Synoptic Discussion - January 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.


NCEI added Alaska climate divisions to its nClimDiv dataset on Friday, March 6, 2015, coincident with the release of the February 2015 monthly monitoring report. For more information on this data, please visit the Alaska Climate Divisions FAQ.


Summary


The Earth's ocean-atmosphere system transitioned into an ENSO-neutral state during January 2017. The weather over the contiguous United States (CONUS) this month was dominated by strong troughs and lows moving in the fast jet stream flow. Cold fronts and surface lows dragged along by the upper-level troughs brought areas of precipitation to much of the country. Precipitation was above normal in most areas of the CONUS, improving drought conditions. Only a few parts of the country experienced a drier-than-normal month. The troughs tended to congregate in the western CONUS and spread their associated cold fronts east across the country. The result was a colder-than-normal month from the Pacific Northwest to Northern High Plains, with near to below-normal temperatures in the Southwest and warmer-than-normal temperatures from most of the Plains to the East Coast. The upper-level circulation, temperature, and precipitation anomaly patterns suggested the atmospheric drivers originating in the Pacific Ocean had the greatest 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, January is the middle month of climatological winter which is the time of year when solar heating is at its minimum due to the low sun angle, and an expanded circumpolar vortex results in the furthest southern extent of the jet stream. Polar air masses dominate the weather over the contiguous U.S. (CONUS), and the warm, dry subtropical high pressure belts have their least influence.

A strong westerly flow dominated the upper-level circulation across the CONUS during January 2017. Troughs and cutoff lows moving in the jet stream flow dragged cold fronts and surface low pressure systems with them which brought areas of precipitation to much of the country. Only a few areas of the CONUS had a drier-than-normal month and included parts of the Pacific Northwest, Mid-Mississippi Valley, Rio Grande Valley, and East Coast. The rest of the country was wetter than normal with nine states in the West, Central Plains, Upper Midwest, and Southeast having the tenth wettest, or wetter January in the 1895-2017 record.

The precipitation improved drought conditions across much of the CONUS, decreasing the national moderate-to-exceptional drought footprint from 22.5 percent at the end of December to 14.6 percent at the end of January (from 18.8 percent to 12.2 percent for all of the U.S.). January began with several large wildfires burning in the Southern Plains, mostly in Oklahoma (wildfire maps for January 1, 10, 13, 26, 31). Widespread rains at mid-month helped reduce the number of fires (precipitation anomaly maps for weeks 1, 2, 3, 4).

The upper-level troughs and lows tended to favor the western CONUS. Their associated cold fronts and moist Pacific air masses kept temperatures cooler than normal for most of the month (temperature anomaly maps for weeks 1, 2, 3, 4). For the month as a whole, temperatures were cooler than normal across much of the West into the Northern Plains, but warmer than normal in the Southern Plains and from the Mississippi River to the East Coast where downstream ridging in the jet stream contributed to a southerly flow which brought in milder Gulf of Mexico air. Twenty states from the Southern Plains to Northeast had a top ten warmest January. The REDTI (Residential Energy Demand Temperature Index) for January 2017 ranked ninth lowest in the 123-year record for January. This was due to above-normal temperatures in the high-population centers of the South, Midwest, and East which reduced heating demand. There were 5,849 record warm daily high (2,299) and low (3,550) temperature records. This was about two and a half times the 2,318 record cold daily high (1,266) and low (1,052) temperature records that were associated with the air masses behind the cold fronts. The combination of warm and cold extremes resulted in the 18th warmest January, nationally, in the 1895-2017 record.

In spite of the downstream ridging in the jet stream, cold fronts and low pressure systems still moved across the eastern CONUS. Some of the fronts and surface lows were very strong and combined with upper-level lows to create unstable air and dynamics favorable for severe weather, especially on the 2nd and 20th-22nd. Based on preliminary data, there were 138 tornadoes in January 2017 (through January 27), which is four times the January average of 35. Most occurred in the Southern Plains to Southeast where Gulf of Mexico moisture was drawn in to the powerful upper-level troughs and surface fronts and low pressure systems.

Below-freezing air behind the cold fronts and surface lows caused precipitation to fall as snow instead of rain. The national snow cover area hovered around 40 to 45 percent of the CONUS for much of the month, with generally below-freezing air maintaining the snow cover in the West and North. An especially potent front and low pressure system spread snow across the Southern Plains and Southeast early in the month, with the CONUS snow cover area peaking at about 66 percent on the 8th. Based on the 1966-2017 satellite record, January 2017 had the 13th largest January snow cover area when averaged across the month and across North America.

The tropical Pacific ocean-atmosphere system transitioned to ENSO-neutral conditions during January, but La Niña-like tropical convection patterns lingered. La Niña typically suppresses hurricane activity in the central and eastern Pacific basins, and enhances it in the Atlantic basin. However, the tropical cyclone season in the North Atlantic and eastern tropical Pacific oceans normally ends in November. During January 2017, no tropical systems developed in these two basins. But a couple tropical disturbances developed in the western tropical Pacific which brought abundant rainfall to U.S.-Affiliated Pacific Islands. None of these developed into tropical storms or hurricanes (typhoons).

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. Temperature and precipitation extremes occurred during the month in many areas, and several regions had component extremes in the top ten category, but none of the regions had a top ten integrated regional CEI. The Northeast region had the fourth most extreme warm minimum temperature component and tenth most extreme warm maximum temperature component. The East North Central region had the fifth most extreme wet spell component and sixth most extreme warm minimum temperature component. The Central region had the sixth most extreme warm minimum temperature component and ninth most extreme days with precipitation component. The Southeast region had the eighth most extreme warm minimum temperature component. The South region had the seventh most extreme warm minimum temperature component. The Southwest region had the eighth most extreme days with precipitation component. The West region had the fourth most extreme days with precipitation component and seventh most extreme one-day heavy precipitation component. When aggregated across the nation, January 2017 had the fourth most extreme warm minimum temperature component and tenth most extreme days with precipitation component, but only the 24th most extreme national January CEI in the 108-year record.

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

The upper-level circulation pattern, averaged for the month, consisted of below-normal upper-level (500-mb) height anomalies over the western CONUS, resulting from troughs frequently moving across the area, and above-normal height anomalies over the eastern CONUS. The below-normal heights reflected a weakening of the Aleutian Low and an increase in cyclonic activity over western North America. Of the circulation indices usually discussed on this page, the teleconnections aggregated for a negative PNA and positive WP most closely match the January 2017 500-mb circulation anomalies.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

Precipitation was above normal across much of the CONUS. Pockets of below-normal precipitation could be found in the Northwest, mid-Mississippi Valley, Rio Grande Valley, and East Coast. Hawaii was drier than normal and a mixed anomaly pattern was seen over Alaska and Puerto Rico.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

January 2017 was cooler than normal across much of the West into the Northern Plains, and warmer than normal in the Southern Plains and from the Mississippi River to the East Coast. Alaska was warmer than normal in the north and cooler than normal in the south central coastal region.

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 couplets could be seen (trough or below-normal heights over the western CONUS, southern Europe, western Siberia, and western North Pacific coupled with ridge or above-normal heights over eastern North America/North Atlantic, eastern Asia, northern North Pacific) as well as north-south couplets (southern Europe/northern Europe). The below-normal heights over the Arctic could conceivably be linked to several of the above-normal/below-normal height pairs to the south of the Arctic. These couplets illustrate the wave nature of the atmosphere. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (in Europe), below-normal snow cover (over parts of eastern Asia), and above-normal surface temperatures over much of Canada, the eastern CONUS, northern Europe, and eastern Asia, and in parts of the North Atlantic. The areas of below-normal 500-mb heights were associated with upper-level troughing, near- to below-normal surface temperatures over northwest CONUS and southern Europe, above-normal precipitation over the southwestern CONUS, and above-normal snow cover over the southwestern CONUS and southern to eastern Europe. With the Americas and much of Africa and Asia having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the January 2017 global temperature was above normal, but the cooler-than-normal temperatures over parts of Europe and North Africa and parts of the oceans had a moderating effect.


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 January 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in January was traced mostly to atmospheric drivers originating over the North Pacific and equatorial Pacific Oceans, with possibly some influence from the Arctic. The precipitation teleconnections for all of the circulation indices were either weak or not strongly correlated with the observed precipitation anomaly pattern. However, there was some agreement between the observed pattern and that expected with this month's MJO phases and with the PNA. The temperature anomaly pattern appears to be an integration of the patterns associated with the PNA, WP, and AO drivers, with influence from the MJO some weeks, and possible some residual influence from La Niña. The NAO temperature anomaly pattern agrees east of the Rockies, but that may have been coincidence since the upper-level circulation pattern isn't a good match. The circulation anomaly pattern fits very well as an integration of the patterns associated with the PNA and WP drivers.

This month illustrates how the weather and climate anomaly patterns can be strongly influenced by atmospheric drivers (or modes of atmospheric variability) originating in the Pacific Ocean, with Arctic drivers possibly playing a role.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for January 2017, published online February 2017, retrieved on March 1, 2017 from http://www.ncdc.noaa.gov/sotc/synoptic/201701.