Synoptic Discussion - July 2015


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


July 2015 was characterized by an active upper-level circulation pattern over the contiguous United States (CONUS), with several weather systems moving in the jet stream flow. Warmer- and drier-than-normal weather dominated the Pacific Northwest and the Southern Plains to Southeast. These regions fell under the influence of upper-level ridges caused by the North Pacific (in the Northwest) and North Atlantic (in the South) subtropical high pressure centers. El Niño-enhanced convection and tropical cyclones over the eastern equatorial Pacific funneled moisture into the Southwestern CONUS, contributing to above-normal precipitation and below-normal temperatures in between these high pressure centers. Cool fronts and low pressure systems, moving along the storm track east of the Rockies, brought above-normal precipitation and cooler-than-normal temperatures to much of the Central Plains to Appalachians. As a result, drought expanded in the Southeast and intensified in the Northwest. Nationwide, the month was wetter than normal with temperatures near average. Continued dryness (and heat in the Northwest) set the stage for numerous large wildfires in Alaska and the Pacific Northwest. The North Atlantic High inhibited severe weather in the South while El Niño inhibited the formation of Atlantic tropical cyclones. A near-average number of tornadoes occurred in July, mostly along the storm track east of the Rockies. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather and climate of July 2015 were the result of influences from multiple atmospheric drivers originating over the Tropical and North Pacific, North Atlantic, and Arctic oceans, with the El Niño playing a dominant 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.
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, July is in the heart 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 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 moving into the contiguous United States (CONUS) more frequently.

During July 2015, the North Pacific and North Atlantic subtropical high pressure centers exerted control over the nation's weather, but their influence was tempered by the El Niño in the West and jet stream weather systems in the East. The North Pacific High tended to create an upper-level ridge over the northeastern North Pacific Ocean and western North America. But the moderate to strong El Niño enhanced the formation of tropical cyclones over the eastern tropical Pacific which fed energy and moisture into the southwestern CONUS, and this was manifested as a trough over the Southwest. Weather systems moving in the very active jet stream flow distorted the ridge at the higher latitudes. Consequently, warmer- and drier-than-normal weather dominated much of the Pacific Northwest, but moisture from remnants of Hurricane Dolores, and from jet stream fronts and lows, contributed to wetter- and cooler-than-normal weather over the Southwest and other parts of the West. Washington ended the month with the fourth warmest July in the 1895-2015 record. The persistent heat, dryness, and lack of thick winter mountain snowpack intensified and expanded drought and abnormal dryness and laid the foundation for an explosion of large wildfires across the Far West (maps for July 1, 6, 10, 15, 24, 31) and Alaska. As the month wore on, troughs in the jet stream brought cooler and wetter weather to Alaska which helped shrink the wildfires in the "Last Frontier" State (maps for July 1, 10, 20, 29).

The North Atlantic High (also known as the Bermuda High) brought warmer- and drier-than-normal weather to much of the Southern Plains and Southeast, where drought and abnormal dryness expanded. Louisiana finished the month with the ninth driest and third warmest July on record. El Niño contributed to unfavorable atmospheric conditions (increased vertical wind shear) which inhibited the formation of tropical storms and hurricanes over the Caribbean, Gulf of Mexico, and Atlantic Ocean. The lack of rainfall from tropical systems helped intensify the dryness across the southern CONUS and much of the Caribbean.

The jet stream was unusually active this summer and extended farther south than usual in the eastern CONUS. Cool fronts and low pressure systems moving in the jet stream flow bumped into the Bermuda High, with some fronts penetrating into the Southeast. Showers and thunderstorms along the resulting storm track brought above-normal precipitation to many areas from the Central and Northern Plains to the Ohio Valley and Central Appalachians, and gave Kentucky the wettest July on record. The rain and fronts kept monthly temperatures cooler than normal from the Central Rockies to the Northeast. Severe weather also occurred along the storm track, but with the Bermuda High competing for dominance, only 122 tornadoes occurred (according to preliminary reports), which is slightly below the July average of 134.

When the regional variations of temperature and precipitation anomalies are averaged together across the CONUS, July 2015 ranked as the 14th wettest and 56th warmest July in the 121-year record. The rain mostly fell in areas that were not in drought, so the national drought footprint expanded, with 27.1 percent of the CONUS experiencing moderate to exceptional drought at the end of July, compared to 25.9 percent at the end of June. Record warmth won out over record cold temperatures, but not by much. By the end of the month, there were 3,915 record warm daily high (1,038) and low (2,877) temperature records, which is about one and a half times the 2,662 record cold daily high (2,021) and low (641) temperature records. The REDTI (Residential Energy Demand Temperature Index) for July 2015 ranked 38th highest for July, illustrating how the unusual warmth in the heavily-populated areas of the Northwest and South increased cooling demands while the large area of well-below-normal temperatures in the less-populated Intermountain West had a smaller effect on lowering cooling demands.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Several parts of the U.S. experienced temperature and precipitation extremes during July, including the East North Central region (seventh highest occurrence of heavy one-day precipitation events), Central region (fifth highest days with precipitation and sixth highest Palmer wet spell components), and South region (fifth highest Palmer wet spell and eighth highest warm minimum temperature components), but they were not enough to rank the regional CEI's into the top ten category. When the extremes for all of the components were aggregated together, the two westernmost regions had several extreme components which combined to yield extremely high regional CEI's. The Northwest region had the third most extreme July CEI on record due to the fourth most extreme warm minimum and ninth most extreme warm maximum temperature components, fifth most extreme one-day precipitation component, and eighth most extreme drought component. The West region had the fourth most extreme July CEI on record due to the most extreme days with precipitation component, fourth most extreme drought component, and eighth most extreme warm minimum temperature component. (It's ironic that the days with precipitation component contributed so heavily to the regional CEI. Since this is the dry season for the West region, any precipitation will not have much effect on drought, but any precipitation that falls will be highly unusual in itself simply because it is the dry season.) When aggregated across the nation, July 2015 had only the 26th most extreme national July CEI on record.

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

The movement of low pressure troughs and remnants of tropical cyclones across the southwestern CONUS resulted in a pattern of below-normal 500-mb height anomalies over the southwestern CONUS and adjacent eastern Pacific Ocean. Further north, in spite of the movement of short-wave troughs and lows through the upper-level circulation, a long-wave ridge pattern with above-normal heights dominated across the northeastern Pacific and into the adjacent Pacific Northwest and southwestern Canada, reflecting the influence of the North Pacific High pressure system. A high pressure ridge with above-normal 500-mb height anomalies dominated the Southern Plains, reflecting the influence of the North Atlantic High. Below-normal heights extended from the north-central Pacific, across Alaska and into northwest Canada, and from the northeastern Atlantic to northwest Russia. Above-normal heights covered the Arctic Ocean and Greenland.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Parts of the Pacific Northwest, Southern Plains, Southeast, and Great Lakes regions were drier than normal during July 2015. Precipitation was above normal across much of the West and Central Plains to Ohio Valley. A mixed pattern of precipitation anomalies characterized some of these areas and the rest of the CONUS, as well as Alaska and much of the Hawaiian Islands. July 2015 was the wettest July on record for Kentucky and second wettest for California, although this is the dry season for the Far West and it doesn't take much rain to give California a near-record wet month. July was drier than normal across most of Puerto Rico as an El Niño-enhanced dry spell afflicted much of the Caribbean.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

July 2015 temperatures averaged warmer than normal across parts of the Pacific Northwest, Southern Plains, and Southeast; southern and western coastal Alaska; and most of Hawaii. Temperatures were colder than normal across much of the West and Central Plains to Northeast.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
Northern Hemisphere monthly upper-level circulation pattern and anomalies.
Global Linkages: The upper-level circulation anomaly pattern over North America was part of a long-wave pattern that stretched across the Northern Hemisphere. Above-normal 500-mb heights occurred across the Arctic Ocean and Greenland, with below-normal heights across the North Atlantic and in the north central North Pacific, which corresponds well with the pattern expected with a negative Arctic Oscillation (AO) index. Over Europe and western Asia, below-normal heights at high latitudes and above-normal heights at low latitudes indicated an enhanced jet stream over northern Europe into Asia. The above-normal 500-mb heights were associated with upper-level ridging, above-normal surface temperatures, and below-normal precipitation over the northwestern CONUS, southern Europe, and southwestern Asia. The below-normal 500-mb heights were associated with upper-level troughing, below-normal surface temperatures, and above-normal precipitation over the southwestern CONUS, northern Europe, and northeastern Asia. With large portions of the continents having warmer-than-normal temperatures, and large portions of the equatorial Pacific Ocean (due to El Niño) and eastern North Pacific Ocean having warmer-than-normal sea surface temperatures , the July 2015 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 July 2015 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in July reflected influences from several atmospheric drivers. The El Niño funneled energy and moisture from the eastern equatorial Pacific into the U.S. Southwest and inhibited development of North Atlantic tropical cyclones. The MJO worked in conjunction with (constructively interfered with) El Niño to enhance development of tropical cyclones and other convection in the eastern tropical Pacific which fed moisture into the southwestern CONUS. Most of the indices showed some teleconnection with temperature over some parts of the CONUS during July. The July 2015 temperature anomaly pattern matched well with the teleconnections for all of them (ENSO, MJO, AO, NAO, and WP), indicating that the atmosphere and oceans (Arctic, North Atlantic, North Pacific, and tropical Pacific) worked in conjunction to influence July's temperature pattern over the CONUS. Precipitation is not well-correlated with most of the indices during the summer, but there was good agreement (where teleconnections exist) with the PNA, AO, and NAO, hinting that these drivers had some influence. The AO, NAO, and WP showed reasonable agreement with the circulation anomalies over the oceans and higher latitudes, but not over North America, suggesting that these drivers were important for July's weather over the Northern Hemisphere but that the other drivers (e.g., the tropical Pacific) were influencing the weather over North America more than the North Atlantic, North Pacific, or Arctic drivers were.

This month illustrates how the weather and climate anomaly patterns can reflect the combined influence of several atmospheric drivers (or modes of atmospheric variability), but how those originating from the Tropical Pacific can have a dominating role.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for July 2015, published online August 2015, retrieved on September 3, 2015 from http://www.ncdc.noaa.gov/sotc/synoptic/201507.