October 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 October 2018. The upper-level circulation was quite active this month with shortwave ridges and troughs migrating through the jet stream flow over the contiguous United States (CONUS). All of this activity took place within a bigger picture wherein a large broadscale/long-wave trough/ridge pattern shifted during the month. Fronts and low pressure systems traversed the country throughout the month, with temperatures switching from above normal to below normal and back again. When averaged across the month, October 2018 was generally colder than normal in the central and most of the western U.S., and warmer than normal in the Southeast to Mid-Atlantic. The fronts and lows brought above-normal precipitation to a broad swath of the central CONUS, from Texas to the Great Lakes. The remnants of tropical cyclones from the East Pacific contributed moisture to the fronts. Another swath of above-normal precipitation across the Southwest, and a third in the Southeast to Mid-Atlantic region, were also the result of moisture from tropical systems or their remnants. The rain in many areas fell where it was needed most, replenishing moisture supplies and helping to contract drought and abnormally dry areas. In addition to precipitation, some of the fronts and lows brought severe weather, with tornado activity roughly double the long-term average for October. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather during October reflected the influence of atmospheric drivers originating in the Pacific Ocean, with a moderating influence of Arctic and North Atlantic drivers, and tropical cyclones playing an important part. 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, October marks the middle 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 October 1-9, 2018.

500-mb mean circulation for the CONUS for October 10-15, 2018.

500-mb mean circulation for the CONUS for October 16-27, 2018.

500-mb mean circulation for the CONUS for October 28-31, 2018.

The highly-amplified upper-level circulation during October 2018 was true to form, with short-wave ridges, troughs, and lows migrating through the long-wave pattern, and the long-wave pattern itself shifting throughout the month. A long-wave trough dominated the West during roughly the first third of the month with a long-wave ridge (and Bermuda High) dominating the East and Southeast. The long-wave trough migrated toward the north central CONUS at mid-month with a ridge building along the West Coast and the Bermuda High hanging on in the Southeast. During the last half of the month, the trough shifted to the East with the ridge expanding across the West. The last few days of the month saw the pattern shift again, entering a state of flux.

500-mb mean circulation for the CONUS for October 1-9, 2018.

500-mb circulation anomalies for the CONUS for October 1-9, 2018.

Temperature anomalies (departure from normal) for the CONUS for October 1-8, 2018.

Precipitation anomalies (percent of normal) for the CONUS for October 1-8, 2018.

October 1-9: The western trough directed cool Pacific air masses and moisture into the West and cold Canadian air masses into the northern to central Plains. The Pacific fronts brought above-normal precipitation to much of the West during this period, while Gulf of Mexico moisture was drawn up to feed storms along the cold fronts in the Plains to Great Lakes. One storm system was particularly effective at producing severe weather in the Midwest during October 8^th-9^th. The upper-level ridge in the East worked in concert with the Bermuda High to block the fronts, keeping much of the country from the Lower Mississippi Valley to the Mid-Atlantic region warmer and drier than normal. However, by the end of this period, both the Bermuda High and western trough set up a southerly wind flow pattern which pushed Hurricane Michael toward the eastern Gulf of Mexico and a rendezvous with the Florida panhandle.

500-mb mean circulation for the CONUS for October 10-15, 2018.

500-mb circulation anomalies for the CONUS for October 10-15, 2018.

Temperature anomalies (departure from normal) for the CONUS for October 10-16, 2018.

Precipitation anomalies (percent of normal) for the CONUS for October 10-16, 2018.

October 10-15: This period began with Hurricane Michael smashing through the Florida panhandle and rushing across Georgia and the Carolinas, leaving rain and destruction in its wake. The long-wave upper-level trough shifted eastward, taking up residence over the central CONUS and funneling cold Canadian air masses across the Plains and into the East. The country saw the first notable taste of snow as snow cover in the Plains briefly expanded the national coverage to about 16.7 percent of the CONUS. The fronts and their surface lows gave much of the Plains above-normal precipitation. The eastern ridge was gradually pushed further east, but the Bermuda High struggled to keep its grip on the Southeast, with temperatures averaging above normal in these areas. Michael and fronts that pushed their way to the East Coast gave much of the area above-normal precipitation for this period, but some areas were left drier than normal. As the long-wave trough migrated east, a ridge set up over the West Coast. The resulting northwesterly flow kept precipitation drier than normal across much of the West, and below-normal temperatures lingered. Upper-level lows and troughing persisted across the Southwest, in spite of the western ridge. The southwesterly flow along the southern boundary of the troughs helped pull East Pacific Hurricane Sergio into northern Mexico, with moisture from Sergio's remnants crossing the border and helping bring above-normal precipitation to parts of the Southwest and southern Plains to Midwest.

500-mb mean circulation for the CONUS for October 16-27, 2018.

500-mb circulation anomalies for the CONUS for October 16-27, 2018.

Temperature anomalies (departure from normal) for the CONUS for October 15-28, 2018.

Precipitation anomalies (percent of normal) for the CONUS for October 15-28, 2018.

October 16-27: The long-wave trough shifted further east, battling with the Bermuda High for control over the Southeast but winning in the Northeast. The long-wave ridge moved over more of the West, but cutoff lows and short-wave troughs took their time leaving the Southwest. The ridge kept much of the West warmer and drier than normal, except for the Southwest where the troughing brought cooler- and wetter-than-normal weather. Like Sergio, moisture from the remnants of East Pacific Hurricane Willa was pulled into the southern states by the Southwest troughs, contributing to the above-normal precipitation in the Southwest and feeding fronts across the southern Plains. The northwesterly flow between the western ridge and eastern trough funneled cold Canadian air into the southern Plains, Midwest, and Northeast. Above-normal precipitation fell along frontal boundaries from the Tennessee Valley to Appalachians, but the northwesterly flow kept much of the Plains, Midwest, and Southeast drier than normal.

October 28-31: The long-wave pattern shifted again during the last four days of the month, with short-wave troughs propagating across the northern tier states, bringing areas of above-normal precipitation to the Northwest, northern Plains, and Northeast. One trough dug into the Southwest, generating above-normal precipitation for the Southwest and southern Plains but setting the stage for violent weather. Warmer-than-normal temperatures spread from the West to Plains states as the cold Canadian air masses shifted to the East and out to sea.

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

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

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

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

The long-wave circulation pattern averaged across the entire month shows the complex aggregated effect of the competing long-wave regimes, with the migratory short-wave troughs and ridges poured into the mix. Above-normal heights with ridging is evident along the West Coast and in the Southeast, while below-normal heights with troughing dominated in the Southwest and over the north central to northeast states. Monthly temperatures were above normal in the Southeast and along parts of the West Coast, but they were below normal across the rest of the CONUS. 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, as well as tropical systems. The dry areas resulted from persistence of upper-level ridging 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.

• While some areas averaged extremely cold (northern to central Plains) and extremely warm (Southeast and coastal California) for this time of year, only a few states had extreme statewide temperature ranks. North Dakota had the 12^th coldest October in the 1895-2018 record, South Dakota ranked 13^th coldest, and Minnesota 14^th coldest, while top ten warmest states included Florida (fourth warmest), Georgia (seventh warmest), and South Carolina (eighth warmest). The cold extremes were a little bit more widespread, resulting in the CONUS ranking 44^th coldest. On a statewide basis, Texas had the wettest October in the historical record, with seven other states in the Southwest, southern to central Plains, and Midwest having a top ten wettest rank. The states with the driest ranks were Florida (24^th driest) and California (40^th driest). With wet conditions dominating the country, October 2018 ranked as the sixth wettest October on record, nationally.
• When daily temperature records are examined, the extreme warmth and extreme cold nearly balanced each other out. When integrated across the month, there were 4,871 record warm daily high (1,302) and low (3,569) temperature records. This was just 1.1 times the 4,320 record cold daily high (3,139) and low (1,181) temperature records.
• As noted earlier, October is in the fall 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. The circulation pattern was highly meridional this month, with cold outbreaks occurring frequently in the central part of the country. Temperatures averaged across the country gave the CONUS the 44^th coolest October in the 1895-2018 record. With much of the below-normal temperatures occurring across the sparsely-populated West and northern Plains, and the month averaging warmer than normal in the higher population areas of the South and Mid-Atlantic regions, the national REDTI (Residential Energy Demand Temperature Index) value for October 2018 ranked near the middle of the 124-year historical record at 62^nd lowest October REDTI.
• Much of the precipitation during October fell on areas that were in drought or abnormally dry at the end of September, resulting in welcome drought improvement, especially east of the Rockies. Drought and abnormal dryness contracted across the West, Plains, Great Lakes, Northeast, and parts of the Southeast, and in Hawaii and Puerto Rico. Abnormal dryness expanded in other parts of the Southeast and a little in the southern Plains. Contraction outweighed expansion this month, so at the national level drought contracted from 29.0 percent of the CONUS at the end of September to 22.0 percent of the CONUS at the end of October (from 24.6 percent to 18.8 percent for all of the U.S.).
• The strong upper-level trough, that moved into the central CONUS beginning October 7^th, lingered for several days. The southerly flow on its front side drew in Gulf of Mexico moisture, while the northerly flow on its back side pulled in cold Canadian air. The Canadian air with the first cold front was below freezing, leaving behind a blanket of snow across the central Rockies to northern Plains which amounted to about 15 percent of the CONUS. A second front a couple days later brought the snow cover area to a peak of about 16.7 percent. It is early in the season, so the snow cover quickly melted when the trough migrated east and above-freezing air replaced the colder Canadian air. The month ended with snow covering about 5 percent of the CONUS — in parts of the Rockies and northern New England.
• October began with dozens of large wildfires burning across the West. Widespread rain and cooler temperatures during the first two weeks of the month helped quench most of the fires, so that only a handful were burning during the last half of the month (wildfire maps for October 4, 12, 19, 25, 31).
• 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, both as short-wave troughs and closed lows traversed the country and as the long-wave pattern shifted. The number of tornadoes for October 2018 (139 based on preliminary data) was roughly twice the October average of 61. Most of the tornadoes and other severe weather occurred during three periods: at the beginning of the month in the Northeast, around the 7^th-11^th in the Midwest, and at the end of the month in the southern Plains to Lower Mississippi Valley. A strong upper-level trough moved slowly across the central part of the CONUS during October 7^th-11^th. It was associated with low pressure and a cold front at the surface. The southerly flow at the surface and aloft funneled Gulf of Mexico moisture into the Midwest which provided the energy reserve of latent heat. The southerly flow and divergence in the upper-levels enhanced the uplift at the surface caused by the contrasting air masses (front) and convergence (surface low) to result in numerous outbreaks of tornadoes and other severe weather during these five days, especially during the 8^th and 9^th in the central Plains to Mid-Mississippi Valley. The tornadoes on the 2^nd were associated with a stationary front and low pressure system in the Northeast which provided the instability trigger. The tornadoes on the 31^st occurred along a cold front and surface low which moved into Texas and across the Lower Mississippi Valley in association with a strong upper-level trough.

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 October 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 busy this October, with four tropical systems (Hurricanes Leslie, Michael, and Oscar, and Tropical Storm Nadine) active during the month. Leslie began as a subtropical storm in the central North Atlantic in September and continued into October, lasting about a month. Leslie led a strange life, embedded at birth within an upper-level low with very light steering currents then circling aimlessly in the central Atlantic, pulled to and fro by various mid-latitude troughs. At one point it transitioned to a mid-latitude frontal low, then became cut-off from the mid-latitude flow, then became subtropical again, then took on the circulation features and deep-layer warm core structure of a tropical system, finally strengthening into a tropical storm near the end of September. In early October, Leslie made the big leagues when it strengthened into a hurricane. It weakened back to tropical storm strength after a few days, then intensified back into a hurricane. Finally pulled into the westerlies, Leslie rushed to the northeast as it transitioned into an extratropical cyclone and took aim at the Iberian peninsula in mid-October, threatening the Europeans with gale-force to near-hurricane force winds and heavy rain. Hurricane Michael was born in the western Caribbean Sea, brushing the Yucatan and bringing heavy rains and strong winds to western Cuba as it was drawn north toward the CONUS. The northerly flow on the west side of the Bermuda High and east side of a central U.S. trough accelerated Michael across the eastern Gulf of Mexico, where low wind shear and very warm sea surface temperatures led to its rapid intensification. It struck the Florida panhandle as a high end category 4 tropical cyclone, bringing heavy rain and widespread devastation. The strong winds associated with the upper-level trough pushed Michael rapidly across Georgia and the Carolinas, and it transitioned to a baroclinic post-tropical gale-force low as it became embedded within the trough's associated frontal zone. Carried by the westerlies, Michael left the CONUS along the Virginia coast, eventually dissipating over the eastern Atlantic. Tropical Storm Nadine formed in the eastern tropical Atlantic and moved northwest under the influence of a broad upper-level trough over the central Atlantic (the same one that played with Leslie). After a few days, Nadine succumbed to increasing wind shear, drier air, and slightly cooler SSTs, eventually dissipating in mid-October. Hurricane Oscar began as a tropical/subtropical disturbance in late October in the central Atlantic and quickly tangled with a developing upper-level low. After disengaging with the low, Subtropical Storm Oscar moved west on the southern side of an upper-level ridge, until it bumped into a mid-latitude trough exiting the U.S. and moving across the western Atlantic. Oscar intensified into a hurricane over the central Atlantic then was steered north by the upper-level trough. Oscar was still at hurricane strength when it became embedded within a frontal zone associated with the trough at the end of the month. Oscar maintained its strength as a powerful hurricane-force extratropical low as it rushed past Iceland in early November within the fast-moving mid-latitude westerlies.
• Five tropical systems were active in the Eastern North Pacific basin during October 2018, including Hurricanes Rosa, Sergio, and Willa, and Tropical Storms Tara and Vincente. Rosa and Sergio came to life in late September. Hurricane Rosa formed off the coast of southern Mexico and began moving west along the southern side of a ridge to the north, but was eventually pulled north by an upper-level trough moving across the western CONUS. Rosa had weakened to tropical storm strength by October 1^st as it bore down on Baja California. Moisture from the remnants of Rosa was pulled into the trough moving over the Southwest U.S. in early October and eventually fed a frontal system over the southern Plains. Hurricane Sergio followed a similar path to Rosa a week or two later — steered first by the easterly trades on the southern side of the subtropical ridge over Mexico, then pulled north by a trough over California. Moisture from the remnants of Sergio moved across northern Mexico and was caught up in the trough to feed storm systems over the U.S. Southwest, southern Plains, and Midwest. Like Rosa and Sergio, Tropical Storm Tara developed off the coast of southwest Mexico, but due to weak steering currents, this small cyclone stayed close to the coast. After only about four days, Tara moved inland and dissipated, dumping locally heavy rainfall across parts of southwest Mexico. Hurricane Willa developed at mid-month off the southwest coast of Mexico and moved north along the western edge of a subtropical ridge that extended from the Gulf of Mexico across mainland Mexico. Like Rosa and Sergio, Willa was pulled to the northeast by a trough over the western CONUS, making landfall over western Mexico. Moisture from the remnants of Willa fed the trough and its surface low and front over Texas and the Gulf of Mexico coast. Tropical Storm Vicente, another small tropical cyclone, developed in the last half of the month off the coast of Central America just a couple days after Willa. The easterly flow on the south side of the Mexican ridge pushed Vincente westward at first, but then it was pulled north when it encountered the eastern portion of Willa's circulation. Vincente came ashore in southern Mexico close to where Tara made landfall, leaving heavy rain and localized flooding in its wake. Several other low pressure centers developed in the eastern North Pacific off the coast of Mexico and Central America late in the month. Most never developed further, with one exception. A surface low at the end of the month would eventually develop in early November into Tropical Storm Xavier.
• One tropical system (Hurricane Walaka) was active in the Central North Pacific basin during October. Walaka developed in the central Pacific near the end of September well south of Hawaii and tracked west along the southern periphery of the North Pacific High, until a deep upper-level low in the central North Pacific pulled it north. Walaka was in a favorable environment of low vertical wind shear, high ocean heat content, warm SSTs, and a deep moist air mass, which led to its intensification to category 5 status. Its northward track occurred well west of Hawaii, so Walaka never posed a direct threat to the main islands, although Hurricane Warnings were issued for portions of the Papahanaumokuakea Marine National Monument. Another sharp upper-level trough moving across the North Pacific picked up the tropical cyclone, with Walaka eventually transitioning into a powerful extratropical low. In early October, Walaka was absorbed into the upper-level low and its associated surface cold front and raced into the Gulf of Alaska as a gale-force low (surface weather maps for October 3, 4, 5, 6, 7).
• Two typhoons (Kong-Rey and Yutu) and a couple tropical disturbances (95W and 96W) were active in the western tropical Pacific during October. Three originated within the waters of the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia). Typhoon Kong-Rey developed as a large but weak circulation in eastern Micronesian waters near the end of September. It tracked west-northwest across the USAPI, passing near the Marianas as a tropical storm. Kong-Rey strengthened to a typhoon over the Philippine Sea, briefly attaining super typhoon strength. The North Pacific High guided its path — the easterly trades on the southern side of the High pushed Kong-Rey westward across the USAPI waters, then the High steered it northward as the tropical cyclone moved along the High's western edge. Kong-Rey moved north across the East China Sea, brushed the southern coast of South Korea, and moved across northern Japan, where it became embedded in a mid-latitude trough and transitioned into an extratropical system, adding its energy and moisture to the westerlies in early October. Super Typhoon Yutu was born as a disturbance over the waters of eastern Micronesia in mid-October. It was in a favorable environment for development with high SSTs, weak vertical wind shear, and good upper-level outflow, so Yutu rapidly reached typhoon status. The easterly winds along the southern periphery of the North Pacific High steered the cyclone westward across the USAPI, with it taking direct aim at the Marianas Islands. Yutu strengthened to super typhoon status east of the Marianas, making a direct hit on the islands of Saipan and Tinian, with its estimated 180 mph winds causing catastrophic damage. High vertical wind shear and diminished poleward outflow weakened Yutu down to typhoon status as it tracked across the Philippine Sea. A strengthening of the North Pacific High in this area pushed Yutu in a more westerly to southwesterly direction just as it bore down on the Philippines, with Typhoon Yutu raking the northern island of Luzon. A midlatitude trough over China pulled Yutu to the north as it left the Philippines, then the steering currents weakened. Yutu stalled out over the South China Sea, where increasing vertical wind shear and cooler SSTs led to its dissipation in early November. Tropical Disturbance 95W formed south of Okinawa in early October and lasted just a couple days. Tropical Disturbance 96W formed in mid-October east of the Marianas and drifted to the north, dissipating a couple days later. Neither disturbance posed a threat to the USAPI.

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

Map of monthly precipitation anomalies. Monthly precipitation was drier than normal across Alabama, Florida, and Mississippi; much of Puerto Rico; and parts of the West Coast, northern High Plains, Ohio Valley, and southeast to central interior Alaska. October was wetter than normal across Hawaii, other parts of Alaska, much of the Great Lakes, along the Appalachian Chain, parts of the Pacific Northwest and northern Plains, and most of the Southwest and southern to central Plains.

Map of monthly temperature anomalies. Monthly temperatures were warmer than normal across the Southeast, Lower Mississippi Valley, Mid-Atlantic States, and Alaska, near to warmer than normal along the West Coast, and cooler to much cooler than normal across the interior West to Great Plains, Great Lakes, and northern New England.

Northern Hemisphere monthly upper-level circulation pattern and anomalies (cylindrical projection).

Northern Hemisphere monthly upper-level circulation anomalies (polar stereographic projection).

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 October 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 October 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 October transitioning from phase 1 to phase 2, then became incoherent during the last half of the month, except for the last couple days when it re-emerged weakly in phase 8. Other aspects of the tropical ocean-atmosphere system (referred to in the CPC reports as the background or low frequency [e.g., La Niña or El Niño] state, Kelvin and Rossby waves, and tropical cyclone activity) also appeared to play a role in influencing the month's climate (especially Kelvin waves and tropical cyclone activity) (MJO updates for October 8, 15, 22, 29, and November 5).
  • 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 October (September-November) 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 October 2018, the MJO spent some of the time in phase 1 (October 1-9) and phase 2 (October 10-14). The teleconnections for phase 1 show some agreement (in the West and Southeast) with the monthly temperature anomaly pattern and the weekly temperature anomaly pattern for October 1-8, but not in the northern Plains and Northeast. The temperature anomaly patterns for the month or next week (October 10-16) do not agree with the teleconnection pattern for phase 2. The teleconnections for phases 1 and 2 have considerable disagreement with the monthly precipitation anomaly pattern and the weekly precipitation patterns for October 1-8 and 10-16.
• 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 near zero for the first half of the month, then weakly positive for 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 warm temperature anomalies over Alaska, western Canada, and the U.S. West Coast, and cold temperature anomalies over Texas to the Southeast; is weakly associated with below-normal precipitation in the Great Lakes; and has 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.
  • Comparison to Observed: The October 2018 monthly upper-level circulation anomaly pattern agrees with the teleconnections for a positive PNA over the North Pacific and western North America, but appears shifted a bit, however it does not agree over the Southeast CONUS. The monthly temperature anomaly pattern agrees over Alaska, northwest Canada, and the U.S. West Coast, but not over the rest of North America. The monthly precipitation anomaly pattern does not agree with the teleconnections for a positive PNA, but the weekly precipitation anomaly pattern for October 15-28 (when the PNA was positive) does agree in the Great Lakes.
• 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 most of the month, then turned slightly negative, 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 AO in October (September-November in the AO Quarterly Composites table) is typically associated with below-normal upper-level circulation anomalies over the Arctic Ocean and Greenland into northern Alaska and the northern North Atlantic; above-normal upper-level circulation anomalies over the Great Lakes to eastern Canada, over the North Atlantic extending into Europe, and over parts of the North Pacific; above-normal temperatures across the northern Plains; near-normal temperatures elsewhere in the CONUS; wetter-than-normal conditions in western Washington; and drier-than-normal conditions in northern California, across the southern Plains to Tennessee Valley, and from the northern Plains to Northeast.
  • Comparison to Observed: The October 2018 monthly upper-level circulation anomaly pattern agrees with the teleconnections for a positive AO over eastern North America, across the North Atlantic, to Europe, but the anomalies appear shifted (especially shifted out of the western Arctic — Beaufort, Chukchi, and East Siberian Seas); however, the observed anomalies are opposite over the North Pacific. There is little agreement between the monthly precipitation anomaly pattern and the teleconnections, and the monthly temperature anomaly pattern is opposite.
• 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 most of the month, then turned slightly negative, and averaged positive for the month. The 3-month-averaged index was strongly positive.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a positive NAO during this time of year (October on the quarterly teleconnection maps) is associated with above-normal upper-level circulation anomalies from the eastern CONUS, across the Atlantic, to Europe; below-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.
  • Comparison to Observed: The October 2018 monthly upper-level circulation anomaly pattern is a good match with the teleconnections for a positive NAO, although the negative anomalies over Greenland and northern Canada extend further south over eastern Canada. There are no 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.
  • 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 October 2018 monthly upper-level circulation anomaly pattern does not match the teleconnections for a negative WP, but the temperature anomaly pattern agrees well over the CONUS where teleconnections exist. The monthly precipitation anomaly pattern matches in the Great Lakes but not elsewhere.
• 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 October 2018 monthly EP-NP index was positive.
  • 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 October 2018 monthly upper-level circulation and temperature anomaly patterns are a good match with the teleconnections for a positive EP-NP, although a little shifted. 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 October 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggests the atmospheric driver behind the EP-NP pattern had the greatest influence on this month's weather, but other drivers may have been influential, including tropical cyclones. 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, but seemed shifted a little, possibly due to the influence of the WP and MJO (for temperature) and PNA, AO, and NAO (for circulation). For temperature, WP's teleconnections include cold in the northern Plains, EP-NP is cold east of the Rockies (except the Southeast), and MJO (during the first week) has warmth in the East (which could account for the warmth in the Southeast in the monthly map). This combination best explains the observed temperature anomaly pattern. For circulation, the EP-NP captures the overall anomaly pattern over North America, but it is shifted due to the influence of the other drivers. NAO and AO pulled the eastern North America anomalies a little to the north, while EP-NP and PNA capture the anomaly pattern over the North Pacific (Aleutian Low region). Both the PNA and EP-NP teleconnections somewhat match the observed precipitation anomaly pattern, but tropical cyclones (a wild card) seemed to have the greatest impact on the precipitation pattern.

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 moderating influence of drivers originating in the North Atlantic and Arctic, and how precipitation can be strongly influenced by the wild card of tropical cyclones.

Citing This Report

Metadata