December 2018 Synoptic Discussion

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 December 2018. Long-wave ridging dominated the upper-level circulation at the higher latitudes across the contiguous United States (CONUS), while short-wave ridges and troughs migrated through the jet stream flow in the lower latitudes. Fronts and low pressure systems traversed the country throughout the month, bringing above-normal precipitation to many areas east of the Rockies. Above-normal temperatures dominated the month beneath the upper-level ridging, helping to keep heating demand lower than average and reduce snow cover area during the middle of the month, but December was bracketed by troughs and increased snow cover at the beginning and ending weeks. The precipitation contracted drought and abnormally dry areas in the West and Plains, but drought and abnormal dryness expanded in other areas, with only a small decrease in the national drought footprint. In addition to precipitation, some of the fronts and lows brought severe weather, with tornado activity more than double the long-term average for December. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather during December reflected the influence of atmospheric drivers originating in the Pacific Ocean and North America. See below for details.

Synoptic Discussion

Animation of daily upper-level circulation 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 North America for December 1-5, 2018.

500-mb mean circulation for North America for December 6-23, 2018.

500-mb mean circulation for North America for December 24-31, 2018.

December 1-5: The month began with a large upper-level trough across the north central part of the CONUS. It's associated surface low and frontal system brought above-normal precipitation and severe weather to the central Plains to Great Lakes, and across the Southeast. An upper-level ridge dominated the circulation over Alaska and northwestern Canada.

500-mb mean circulation for North America for December 1-5, 2018.

500-mb circulation anomalies for North America for December 1-5, 2018.

Temperature anomalies (departure from normal) for the CONUS for December 1-5, 2018.

Precipitation anomalies (percent of normal) for the CONUS for December 1-5, 2018.

December 6-23: During the middle half of the month, the long-wave ridge shifted east to take up residence over central North America. But the jet stream remained very active, with Pacific short-waves and closed lows moving through the ridge. Their surface lows tracked preferentially across the southern Plains to Southeast during December 6-11; across the southern Plains, Midwest, Southeast, and Northeast during December 12-18; and from the Southeast to Northeast during December 20-22, leaving above-normal precipitation in their wake. Pacific fronts also brought above-normal precipitation to parts of the Pacific Northwest during this time.

500-mb mean circulation for North America for December 6-23, 2018.

500-mb circulation anomalies for North America for December 6-23, 2018.

Temperature anomalies (departure from normal) for the CONUS for December 10-23, 2018.

Precipitation anomalies (percent of normal) for the CONUS for December 10-23, 2018.

December 24-31: Troughing dominated the western CONUS during the last week of December, kicking up a ridge over the East. This shift in the jet stream enhanced development of two storm systems across the central CONUS. They dropped above-normal precipitation across the Plains to western Great Lakes, and their trailing fronts brought above-normal precipitation to the South and up the Mid-Atlantic Coast. Temperatures in the Plains were below freezing, so much of the precipitation there fell as snow, increasing the national snow cover area.

500-mb mean circulation for North America for December 24-31, 2018.

500-mb circulation anomalies for North America for December 24-31, 2018.

Temperature anomalies (departure from normal) for the CONUS for December 25-31, 2018.

Precipitation anomalies (percent of normal) for the CONUS for December 25-31, 2018.

The full monthly circulation pattern reflected the dominance of the mid-month pattern, but tempered in the West by the circulation anomalies at the beginning and end of the month.

500-mb mean circulation for North America for December 2018.

500-mb circulation anomalies for North America for December 2018.

Temperature anomalies (departure from normal) for the CONUS for December 2018.

Precipitation anomalies (percent of normal) for the CONUS for December 2018.

The long-wave circulation pattern, averaged for the month, consisted of a weak upper-level ridge over western North America, with a weakened trough over eastern North America and dominance of troughing over the south central CONUS. Above-normal monthly temperatures overspread most of the CONUS, with the warmest anomalies over the northern Plains to Great Lakes. Some near to colder temperature anomalies in the West and South reflected the frequent troughing there. The precipitation anomaly pattern for the month (the wet areas) represented an additive result of precipitation from the individual frontal passages and low pressure systems. The dry areas resulted from persistence of upper-level ridging, or areas that missed out on the precipitation purely by chance. Consequently, above-normal precipitation from the storms in the central, southern, and southeastern portions of the CONUS were reflected at the monthly scale. The circulation during this month was also reflected in severe weather, drought, snow, and regional records.

• While some areas averaged extremely warm (northern Plains to Great Lakes) for this time of year, no states had extreme (top ten warmest) statewide temperature ranks. Michigan had the warmest rank at 16^th warmest December in the 1895-2018 record. On a statewide basis, Georgia had the wettest December in the 124-year record. Eleven other states in the South, Mid-Atlantic, and Plains were top ten wettest.
• When daily temperature records are examined, the extreme warmth dominated the extreme cold. When integrated across the month, there were 1,733 record warm daily high (688) and low (1,045) temperature records. This was more than five times the 338 record cold daily high (253) and low (85) temperature records.
• Temperatures averaged across the country gave the CONUS the 21^st warmest December in the 1895-2018 record. With the unusual warmth concentrated in the high population centers of the country during this normally very cold time of year, heating demand was less than normal. Consequently, the national REDTI (Residential Energy Demand Temperature Index) value for December 2018 ranked as the 15^th lowest December REDTI in the 124-year record.
• Some of the precipitation during December fell on areas that were in drought or abnormally dry at the end of November, resulting in welcome drought improvement, especially in the central Plains and along the peripheral edges of the western drought. Drought and abnormal dryness contracted in these areas, but expanded in other parts of the West and Plains, and in southern Florida, Hawaii, and Puerto Rico. Contraction was a little more than expansion this month, so at the national level drought contracted from 22.2 percent of the CONUS at the end of November to 21.9 percent of the CONUS at the end of December (from 18.9 percent to 18.7 percent for all of the U.S.).

Daily snow cover area across the CONUS for the month.
• December began with snow cover over the Rockies and northern Plains, Midwest, and Northeast. The upper-level trough and its associated surface lows and fronts at the beginning of the month expanded snow across the Rockies and into parts of the southern Plains and Southeast. Snow cover area hovered around 40-45 percent of the CONUS during the first ten days, peaking at about 46.7 percent on the 10^th. The shift to a more ridge-like pattern across the central CONUS during the middle of the month warmed the air masses and contracted the snow cover area to a monthly minimum of about 17.5 percent of the CONUS by December 21^st. When the long-wave trough built over the West during the last week of the month, colder air from the north and Gulf of Mexico moisture from the south fed the storm systems which tracked across the Midwest, expanding the snow cover in the West and Plains, with snow cover peaking during this period at about 40.1 percent on the 29^th. Based on satellite observations, the monthly snow coverage across the CONUS was 1.16 million sq. mi. (3.004 million sq. km.), 22 thousand sq. mi. (57 thousand sq. km.) below average, and ranked near the middle of the 53-year record at 25^th largest December snow cover area. Snow cover was above normal in parts of the West, central Plains, Great Lakes, and East Coast, but below normal in other parts of the West, Plains, and Northeast, and much of the Midwest.
• 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, with its frontal systems and surface lows, occurred frequently, with the number of tornadoes for December 2018 (61 based on preliminary data) was more than twice the December average of 24. Most of the tornadoes occurred on December 1^st in Illinois as a strong low pressure and frontal system moved across the Midwest. The rest of the tornadoes occurred in the southern Plains to Southeast.

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 December 2018.

• The Atlantic hurricane season runs from June 1^st through November 30^th and the Eastern North Pacific (ENP) hurricane season runs from May 15^th through November 30^th.
• No tropical systems developed in the North Atlantic or Eastern North Pacific during December 2018. One system (tropical disturbance 94W) formed in the Central North Pacific and quickly moved into the western North Pacific before dissipating.
• Two tropical depressions and two tropical disturbances were active in the western tropical Pacific during December. One (Tropical Depression 35W) formed within the waters of the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia). The two tropical disturbances (99C and 94W) formed in mid-December and weakened or dissipated before they could move into USAPI waters. Tropical Depression 35W formed in western Micronesia during the last half of December and fell apart as it crossed the Philippines near the end of the month. Tropical Depression 36W formed near the end of the month well west of the USAPI and moved toward Southeast Asia.

North America monthly upper-level circulation pattern and anomalies. The upper-level circulation pattern during December, when averaged for the month, consisted of a weak upper-level ridge over western North America, with a weakened trough over eastern North America and dominance of troughing over the south central CONUS. This was reflected by above-normal height anomalies across the northern Plains to Northeast and below-normal height anomalies over the south central CONUS.

Map of monthly precipitation anomalies. Monthly precipitation was drier than normal across most of Puerto Rico and Hawaii, parts of Alaska, most of the western CONUS, and parts of the Great Lakes, southern Florida, and extreme southern Texas. December was wetter than normal over other parts of Alaska and most of the Great Plains to South and East.

Map of monthly temperature anomalies. Monthly temperatures were warmer than normal in most of Alaska and across most of the CONUS, especially in the northern Plains to western Great Lakes. December temperatures were near to cooler than normal in parts of the West, southern Plains, and New England.

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:

• El Niño Southern Oscillation (ENSO)
  • Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
  • Status: The ocean and atmosphere system reflected an ENSO-neutral state during December 2018. SSTs were above average across the equatorial Pacific Ocean, suggesting El Niño conditions were imminent, but the atmosphere was not responding to the oceanic conditions, so officially ENSO-neutral conditions continued for now with El Niño conditions considered likely in the next few months.
  • Teleconnections (influence on weather): The NWS CPC has no teleconnections for ENSO-neutral conditions. However, to the extent teleconnections are known, the typical temperature and precipitation patterns associated with El Niño during December include below-normal temperatures from the Southwest to Southeast; above-normal temperatures in the Pacific Northwest; above-normal precipitation in California, parts of the Southwest, and from the southern Plains to Southeast then up the Atlantic Coast; and below-normal precipitation in the Pacific Northwest and Midwest.
  • Comparison to Observed: The December 2018 temperature and precipitation anomaly patterns do not match the teleconnections for La Niña. The temperature anomaly pattern does not match the teleconnections for El Niño, but the precipitation anomaly pattern mostly does.
• Madden-Julian Oscillation (MJO)
  • Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. The MJO can enter periods of little or no activity, when it becomes neutral or incoherent and has little influence on the weather. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
  • Status: The MJO Wheeler-Hendon index indicated that the MJO transitioned rapidly through phases 1-3 during the first half of the month, and spent the last two weeks in phases 4 then 5. The atmospheric anomalies during December largely reflected intra-seasonal variability related to the MJO, as opposted to El Niño, but other modes of coherent subseasonal tropical variability, such as Rossby wave activity, interfered with the MJO (especially during the last half of the month (MJO updates for December 3, 10, 17, 24, and 31).
  • Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase; there is also a lagged component associated with the teleconnections. To the extent teleconnections are known, the December (November-January) teleconnections for temperature are shown here and for precipitation are shown here.
  • Comparison to Observed: The MJO is transitory and can change phases (modes) within a month, so it is usually more closely related to weekly weather patterns than monthly. In the case of December 2018, the monthly temperature anomaly pattern is a good match for the teleconnections for phases 3-5. The monthly precipitation anomaly pattern does not match the teleconnections for any of the phases. The weekly (December 13-19 and 20-26) and bi-weekly (December 18-31) temperature anomaly patterns are a good match for the teleconnection patterns for the phases (4-5) the MJO was in during the last half of the month. There is some agreement during these weeks (December 13-19 and 20-26) between the precipitation anomaly pattern and the teleconnection patterns for phases 4-5.
• The Pacific/North American (PNA) pattern
  • Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
  • Status: The daily PNA index was positive for most of the month, and averaged positive for the month as a whole.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive PNA for this time of year (January on the quarterly teleconnection maps) is associated with below-normal upper-level circulation anomalies over the northern North Pacific and the southeast U.S., and above-normal circulation anomalies over western North America; warm temperature anomalies over Alaska, western Canada, the U.S. West Coast and northern Plains, and cold temperature anomalies over New Mexico to the Southeast and Northeast; below-normal precipitation in the Great Lakes, Pacific Northwest, and central Rockies; and near-normal precipitation elsewhere.
  • Comparison to Observed: The December 2018 monthly upper-level circulation anomaly pattern does not agree with the teleconnections for a positive PNA. The monthly temperature and precipitation anomaly patterns agree in some areas but disagree over other larger areas. But the weekly temperature anomaly pattern for the second week agrees quite well.
• The Arctic Oscillation (AO) pattern
  • Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
  • Status: The daily AO index was positive for the first half of the month, then oscillated around zero (neutral) during the second half, averaging near zero (neutral) (but slightly positive) for the month. The 3-month-averaged index was near zero.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a neutral AO in December (November-January in the AO Quarterly Composites table) is typically associated with a near-normal circulation pattern (zero anomalies) across most of the Northern Hemisphere (a positive AO has below-normal upper-level circulation anomalies over the Arctic Ocean into Alaska and northwest Canada, and above-normal anomalies over the North Pacific, and eastern North America extending across the North Atlantic to Western Europe); above-normal temperatures from the Pacific Northwest to central Plains (a positive AO has extensive above-normal temperatures east of the Rockies); wetter-than-normal conditions in northern California and the Pacific Northwest Coast; and drier-than-normal conditions from the central Plains and Midwest to Southeast (positive AO also has dry anomalies in the Northeast).
  • Comparison to Observed: The December 2018 monthly upper-level circulation anomaly pattern does not match the teleconnections for a neutral or positive AO. The monthly temperature anomaly pattern is a good match for a positive AO and reasonable for a neutral AO, as is the weekly temperature anomaly pattern for the second week (December 10-16) when the daily AO was most positive during the month. The monthly precipitation anomaly pattern does not match the teleconnections for a positive or neutral AO, nor do the weekly anomaly patterns.
• The North Atlantic Oscillation (NAO) pattern
  • Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
  • Status: The daily NAO index was positive for the first half of the month, and near zero (neutral) for the second half, averaging positive for the month. The 3-month-averaged index was also positive.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (January on the quarterly teleconnection maps) is associated with above-normal upper-level circulation anomalies across the CONUS from the Rockies to East Coast, across the Atlantic, to Europe; below-normal circulation anomalies over northern Canada to Greenland; above-normal temperatures across the southern Plains to Southeast, and northern Plains to Northeast; and below-normal precipitation in the Far West, but there are few precipitation teleconnections.
  • Comparison to Observed: The December 2018 monthly upper-level circulation anomaly pattern, and weekly anomaly patterns, do not agree with the teleconnections for a positive NAO. The monthly temperature anomaly pattern is a good match, but the weekly pattern, for the week when the NAO was strongest, is opposite, so the monthly agreement may be a coincidence. The monthly precipitation anomaly pattern has some agreement in the West where the few teleconnections exist.
• The West Pacific (WP) pattern
  • Description: The WP teleconnection pattern is a primary mode of low-frequency variability over the North Pacific and reflects zonal and meridional variations in the location and intensity of the (East Asian) jet stream in the western Pacific.
  • Status: The monthly WP index was near zero (slightly positive) (daily index values are not available).
  • Teleconnections (influence on weather): The NWS CPC has no teleconnections for a neutral (near zero) WP. However, to the extent teleconnections are known, a positive WP during this time of year (January on the quarterly teleconnection maps) is typically associated with below-normal circulation anomalies over the western CONUS and eastern Siberia; above-normal circulation anomalies over the eastern CONUS and southeast Canada; below-normal temperatures over California and Nevada; above-normal temperatures over the Mid- to Upper Mississippi Valley, Great Lakes, Northeast, and southeast Canada; and above-normal precipitation from Oklahoma to the Ohio Valley.
  • Comparison to Observed: The December 2018 monthly upper-level circulation anomaly pattern does not match the teleconnections for a positive WP. The temperature anomaly pattern has some agreement, although this may be a coincidence since the WP index is only slightly positive (really, near zero). The monthly precipitation anomaly pattern agrees where there are teleconnections, but has large anomalies where the teleconnections show zero correlation.
• The Tropical/Northern Hemisphere (TNH) pattern
  • Description: The TNH teleconnection pattern reflects large-scale changes in both the location and eastward extent of the Pacific jet stream, and also in the strength and position of the climatological mean Hudson Bay Low, and is dominant in the winter months. The pattern significantly modulates the flow of marine air into North America, as well as the southward transport of cold Canadian air into the north-central United States.
  • Status: The TNH index was negative during December (daily index values are not available).
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative TNH is typically associated with upper-level circulation anomalies consisting of above-normal 500-mb geopotential heights over the eastern two-thirds of Canada and the north central U.S., and below-normal heights over the northeastern Pacific and Gulf of Mexico into the western North Atlantic; warmer-than-normal temperatures for much of the country as well as eastern Canada, and near-normal temperatures in the southern and eastern CONUS; and wetter-than-normal conditions in the West, and slightly drier-than-normal conditions in the Tennessee Valley.
  • Observed: The December 2018 monthly upper-level circulation anomaly pattern agrees very well with the teleconnections for a negative TNH, with the agreement especially strong during the first two-thirds of the month (December 1-22, 9-15, 11-22). The monthly temperature anomaly pattern has some agreement, but the bi-weekly anomaly pattern for the middle of the month (December 11-24) agrees quite well. The monthly precipitation anomaly pattern does not match the teleconnections.

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed December 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggests the atmospheric drivers behind the Pacific (MJO, ENSO, PNA) and North American (TNH) teleconnection patterns had the greatest influence on this month's weather, but other drivers may have been influential. The circulation seems to have been most strongly affected by the TNH driver, especially during the middle of the month. Temperatures for December seem to have been strongly influenced by the TNH driver, especially during the middle of the month, with reinforcement from the AO in the middle of the month and the MJO during the last half of the month. Precipitation seems to have been largely influenced by the MJO during the last half of the month. Even though ENSO was technically neutral, SST anomalies in the equatorial Pacific were positive (which reflect an El Niño) and the monthly precipitation anomalies over the CONUS matched the teleconnections for El Niño in many areas.

This month illustrates how the atmospheric circulation for the month can reflect the combined influence of atmospheric drivers (or modes of atmospheric variability) originating in the Pacific and North America.

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