Synoptic Discussion - December 2017


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 La Niña state during December 2017. The upper-level circulation pattern experienced several shifts over the contiguous United States (CONUS) during the month, changing back and forth between a westerly zonal flow and ridge/trough 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 very cold note. This variability resulted in a confused-looking monthly temperature anomaly pattern which masked the weekly extremes. Warmer-than-normal temperatures slightly dominated at the monthly scale, especially from the Southwest to northern Plains. Both the westerly pattern, and the northwesterly flow over the central CONUS associated with the ridge/trough pattern, blocked moist air masses, so drier-than-normal conditions dominated the month; they also inhibited the development of severe weather. The lack of precipitation resulted in a significant expansion of drought and abnormal dryness, especially from the Southwest to Mid-Mississippi Valley, and from the Southeast to Northeast. 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), Arctic (AO), and Hudson Bay (TNH) had a controlling influence on the month's weather, some throughout the entire month (the La Niña "base state") and others during various weeks (MJO, PNA, AO, TNH). 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, December is the first month of climatological winter which is the time of year when 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 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 December 1-5, 2017, showing a zonal flow
500-mb mean circulation for the CONUS for December 1-5, 2017, showing a zonal flow.
500-mb mean circulation for the CONUS for December 6-12, 2017, showing a long-wave ridge in the West and trough in the East
500-mb mean circulation for the CONUS for December 6-12, 2017, showing a long-wave ridge in the West and trough in the East.
500-mb mean circulation for the CONUS for December 13-22, 2017, showing mostly a zonal flow with some elongation
500-mb mean circulation for the CONUS for December 13-22, 2017, showing mostly a zonal flow with some elongation.
500-mb mean circulation for the CONUS for December 23-31, 2017, showing the dominance of a long-wave trough in the East with a long-wave ridge along the West Coast
500-mb mean circulation for the CONUS for December 23-31, 2017, showing the dominance of a long-wave trough in the East with a long-wave ridge along the West Coast.

The upper-level circulation was very active this month with several short-wave troughs migrating across the CONUS in the jet stream flow. The overall general (or "long-wave") circulation pattern, through which the short-wave troughs migrated, underwent several shifts during the month. A zonal circulation pattern held sway during the first week of December. A zonal circulation is generally a west-to-east flow that keeps cold air masses bottled up to the north and warmer air masses concentrated to the south. It reduces north-south movement of air masses and inhibits the atmosphere's efforts to transfer heat from the tropics towards the poles to balance out the distribution of heat across the Earth. The circulation pattern shifted to a meridional state the following week. A meridional circulation consists of large undulations of the jet stream to the north and south, frequently resulting in stable long-wave troughs and ridges. Troughs transport colder air masses from the poles southward, and ridges transfer heat from the low latitudes to higher latitudes, thus balancing out the Earth's surface heat in the long run. A generally zonal flow followed during roughly the third week, with some undulations due to closed lows frequently passing over the Southwest. The month ended with the Hudson Bay Low expanding southward into the CONUS, intensifying a long-wave trough which dominated the central and eastern parts of the country. Each of these patterns was characterized by specific weather phenomena, temperature anomalies, precipitation anomalies, and impacts.


500-mb circulation anomalies for the CONUS for December 1-5, 2017
500-mb circulation anomalies* for the CONUS for December 1-5, 2017.
Temperature anomalies (departure from normal) for the CONUS for December 1-5, 2017
Temperature anomalies (departure from normal) for the CONUS for December 1-5, 2017.
Precipitation anomalies (percent of normal) for the CONUS for December 1-5, 2017
Precipitation anomalies (percent of normal) for the CONUS for December 1-5, 2017.

A generally westerly flow in the jet stream held sway during the first week, keeping colder air masses bottled up north of the Canadian border, resulting in a warmer-than-normal week for much of the CONUS. Pacific weather systems mostly kept to a northerly track, bringing above-normal precipitation to the central and northern Rockies, northern Plains, and western Great Lakes, and laying down a mountain snowpack across the higher elevations of the Pacific Northwest and northern Rockies. A front near the end of this period tapped Gulf of Mexico moisture to drop above-normal precipitation across parts of coastal Texas and the Lower Mississippi Valley. The rest of the CONUS was moisture-starved and drier than normal.


500-mb circulation anomalies for the CONUS for December 6-12, 2017
500-mb circulation anomalies for the CONUS for December 6-12, 2017.
Temperature anomalies (departure from normal) for the CONUS for December 6-12, 2017
Temperature anomalies (departure from normal) for the CONUS for December 6-12, 2017.
Precipitation anomalies (percent of normal) for the CONUS for December 6-12, 2017
Precipitation anomalies (percent of normal) for the CONUS for December 6-12, 2017.

During the following week, a western ridge/eastern trough pattern dominated the upper-level circulation. This pattern deflected storm systems away from the West and enabled colder Canadian air masses to plunge south into the eastern CONUS. The western ridge, and associated chinook wind patterns along the east slopes of the Rockies, brought above-normal temperatures to the northern and central Plains. Even though a ridge dominated the circulation pattern over the West, the mountain snowpack likely contributed to below-normal temperatures in the interior West, as did cold pooling from strong temperature inversions. With no Pacific moisture or storms, the West was dry, and a northwesterly flow in the upper levels of the atmosphere kept moisture out of the central CONUS. Weather systems moving along cold fronts in the Deep South tapped Gulf of Mexico moisture to bring above-normal precipitation to southern Texas, across the Gulf coast, to the southern Atlantic Coast states. The air was below freezing with these air masses, so a swath of snow was deposited across southern Texas and the Southeast; this far south, the snowpack melted after just a few days.


500-mb circulation anomalies for the CONUS for December 13-22, 2017
500-mb circulation anomalies for the CONUS for December 13-22, 2017.
Temperature anomalies (departure from normal) for the CONUS for December 15-21, 2017
Temperature anomalies (departure from normal) for the CONUS for December 15-21, 2017.
Precipitation anomalies (percent of normal) for the CONUS for December 15-21, 2017
Precipitation anomalies (percent of normal) for the CONUS for December 15-21, 2017.

The upper-level circulation pattern flattened out again roughly during December 13-22, although a ridge tried to hold on over the West. Pacific fronts and weather systems penetrated the western ridge, flattening it, with some of them moving across the southern states, while others tracked across the north. The western ridge had enough strength to keep much of the West drier than normal. The Pacific systems dried out as they crossed the Rockies, and the westerly flow blocked Gulf of Mexico moisture, so the central Plains was drier than normal during this period. But the Pacific systems that tracked across the north brought above-normal precipitation to the northern parts of the Pacific Northwest and northern Plains as well as the central Rockies. The southern weather systems brought Gulf of Mexico moisture up against cold fronts lying in the South, resulting in above-normal precipitation from the southern Plains to southern Appalachians. A low pressure system and cold front on the 19th-20th generated tornadoes across the Lower Mississippi Valley to Southeast. While some precipitation did fall across the Midwest to Northeast, the weather systems largely tracked north or south of this region so this period was mostly drier than normal here. The westerly flow kept the really cold air bottled up in Canada, so most of the CONUS was warmer than normal; the warm temperature departures were especially significant across the northern Plains.


500-mb circulation anomalies for the CONUS for December 23-31, 2017
500-mb circulation anomalies for the CONUS for December 23-31, 2017.
Temperature anomalies (departure from normal) for the CONUS for December 25-31, 2017
Temperature anomalies (departure from normal) for the CONUS for December 25-31, 2017.
Precipitation anomalies (percent of normal) for the CONUS for December 25-31, 2017
Precipitation anomalies (percent of normal) for the CONUS for December 25-31, 2017.

By December 23, a huge omega block-type ridge had developed in the upper-level circulation along the western coast of North America. This helped displace, or intensify, the Hudson Bay Low, which set the stage for the return of a western ridge/eastern trough pattern over the CONUS. The associated due-northerly flow in the upper atmosphere funneled very cold Arctic air masses across Canada and into the heart of the CONUS. The pattern became entrenched, sending wave after wave of brutally cold air across the central and eastern CONUS for the rest of December and well into January. The western ridge kept the Southwest warmer than normal but also dry. Weather systems moving in this convoluted flow pattern brought above-normal precipitation to the northern parts of the Pacific Northwest and into the northern Plains. Precipitation fell across the Great Lakes to Northeast, and along the Gulf Coast states, but it was still generally below normal for the period. The central Plains to Mid-Atlantic states were caught in between these storm tracks and received little to no precipitation. With below-freezing air, the northern weather systems laid down a snowpack from the northern and central Rockies, across the Plains and Ohio Valley, to the Northeast, and the relentless blasts of arctic air behind each new cold front kept most of the snowpack from melting.


500-mb mean circulation for the CONUS for December 2017
500-mb mean circulation for the CONUS for December 2017.
500-mb circulation anomalies for the CONUS for December 2017
500-mb circulation anomalies for the CONUS for December 2017.
Temperature anomalies (departure from normal) for the CONUS for December 2017
Temperature anomalies (departure from normal) for the CONUS for December 2017.
Precipitation anomalies (percent of normal) for the CONUS for December 2017
Precipitation anomalies (percent of normal) for the CONUS for December 2017.

When conditions are averaged across the entire month, the circulation anomalies associated with the ridge/trough weeks tended to dominate the monthly pattern, while the monthly temperature anomaly pattern reflected a chaotic merging of the anomaly patterns of the various weeks. The precipitation anomaly pattern for the month represented an additive result of the anomaly patterns for the various weeks. 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 December 2017.

  • 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 December. No tropical cyclones formed in, or moved into, the central North Pacific.
  • Two named tropical systems (Tropical Storm Kai-Tak and Typhoon Tembin) and one tropical disturbance were active in the western Tropical Pacific near the U.S.-Affiliated Pacific Islands (USAPI) in Micronesia, but none of them had any significant effect on the USAPI. Tropical Storm Kai-Tak and Typhoon Tembin originated as areas of convection over western Micronesia at mid-month, but didn't intensify until they crossed into the Philippine Sea. Both tropical cyclones moved over the Philippines before traversing the South China Sea and encountering Southeast Asia. Another area of convection (99W) developed over western Micronesia at the end of the month; it developed into Tropical Storm Bolaven in early January after crossing the Philippines.
Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

December was wetter than normal across Montana and Wyoming, parts of the Upper Mississippi Valley and western Great Lakes, parts of the southern Plains to Lower Mississippi Valley, and parts of the coastal Southeast, but drier-than-normal weather dominated most of the CONUS. Alaska had a mixed precipitation anomaly pattern, Hawaii was mostly drier than normal, and December was drier than normal in San Juan, Puerto Rico.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

Monthly temperatures were warmer than normal in the southwestern CONUS and in Alaska, near to warmer than normal across most of the Plains and Southeast, and cooler than normal in parts of the Pacific Northwest and northern Rockies, and from the Great Lakes 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 (500-mb) circulation anomaly pattern over North America was part of a complex long-wave pattern that stretched across the Northern Hemisphere. East-west trough/ridge pairs (or couplets) and anomaly couplets could be found in the monthly maps, especially over Eurasia, North America/North Pacific, North America/North Atlantic/Europe, and Siberia/North Pacific; and latitudinally (eastern North America/Arctic, Western Europe/Arctic, and Arctic/eastern Siberia to Sea of Japan). The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation (over western North America, parts of Russia); below-normal snow cover (over parts of Asia, western Europe, southwestern CONUS); above-normal surface temperatures over the southwestern CONUS, Alaska, western Russia, and extreme northern Siberia; and warm SST anomalies in the eastern North Atlantic and parts of the North Pacific, South Pacific, South Atlantic, and Indian Oceans. The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over southeastern Canada to northeastern CONUS, Sea of Japan coastal areas, Scandinavia; cool SST anomalies in parts of the central North Pacific and Sea of Okhotsk; above-normal precipitation over Scandinavia and Eastern Europe to western Russia; and above-normal snow cover over northeastern CONUS, Scandinavia, and Sea of Japan coastal areas. Parts of eastern North America, Eurasia, northern Africa, and the Arabian peninsula were near to cooler than normal, and parts of the equatorial Pacific, eastern South Pacific, and South Atlantic had cooler-than-normal SST anomalies. But with large parts of the continents still having warmer-than-normal temperatures, and large portions of the Atlantic, Pacific, and Indian Oceans having warmer-than-normal sea surface temperatures, the December 2017 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:


Circulation indices and their agreement with the temperature and precipitation anomaly patterns, by week.
Week Temperature Precipitation
December 3-9 MJO MJO
December 10-16 MJO, PNA MJO
December 17-23 AO MJO
December 24-30 MJO, TNH

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed December 2017 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggest that La Niña had some influence on the weather over the CONUS in December, but that it shared this influence with other drivers originating in other parts of the world. There was some agreement between the observed precipitation anomalies and the teleconnections for La Niña, but the temperature anomalies did not match. The other equatorial driver (associated with the MJO), however, had good agreement between the observed weekly anomalies and the corresponding teleconnections for most weeks. The PNA, NAO, WP, and TNH were all positive for the month. The observed anomalies showed little agreement with the teleconnections for the NAO and WP, so the drivers behind these indices likely had little influence on this month's weather. The upper-level circulation anomaly patterns had some similarity to the teleconnections for the PNA and TNH, which suggested the Pacific jet stream and Hudson Bay Low strongly influenced the circulation over North America. There was also a good match between the observed precipitation anomalies and the teleconnections for the PNA and TNH. The AO was negative for the first half of the month and positive for the second half. There was little agreement between the AO teleconnections and the observed upper-level circulation anomalies, and just some agreement with the observed precipitation anomalies. The temperature anomalies showed little agreement with the AO teleconnections, except for the third week of the month. Likewise, the observed temperature anomalies generally did not match the teleconnections for the PNA and TNH, except for certain weeks. The PNA matched well during the second week (when the PNA index was strongest) and the TNH matched during the last week.

This month illustrates several things. First, it illustrates how the weather and climate anomaly patterns can be influenced by atmospheric drivers (or modes of atmospheric variability) originating in the equatorial Pacific Ocean, North Pacific Ocean, and northeast North America (Hudson Bay Low). Second, it demonstrates how the circulation anomaly patterns can result from a complex interaction of many drivers operating at different time scales, and how some drivers can have influence at certain times of the month while others have influence at other times of the month. Third, it shows the importance of selecting an appropriate time scale to examine and how signals can be masked when averaging over a monthly time scale.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for December 2017, published online January 2018, retrieved on December 15, 2018 from https://www.ncdc.noaa.gov/sotc/synoptic/201712.

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