Synoptic Discussion - October 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 entered into a weak La Niña state during October 2017. The upper-level circulation pattern underwent a couple significant shifts over the contiguous United States (CONUS) at mid-month and a little later in the month, changing from a predominantly trough-West/ridge-East pattern during the first half of the month to a predominantly zonal flow for the middle third of the month, ending with mostly a ridge-West/trough-East pattern for the last third of the month. The temperature anomaly pattern concurrently switched from a cold-West/warm-East pattern during the first half of the month to a cold-South-and-Southeast/warm-West-and-North pattern for the last half of the month. Fronts and low pressure systems brought above-normal precipitation to the Pacific Northwest, central Plains to Great Lakes, and, with help from the remnants of Hurricane Nate and Tropical Storm Philippe, to the Southeast to New England. The weather systems mostly missed much of the rest of the West, far northern and southern Plains, Lower Mississippi Valley, and parts of the coastal Southeast, where it was drier than normal. The precipitation helped shrink drought and abnormal dryness, especially in the Northwest, Midwest, and Northeast, while drought and abnormal dryness expanded where it was dry in the Southwest and Lower Mississippi Valley. The upper-level circulation, temperature, and precipitation anomaly patterns suggested that the atmospheric drivers originating in the Pacific likely had an influence on the month's weather. 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, 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 1-16, 2017, showing a long-wave trough in the West and ridge in the East
500-mb mean circulation for the CONUS for October 1-16, 2017, showing a long-wave trough in the West and ridge in the East.
500-mb mean circulation for the CONUS for October 17-23, 2017, showing mostly a zonal flow
500-mb mean circulation for the CONUS for October 17-23, 2017, showing mostly a zonal flow.
500-mb mean circulation for the CONUS for October 23-31, 2017, showing a long-wave ridge in the West and trough in the East
500-mb mean circulation for the CONUS for October 23-31, 2017, showing a long-wave ridge in the West and trough in the East.

During October 2017, the atmospheric circulation continued in a highly meridional state. A meridional circulation pattern transfers heat from the low latitudes to higher latitudes and transports colder air masses from the poles southward, which helps to balance the distribution of heat across the Earth. But the circulation behaved in an unusual way by "flipping" modes a couple times during the month, changing from a stable long-wave trough-West/ridge-East pattern for the first half of the month to a pattern consisting of a ridge in the West and trough in the East for the last third of the month, with a variable (mostly zonal) pattern in between. Each of these patterns was characterized by specific weather phenomena, temperature anomalies, precipitation anomalies, and impacts.


500-mb circulation anomalies for the CONUS for October 1-16, 2017
500-mb circulation anomalies for the CONUS for October 1-16, 2017.
Temperature anomalies (departure from normal) for the CONUS for October 1-16, 2017
Temperature anomalies (departure from normal) for the CONUS for October 1-16, 2017.
Precipitation anomalies (percent of normal) for the CONUS for October 1-16, 2017
Precipitation anomalies (percent of normal) for the CONUS for October 1-16, 2017.

Cold fronts and surface low pressure systems dived into the trough over the West during the first half of the month, frequently stalling out as they moved into the Plains. They picked up Gulf of Mexico moisture after they crossed the Rockies, resulting in above-normal precipitation from the central Plains to Great Lakes. The remnants of Hurricane Nate became entwined along a cold front (that penetrated into the Southeast on October 8th) and moved up the Appalachians, resulting in above-normal precipitation for that region. But much of the West, southern and northern Plains, and East Coast were dry. The colder Pacific and Canadian air that sank over the West beneath the trough kept temperatures colder than normal. In the East, a southerly flow of warm air combined with subsidence beneath the ridge to bring warmer-than-normal temperatures.


500-mb circulation anomalies for the CONUS for October 17-23, 2017
500-mb circulation anomalies for the CONUS for October 17-23, 2017.
Temperature anomalies (departure from normal) for the CONUS for October 17-23, 2017
Temperature anomalies (departure from normal) for the CONUS for October 17-23, 2017.
Precipitation anomalies (percent of normal) for the CONUS for October 17-23, 2017
Precipitation anomalies (percent of normal) for the CONUS for October 17-23, 2017.

Fronts and surface lows moved across the CONUS in a more westerly flow during the middle of the month, but the air masses were not as cold so temperatures were near to above normal for most of the country. The absence of cold air masses was especially noticeable in the northern Plains to Great Lakes, where temperatures were much warmer than normal. Pacific fronts brought above-normal precipitation to the Pacific Northwest, and Gulf of Mexico moisture was drawn along a cold front bringing above-normal precipitation to parts of the South to Midwest, but much of the rest of the country was drier than normal during this period.


500-mb circulation anomalies for the CONUS for October 23-31, 2017
500-mb circulation anomalies for the CONUS for October 23-31, 2017.
Temperature anomalies (departure from normal) for the CONUS for October 25-31, 2017
Temperature anomalies (departure from normal) for the CONUS for October 25-31, 2017.
Precipitation anomalies (percent of normal) for the CONUS for October 25-31, 2017
Precipitation anomalies (percent of normal) for the CONUS for October 25-31, 2017.

When the upper-level circulation pattern shifted to a ridge in the West and trough in the East during the last third of the month, cold Canadian air masses were funneled far to the South, plunging into the Gulf of Mexico and northern Mexico. As a result, below-normal temperatures dominated much of the country east of the Rockies. The storm track frequently brought surface lows across the Great Lakes. The warm southerly flow ahead of the lows kept New England warmer than normal for much of this period. The surface lows and fronts pulled in Atlantic moisture, including the remnants of Tropical Storm Philippe near the end of the month, giving much of the Mid-Atlantic to Northeast wetter-than-normal weather. The ridge in the West kept the weather warmer than normal. With the ridge driving weather systems to the north, most of the western CONUS was drier than normal, and the northwesterly flow aloft kept Gulf of Mexico moisture out of the central CONUS, with drier-than-normal Canadian air masses dominating there.


The regional and meridional characteristics of the atmospheric circulation and temperature and precipitation anomalies become hard to identify at the monthly level because the averaging process masks out the extremes that occurred during the three parts of the month.

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 entered into a weak La Niña state during October 2017. The Atlantic basin saw more active tropical cyclone activity than average while the East Pacific basin had near to slightly less than average tropical cyclone activity. But this is late in the season for both of these basins.

  • The Atlantic hurricane season runs from June 1st through November 30th and three tropical systems (Hurricanes Nate and Ophelia, and Tropical Storm Philippe) were active in the North Atlantic during October. Two of the systems interacted with cold fronts and upper-level troughs over the eastern CONUS. Hurricane Nate formed early in the month in the western Caribbean and brushed Central America and Mexico before entering the Gulf of Mexico and taking aim at the Mississippi coast. After bringing rain to the Southeast, Nate's remnants were swept up in a cold front and rapidly moved to the northeast. Hurricane Ophelia developed at mid-month in the central North Atlantic and eventually made its way to the British Isles where it took aim at Ireland. Tropical Storm Philippe originated in the western Caribbean near the end of the month. It crossed Cuba and the southern tip of Florida before encountering a cold front along the eastern seaboard. Moisture from Philippe's remnants was drawn in to the frontal system and contributed soaking rains which alleviated drought and dryness in the Northeast.
  • The Eastern North Pacific hurricane season runs from May 15th through November 30th and three tropical systems (Tropical Storms Ramon and Selma and Tropical Disturbance 91E) were active during October. Both Ramon and Selma formed in the southern portion of the region and were short-lived. Ramon formed early in the month and brushed southern Mexico before dissipating. Selma formed near the end of the month and brought rain to Central America when it came ashore at El Salvador. Tropical Disturbance 91E formed at mid-month well west of Mexico and lasted only a couple days before dissipating. All of these systems were too far south to interact with the circulation over the CONUS.
  • No tropical cyclones formed in, or moved into, the Central North Pacific.
  • Three named tropical systems (Typhoons Khanun, Lan, and Saola) were active in the western Tropical Pacific near the U.S.-Affiliated Pacific Islands (USAPI) in Micronesia, but (other than rain) none of them had any significant effect on the USAPI. A couple other tropical disturbances briefly appeared within USAPI waters but quickly dissipated before developing further. Khanun developed west of the USAPI and eventually made landfall over Southeast Asia. Lan spent several days in the Philippine Sea, just west of the USAPI, bringing waves and rain to Yap and Palau, before moving north towards Japan. Saola formed over western Micronesia, passing near the Marianas as a tropical storm. Saola intensified into a typhoon over the Philippine Sea before moving north and brushing Japan. Saola's remnants were swept up in the westerlies as an extratropical low, eventually contributing rain to parts of North America.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. During October 2017, the meridional circulation resulted in some extreme weather, but the flipping of the circulation during the month tended to dampen the effect of the extreme weather on the cumulative CEI. Several regions had top ten or near-top ten CEI components. Only the Northeast region — with its most extreme warm minimum temperature component, fifth most extreme warm maximum temperature component, and 19th most extreme 1-day precipitation component — ranked in the top ten category regionally, having the fifth most extreme October CEI in the 1910-2017 record. Even with the second most extreme 1-day precipitation component, fifth most extreme wet spell component, eighth most extreme days with precipitation component, and tenth most extreme warm minimum temperature component, the East North Central region had only the 16th most extreme October CEI. The Southwest region had the eighth most extreme days without precipitation component, and the Northwest region had the sixth most extreme 1-day precipitation component, but neither of these regions had a regional CEI that was even in the top 30 category. None of the national CEI components ranked in the top ten category, and the October CEI for the nation ranked only 34th most extreme out of 108 years.

North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern during October changed dominant modes several times during the month. During the first half of the month, it mostly consisted of a trough in the West and ridge in the East. During the middle third of the month, it mainly consisted of a more westerly or zonal flow with mostly above-normal height anomalies. During the last third of the month, it mainly consisted of a trough along the Mississippi Valley and ridges along the West Coast and from New England to the Canadian Maritime Provinces. When these contrasting patterns are averaged over the month, the pattern consists of a weak trough from central Canada into the central CONUS with positive height anomalies, or ridges, along the West and East Coasts. Of the circulation indices usually discussed on this page, the teleconnections for the PNA and EP-NP come closest to the October 2017 500-mb circulation anomalies over the CONUS.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

The October precipitation anomaly pattern was driven largely by the precipitation along fronts and upper-level troughs. During the first half of the month, precipitation fell from the central Plains to Great Lakes, in the central Gulf of Mexico coast, and along the Appalachians. During the second half of the month, precipitation was above normal in the Pacific Northwest and in the eastern third of the CONUS. When the two halves of the month are combined, these areas were wetter than normal, while much of the Southwest, southern and northern Plains, Lower Mississippi Valley, and parts of the coastal Southeast were drier than normal. The month was wetter than normal across most of Alaska and Hawaii.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The monthly temperature anomaly map represents a mixture of two dichotomous patterns. During the first half of the month, temperatures were much warmer than normal across the eastern half of the CONUS, beneath the predominant upper-level ridge, and much below normal across the West, beneath the predominant upper-level trough. The pattern shifted at mid-month, with troughs funneling cold fronts into the eastern CONUS which brought below-normal temperatures to the South, while above-normal temperatures dominated in the western and northern states. When averaged together, the monthly temperature anomaly pattern across the CONUS consisted of below-normal temperatures dominating in the Pacific Northwest to central Rockies and across parts of the southern Plains, with above-normal temperatures prevalent east of the Mississippi, especially in the Northeast, and in parts of the Southwest. Alaska was mostly warmer than normal for the month.

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, the North Atlantic, and the North Pacific; and North Atlantic/Western Europe, North Atlantic/North America, and North Pacific/North America. There were also significant differences in the upper-level circulation anomaly patterns at shorter time scales (first half of the month, middle third, and last third). The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes; below-normal precipitation (over western Europe and northwest Siberia); above-normal surface temperatures over eastern North America, western Europe, and northwest Siberia; and warm SST anomalies in the North Atlantic and parts of the North Pacific. The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over central Canada and much of Russia; cool SST anomalies in parts of the western North Pacific; above-normal precipitation over central Canada, eastern Europe, western Russia, and eastern Siberia; and above-normal snow cover over much of Canada and Russia. Large parts of Asia and North America 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 and Pacific Oceans having warmer-than-normal sea surface temperatures, the October 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 October 2017 monthly temperature, precipitation, and circulation anomaly patterns, suggest that it was difficult to trace the weather over the CONUS in October to one specific atmospheric driver. Rather, the weather was influenced by most if not all of the drivers to some degree. The PNA had the best match for temperature, precipitation, and upper-level circulation, both on a monthly basis and especially when the month was broken down into halves matching specific circulation regimes. The monthly upper-level circulation anomaly pattern was best explained by a combination of the EP-NP and PNA teleconnections. The AO and NAO upper-level circulation teleconnections have some agreement over the CONUS, but that may be a coincidence since there is little agreement in other parts of the world. The PNA teleconnections best explained the temperature anomaly patterns for the first and second halves of the month, although the MJO teleconnections matched the mid-month pattern. The temperature anomaly pattern over the CONUS, for the month as a whole, was best explained by the EP-NP teleconnections, while the WP teleconnections were the only ones that were remotely close to the observed anomaly pattern in Alaska. The temperature teleconnections for AO and La Niña also matched the observed anomaly patterns where teleconnections exist, but the AO similarity may have been a coincidence. No teleconnection pattern perfectly matched the observed precipitation anomaly pattern, but there seemed to be elements of several, including La Niña, MJO, PNA, AO, NAO, and EP-NP.

This month illustrates several things. First, it illustrates how the weather and climate anomaly patterns are influenced by atmospheric drivers (or modes of atmospheric variability) originating in the Pacific Ocean. Second, it demonstrates how the anomaly patterns can result from a combination of many drivers. 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 October 2017, published online November 2017, retrieved on August 18, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201710.

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