November 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 November 2018. The upper-level circulation was quite active this month and was locked into a longwave pattern in which a longwave trough dominated the central and eastern contiguous United States (CONUS) while a longwave ridge held sway along the West Coast. Shortwave ridges and troughs migrated through the jet stream flow, with the troughs and their associated fronts and surface lows bringing above-normal precipitation to the northern High Plains, East Coast, and South, and, later in the month, to parts of the West. As a result of the longwave trough, a northwesterly flow dominated the central CONUS, sending wave after wave of cold Canadian air masses into the heart of the nation, keeping temperatures below normal for most regions. The northwesterly flow over the central part of the country and its corresponding ridge out West kept much of the western CONUS and Great Plains drier than normal. The precipitation contracted drought and abnormal dryness in some regions, but drought and abnormal dryness expanded in the persistently dry areas and where impacts increased. In addition to precipitation, some of the fronts and lows brought severe weather, but tornado activity was roughly the long-term average for November. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather during November reflected the influence of atmospheric drivers originating in the Pacific Ocean, with a reinforcing influence of Arctic and North Atlantic drivers. 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, November marks the end of climatological fall (autumn) which is the time of year when solar heating decreases as the sun angle decreases, and an expanding circumpolar vortex forces the jet stream to migrate southward. Polar air masses influence the weather over the contiguous U.S. (CONUS) more, and the warm, dry subtropical high pressure belts influence the weather less.

500-mb mean circulation for the CONUS for November 1-20, 2018, showing a trough over the central CONUS.

500-mb mean circulation for the CONUS for November 21-30, 2018, showing the trough moderating and shifting east.

The upper-level circulation was quite active this month and was locked into a longwave pattern in which a longwave trough dominated the central and eastern contiguous United States (CONUS) while a longwave ridge held sway along the West Coast. Shortwave ridges and troughs migrated through the jet stream flow, but this longwave pattern held for the first two-thirds of the month. The trough moderated a bit and shifted east some during the last third of the month. This overall circulation regime therefore breaks the discussion of the month's weather into two parts.

500-mb mean circulation for the CONUS for November 1-20, 2018.

500-mb circulation anomalies for the CONUS for November 1-20, 2018.

Temperature anomalies (departure from normal) for the CONUS for November 1-20, 2018.

Precipitation anomalies (percent of normal) for the CONUS for November 1-20, 2018.

November 1-20: The trough over the central to eastern CONUS funneled cold Canadian air masses deep into the interior of the nation. The air masses were also dry, and the northwesterly flow kept Gulf of Mexico moisture out of the Great Plains to Great Lakes region. The Canadian cold fronts penetrated into the Deep South and out into the Gulf of Mexico. Low pressure systems developed along the fronts and moved across the Gulf Coast to East Coast, pulling in Gulf of Mexico moisture to bring above-normal precipitation across the Lower Mississippi and Ohio Valleys to the East Coast. The western ridge kept much of the West drier than normal during this period, and directed warmer-than-normal air into Alaska. Pacific weather systems rode over the top of the ridge, dropping precipitation across parts of the Pacific Northwest to northern and central Rockies and northern Plains — the precipitation was even above normal across Montana to the Dakotas, and near normal across parts of the Great Lakes. The Pacific weather systems plunged southward as they moved into the long-wave trough, with some being re-energized and contributing to the precipitation in the eastern CONUS. The dynamics associated with the upper-level short-wave troughs and lows, and lifting caused by their corresponding surface lows and fronts, frequently triggered outbreaks of severe weather. Most of the action occurred in the southern Plains to Southeast, with major outbreaks occurring on November 2^nd, 5^th, and 30^th. Wave after wave of cold Canadian air moved across the central CONUS, with the Canadian air masses spreading past the Rockies into the Great Basin. Only the West Coast and coastal Southeast averaged warmer than normal during this period. With the air below freezing behind the fronts, the frontal low pressure systems generated snow to the north and west of their track with rain south and east. National snow cover area increased dramatically during November 9-13. Snow storms came and went, with the actual regions covered by snow changing with each storm, but the total area covered by snow held fairly steady during November 14-20. With the western ridge keeping weather systems out of California, and high pressure over the Great Basin generating strong Santa Ana-type winds, several large wildfires spread rapidly in the state during this period.

500-mb mean circulation for the CONUS for November 21-30, 2018.

500-mb circulation anomalies for the CONUS for November 21-30, 2018.

Temperature anomalies (departure from normal) for the CONUS for November 24-30, 2018.

Precipitation anomalies (percent of normal) for the CONUS for November 24-30, 2018.

November 21-30: The long-wave trough over the central CONUS shifted further east beginning on November 21, enabling the western ridge to migrate to the east and spread out, or flatten. As the cold Canadian air masses shifted their path to the eastern CONUS, warmer-than-normal air began to dominate in the West and High Plains. With the flattening of the western ridge, Pacific weather systems were able to sweep across the West and into the Great Plains, bringing above-normal precipitation to much of the West Coast and Intermountain West, and from the central Rockies to central Plains. The rain helped firefighters contain then extinguish the California blazes, mountain snowpack increased (especially from California to the central Rockies), and the precipitation brought local relief from the ongoing drought. When the Pacific fronts reached the Plains, they encountered the colder-than-normal Canadian air which still dominated in the East. Below-freezing air in the Plains to East resulted in the weather systems producing snow, which kept the national snow cover area between 20 and 30 percent. The fronts and low pressure systems also triggered severe weather in the South, especially at the end of the month. With the storms mostly tracking across the central Plains, the northern and southern Plains were drier than normal during November 21-30, and the northwesterly flow across the central CONUS associated with the eastern trough blocked Gulf of Mexico moisture from the Great Lakes and much of the Southeast. The weather systems were able to tap Atlantic moisture to spread above-normal precipitation across the Mid-Atlantic to Northeast regions.

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

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

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

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

The pattern which dominated during November 1-20 was reflected in the full monthly pattern. An upper-level trough, with much-below-normal height anomalies, stretched from southeast Canada, across the Great Lakes, into the southern Plains. An upper-level ridge, with above-normal heights, dominated the West Coast and stretched north along the west coast of Canada to Alaska. Below-normal monthly temperatures dominated the CONUS, with above-normal temperatures evident only across the Florida peninsula and where the ridge dominated in the West — along the West Coast and across Alaska. 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 and northwesterly flow, or areas that missed out on the precipitation purely by chance. The circulation during this month was also reflected in severe weather, drought, and regional records.

• The extreme cold east of the Rockies was reflected in cold state temperature ranks. Five states ranked in the top ten cold category for November, including Missouri (fourth coldest in the 1895-2018 record), Arkansas (seventh coldest), Illinois and Oklahoma (both ninth coldest), and Mississippi (tenth coldest). Other "almost-top ten" states included Iowa (eleventh coldest) and Kansas and Maine (twelfth coldest). At the other end of the spectrum, California had the tenth warmest November in the 1895-2018 record and Alaska seventh warmest in its 1925-2018 record. The cold extremes were much more widespread, resulting in the CONUS ranking 27^th coldest for November. On a statewide basis, Delaware, Maryland, and Massachusetts had the wettest November in the 1895-2018 record, with 13 other states in the East ranking in the top ten wettest category. While there were dry areas in parts of the western and Plains states, nearby normal to wet areas kept any state from ranking in the top ten driest category. The wet conditions were widespread and extreme enough to give the nation a rank of 31^st wettest for November.
• When daily temperature records are examined, the extreme cold outweighed the extreme warmth. Integrated across the month, there were 3,447 record cold daily high (1,908) and low (1,539) temperature records. This was more than three times the 1,066 record warm daily high (409) and low (657) temperature records.
• As noted earlier, November marks the end of the Northern Hemisphere climatological autumn, which is the transition period between summer and winter when heating demand in the northern states increases while cooling demand in the southern states can still be significant. With colder-than-normal temperatures dominating most of the country east of the Rockies, especially the high population centers, heating demand was greater than normal. Consequently, the national REDTI (Residential Energy Demand Temperature Index) value for November 2018 ranked as the 25^th highest November REDTI in the 124-year record, very close to the 27^th coldest November ranking by area.
• Much of the precipitation during November fell on areas that were in drought or abnormally dry at the end of October, resulting in welcome drought improvement, while many other areas continued persistently dry. Drought and abnormal dryness contracted across parts of the West, Plains, Northeast, and Southeast. Contraction in the Southwest was partially due to precipitation that fell between November 30^th and the date of the U.S. Drought Monitor (December 4^th), as well as a re-assessment due to longer-term precipitation. The contraction was balanced by expansion in parts of the Far West, southern to central Plains, Southeast, Hawaii, and Puerto Rico. With contraction and expansion essentially balancing each other out this month, the national level drought area changed little, expanding from 22.0 percent of the CONUS at the end of October to 22.2 percent of the CONUS at the end of November (from 18.8 percent to 18.9 percent for all of the U.S.).
• The strong upper-level long-wave trough, which dominated the central to eastern CONUS for much of the month, funneled below-freezing air into the CONUS east of the Rockies. Weather systems moving along these cold fronts generated snow cover across the central to northern Plains, Ohio Valley to Great Lakes, and into the Northeast. When the western ridge weakened late in the month, Pacific weather systems increased the mountain snow cover in the West. Only about 6 percent of the CONUS was covered by snow when November began, but the national snow cover area rapidly increased during the second week when snowstorms dumped snow across the central Plains to Midwest. The national snow coverage hovered around 25 to 30 percent during the middle of the month, dipped to just below 20 percent on the 22^nd, but peaked at about 31 percent as the month ended. Based on satellite observations, the monthly snow coverage across the CONUS was 831 thousand sq. mi. (2.153 million sq. km.), 356 thousand sq. mi. (921 thousand sq. km.) above normal, and ranked as the third largest November snow cover area in the 1966-2018 satellite record. The above-normal snow cover across most of the CONUS and much of Canada resulted in the largest November snow cover area in the 53-year record for North America.
• November began with just one large wildfire burning in southwest Oregon. As the month progressed, dry weather and strong winds gave rise to several large wildfires in California. They persisted during the second and third weeks of the month, until the western ridge broke down and Pacific fronts and low pressure systems brought locally heavy rains which helped firefighters contain the fires. By November 30^th, most of the large wildfires in the West were extinguished, but several had broken out in Oklahoma (wildfire maps for November 1, 9, 12, 23, 28, 30).
• 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. In November, a longwave northwesterly flow dominated in the central part of the country, but several upper-level troughs and lows migrated through this pattern, dragging surface fronts and lows with them, and it was these systems that triggered the severe weather. The number of tornadoes for November 2018 (56 based on preliminary data) was close to the November average of 58. Most of the tornadoes and other severe weather occurred during three periods: on November 2^nd in the Northeast, and on the 5^th and 30^th 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 November 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.
• The North Atlantic basin was quiet this November, which is not uncommon. The only system of note was Hurricane Oscar, which developed in October. Oscar moved north and became embedded in the mid-latitude westerlies by the end of October. In early November, Post-Tropical Cyclone Oscar had transitioned into a powerful hurricane-force extratropical low attached to a frontal zone which was headed toward the Norwegian Sea.
• Two tropical systems were active in the Eastern North Pacific basin during November 2018, including Tropical Storm Xavier and a tropical disturbance. Xavier was being noticed as an area of low pressure at the end of October. This low pressure area developed into a tropical disturbance, then tropical depression, then tropical storm in early November. Tropical Storm Xavier lasted about a week. It was drawn toward the southwestern Mexico coast by a passing mid-latitude trough, before a subtropical ridge took over and turned it westward away from land; increasing shear and drier air eventually dissipated it. Near the end of the month, a weak disturbance formed southwest of Mexico, far out to sea, and lasted barely a day before dissipating.
• No tropical cyclones were active in the Central North Pacific basin during November, but a tropical disturbance (Invest 98C) developed early in the month southwest of Hawaii. Guided westward by the easterly flow on the southern side of the North Pacific High, 98C crossed over into Western North Pacific waters on November 7.
• Three typhoons (Yutu, Usagi, and Man-Yi), one tropical storm (Toraji), and a tropical disturbance (93W) were active in the western tropical Pacific during November. Three originated within, or crossed, the waters of the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia). Typhoon Usagi originated as a tropical disturbance (98C) in the Central North Pacific. The easterly trades on the south side of the North Pacific High guided 98C across USAPI waters as a disturbance and dry air in the upper levels of the atmosphere kept it from developing further in this region. 98C finally strengthened into Tropical Storm Usagi over the Philippine Sea, then into a typhoon over the South China Sea, before meeting its end when it made landfall over southern Vietnam. Super Typhoon Yutu developed in October and devastated Tinian and Saipan when it crossed the Marianas near the end of that month. Yutu had weakened to tropical storm strength by the first of November. It meandered in the South China Sea under weak steering currents for a couple days before cooler SSTs and increasing wind shear dissipated it in early November. Tropical Storm Toraji developed in the South China Sea, survived its encounter with Vietnam, re-emerged as a tropical disturbance in the Gulf of Siam, then encountered land again near the Thailand/Malaysia border. Typhoon Man-Yi developed as a disturbance in eastern Micronesian waters and moved westward under the easterly steering currents on the south side of the North Pacific High. It intensified to tropical storm strength in western Micronesia, then passed between Guam and Yap as a typhoon. Man-Yi took a more northerly track in the Philippine Sea as it moved along the western end of the North Pacific High, and increasing vertical wind shear weakened it before now-Tropical Depression Man-Yi was swept up in the mid-latitude westerlies to become an extratropical low. Tropical Disturbance 93W developed near the end of the month in eastern Micronesian waters. Unfavorable upper-level conditions, including moderate vertical wind shear and weak divergence aloft, inhibited development, with 93W dissipating as an organized system (opening up into a wave trough) at the end of the month.

North America monthly upper-level circulation pattern and anomalies. The upper-level circulation pattern during November, when averaged for the month, consisted of above-normal height anomalies with ridging across the West Coast extending into Alaska, and below-normal height anomalies with troughing across the central to eastern CONUS.

Map of monthly precipitation anomalies. Monthly precipitation was above normal across the Southeast and Northeast; parts of the Ohio Valley, Alaska, and northern Rockies to northern Plains; and the central third of California. November was drier than normal across the rest of the West and Great Plains; much of the Great Lakes to Mid-Mississippi Valley, Hawaii, Puerto Rico, and southern Florida; and parts of Alaska.

Map of monthly temperature anomalies. Monthly temperatures were colder than normal across most of the CONUS from the Intermountain West to the East Coast, and warmer than normal along the West Coast, much of Florida, and all of Alaska.

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 November 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.
  • Comparison to Observed: The November 2018 temperature and precipitation anomaly patterns do not match the teleconnections for El Niño or La Niña.
• 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 spent the first half of November moving through phases 1 through 3, then became incoherent for about a week, ending the month weakly in phases 7 and 8. Other aspects of the tropical ocean-atmosphere system (referred to in the CPC reports as the background or low frequency (or base) [e.g., La Niña or El Niño] state, and Kelvin and Rossby waves) also appeared to play a role in influencing the month's climate (especially Rossby waves and Kelvin waves) (MJO updates for November 5, 12, 19, 26, and December 3).
  • 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 November (October-December) 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 November 2018, the monthly temperature and precipitation anomaly patterns do not agree with the teleconnections for any of the MJO phases. The weekly temperature anomaly patterns do not agree with the corresponding MJO teleconnections for those weeks. There is some hint of agreement between the precipitation anomaly pattern for the last week (November 25-December 1) and the teleconnections for phases 7 and 8 (especially in the West), but not for the other weeks.
• 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 negative in the first half of the month, then went positive in the last half of the month, and averaged weakly 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 (October 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, and the U.S. West Coast, and cold temperature anomalies over Texas to the Southeast; and is weakly associated with below-normal precipitation in the Great Lakes.
  • Comparison to Observed: The November 2018 monthly upper-level circulation anomaly pattern agrees with the teleconnections for a positive PNA over western North America, but appears shifted over the North Pacific and eastern North America; however, there is better agreement in these areas for the last week when the daily PNA was positive. The monthly temperature and precipitation anomaly patterns agree over the CONUS where teleconnections exist. The weekly temperature anomaly pattern for the last week (when the PNA was positive) 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 neutral for the first half of the month, then turned strongly negative during the second half, and averaged negative for the month. The 3-month-averaged index was positive but decreasing.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO in November (October-December in the AO Quarterly Composites table) is typically associated with below-normal upper-level circulation anomalies over the eastern CONUS, extending across the North Atlantic, to Western Europe, and over the North Pacific; positive upper-level circulation anomalies over Greenland and the Arctic; below-normal temperatures across the northern Plains to the Northeast and Southeast; wetter-than-normal conditions in northern California and New England; and drier-than-normal conditions from the southern Plains and Midwest to Southeast.
  • Comparison to Observed: The November 2018 monthly upper-level circulation anomaly pattern is a good match with the teleconnections for a negative AO over North America, the North Atlantic, and the Arctic, but not over the Pacific. However, the upper-level circulation anomaly pattern for the last week of the month, when the AO was strongly negative, is a very good match in most places where there are teleconnections. The monthly temperature anomaly pattern is generally a good match, but the last week is an even better match. The monthly precipitation anomaly pattern has some agreement with the teleconnections, but (like for circulation and temperature) the last week has even better agreement.
• 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 near zero (neutral) for most of the month, then turned slightly negative at the end of the month, averaging slightly negative (near zero) for the month. The 3-month-averaged index was strongly positive.
  • Teleconnections (influence on weather): There are no teleconnections for a neutral (zero) NAO. But, to the extent teleconnections are known, a negative NAO during this time of year (October on the quarterly teleconnection maps) is associated with below-normal upper-level circulation anomalies from the eastern CONUS, across the Atlantic, to Europe; and above-normal circulation anomalies over northern Canada to Greenland. There are very few to no teleconnections over the CONUS or much of North America for temperature and precipitation for this time of year, except some hint of dryness in the Ohio Valley.
  • Comparison to Observed: The November 2018 monthly upper-level circulation anomaly pattern is a good match with the teleconnections for a negative NAO, with the upper-level circulation anomaly pattern for the last week of the month, when the daily NAO was negative, an even better match. There are no significant teleconnections for the monthly temperature and precipitation anomaly patterns to compare to.
• 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 negative (daily index values are not available).
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative WP during this time of year (October on the quarterly teleconnection maps) is typically associated with below-normal circulation anomalies over central Canada to the north central CONUS, and across the western North Pacific; above-normal circulation anomalies over the Pacific just west of the CONUS and over eastern Siberia; below-normal temperatures from northern Alaska, across western Canada, to the north central CONUS; and above-normal precipitation across the Pacific Northwest, southwestern Canada to the southern Prairies, Great Lakes, and Lower Mississippi Valley.
  • Comparison to Observed: The November 2018 monthly upper-level circulation anomaly pattern does not match the teleconnections for a negative WP. The temperature anomaly pattern agrees well over the CONUS where teleconnections exist (northern Plains), although this may be a coincidence since the Alaska anomalies don't match. The monthly precipitation anomaly pattern shows little agreement.
• 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: The November 2018 monthly EP-NP index was positive (daily index values are not available).
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive EP-NP index this time of year (October on the quarterly teleconnection maps) is typically associated with below-normal upper-level circulation anomalies over eastern Canada extending into the eastern CONUS, and over the North Pacific; above-normal upper-level circulation anomalies across Alaska and the Canadian West Coast, and from the Caribbean into the North Atlantic; warmer-than-normal temperatures across Alaska and the U.S. West Coast; below-normal temperatures across eastern Canada and the CONUS east of the Rockies (except for the Southeast); above-normal precipitation over southern Alaska and below-normal precipitation over the Pacific Northwest and western Canada (otherwise there are few precipitation teleconnections).
  • Comparison to Observed: The November 2018 monthly upper-level circulation and temperature anomaly patterns are a very good match with the teleconnections for a positive EP-NP. The monthly precipitation anomaly patterns are also a good match, although there are few precipitation teleconnections.

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed November 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggests that several atmospheric drivers influenced the weather this month, but the atmospheric driver behind the EP-NP pattern had the greatest influence. The equatorial Pacific was in an ENSO-neutral state and did not have an influence. The EP-NP had the best overall agreement with the observed anomaly patterns for all three variables (circulation, temperature, and precipitation), for both the monthly observed patterns and sub-monthly observed patterns. The NAO, AO, and PNA showed some degree of good agreement with the monthly circulation patterns, but they were especially in agreement for the last week of the month (for circulation). The PNA and AO were in good agreement for temperature during the last week. The AO had good agreement for precipitation during the last week, and the MJO had some agreement for precipitation during the last week.

This month illustrates how the atmospheric circulation for the month can reflect the influence of atmospheric drivers (or modes of atmospheric variability) originating in the North Pacific, with a reinforcing influence of drivers originating in the North Atlantic and Arctic, and to a lesser degree from the equatorial Pacific.

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