January 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 a La Niña state during January 2018. Like last month, the upper-level circulation pattern experienced several shifts over the contiguous United States (CONUS) during January, 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 warm note nationally. This variability resulted in a monthly temperature anomaly pattern with warm anomalies dominating in the West and cold anomalies dominating in the East. 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 dryness was especially pronounced in the southern Plains. The lack of precipitation resulted in a significant expansion of drought and abnormal dryness, especially across much of the West, southern Plains, and Southeast. 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 and EP-NP), and Arctic (AO) 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, EP-NP). 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, January is in the middle of climatological winter 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 January 1-5, 2018, showing a long-wave ridge in the West and trough in the East.

500-mb mean circulation for the CONUS for January 6-11, 2018, showing a zonal flow.

500-mb mean circulation for the CONUS for January 12-17, 2018, showing a long-wave ridge in the West and trough in the East.

500-mb mean circulation for the CONUS for January 18-31, 2018, showing a zonal flow.

The upper-level circulation was very active this month and experienced four general (or "long-wave") regimes. During most of the first half of the month, an upper-level ridge dominated the West with a trough over the East. This meridional pattern was interrupted briefly by a westerly (or zonal) flow from about January 6-11. During the second half of the month, a westerly flow dominated the circulation 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 January 1-5, 2018.

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

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

The western ridge/eastern trough pattern during roughly January 1-5 was a continuation of the pattern that had become established during the last part of December. The ridge deflected Pacific weather systems to the north into western Canada, keeping much of the western CONUS drier than normal, the Southwest warmer than normal, and mountain snowpack well below normal. The eastern trough funneled cold arctic air masses across the Plains to the Deep South and East Coast. The air masses were dry, and the northwesterly flow aloft blocked Gulf of Mexico moisture, so most of the CONUS east of the Rockies was drier than normal.

500-mb circulation anomalies* for the CONUS for January 6-11, 2018.

Temperature anomalies (departure from normal) for the CONUS for January 6-12, 2018.

Precipitation anomalies (percent of normal) for the CONUS for January 6-12, 2018.

• * A westerly flow shows up on the 500-mb anomaly map as below-normal heights in western Canada and above-normal heights across the central and eastern CONUS because the climatological ("normal") pattern is a ridge across western North America and trough in the East. A westerly flow is manifested by a weakened western ridge and weakened eastern trough, which is what this anomaly pattern reflects.

The ridge/trough patterned weakened during January 6-11, allowing a westerly flow to take hold and carry Pacific weather systems across the Lower 48 States. With milder air masses, temperatures were generally warmer than normal, especially in the West. The arctic air masses over the East were slow to be pushed out, leaving behind a colder-than-normal temperature footprint across the East Coast. The Pacific systems brought above-normal precipitation to much of the West, but were dried out as they crossed the Rockies resulting in below-normal precipitation for much of the Plains. They picked up Gulf of Mexico moisture as they traveled further east, with the storm track bringing above-normal precipitation to the Lower Mississippi Valley to eastern Great Lakes, but below-normal precipitation across the East Coast.

500-mb circulation anomalies for the CONUS for January 12-17, 2018.

Temperature anomalies (departure from normal) for the CONUS for January 12-18, 2018.

Precipitation anomalies (percent of normal) for the CONUS for January 12-18, 2018.

The ridge/trough pattern became established again during roughly January 12-17, once again blocking Pacific weather systems from the West and funneling cold arctic air masses across the Plains and into the heart of the CONUS. The ridge kept the West warmer than normal, and the trough brought colder-than-normal weather to much of the CONUS east of the Rockies. Some of the Pacific systems and "short-waves" moving in the ridge/trough flow brought above-normal precipitation to parts of the West and Plains, but the overall precipitation pattern was drier than normal beneath the ridge in the West and northwesterly flow in the central CONUS. The storm track converged in the Northeast, where precipitation was above normal.

500-mb circulation anomalies for the CONUS for January 18-31, 2018.

Temperature anomalies (departure from normal) for the CONUS for January 18-31, 2018.

Precipitation anomalies (percent of normal) for the CONUS for January 18-31, 2018.

The ridge/trough pattern flattened into a generally westerly flow during the last half of the month. This allowed Pacific weather systems to track across the CONUS, but most of them followed a storm track across the northern half of the country, dropping above-normal precipitation over the Pacific Northwest and central Rockies to western Great Lakes, and leaving the Southwest and southern Plains in a dry pattern. Some of the systems tapped Gulf of Mexico moisture to bring swaths of above-normal precipitation to the South, but the precipitation that fell was mostly below normal along and east of the Mississippi River. One of the systems had enough energy and instability to generate severe weather in the Lower Mississippi Valley. The westerly flow blocked arctic air masses and allowed a milder southerly flow to dominate, bringing above-normal temperatures to most of the CONUS, especially along the northern tier states. Above-freezing daytime temperatures in the central Plains helped to melt the snow cover that had built up in the region earlier in the month.

500-mb mean circulation for the CONUS for January 2018.

500-mb circulation anomalies for the CONUS for January 2018.

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

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

When conditions are averaged across the entire month, a western ridge/eastern trough pattern was evident in the upper-level circulation with the circulation anomalies associated with the western ridge dominating. The ridge/trough pattern was also evident in the monthly temperature anomaly pattern, with the West warmer than normal and East (especially Southeast) cooler than normal. 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 southern 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.

• Monthly temperatures were unusually warm across the West, where nine states had the tenth warmest, or warmer, January in the 1895-2018 record. The cold snaps in the central and northeastern parts of the country were counterbalanced by warm periods, so that the statewide monthly temperatures were near average, but cold dominated in the South and Southeast to Mid-Atlantic where 16 states ranked in the cool third of the historical record.
• When daily temperature records are examined, the extreme cold was a little more extreme, frequent, and/or widespread than the extreme warmth. When integrated across the month, there were 5,941 record cold daily high (3,190) and low (2,751) temperature records. This was about 1.28 times the 4,654 record warm daily high (2,140) and low (2,514) temperature records.
• As noted earlier, January is in the middle of the Northern Hemisphere climatological winter, the coldest season when heating demand peaks in the United States. Temperatures averaged across the month and across the CONUS gave January 2018 a rank of 35^th warmest. But most of the warmth occurred in the sparsely populated West. The higher-population centers of the Midwest, South, and Northeast were mostly near to colder than normal for the month. This gave the REDTI (Residential Energy Demand Temperature Index) value for January 2018 a value which tied with January 1960 as the 59^th lowest (warmest) in the 124-year historical record for January.
• In spite of widespread areas with below-normal precipitation, scattered areas of above-normal precipitation occurred across the West such that only two western states (Colorado and New Mexico) were dominated by below-normal precipitation and ranked in the dry third of the historical record. The dryness was more widespread in the Plains to Southeast and Midwest. Altogether, 15 states ranked in the dry third of the historical record and eight in the wet third. Two states were in the top ten driest category — Alabama (ninth driest January) and New Mexico (tenth driest). About an eighth (12.2 percent) of the country was very dry (had January precipitation in the driest tenth of the historical record). When averaged across the CONUS, January 2018 ranked as the 21^st driest January in the 1895-2018 record.
• The above-normal precipitation that fell across Montana to Iowa and parts of Louisiana and the Carolinas helped to contract drought and abnormal dryness in those areas. Drought also improved in Hawaii. But below-normal precipitation dominated much of the CONUS. This continued a pattern of dryness which has persisted for the last two to three months, and even longer in some areas. As a result, drought and abnormal dryness expanded across the West, Southern Plains, and Southeast on the U.S. Drought Monitor (USDM) map. Expansion outweighed contraction, so at the national level drought expanded from 27.7 percent of the CONUS at the end of December to 38.4 percent of the CONUS at the end of January (from 23.2 percent to 32.1 percent for all of the U.S.). The continued dry conditions in the southern Plains set the stage for the development of several large wildfires by the end of the month (wildfire maps for January 1, 23, 28, 30).
• January began with 41.9 percent of the CONUS experiencing a snow cover. The upper-level ridge in the West during the first week of the month kept the western snowpack limited mainly to the Pacific Northwest and northern Rockies, while the trough in the East maintained the snow cover across the central and northern Plains to the Northeast. When the circulation pattern shifted during the second week, the westerly flow brought warmer (above-freezing) air to the central Plains, melting the snow cover there and reducing the CONUS snow coverage to about 32.9 percent by the 12^th. The re-establishment of the eastern trough the following week brought colder (below-freezing) air masses across the Plains and into the Deep South, where cold fronts tapped Gulf of Mexico moisture to lay down swaths of snow across the southern Plains to Southeast, and bringing the areal extent of snow coverage to a peak of about 53.7 percent of the CONUS by the 18^th. The switch of the upper-level circulation back to a westerly flow during the last half of the month brought warmer air to the East, melting the snow cover. But Pacific weather systems moving in the westerlies were able to cross the Southwest, increasing the snowpack in some areas. The late-month snow was not enough, however, to make up for the lack of snow due to the dominance of the western ridge for much of the month. By January 31^st, the CONUS snow coverage had decreased to 25.8 percent, which was the low point 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. With an upper-level ridge (over the West), northwesterly flow (over the central CONUS), and westerly flow dominating the upper-level circulation this month, severe weather was inhibited, with only 16 tornadoes reported this month (based on preliminary reports through January 27), which is below the January average of 35. Most of the tornadoes developed on January 21-22 in the Lower Mississippi Valley when a closed low moving in the upper-level westerly circulation dragged a cold front and surface low across the area, providing the dynamical uplift and energy needed to trigger violent weather. The development of tornadoes was inhibited during most of this month by the northwesterly flow across the central CONUS associated with the frequent long-wave troughs occurring across the East.

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 January 2018.

• The Atlantic hurricane season runs from June 1^st through November 30^th and the Eastern North Pacific hurricane season runs from May 15^th through November 30^th. No tropical storms or hurricanes developed in these basins during January 2018. No tropical cyclones formed in, or moved into, the central North Pacific.
• A tropical storm and two tropical disturbances developed in the western North Pacific in or near the U.S.-Affiliated Pacific Islands (USAPI) in Micronesia, but none of them had any significant effect on the USAPI. Tropical Storm Bolaven originated as a disturbance in the western USAPI early in the month, but didn't develop into a tropical depression until it crossed the Philippines and into a tropical storm until it reached the South China Sea. Two other disturbances (Invest 91W and 92W) developed in USAPI waters at the middle and end of the month, respectively, but both quickly dissipated.

North America monthly upper-level circulation pattern and anomalies. The upper-level circulation pattern during January, when averaged for the month, consisted of a ridge over the western CONUS and trough over the eastern CONUS. The ridge was the most significantly stronger than normal in the Southwest where the 500-mb height anomalies were largest. Above-normal height anomalies extended across much of North America.

Map of monthly precipitation anomalies. January was wetter than normal across parts of the Pacific Northwest to central Plains and Upper Midwest, across most of the Northeast, and a few parts of the Great Basin and Southeast. But drier-than-normal weather dominated, especially across the Southwest, southern Plains, Southeast, Midwest, and parts of the northern Plains. Alaska and Hawaii were mostly drier than normal, while San Juan, Puerto Rico, was wetter than normal.

Map of monthly temperature anomalies. Monthly temperatures were warmer than normal across the western CONUS and in Alaska, and cooler than normal from the Plains to East Coast.

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, and Europe/North Atlantic; and latitudinally (North Pacific/Arctic/North Atlantic). Especially notable was the dominance of above-normal height anomalies over the Arctic which spread equatorward in four directions. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation (over southwestern North America, western Russia); below-normal snow cover (over parts of western CONUS); above-normal surface temperatures over western North America, northwestern Russia, and eastern Siberia; and warm SST anomalies in the Bering Sea, 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; near- to below-normal surface temperatures over southeastern CONUS and Mongolia; cool SST anomalies in parts of the northeastern North Pacific; above-normal precipitation over western Europe and eastern Asia; and above-normal snow cover over Mongolia to northern China and the Sea of Japan coastal areas. Parts of southeastern North America, central Asia, central Africa to the southern Arabian peninsula, and northwest Australia 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 large parts of the continents still 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 January 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:

• 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: La Niña conditions were present during January 2018 with below-average SSTs across the central and eastern equatorial Pacific Ocean.
  • Teleconnections (influence on weather): To the extent teleconnections are known, the typical temperature and precipitation patterns associated with La Niña during January include above-normal temperatures across the southern Plains to Southeast; below-normal temperatures across the West and the northern Plains to Upper Midwest; near-normal temperatures from the central Plains to East Coast; above-normal precipitation in the Pacific Northwest to northern Rockies and northern California, and in the Tennessee to Ohio Valleys; and below-normal precipitation across most of the Southwest, southern Plains, and coastal Southeast to Mid-Atlantic region.
  • Comparison to Observed: The January 2018 temperature anomaly pattern does not agree with the teleconnections for La Niña; in fact, it is opposite. The January 2018 precipitation anomaly pattern agrees in the West, southern Plains, and coastal Southeast, but not in the Tennessee to Ohio Valleys.
• 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 began the month in phase 2, transitioned through phases 3, 4, and 5, ending the month in phase 6. During part of the month, the MJO destructively interfered with the La Niña, which reduced its influence, while it constructively interfered with it at other times. Other aspects of the tropical ocean-atmosphere system (referred to in the CPC reports as the background or low frequency [i.e., La Niña] state, Kelvin and Rossby waves, and tropical cyclone activity) also appeared to play a role in influencing the month's climate (especially Rossby waves and the base state, as well as Kelvin waves) (MJO updates for January 8, 15, 22, 29, and February 5).
  • Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the January (December-February) 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 January 2018, the MJO transitioned through five different phases throughout the month, with some of the phases having opposite teleconnections to preceding phases. Consequently, the monthly temperature anomaly pattern does not match the temperature anomaly patterns associated with the MJO phases experienced this month. The weekly temperature anomaly patterns, however, do show some agreement with the corresponding MJO phase teleconnections for December 31-January 6 (phase 2) (agrees except in the West), January 7-13 (phases 2-3) (agrees with phase 3 teleconnections), and especially January 21-27 (phases 4-5) (very good match). Neither does the monthly precipitation anomaly pattern match the precipitation anomaly patterns associated with the MJO phases experienced this month. But the weekly precipitation anomaly patterns show some agreement. January 7-13 (phases 2-3) was wet in parts of the West and central Plains; January 21-27 (phases 4-5) was wet into the Mid-Mississippi Valley and from the central Plains to western Great Lakes; and January 28-February 3 (phase 6) was dry in California and wet in Montana to Wyoming.
• 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 the first half of the month, negative for the second half, averaging near zero for the month as a whole. The 3-month-averaged index was negative.
  • Teleconnections (influence on weather): No teleconnections exist for a zero (neutral) PNA. However, 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 temperatures across the southern Plains to East Coast; above-normal temperatures across the West Coast and northern Plains to Alaska; below-normal upper-level circulation anomalies over southeastern North America; above-normal upper-level circulation anomalies over western North America; and below-normal precipitation in the Northwest, Great Basin, and most of the CONUS along and east of the Mississippi River.
  • Comparison to Observed: The January 2018 monthly upper-level circulation anomaly pattern over North America is similar to the teleconnections for a positive PNA, but the weekly anomaly patterns are a much better match, especially the patterns for January 1-5 and 12-17, which is when the PNA was positive. The monthly temperature anomaly pattern generally matches. The monthly precipitation anomaly pattern is a good match except in Washington State.
• 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 oscillated back and forth frequently between positive and negative, averaging negative for the month. The 3-month-averaged index was near zero.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO in January (December-February in the AO Composites table) is typically associated with above-normal upper-level circulation anomalies over the Arctic Ocean and Greenland; below-normal upper-level circulation anomalies over the eastern half of the CONUS; below-normal temperatures across most of the CONUS east of the Rockies; near-normal temperatures over the West and southern Plains; and drier-than-normal conditions from the southern Plains to Ohio Valley, and Southwest, with a few spots of wetter-than-normal conditions in northern California and New England.
  • Comparison to Observed: The January 2018 monthly upper-level circulation anomaly pattern matches over the Arctic, but not over North America or the North Atlantic. The monthly temperature anomaly pattern shows good agreement with the teleconnections east of the Rockies. The monthly precipitation anomaly pattern matches where teleconnections exist.
• 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 throughout the month, and averaged positive for the month. The 3-month-averaged index was 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 over the central to eastern CONUS, extending across the North Atlantic to Western Europe; above-normal temperature anomalies from the Plains to East Coast; below-normal circulation anomalies over northern Canada to Greenland; and drier-than-normal precipitation anomalies in California to Nevada, but the precipitation teleconnections are weak.
  • Comparison to Observed: The January 2018 precipitation anomaly pattern agrees with the teleconnections over California and Nevada, but that may be a coincidence since the precipitation teleconnections are weak. The upper-level circulation and temperature anomaly patterns show no agreement to the teleconnections.
• 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 positive, as was the three-month average WP index.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive WP during this time of year (January on the quarterly teleconnection maps) is typically associated with above-normal temperatures from the Midwest to Northeast; below-normal temperatures in the Southwest; above-normal circulation anomalies over eastern North America; below-normal circulation anomalies over the western CONUS; and above-normal precipitation from Oklahoma to the Ohio Valley.
  • Comparison to Observed: The January 2018 monthly upper-level circulation, temperature, and precipitation anomaly patterns do not match the teleconnections for a positive WP.
• 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 slightly negative during January.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative TNH is typically associated with warmer-than-normal temperatures for much of the country as well as eastern Canada, wetter-than-normal conditions in the West, and slightly drier-than-normal conditions in the Tennessee Valley. The upper-level circulation anomalies for a negative TNH are 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.
  • Observed: The January 2018 monthly upper-level circulation anomaly pattern agrees with the teleconnections for a negative TNH over the northeastern North Pacific and eastern Canada, but not over the southwestern CONUS and Gulf of Mexico to North Atlantic. The monthly temperature anomaly pattern does not agree, although there is some similarity with the weekly temperature anomaly patterns for the zonal weeks (the second week and last half of the month), but this may be a coincidence since the circulation pattern is not consistent. The monthly precipitation anomaly pattern agrees over the Tennessee Valley but not over the West, but that also may be a coincidence.
• The East Pacific-North Pacific (EP-NP) pattern
  • Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns (geopotential height anomalies) over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
  • Status: North Pacific SSTs warmed during January in the northwestern North Pacific (from Japan to the Bering Sea) and central North Pacific, but cooled in between those warming areas and in the southeastern North Pacific (off the coast of Mexico and Central America). These trends decreased the magnitude of both the warm and cold SST anomalies compared to December. The average sea level pressure (SLP) field map and SLP anomaly field map indicated that the North Pacific's Aleutian Low was weaker than normal and shifted (split) to the west and east, and the North Atlantic's Icelandic Low was also weaker and shifted. The average SLP field map and SLP anomaly field map over North America reflected the frequent passage of cold Arctic air masses, and their associated high pressure centers, across the eastern CONUS. The January 2018 monthly EP-NP index was positive, with the 3-month running mean slightly positive.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index this time of year (between October and April on the quarterly teleconnection maps) is typically associated with cooler-than-normal temperatures across the CONUS from the Great Plains to East Coast (except the Southeast) and across central to eastern Canada; above-normal temperatures over Alaska; above-normal upper-level circulation anomalies (stronger upper-level ridge) over Alaska and western Canada; and below-normal upper-level circulation anomalies across central to eastern North America (stronger upper-level trough) and over the central North Pacific. The precipitation teleconnections are weak but suggest drier-than-normal conditions over the Pacific Northwest and wetter-than-normal over Wyoming.
  • Comparison to Observed: The January 2018 monthly upper-level circulation anomaly pattern shows little agreement with the teleconnections associated with a positive EP-NP, but there is very good agreement with the anomaly patterns for the weeks of January 1-5 and 12-17. The monthly temperature anomaly pattern is a very good match, as are the weekly temperature anomaly patterns for December 31-January 6 and January 14-20, while the monthly precipitation anomaly pattern generally agrees where the few teleconnections exist.

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed January 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggest that La Niña had some influence on the weather over the CONUS in January, especially for precipitation, 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 observed anomalies showed little agreement with the teleconnections for the NAO, WP, and TNH, so the drivers behind these indices likely had little influence on this month's weather. The observed upper-level circulation anomaly patterns were a good match for the PNA and EP-NP during certain weeks, and the temperature anomaly patterns were a good match for the month and certain weeks, which indicated that the North Pacific drivers affecting the jet stream had a strong influence on the circulation over North America. The daily AO index oscillated between positive and negative frequently throughout the month, but the monthly value was negative and there was good agreement between the observed temperature and precipitation anomalies with the monthly teleconnections. This suggests that the Arctic drivers behind the AO had some influence on the month's weather, possibly being in synch with the North Pacific drivers. It's also possible that the agreement was a coincidence, but the widespread positive 500-mb height anomalies across the Arctic suggest otherwise.

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 Arctic Ocean. 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.

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