Synoptic Discussion - October 2014


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.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Summary


October 2014 was characterized by an active jet stream along the U.S.-Canadian border, with frequent vigorous short-wave troughs and ridges migrating through the upper-level circulation over the contiguous United States (CONUS). The competition between cold fronts cast southward by the jet stream and warm air masses associated with ridges and subtropical high pressure systems produced a net result of warmer-than-normal monthly temperatures across most of the country, with near- to below-normal temperatures beneath a dominating trough over the Great Lakes region. Fronts and surface low pressure systems, associated with the jet stream troughs, brought above-normal rainfall to parts of the country east of the Plains, along with areas of severe weather, but drier-than-normal weather beneath high pressure dominated much of the West, Plains, and coastal Southeast. The net result was a slight contraction in the national drought footprint. The upper-level circulation pattern, and temperature and precipitation patterns, suggest that the weather and climate of October 2014 were influenced, in part, by modes of variability associated with conditions in the North Pacific, North Atlantic, and Arctic Oceans, but that normal random variability of the atmosphere may have played a dominant role. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.

In the Northern Hemisphere, October is in 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 begin to influence the weather more, and the warm, dry subtropical high pressure belts influence the weather less. In October 2014, the jet stream hung back, staying active along the U.S.-Canadian border but sending frequent strong short-wave troughs over the CONUS, which frequently amplified the circulation into a strong meridional flow. Strong short-wave ridges were pushed up on either side of the troughs. The North Pacific and North Atlantic subtropical high pressure belts lingered in the southern CONUS and reinforced the short-wave ridges with warm air.

Animation of daily surface fronts and pressure systems for the month
Animation of daily surface fronts and pressure systems for the month.

These upper-level weather systems generated low pressure systems with their associated cold fronts at the surface — warm air flowed ahead of the lows, with cooler air surging southward behind them. The alternating short-wave troughs and ridges moving in the jet stream flow caused the temperature anomaly pattern to change week to week (weeks 1, 2, 3, 4, 5). This seesaw of warm and cold fronts produced 712 record cold daily high (506) and low (206) temperature records but five times as many (3,562) record warm daily high (1,484) and low (2,078) temperature records. This was reflected in the national monthly average temperature which ranked October 2014 as the fourth warmest October in the 1895-2014 record. It was also reflected in the 13th lowest REDTI (Residential Energy Demand Temperature Index) for October, indicating that the widespread above-normal temperatures reduced the energy needed to heat homes.

The fronts and surface low pressure systems associated with the jet stream troughs brought frequent days with rain and above-normal monthly total rainfall to the coastal Northwest and parts of the country east of the Plains. Subsiding air beneath the upper-level ridges and subtropical highs dominated much of the West, Great Plains, and coastal Southeast, reducing the number of rain days and keeping precipitation below normal. Drought contracted where it rained and expanded or intensified where it didn't, but the net change in drought area was a contraction of the moderate to exceptional national drought footprint compared to the end of September.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. While the aggregated national CEI for October ranked slightly above average, the regional CEI reflected the extreme weather out west. The second highest warm minimum temperature component, fourth most extreme drought component, and 11th highest warm maximum temperature component contributed to the fourth most extreme October CEI for the West region.

Areas of severe weather developed ahead of the troughs, especially during the first half of the month, with tornado outbreaks concentrated in the Midwest to Deep South. The preliminary count of 69 tornadoes compared to the average count of 61 tornadoes for October. The circulation pattern also helped to deflect the three North Atlantic tropical systems (Hurricanes Fay and Gonzalo and Tropical Storm Hanna) away from the CONUS.

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

When integrated across the month, the atmospheric circulation indicated a pattern of above-normal 500-mb heights (stronger-than-normal long-wave ridge) over the western CONUS, and another area of above-normal heights over north central and northeastern Canada and into the North Atlantic. The circulation pattern consisted of below-normal 500-mb heights (stronger-than-normal long-wave trough) over the eastern CONUS and northeastern North Pacific. This anomaly pattern masks the large variability in the circulation which resulted from the numerous short-wave ridges and troughs that migrated through the westerly flow.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Above-normal precipitation fell across parts of the Midwest, Tennessee Valley, Southern to Central Appalachians, New England, and coastal Northwest. Precipitation was below normal across the coastal Southeast and large parts of the West and Great Plains. October was drier than normal over much of Alaska and the northeast coasts of several of the Hawaiian Islands, but tropical system Ana contributed to above-normal rainfall across the rest of Hawaii.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

October temperatures were warmer than normal across most of the CONUS. Near- to below-normal temperatures occurred over the Great Lakes and Florida. The monthly temperature pattern was mixed over Alaska, but generally was warmer than normal in the north and in the panhandle, and cooler than normal in the southwest.

Global Linkages: The upper-level circulation over North America is part of the hemispheric mid-latitude westerly circulation. The circulation anomaly pattern over North America matches up with the anomaly pattern over Eurasia. The October mid-latitude (jet stream) pattern across the Northern Hemisphere consisted of four pairs of negative and positive anomalies — one pair spanned the North Atlantic and Europe, another covered Asia and the western North Pacific, and the remaining two pair stretched across North America from the eastern North Pacific to the western North Atlantic. Above-normal 500-mb heights, associated with ridging in the upper atmosphere, were reflected by above-normal temperatures at the surface over Europe, northeastern Siberia, and much of North America. Below-normal 500-mb heights, associated with troughs in the upper atmosphere, were reflected by below-normal temperatures and above-normal precipitation at the surface over central Siberia, and above-normal snow cover across much of Siberia.


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:


Upper-level circulation pattern and anomalies averaged for the last three months
Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed October 2014 temperature, precipitation, and circulation patterns, suggest that the weather over the CONUS in October was related to the normal chaotic nature of the transition between seasons, but there were hints that the jet stream and ocean-atmosphere interactions over the North Pacific, North Atlantic, and Arctic exerted some limited influence. The MJO was mostly weak or incoherent and ENSO was neutral, but equatorial Pacific oceanic conditions were slowly evolving toward an El Niño state. The anomaly patterns suggest that Arctic (AO), Atlantic (NAO), and Pacific (PNA) drivers had some influence on the circulation, but the competition resulted in an upper-level circulation pattern that didn't perfectly match any of the teleconnections. The October temperature anomaly pattern over the CONUS did not match any of the teleconnections, except those for the WP over the Great Lakes, which suggests the WP had an influence on temperature. Monthly precipitation anomaly patterns are usually more chaotic than temperature patterns, but the limited agreement with the teleconnections suggested the EP-NP, WP, AO, and perhaps even the MJO may have contributed to the October precipitation anomalies; but, on the other hand, these agreements may have been pure coincidence.

This month illustrates how weather and climate anomaly patterns can be the manifestation of normal (random) atmospheric variability, but also how they reflect influences from several atmospheric drivers (or modes of atmospheric variability).

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for October 2014, published online November 2014, retrieved on November 23, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/.