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.


Summary


The Earth's ocean-atmosphere system continued in an ENSO-neutral state during August 2018. Like the last several months, the upper-level circulation was quite active this month with shortwave ridges and troughs migrating through the jet stream flow over the contiguous United States (CONUS). All of this activity took place within a bigger picture wherein a large broadscale/long-wave ridge dominated the overarching circulation over the CONUS, except this month troughs tended to counteract the ridge in the central CONUS, lowering the 500-mb heights and creating a trough in the midst of the long-wave ridge. The resulting ridges and above-normal heights along the East and West Coasts were responsible for above-normal monthly temperatures, while the trough in the central CONUS and its associated frequent cold fronts brought near- to cooler-than-normal temperatures to the region stretching from the northern Plains to Southeast. The summer monsoon brought above-normal precipitation to parts of the Southwest, but the upper-level ridge kept most of the West drier than normal and encouraged the spread or persistence of dozens of large wildfires, while the upper-level trough and fronts brought above-normal precipitation to parts of the central and southern Plains, stretching across the Midwest to the Mid-Atlantic and Northeast coast. Hurricane Lane brought drenching rains to much of Hawaii. The rains helped contract drought and abnormal dryness in parts of the Southwest, southern to central Plains, and Northeast, and other parts of the Great Lakes, Mid-Mississippi Valley, and Hawaii. But the ridge and resulting circulation pattern kept rain out of the Pacific Northwest to central Rockies and parts of the northern and southern Plains, Mid-Mississippi Valley, and Great Lakes, where drought and abnormal dryness developed, expanded, or intensified. Drought expansion outpaced contraction, but just barely, so the national drought footprint grew a bit when compared to the end of July. The active troughs and lows, and their associated fronts, generated severe weather every day, but they were working against the broadscale ridge which generally inhibited severe weather, so the total (preliminary) tornado count was just a little below the long-term average. Numerous tropical cyclones developed in the Pacific basin, with several turning north and reaching the mid-latitudes to add their energy and moisture to the westerly circulation. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather during August reflected the influence of atmospheric drivers originating in the Pacific Ocean, with tropical cyclones playing an important part. 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, August marks the end of climatological summer which is the time of year when solar heating is at maximum with the high sun angle, arctic air masses are weakest, and the circumpolar vortex and jet stream have retreated far northward. Polar air masses influence the weather over the contiguous U.S. (CONUS) less, and the warm, dry subtropical high pressure belts dominate the weather.

During August 2018, however, the jet stream wanted to play a little harder than usual. The upper-level circulation was very active, with subtropical high pressure dominating the southern tier states and numerous troughs and low pressure systems migrating across the northern tier states. The upper-level weather systems brought surface fronts and low pressure systems along with them. A few of the fronts penetrated into the western CONUS, but drier- and warmer-than-normal weather was the rule as upper-level ridging dominated the West for much of the month. Monsoon showers gave parts of the Southwest above-normal precipitation, but the western ridging inhibited the monsoon in other parts, resulting in below-normal precipitation for the month. The fronts frequently plunged deep into the Southeast, bringing cooler Canadian air behind them which gave much of the Plains to Southeast a near- to cooler-than-normal month. The fronts and their surface lows tapped moisture from the Gulf of Mexico and Atlantic. The general lifting of the atmosphere along the fronts and with the lows wrung out this moisture across parts of the central to southern Plains and much of the CONUS along and east of the Mississippi, where monthly precipitation totals were above normal. The upward motion with these weather systems also triggered severe weather. Many of the lows and troughs tracked across the Great Lakes. This gave the Northeast a frequent warm southerly air flow. This southerly circulation, and frequent upper-level ridging, resulted in a warmer-than-normal month for the Northeast.

500-mb mean circulation for the CONUS for August 2018
500-mb mean circulation for the CONUS for August 2018.
 500-mb circulation anomalies for the CONUS for August 2018
500-mb circulation anomalies for the CONUS for August 2018.
 Temperature anomalies (departure from normal) for the CONUS for August 2018
Temperature anomalies (departure from normal) for the CONUS for August 2018.
 Precipitation anomalies (percent of normal) for the CONUS for August 2018
Precipitation anomalies (percent of normal) for the CONUS for August 2018.

The long-wave circulation pattern averaged across the entire month shows the complex aggregated effect of the upper-level troughs migrating through the broadscale ridge. Above-normal heights with ridging is evident in the monthly mean along the West and East Coasts and adjacent Pacific and Atlantic Oceans, with inhibited ridging or slight troughing in the central CONUS along with near-average (zero) height anomalies. Monthly temperatures were warmer than normal beneath the ridges in the West and Northeast, especially in the Northeast where the height anomalies were greatest, and near to cooler than normal from the northern Plains to Southeast where the height anomalies were near zero, troughs frequented, and cooler Canadian air dominated. The precipitation anomaly pattern for the month (the wet areas) represented an additive result of rainfall from the individual frontal passages and low pressure systems, as well as convective and monsoon showers. The dry areas resulted from persistence of upper-level ridging or areas that missed convective showers purely by chance. The circulation during this month was also reflected in severe weather, drought, and regional records.

  • The extreme warmth was reflected in the statewide temperature ranks. In addition to being the 17th warmest August on record nationally, August 2018 ranked in the top ten warmest category for 15 states in the Southwest and Northeast. These included the warmest August in the 124-year record for Delaware, Connecticut, Rhode Island, Massachusetts, and New Hampshire. On a statewide basis, California had the tenth driest August while three states (Arkansas, New York, and Pennsylvania) ranked in the top ten wettest category. Nationally, August 2018 ranked as the 20th wettest August in the 1895-2018 record.
  • When daily temperature records are examined, the extreme warmth overwhelmed the extreme cold. When integrated across the month, there were 4,079 record warm daily high (1,320) and low (2,759) temperature records. This was over two times the 1,741 record cold daily high (1,233) and low (508) temperature records.
  • As noted earlier, August marks the end of the Northern Hemisphere climatological summer, the time of year when temperatures reach their peak seasonal warmth. The areally-averaged temperature across the CONUS resulted in the 17th warmest August, nationally. The spatial pattern of temperature anomalies included some near- to cooler-than-normal temperatures in the northern Plains to Southeast, but much of the abnormal warmth occurred in the high-population areas of the Northeast and southern California. When cooling degree days across the country are weighted by population, the result was a national REDTI (Residential Energy Demand Temperature Index) value for August 2018 that ranked as the seventh highest August REDTI in the 124-year record. The August REDTI for much of the last two decades has been well above average, reflecting an increase in energy demand for cooling. The persistent warmth of the last three months resulted in the fourth warmest June-August on record. The 3-month REDTI for June-August 2018 also ranked as the fourth highest, due in part to large temperature anomalies and ranks being concentrated in the Southwest, South, Midwest, and Northeast. The summer REDTI for the last two decades has equaled or exceeded the REDTI warmth (reflecting cooling demand) at the peak of the 1930s and 1950s heat waves.
  • Some of the precipitation during August fell on areas that were in drought at the end of July and contracted drought and abnormal dryness, while other drought areas continued quite dry. Drought and abnormal dryness developed, expanded, or intensified in the Pacific Northwest to central Rockies and parts of the northern and southern Plains, Mid-Mississippi Valley, Great Lakes, Hawaii, and Puerto Rico, while it contracted in parts of the Southwest, southern to central Plains, and Northeast, and other parts of the Great Lakes, Mid-Mississippi Valley, and Hawaii. Expansion outweighed contraction, but just barely, so at the national level drought expanded from 34.1 percent of the CONUS at the end of July to 34.4 percent of the CONUS at the end of August (from 29.0 percent to 29.2 percent for all of the U.S.).
  • The hot and dry weather in the West maintained conditions favorable for the continuation of wildfires. More than a hundred large wildfires burned across much of the West (wildfire maps for August 1, 8, 15, 22, 30) and Alaska (wildfire maps for August 1, 8, 18, 30) throughout the month. Large wildfires also burned in parts of Texas, especially near the end of the month.
  • The atmospheric circulation needed to create the instability and dynamics favorable for severe weather consists largely of a southwesterly flow across the central part of the CONUS, which funnels moist Gulf of Mexico air and its latent heat energy into the mix. Surface fronts provide additional atmospheric lifting. The changing direction of the circulation around surface lows and the upper-level troughs and lows above them adds spin to the rising air, which enhances the formation of tornadoes. This upper-level circulation pattern occurred frequently as the short-wave troughs and closed lows, and their associated surface fronts and lows, moved across the country, with some type of severe weather (hail, strong winds, or tornadoes) occurring every day. But these weather systems were fighting against the broadscale ridge and its predominantly descending stable air, so the number of tornadoes for August 2018 (73 based on preliminary data) was less than the August average of 83, but not by much. The tornadoes mostly occurred from the east of the Rockies as the fronts and lows traversed these areas.

Typically tropical cyclone activity is enhanced in the Eastern North Pacific and inhibited in the North Atlantic during El Niños, and inhibited in the Eastern North Pacific and enhanced in the North Atlantic during La Niñas, due mostly to changes in vertical wind shear during the two extreme events. The relationship is unclear during ENSO-neutral events. Warm sea surface temperatures (SSTs) fuel tropical cyclones while vertical wind shear tears them apart. The tropical Pacific Ocean was in an ENSO-neutral state during August 2018.

  • The Atlantic hurricane season runs from June 1st through November 30th and the Eastern North Pacific (ENP) hurricane season runs from May 15th through November 30th.
  • Three tropical cyclones (Tropical Storms Debby and Ernesto, and Tropical Depression 6) developed in the North Atlantic basin during August 2018. Debby developed as a subtropical (extratropical) storm early in the month in the central portions of the North Atlantic, took on tropical characteristics to become Tropical Storm Debby, then quickly merged with the mid-latitude circulation after just a day or two. Like Debby, Tropical Storm Ernesto developed in the central portions of the North Atlantic at mid-month as a subtropical depression and, like Debby, Ernesto quickly moved north to merge with a frontal system a couple days later. Tropical Depression 6 formed at the end of the month off the coast of Africa south of the Cape Verde Islands; during September, TD 6 would eventually intensify into Hurricane Florence.
  • Eight tropical systems were active in the Eastern North Pacific basin during August 2018. All developed in the eastern tropical Pacific some distance off the coast of Mexico. Three (Tropical Storm Ileana, Hurricane John, Tropical Storm Kristy) stayed in the eastern North Pacific, tracking northwest parallel to the Mexican coast. Three of the eight systems (Hurricanes Hector, Lane, and Miriam) took a westerly track along the southern periphery of the North Pacific High, eventually moving into the Central North Pacific basin. Two (Hurricane Norman and Tropical Disturbance 91E) developed near the end of the month and were still active into early September. Ileana hugged the coast of Mexico and moved north until it was torn apart by the larger nearby Hurricane John. John and Kristy eventually fell apart when they moved into an unfavorable environment (cooler waters, dry stable air mass, and wind shear). Like Hector, Lane, and Miriam, Norman was being steered west along the southern periphery of the North Pacific High. Tropical Disturbance 91E, which formed off the Mexican coast on August 31, later developed into Tropical Depression 17E then Hurricane Olivia in early September.
  • Five tropical systems were active in the Central North Pacific basin during August. Three (Hector, Lane, and Miriam) were hurricanes which developed in the eastern tropical Pacific and moved westward, steered by the North Pacific High. Two others (Tropical Disturbances 94C and 95C) were short-lived disturbances which formed southwest (94C) and southeast (95C) of Hawaii and quickly dissipated. Hurricane Hector moved south of Hawaii, steered westward by the North Pacific High, and eventually turned north along the southwestern periphery of the North Pacific High to become absorbed in the westerly circulation of the mid-latitudes. Hurricane Lane moved south of Hawaii, steered by the North Pacific High, until it was pulled northward toward Hawaii by an upper-level trough that was northwest of the state. Lane weakened as it neared Hawaii, but still inundated the state with heavy rain before it moved further west. Lane was eventually absorbed by a large extratropical low in the central North Pacific. Hurricane Miriam moved west, under the influence of the North Pacific High, then was pulled north by an upper-level trough or low in the westerlies, but stayed east of Hawaii. Miriam was eventually dissipated in early September by strong vertical wind shear east of an upper-level trough.
  • There was a lot of tropical activity in the western tropical Pacific during August, with ten tropical cyclones and one tropical disturbance. Several of them (Typhoons Jongdari, Shanshan, Leepi, Soulik, Cimaron, and Jebi, and Tropical Storm Yagi) were active in or near the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia). Those which formed in or near the USAPI tracked westward along the southern periphery of the North Pacific High then turned in a northerly or northwesterly direction along the western edge of the North Pacific High, with some (Shanshan, Soulik, Cimaron, Jebi) reaching the mid-latitudes and becoming absorbed in the westerly circulation. Typhoon Jongdari, which formed in July within USAPI waters, had weakened into a tropical depression over Japan by the 1st of August. It meandered west across the East China Sea over the next couple days before making landfall over China. Tropical Storm Yagi formed as a depression near the western USAPI, eventually tracking northwestward to make landfall over China. Typhoon Shanshan formed over north central USAPI waters and tracked just north of the Marianas Islands as a typhoon in early August. After side-swiping Japan, Shanshan turned northeastward and added its energy and moisture to the mid-latitude westerly flow. Typhoon Leepi also formed near the Marianas about a week after Shanshan and tracked north of the island chain as a tropical storm. Leepi moved across southern Japan before falling apart over South Korea. Typhoon Soulik formed at mid-month over eastern Yap State waters in the Federated States of Micronesia (in the central USAPI), and moved over the Marianas as it was strengthening into a tropical storm. Typhoon Soulik tracked south of Japan before turning north and crossing South Korea. Soulik became embedded in the westerly flow and was absorbed into a cold front over the Sea of Japan. Typhoon Cimaron had its origin at mid-month as a tropical disturbance just north of the Marshall Islands in the eastern USAPI and tracked westward as a tropical depression in the northern USAPI waters before strengthening into a tropical storm east of the Marianas. Cimaron tracked across the northern Marianas at severe tropical storm to typhoon strength, struck southern Japan as a typhoon, then got swept up in the westerly flow and tracked quickly eastward as an extratropical low associated with a cold front. Typhoon Jebi, like Cimaron, originated and tracked across the northern USAPI waters near the end of the month. Jebi reached typhoon strength east of the Marianas and tracked northwestward across the far northern Marianas Islands as a Super Typhoon (category 5 storm) as the month ended. Jebi struck southern Japan as a typhoon in early September and became absorbed in a mid-latitude trough, adding its energy and moisture to the westerly flow. Other tropical cyclones which formed well west of the USAPI included Tropical Storm Bebinca, which made landfall on Vietnam; Tropical Storm Rumbia, which formed east of Taiwan and made landfall on China; and Tropical Depression 24W, which met its end west of Taiwan over China. Tropical disturbance 93W formed near Okinawa but dissipated a few days later.
North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern during August, when averaged for the month, consisted of high pressure across the southern tier states with ridging centered in the Southwest and above-normal 500-mb heights along the West and East coasts. A slight trough with near-zero 500-mb height anomalies dominated the central CONUS east of the Rockies. A trough with below-normal heights was anchored over northern Canada.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Monthly precipitation was drier than normal across much of the West and northern Plains, parts of the central and southern Plains, and southern and eastern Puerto Rico. August was wetter than normal across much of the Mississippi Valley to East Coast, parts of the southern and central Plains and Southwest, most of Hawaii and Alaska, and northwest Puerto Rico.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

Monthly temperatures were warmer than normal across the Northeast, Great Lakes, much of the West, and southern and western Alaska. August temperatures were near to cooler than normal from the northern Plains to Southeast, and in northern Alaska.

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. The sinusoidal nature of the jet stream was evident in east to west couplings of ridges and troughs, but above-normal height anomalies (representing stronger-than-normal ridges) were more prevalent than below-normal height anomalies (representing stronger-than-normal troughs, or weaker-than-normal ridges). There was just one region of below-normal height anomalies in the mid-latitudes — over the Kamchatka Peninsula north of Japan — a region where many Pacific tropical cyclones were absorbed into the westerly flow — compared to four regions of above-normal height anomalies (over the eastern Pacific/western North America, eastern North America/western Atlantic, western Russia, and northern China/Mongolia). A polar view of height anomalies reveals mostly above-normal heights at mid-latitudes surrounding a partial ring of below-normal heights over northern Canada, Iceland, and the Kamchatka area, with above-normal heights intruding into the Arctic from northeastern Russia.

The upper-level circulation and its anomalies are associated with the Sea Level Pressure (SLP) pattern and its anomalies which reflect the semi-permanent centers of action of SLP. The above- and below-normal upper-level height anomalies appear to be associated with above- and below-normal SLP anomalies. The locations of these anomalies with respect to the long-term climatology suggests that the North Pacific High was stronger than normal and shifted to the north, weakening the Aleutian Low (although this is the time of year when the Aleutian and Icelandic Lows are their weakest). The North Atlantic High seemed stronger than normal and extended more to the west (over the eastern CONUS) and to the east (over Europe); the Icelandic Low also seemed a bit stronger than normal.

During August 2018, above-average sea surface temperature (SST) anomalies dominated the North Pacific, while in the North Atlantic there was a dipole of above-average SST anomalies off the North American coast and below-normal SST anomalies south of Iceland. The map of the change in SST anomalies from the end of July to the end of August showed significant cooling in the vicinity of Japan — where many of the Pacific tropical cyclones transitioned into extratropical lows and became absorbed into the westerlies — and warming southeast and northwest of there, and cooling south of the Gulf of Alaska; warming SST anomalies seemed to dominate much of the North Atlantic.

The above-normal 500-mb heights were associated with upper-level ridging, or with weakened troughs, at the mid-latitudes; below-normal precipitation (over the western CONUS, much of Europe, and northeast Russia); above-normal surface temperatures (over western and eastern North America, most of Europe, and China to northeast Russia); and warm SST anomalies that were getting warmer (across much of the North Atlantic and North Pacific). The areas of below-normal 500-mb heights were associated with upper-level troughing, or with weakened ridges; near- to below-normal surface temperatures (over central North America and central Russia); cool SST anomalies (in the northern North Atlantic); and above-normal precipitation (over eastern CONUS and central Russia). Parts of all of the continents were near to cooler than normal, and parts of the Pacific, Atlantic, and Indian Oceans had cooler-than-normal SST anomalies. But with most of Africa and Eurasia, and much of South America, having warmer- to much-warmer-than-normal temperatures, and large portions of the Atlantic, Pacific, and Indian Oceans having warmer-than-normal sea surface temperatures, the August 2018 global temperature was still 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 August 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, indicates a weak correlation to any of the atmospheric drivers. The equatorial Pacific was in an ENSO-neutral state and did not have an influence; the MJO was incoherent during part of the month, but the temperature and precipitation patterns did not match the phases it was in during the other part, so the MJO did not appear to be influential; the PNA, EP-NP, and PT teleconnections did not match the observed anomalies and thus did not have an influence. The AO and NAO circulation teleconnections seemed to match the observed circulation anomalies, in large part, but anomaly agreement was not manifested in the temperature or precipitation fields. There was some agreement for the NAO, but there was also disagreement so the agreement may have been a coincidence. For the AO, the teleconnections are strongest in winter and weakest in summer. For the WP, there are no precipitation teleconnections in August, but the circulation and temperature teleconnections generally matched the observed anomalies, which suggests the atmospheric drivers originating in the mid-latitude North Pacific had the greatest influence on the August weather in the CONUS. This is consistent with analyses by CPC in which several Pacific tropical cyclones migrated northward later in their life span to be absorbed in the mid-latitude circulation; with the energy and moisture from the remnants of these tropical cyclones added to the westerly jet stream flow, they likely had a downstream effect on the weather over North America.

This month illustrates how the atmospheric circulation for the month can reflect the influence of atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific, especially the influence of tropical cyclones on the mid-latitude circulation.

Citing This Report

NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for August 2018, published online September 2018, retrieved on July 27, 2024 from https://www.ncei.noaa.gov/access/monitoring/monthly-report/synoptic/201808.