Synoptic Discussion - July 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


July is in the middle of climatological summer in the Northern Hemisphere, which is the season of maximum solar heating and warmest temperatures. This season is normally characterized by a contraction of the circumpolar vortex and dominance of the subtropical high pressure centers, but July 2014 was characterized by an active jet stream with high amplitude meridional flow. Cooler-than-normal temperatures dominated the central and eastern U.S. beneath a strong upper-level long-wave trough, while warmer- and drier-than-normal weather prevailed across much of the West due to an intense upper-level ridge. Monsoon showers dumped moisture over the southwestern U.S., while weather systems moving through the eastern trough brought above-normal rainfall to parts of the Southern Plains and East Coast. The upper-level circulation pattern inhibited precipitation over the Northern and Central Plains to Midwest, reduced the occurrence of severe weather, and contributed to large wildfires across the West. Drought contracted where it rained, and expanded where it didn't, resulting in a national drought footprint about the same as a month ago. The upper-level circulation pattern suggested the combined influence of the East Pacific-North Pacific (EP-NP) and North Atlantic (NAO) modes of variability associated with conditions in the North Pacific and North Atlantic oceans. The warmer-than-normal temperatures in the West can be traced to the NAO's teleconnections while the cooler-than-normal temperatures east of the Rockies can be related to the EP-NP's teleconnections. 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, July is in the middle of climatological summer which is the time of year when solar heating 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 previous 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 July 2014, the North Pacific subtropical high pressure center exerted influence over the western states and occasionally the southern tier states, but a strong upper-level trough dominated the eastern CONUS. A vigorous upper-level circulation sent several weather systems rippling across the northern and eastern CONUS throughout the month.

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 western ridge produced over 2500 (2596) record warm daily high (720) and low (1876) temperature records, but the eastern trough generated more than twice as many (5508) record cold daily high (3333) and low (2175) temperature records. This was reflected in the national monthly average temperature which ranked July 2014 as the 48th coolest July in the 1895-2014 record. The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The cold air masses were so persistent and extreme that they gave the Central region the second most extreme CEI component rank in the 1910-2014 record for the cold maximum and cold minimum temperature components. The East North Central region had the fourth most extreme cold maximum temperature component on record. On the other hand, the western ridge gave the Northwest region the second most extreme warm minimum temperature component and third most extreme warm maximum temperature component, and the West region the second most extreme warm minimum temperature component and eighth most extreme warm maximum temperature component. With below-normal temperature anomalies centered over the high population areas of the central and eastern CONUS, the energy needed to cool the nation, as a whole, was down with the July 2014 REDTI (Residential Energy Demand Temperature Index) ranking as the 33rd lowest July REDTI in the 120-year record.

The western ridge and eastern trough set up a predominantly northwest flow in the upper atmosphere over the central CONUS, which inhibited the development of severe weather. The preliminary count of 85 tornadoes ranked below the average count of 134 tornadoes for July.

The dry and stable air masses delivered by the upper-level northwest flow also resulted in drier-than-normal weather for much of the Northwest, Northern Rockies, Northern and Central Plains, and Midwest. Short-wave weather systems traversing the upper-level flow tapped into Gulf of Mexico moisture as they reached the Southern Plains, dropping above-normal precipitation which helped contract drought areas. As the weather systems curved northward on the other side of the long-wave trough, they generated areas of above-normal rainfall along the East Coast. Hurricane Arthur contributed moisture to the region, especially the Northeast, as it merged with a cold front along the coast. Meanwhile, hot and dry weather, along with low humidities, dominated the West, especially parts of the Northwest, beneath the upper-level ridge. This contributed to the expansion of drought areas and the development of large wildfires across the West, especially in the Northwest. Monsoon showers brought above-normal precipitation to parts of the Southwest, helping to contract drought, especially in New Mexico. The net change in drought area was a slight expansion in the moderate to exceptional national drought footprint compared to the end of June.

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 western North America and below-normal 500-mb heights (stronger-than-normal long-wave trough) over eastern North America centered over the Great Lakes.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across much of the Southwest and Northeast and parts of the Southern Plains and East Coast. Precipitation was below normal across the rest of the West and much of the Plains, with patchy dryness from the Midwest to the Southeast. The precipitation pattern was mixed across Alaska, but with wet conditions more evident. The month was generally wetter than normal across Hawaii.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The dominant long-wave circulation pattern of western ridge/eastern trough funneled polar air masses into the CONUS east of the Rockies, resulting in monthly temperature anomalies that were colder than normal in that region, while warmer-than-normal temperatures dominated the West. Alaska averaged mostly colder than normal.

The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The Northern Hemisphere westerlies during July were characterized by a high amplitude meridional pattern over Eurasia as well as over North America. As with North America, unusually warm and dry weather in Eurasia was associated with the ridges, and unusually cold weather occurred in association with the troughs.


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 July 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in July 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 Ocean and North Atlantic Ocean played a role, and the equatorial Pacific may have exerted some limited influence via the MJO mechanism. ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state, which was affecting the MJO. Even though the MJO was weak or incoherent during most of the month, it appeared to have exerted some influence on temperature and precipitation. The PNA, AO, and WP indices showed very little correlation to the July weather patterns. The teleconnections for the EP-NP match July's upper-level circulation and temperature patterns in the eastern CONUS, while the teleconnections for the NAO match July's upper-level circulation and temperature patterns in the western CONUS and precipitation patterns in parts of the central CONUS, but the value of the NAO index indicated it was very weak. The precipitation teleconnections for all of the indices are very weak this time of the year, indicating that random atmospheric processes (i.e., convection from solar heating) played an important role for July precipitation. 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 July 2014, published online August 2014, retrieved on August 29, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/.