Synoptic Discussion - November 2017


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 weak La Niña state during November 2017. The upper-level circulation pattern underwent a shift over the contiguous United States (CONUS) at mid-month, changing from a mostly zonal pattern during the first half of the month to a predominantly ridge-West/trough-East pattern for the last half of the month. The temperature anomaly pattern was predominantly colder than normal in the northern states and warmer than normal in the southern states during the first half of the month, then colder than normal in the East and warmer than normal in the West for the last half, with the monthly pattern being a mixture of the two. Fronts and low pressure systems brought above-normal precipitation to the Pacific Northwest, northern Rockies, and parts of the Midwest. But the western ridge deflected weather systems away from the rest of the West, and a northwesterly flow blocked Gulf of Mexico moisture from most of the Plains to East Coast, so these regions ended the month drier than normal. The precipitation helped contract drought and abnormal dryness in the Northwest, northern Rockies, and parts of the Midwest, but drought and abnormal dryness expanded or intensified in the Southwest, southern Plains, Lower Mississippi Valley, and Southeast where it was dry. The upper-level circulation, temperature, and precipitation anomaly patterns suggested that the atmospheric drivers originating in the equatorial Pacific (i.e., La Niña) likely had a controlling influence on the month's weather, but drivers originating in other regions may also have had an influence. See below for details.


Synoptic Discussion


Animation of daily upper-level circulation for the month
Animation of daily upper-level circulation for the month.
Animation of daily surface fronts and pressure systems 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 1-14, 2017, showing a zonal flow
500-mb mean circulation for the CONUS for November 1-14, 2017, showing a zonal flow.
500-mb mean circulation for the CONUS for November 14-27, 2017, showing a long-wave ridge in the West and trough in the East
500-mb mean circulation for the CONUS for November 14-27, 2017, showing a long-wave ridge in the West and trough in the East.

The upper-level circulation was very active during November 2017 with several short-wave troughs migrating across the CONUS in the jet stream flow. Two general circulation regimes dominated this month. A zonal circulation pattern held sway during the first half of the month. A zonal circulation is generally a west-to-east flow that keeps cold air masses bottled up to the north and warmer air masses concentrated to the south. It reduces north-south movement of air masses and inhibits the atmosphere's efforts to transfer heat from the tropics towards the poles to balance out the distribution of heat across the Earth. The circulation pattern shifted to a highly meridional state during the second half of the month. A meridional circulation consists of large undulations of the jet stream to the north and south, frequently resulting in stable long-wave troughs and ridges. Troughs transport colder air masses from the poles southward, and ridges transfer heat from the low latitudes to higher latitudes, thus balancing out the Earth's surface heat in the long run. Each of these patterns was characterized by specific weather phenomena, temperature anomalies, precipitation anomalies, and impacts.


500-mb circulation anomalies for the CONUS for November 1-14, 2017
500-mb circulation anomalies for the CONUS for November 1-14, 2017.
Temperature anomalies (departure from normal) for the CONUS for November 1-14, 2017
Temperature anomalies (departure from normal) for the CONUS for November 1-14, 2017.
Precipitation anomalies (percent of normal) for the CONUS for November 1-14, 2017
Precipitation anomalies (percent of normal) for the CONUS for November 1-14, 2017.

A generally westerly flow in the jet stream held sway during the first half of the month, with the short-wave troughs and storm track staying mostly across the northern states. Pacific low pressure systems brought above-normal precipitation to the Pacific Northwest and northern Rockies, but dried out by the time they reached the Plains and Upper Midwest. Enough Gulf of Mexico moisture was drawn into the low pressure systems to produce above-normal precipitation in the Ohio Valley to southern Great Lakes, but otherwise the air masses remained mostly dry. Cold Canadian air masses were pulled in behind the surface low pressure systems, but the westerly flow kept the coldest air across the northern states to central Plains, where temperatures averaged below normal for the period. Some cold fronts managed to reach the Gulf of Mexico coast, but they were greatly modified and generally diffuse by the time they reached the coast. The westerly jet stream flow kept temperatures warmer than normal and precipitation mostly drier than normal across the southern states.


500-mb circulation anomalies for the CONUS for November 14-27, 2017
500-mb circulation anomalies for the CONUS for November 14-27, 2017.
Temperature anomalies (departure from normal) for the CONUS for November 14-27, 2017
Temperature anomalies (departure from normal) for the CONUS for November 14-27, 2017.
Precipitation anomalies (percent of normal) for the CONUS for November 14-27, 2017
Precipitation anomalies (percent of normal) for the CONUS for November 14-27, 2017.

During the second half of the month, the circulation pattern shifted into a mostly upper-level ridge over the West with an enhanced trough over the eastern CONUS. Canadian cold fronts beneath the trough routinely plunged south to the Gulf of Mexico coast and beyond, resulting in below-normal temperatures for most of the CONUS east of the Mississippi River. Descending air (subsidence) beneath the upper-level ridge over the West kept skies sunny; this coupled with an associated southerly flow to bring well above-normal temperatures to the western and central CONUS. Pacific weather systems slammed into the western branch of the ridge, bringing above-normal precipitation to the Pacific Northwest, northern Rockies, and northern California. But the storms were directed to the north into Canada, leaving the Southwest and most of the Great Plains drier than normal. With a northwesterly flow over the central part of the country blocking Gulf of Mexico moisture, most of the CONUS from the Plains to East Coast was drier than normal. The exception, once again, was the southern Great Lakes, where enough moisture was squeezed out of the atmosphere by the low pressure weather systems to result in above-normal precipitation.

During the last three days of the month, the long-wave ridge/trough pattern broke down, but warmer-than-normal temperatures continued to dominate and slowly push the cold anomalies further to the east (2-week temperature anomaly maps for November 14-27, 15-28, 16-29, 17-30).


500-mb mean circulation for the CONUS for November 2017
500-mb mean circulation for the CONUS for November 2017.
500-mb circulation anomalies for the CONUS for November 2017
500-mb circulation anomalies for the CONUS for November 2017.
Temperature anomalies (departure from normal) for the CONUS for November 2017
Temperature anomalies (departure from normal) for the CONUS for November 2017.
Precipitation anomalies (percent of normal) for the CONUS for November 2017
Precipitation anomalies (percent of normal) for the CONUS for November 2017.

When conditions are averaged across the entire month, the circulation anomalies associated with the western ridge tended to dominate the monthly pattern, while the monthly temperature anomaly pattern reflected a merging of the anomaly patterns of the two halves of the month. The precipitation anomaly patterns for the two halves were similar and tended to reinforce each other in the monthly precipitation anomaly pattern. The circulation during this month was also reflected in snow, drought, and regional records.

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 weak La Niña state during November 2017. Both the Atlantic and East Pacific basins saw about average tropical cyclone activity. But this is late in the season for both of these basins when average activity is low.

  • The Atlantic hurricane season runs from June 1st through November 30th and one tropical system (Tropical Storm Rina) was active in the North Atlantic during November. Rina stayed well out to sea, moving north and eventually becoming an extra-tropical storm.
  • The Eastern North Pacific hurricane season runs from May 15th through November 30th. No tropical storms or hurricanes developed during November; however, far out in the Pacific west of Mexico, tropical disturbance 94E showed potential for a couple days at mid-month but never developed further.
  • No tropical cyclones formed in, or moved into, the Central North Pacific.
  • Three named tropical systems (Typhoon Damrey and Tropical Storms Haikui and Kirogi) and one tropical disturbance were active in the western Tropical Pacific near the U.S.-Affiliated Pacific Islands (USAPI) in Micronesia, but none of them had any significant effect on the USAPI. Damrey, Haikui, and Kirogi all developed west of the USAPI near the Philippines, then moved west to eventually make landfall on or near Vietnam. The tropical disturbance, 91W, developed within USAPI waters near Pohnpei at mid-month, but quickly dissipated.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Although temperature and precipitation extremes occurred on a regional basis, they were not widespread enough to rank the national CEI in a top ten category for November. Even with the 12th most extreme warm minimum temperature component and 13th most extreme warm maximum temperature component, the national CEI ranked only 53rd most extreme for November. Of the nine CEI regions, only the Southwest region — with its most extreme warm minimum temperature component, third most extreme warm maximum temperature component, and 19th most extreme days without precipitation component — ranked in the top ten category regionally, having the second most extreme November CEI in the 1910-2017 record. The West region had the second most extreme warm minimum temperature component and 12th most extreme warm maximum temperature component; the East North Central region had the fifth most extreme wet spell component; and the Northeast region had the tenth most extreme 1-day precipitation component — but none of these regions had a regional CEI that was in the top ten category.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

November was wetter than normal across most of the Pacific Northwest to northern Rockies and parts of northern California, Florida, and the Midwest. But drier-than-normal weather dominated the rest of the CONUS. Alaska was wetter than normal in the west and north, but drier than normal in the south to southeast. In Hawaii, much of the Big Island was wetter than normal, but drier-than-normal weather dominated the stations on the rest of the islands. In Puerto Rico, November was wetter than normal at San Juan.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

Monthly temperatures were warmer than normal across much of the West and Great Plains, and cooler than normal in the Northeast, Great Lakes, extreme northern Plains, and Mid-Atlantic coast. Alaska was warmer than normal in the north and west, and cooler than normal in the southeast.

Northern Hemisphere monthly upper-level circulation pattern and anomalies
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, North America/North Atlantic/Europe, and Siberia/North Pacific; and latitudinally across North America (CONUS/Canada/Arctic). The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation (over southern CONUS and western Russia); below-normal snow cover (over the CONUS); above-normal surface temperatures over the southwestern CONUS, western Russia, and extreme eastern Siberia; and warm SST anomalies in the northern North Atlantic and north central North Pacific. The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over Canada and central to southeastern Siberia; cool SST anomalies in parts of the western North Pacific (Sea of Okhotsk); above-normal precipitation over Canada, northwest CONUS, and western Europe; and above-normal snow cover over much of Canada, Scandinavia, and northeast China. Large parts of Canada and Russia were near to cooler than normal. But with large parts of the continents still having warmer-than-normal temperatures, and large portions of the Atlantic, Pacific, and Indian Oceans having warmer-than-normal sea surface temperatures, the November 2017 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:


Examination of the available circulation indices and their teleconnection patterns, and comparison to observed November 2017 monthly temperature, precipitation, and circulation anomaly patterns, suggest that La Niña had a strong influence on the weather over the CONUS in November, but that other drivers may have had some limited influence. Both the AO and NAO were positive during the first half of the month and negative during the second half, with the NAO averaging zero (neutral) for the month. The global monthly upper-level circulation anomaly pattern did not match the teleconnections for the AO and NAO, but the upper-level circulation anomaly pattern for the first half of the month was a reasonable match for the teleconnections for the positive phase of the AO and NAO, and the upper-level circulation anomaly pattern for the second half of the month was a reasonable match for the teleconnections for the negative phase of the AO and NAO. So the Arctic and North Atlantic drivers may have had some influence on the circulation on the short (weekly) time scales. But, since the temperature and precipitation anomaly patterns didn't match the AO and NAO teleconnections, other drivers must have been controlling those. The monthly circulation anomaly pattern showed some similarity to the teleconnections for the PNA and WP. The circulation and temperature anomaly patterns for the EP-NP have some similarity to the observed anomaly patterns for the last half of the month. But in each of these cases, the patterns were somewhat shifted. This suggests the North Pacific drivers may have had some broader-scale influence but that their influence was not ironclad. The temperature and precipitation anomaly patterns matched the teleconnections for La Niña very well, but they also matched the teleconnections for the MJO near the end of the month when the MJO was in phase 4. It is possible that the MJO and La Niña were in phase at that time and reinforced each other.

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. Second, it demonstrates how the circulation anomaly patterns can result from a complex interaction of many drivers operating at different time scales. 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.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for November 2017, published online December 2017, retrieved on June 19, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201711.

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