level troughs gave the U.S. the third highest number of days with precipitation CEI component, but the competition between the upper-level systems muted the other components, resulting in a mid-range aggregated national CEI. On a regional basis, however, the highest warm minimum temperature component, ninth highest days with precipitation component, and twelfth highest warm maximum temperature component helped give the Northwest region its sixth most extreme CEI. The third most extreme drought and warm minimum temperature components contributed to the ninth most extreme CEI for the West region. With mixed temperature anomaly pattern across the country, the energy needed to cool the nation, as a whole, was mid-range with the August 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 58th highest August REDTI in the 120-year record.

The migratory nature of the ridges and troughs in the upper-level circulation kept the number of tornadoes below normal, with a preliminary count of 30 tornadoes comparing to the average count of 83 tornadoes for August. It also helped to deflect the two North Atlantic tropical systems (Hurricanes Bertha and Cristobal) away from the CONUS.

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

When integrated across the month, the circulation produced a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over the eastern North Pacific, into western Canada, and across Canada to Greenland. The circulation pattern consisted of below-normal 500-mb heights over southern Alaska and the adjacent parts of the Gulf of Alaska, and across parts of the North Atlantic Ocean. The short-wave ridges and troughs that migrated through the westerly flow resulted in a circulation pattern across the CONUS that averaged near normal.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across much of the Southwest to Northern Rockies and Northern Plains, and across the Central Plains to parts of the Midwest and Mid-Atlantic states. Precipitation was below normal across the extreme West and much of the Southern Plains and Southeast, and parts of the Northeast. The precipitation pattern over Alaska was drier than normal over the west and wetter than normal over the east. The August rainfall pattern was mixed across Hawaii.

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 resulted in a mixed temperature anomaly pattern. Temperatures generally averaged below normal from the Southwest to Northern Plains, and in much of the East, and above normal over the Northwest. August temperatures were near to above normal across California and parts of the Central to Southern Plains, extreme Southeast, and extreme northern New England. Alaska averaged mostly warmer than normal.

Global Linkages: The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. Unlike the CONUS, where short-wave troughs and ridges averaged out the upper-level circulation anomalies, the August 2014 circulation pattern over Eurasia consisted of several large-amplitude deviations in the westerly flow. Above-normal 500-mb heights, associated with ridging in the upper atmosphere, were reflected by above-normal temperatures at the surface over western and eastern Asia. 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 northern Europe and north-central Asia.


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 August 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in August was related to the normal chaotic nature of summer convection, but there were hints that the jet stream and ocean-atmosphere interactions over the North Pacific, Arctic, and North Atlantic Oceans played a role, and the equatorial Pacific may have exerted some limited influence. The MJO was mostly weak or incoherent and the PNA and WP showed little correlation. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state. A weak to moderate El Niño is forecast by the NOAA Climate Prediction Center to develop in the next few months, and there was even some similarity in the August temperature and precipitation patterns to those expected with an El Niño. But there was also some similarity in the August precipitation pattern to that expected with a negative AO and a negative NAO. The August circulation pattern had some similarity to the circulation patterns expected with the Pacific indices, EP-NP and PT, but they seemed to be shifted from what was expected. Likewise, the August temperature pattern had some agreement with that expected with a positive PT, but it, too, seemed shifted in places. The teleconnections for all of the indices are weak this time of year, especially for precipitation and temperature, indicating that random atmospheric processes (i.e., convection from solar heating) play an important role during the summer. This month illustrates how the weather and climate anomaly patterns can be the manifestation of normal (random) atmospheric variability, but also 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 August 2014, published online September 2014, retrieved on September 22, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic.