Synoptic Discussion - February 2018


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 a weakening La Niña state during February 2018. Like the last couple months, the upper-level circulation pattern experienced several shifts over the contiguous United States (CONUS) during February, changing from a ridge/trough pattern to a trough/ridge pattern. This caused extremes in temperatures, with much warmer-than-normal temperatures during some weeks and colder-than-normal temperatures other weeks, with the month ending on a warm note nationally. This variability resulted in a monthly temperature anomaly pattern with warm anomalies dominating in the East and Southwest, and cold anomalies dominating in the central to northern Plains and other parts of the West. The variable circulation pattern resulted in above-normal precipitation across the northern Rockies to Great Lakes and part of the Southwest, with stalled and slow-moving frontal systems bringing much-above-normal precipitation from eastern Texas to the southern portions of the Northeast. But the circulation pattern kept most of the West and central to southern High Plains, and coastal Southeast, drier than normal. The active fronts and low pressure systems generated above-average severe weather during this normally low-severe-weather winter month. Wildfires were a problem, especially in the severely dry parts of the southern Plains and Southwest. Although widespread flooding resulted in the Mississippi to Ohio Valleys from the heavy rains, much of the precipitation fell over drought areas, especially in parts of the Plains, Alaska, and Hawaii, and much of the Mississippi Valley and Southeast to Northeast, so the national drought footprint shrank when compared to the beginning of February. The upper-level circulation, temperature, and precipitation anomaly patterns suggested that the atmospheric drivers originating in the equatorial Pacific (i.e., La Niña and MJO), North Pacific (PNA), North Atlantic (NAO), and Arctic (AO) had a controlling influence on the month's weather. 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, February marks the end of climatological winter which is the time of year when net solar heating is at its minimum due to the low sun angle, and an expanded circumpolar vortex results in the furthest southern extent of the jet stream. Polar air masses normally dominate the weather over the contiguous U.S. (CONUS), and the warm, dry subtropical high pressure belts have their least influence.

500-mb mean circulation for the CONUS for February 1-8, 2018, showing a long-wave ridge in the West and trough in the East
500-mb mean circulation for the CONUS for February 1-8, 2018, showing a long-wave ridge in the West and trough in the East.
500-mb mean circulation for the CONUS for February 9-14, 2018, showing a transitional pattern to a long-wave trough in the West and ridge in the East
500-mb mean circulation for the CONUS for February 9-14, 2018, showing a transitional pattern to a long-wave trough in the West and ridge in the East.
500-mb mean circulation for the CONUS for February 15-28, 2018, showing a long-wave trough in the West and ridge in the East
500-mb mean circulation for the CONUS for February 15-28, 2018, showing a long-wave trough in the West and ridge in the East.

The upper-level circulation was very active this month and experienced two general (or "long-wave") regimes. During roughly the first week of the month, an upper-level ridge dominated the western CONUS with a trough over the East. The relative positions of the ridge axis and trough axis shifted day by day as short-wave troughs or lows moved through the upper-level flow. During the last two weeks, an upper-level trough dominated the West with a ridge over the East. The week in between these two regimes consisted of a transition period with upper-level systems distorting the long-wave pattern. Short-wave troughs and ridges migrated through the upper-level flow throughout the month, dragging surface fronts and low pressure systems with them. Each of these patterns was characterized by specific weather phenomena, temperature anomalies, precipitation anomalies, and impacts.


500-mb circulation anomalies for the CONUS for February 1-8, 2018
500-mb circulation anomalies for the CONUS for February 1-8, 2018.
Temperature anomalies (departure from normal) for the CONUS for February 1-8, 2018
Temperature anomalies (departure from normal) for the CONUS for February 1-8, 2018.
Precipitation anomalies (percent of normal) for the CONUS for February 1-8, 2018
Precipitation anomalies (percent of normal) for the CONUS for February 1-8, 2018.

The western ridge during roughly February 1-8 deflected storm systems away from the West and kept skies clear for the most part. Very little to no precipitation fell across most of the West and temperatures were warmer than normal. Pacific weather systems rode over the top of the ridge, bringing above-normal precipitation to the northern Rockies and northern Plains. Canadian air masses were funneled through the Plains and into the Southeast beneath the eastern trough, bringing colder-than-normal weather to most of the CONUS east of the Rockies. The air masses were generally dry and the northwesterly flow blocked Gulf of Mexico moisture, so the southern Plains were mostly drier than normal. But low pressure systems developed along the fronts as they lay across the Southeast, tapping Gulf of Mexico moisture and bringing above-normal precipitation to much of the area from the Lower Mississippi Valley to the Appalachians and Northeast. With below-freezing Canadian air, precipitation falling along the fronts and low pressure systems laid down a blanket of snow across the central to northern Plains, extending to the Northeast, while much of the West had a sorely deficient mountain snowpack.


500-mb circulation anomalies for the CONUS for February 9-14, 2018
500-mb circulation anomalies* for the CONUS for February 9-14, 2018.
Temperature anomalies (departure from normal) for the CONUS for February 8-14, 2018
Temperature anomalies (departure from normal) for the CONUS for February 8-14, 2018.
Precipitation anomalies (percent of normal) for the CONUS for February 8-14, 2018
Precipitation anomalies (percent of normal) for the CONUS for February 8-14, 2018.

The upper-level circulation pattern shifted during roughly February 9-14, with the trough migrating westward and weather systems beginning to favor the Southwest. Canadian fronts still plunged southward across the Plains and into the Southeast to East Coast, but Pacific fronts and weather systems moved across the West as well. The Canadian air masses kept temperatures below normal across the Plains to Great Lakes during this period; temperatures moderated across the West but still averaged warmer than normal; and a more southerly flow aloft favored warmer-than-normal temperatures in the Southeast. The Four Corners States benefited from Pacific moisture, but the lows that moved across the West mostly came in from a more northerly direction, which meant they were a bit drier than otherwise, so most of the West was still drier than normal. Upslope precipitation occurred as the Canadian fronts ran into the east slopes of the Rockies, and this was enhanced by passing upper-level weather systems, so the northern High Plains of Montana to South Dakota and Nebraska experienced above-normal precipitation. But the rest of the Plains and parts of the Midwest were cut off from the Gulf moisture source, so they were drier than normal. As before, when the fronts reached the Southeast, they slowed down and drew in Gulf of Mexico moisture, resulting in above-normal precipitation from the Lower Mississippi Valley to Northeast. Also as before, the below-freezing Canadian air allowed precipitation to fall as snow, keeping a snowpack across the central to northern Plains and extending into the Northeast. And some of the Pacific systems were cold enough to drop badly-needed snow across parts of the West.


500-mb circulation anomalies for the CONUS for February 15-28, 2018
500-mb circulation anomalies for the CONUS for February 15-28, 2018.
Temperature anomalies (departure from normal) for the CONUS for February 15-28, 2018
Temperature anomalies (departure from normal) for the CONUS for February 15-28, 2018.
Precipitation anomalies (percent of normal) for the CONUS for February 15-28, 2018
Precipitation anomalies (percent of normal) for the CONUS for February 15-28, 2018.

The upper-level trough struggled to maintain its grip across the West during the last two weeks of the month, with short-wave troughs and lows enhancing the long-wave trough as they migrated through the upper-level flow. The frequent passage of Pacific weather systems, along with their cloudiness and cooler air, left the West with below-normal temperatures. While they brought precipitation with them and increased the western mountain snowpack, only some parts of the West were wetter than normal — California especially missed out on the precipitation, racking up the third driest February in the 1895-2018 record. The fronts tended to stall out along the Mississippi and Ohio Valleys as they ran up against the ridge in the East. With a southerly flow feeding abundant Gulf of Mexico moisture into the region, low pressure systems moving along the stalled fronts generated widespread flooding rains and even outbreaks of severe weather. Above-normal precipitation occurred along this storm track from eastern Texas and eastern Oklahoma, to the Midwest and southern portions of the Northeast. The coastal Southeast, as well as the central to southern High Plains, were outside the storm track and ended up drier than normal. In addition to Gulf of Mexico moisture, much warmer tropical air was channeled northward across the eastern CONUS beneath the upper-level ridge, resulting in record to near-record warm temperatures. Below-freezing Canadian air masses kept the snowpack in place across the northern Plains, where temperatures were below normal, but the warmer air to the south and east greatly contracted the snowpack elsewhere east of the Rockies.


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

When conditions are averaged across the entire month, a western trough/eastern ridge pattern was evident in the upper-level circulation with somewhat of a positive tilt into the northern Plains. The circulation anomalies reflected the trough/ridge pattern, with below-normal height anomalies over the northern Plains to Pacific Northwest and above-normal height anomalies across the East Coast to Gulf Coast. The anomaly pattern also had a positive tilt, with the negative anomalies extending into Canada and centered over the northern part of Hudson Bay. The monthly temperature anomaly pattern reflected the monthly circulation anomaly pattern, with above-normal temperatures in the East beneath the positive height anomalies and below-normal temperatures across the northern and central Plains to western Great Lakes associated with the below-normal height anomalies. The monthly temperature anomalies across the West represented an average of the warm temperatures associated with the ridge at the beginning of the month and the cool temperatures associated with the trough during the last half of the month. The precipitation anomaly pattern for the month represented an additive result of the anomaly patterns for the various weeks, with the areas that were much wetter than normal during one or more weeks showing up wet at the monthly time scale, and those areas (especially the Far West and the central to southern High Plains) that were persistently dry every week showing up as quite dry at the monthly scale. The circulation during this month was also reflected in snow, drought, and regional records.

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. The tropical Pacific Ocean continued in a La Niña state during February 2018, but the La Niña was weakening.

  • The Atlantic hurricane season runs from June 1st through November 30th and the Eastern North Pacific hurricane season runs from May 15th through November 30th. No tropical storms or hurricanes developed in these basins during February 2018. No tropical cyclones formed in, or moved into, the central North Pacific.
  • A tropical storm, a tropical cyclone, and a tropical disturbance developed in the western North Pacific and South Pacific in or near the U.S.-Affiliated Pacific Islands (USAPI). Tropical Storm Sanba developed in western Micronesia in early February. Sanba passed near Koror in the Republic of Palau and crossed the Philippines before dissipating at mid-month. Another tropical disturbance (95W) developed in western Yap State (in the Federated States of Micronesia) and southern Palau during the last half of the month, but quickly dissipated. In the South Pacific, Tropical Cyclone Gita developed in early February and brushed American Samoa before intensifying to severe tropical cyclone status. Gita moved south away from the Samoan Islands, its remnants eventually being swept up in the Southern Hemisphere's mid-latitude circulation at mid-month.
Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

February was drier than normal across much of the West, from the southern High Plains to central Plains, along the coastal Southeast, in parts of the northern Plains, and in southeast Alaska. The month was wetter than normal across the northern Rockies and most of the northern Plains, parts of the central to southern Rockies, and most of the country from eastern Texas and eastern Oklahoma, across the Mississippi and Ohio River Valleys, to the Northeast. Hawaii and most of Alaska were wetter than normal. San Juan, Puerto Rico, was wetter than normal.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

Monthly temperatures were warmer than normal from the Lower Mississippi Valley, Ohio Valley, and eastern Great Lakes, to the East Coast, in parts of the Southwest, and western to northern Alaska. February was colder than normal across the western Great Lakes, northern to central Plains, parts of the West, and across southeastern 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. There were three centers of negative height anomalies and three centers of positive height anomalies. These anomaly centers were coupled together both longitudinally (east/west) and latitudinally. North America had a ridge/trough/ridge pattern, while Eurasia had a trough/ridge/trough pattern. The anomaly patterns were also paired across the Arctic Ocean — eastern CONUS-Canada-Arctic Ocean-eastern Asia, and eastern North Pacific-Arctic Ocean-central Asia. Especially notable was the shift of the circumpolar vortex southward over North America and Europe (below-normal height anomalies) with the above-normal height anomalies over much of the Arctic Ocean. The above-normal 500-mb heights were associated with upper-level ridging, or with weakened troughs, at the mid-latitudes; below-normal precipitation (over central Asia and northern Scandinavia); below-normal snow cover (over the eastern CONUS and southern Asia); above-normal surface temperatures (over the eastern CONUS, southern Asia, and northeastern Siberia to western Alaska); and warm SST anomalies (in the Bering Sea, western North Atlantic, and parts of the North Pacific, South Pacific, and Indian Oceans). 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 Canada to the north central CONUS, Western Europe to northwestern Africa, and northeastern China); cool SST anomalies (in parts of the northern and eastern North Atlantic); above-normal precipitation (over the north central CONUS to adjacent southwestern Canada, and southeastern Europe); and above-normal snow cover (over the north central CONUS and eastern Europe). Parts of central North America, northwestern Africa, western Australia, and much of Asia were near to cooler than normal, and parts of the equatorial Pacific, eastern South Pacific, South Atlantic, and Indian Ocean had cooler-than-normal SST anomalies. But with parts of eastern and northwestern North America, southern and northeastern Asia, the eastern half of Australia, and much of South America and Africa 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 February 2018 global temperature was still 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:


MJO agreement with the temperature and precipitation anomaly patterns, by week.
Week Temperature Precipitation
Jan 28 - Feb 3
February 4-10 MJO
February 11-17 MJO MJO
February 18-24
Feb 25 - Mar 3 MJO MJO

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed February 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in February was influenced by a variety of equatorial and mid-latitude atmospheric drivers, possibly acting in concert. La Niña, PNA, AO, NAO, and TNH all had most teleconnections in agreement with the monthly circulation, temperature, and precipitation anomaly patterns in most parts of the CONUS and, in some cases, most of the North Pacific-North America-North Atlantic region. The MJO agreed during some weeks (see adjacent table). The WP matched for precipitation, but not temperature or circulation, which suggests the precipitation match may have been a coincidence. The EP-NP did not appear to be a player this month. For the precipitation teleconnections, La Niña, MJO, PNA, AO, TNH, WP, and even the EP-NP all had above-normal precipitation teleconnections for some or all of the Arkansas to Ohio Valley region.

This month illustrates how the atmospheric circulation for the month can reflect several atmospheric drivers (or modes of atmospheric variability) in a way that the drivers in the equatorial Pacific, North Pacific, North Atlantic, and Arctic Ocean appear to have been harmoniously synchronized. This random congruence or positive reinforcement of drivers might explain why some extreme weather events (such as record precipitation and widespread flooding) occasionally occur.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for February 2018, published online March 2018, retrieved on June 19, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201802.

Metadata