Synoptic Discussion - April 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 was in an ENSO-neutral state during April 2017. The weather over the contiguous United States (CONUS) this month was dominated by a highly variable jet stream circulation which sent many closed lows, fronts and low pressure systems across the country. The weather systems dropped above-normal precipitation across much of the country, especially in the West, Plains, Midwest, and parts of the Southeast, which significantly reduced areas of drought and abnormal dryness. They also triggered bouts of severe weather. Upper-level ridging during the middle of the month, and a stronger than normal Bermuda High off the East Coast, helped keep temperatures warmer than normal in many areas, especially east of the Mississippi River. The upper-level circulation, temperature, and precipitation anomaly patterns suggested the atmospheric drivers originating in the mid-latitudes 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, April marks the middle of climatological spring which is the time of year when solar heating increases with the rising sun angle, arctic air masses are not as cold, and a contracting circumpolar vortex forces the jet stream northward. Polar air masses influence the weather over the contiguous U.S. (CONUS) less, and the warm, dry subtropical high pressure belts influence the weather more.

April 2017, however, did not follow the usual climatological script. A highly variable upper-level circulation sent many troughs and slow-moving closed lows across the CONUS which left bouts of cooler weather, heavy precipitation, and severe storms in their wake, especially at the beginning and end of the month.

Even though deep closed lows affected the southern states at the beginning and end of the month, the upper-level circulation was dominated by ridging during the middle of the month. The ridging drew warmer-than-normal air far to the north (temperature anomaly maps for weeks 1, 2, 3, 4). Averaged over the month, this upper-level ridging combined with a stronger than normal Bermuda High off the East Coast to give much of the CONUS, especially east of the Mississippi River, a warmer than normal April. Eight states, from the Ohio Valley to mid-Atlantic coast, had the warmest April in the 1895-2017 record, with the CONUS ranking eleventh warmest. Cooler-than-normal monthly temperatures in the northwestern CONUS were associated with persistent troughing and below-normal heights in the upper-level circulation across the North Pacific and into the Pacific Northwest.

  • Some of the potent cold fronts brought much cooler-than-normal air behind them (temperature anomaly map for week 4). That, and cloudiness and rain associated with the closed lows, kept daily temperatures low, while the clear skies and southerly flow associated with the upper-level ridging brought warm daily temperatures. When integrated across the month, there were 3,126 record warm daily high (989) and low (2,137) temperature records. This was over three times the 962 record cold daily high (749) and low (213) temperature records.
  • The REDTI (Residential Energy Demand Temperature Index) for April 2017 ranked lowest in the 123-year record for the month. This was due to above-normal temperatures across much of the CONUS which reduced heating demand in the high population centers.
  • Some of the upper-level lows and their associated cold fronts had air that was cold enough for snow to form. The month began with about 15 percent of the CONUS snow-covered, thanks to a large low pressure trough in the jet stream flow. Upper-level ridging brought warmer temperatures which inhibited snow formation and melted existing snow cover, with snow coverage decreasing to a minimum of about 5.6 percent on the 17th. Another upper-level low near the end of the month brought colder temperatures and new snow, with the CONUS snow cover ending the month at about 14 percent.

The occurrence of slow-moving deep closed lows results in extremes of precipitation and temperature, while persistent ridging in the upper atmosphere frequently causes warm temperature extremes. The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The temperature and precipitation extremes that occurred during the month, especially in the eastern CONUS, gave several regions a top ten integrated regional CEI for April 2017. The Northeast region had the most extreme warm minimum and fifth most extreme warm maximum temperature components, and most extreme 1-day precipitation component, which contributed to the most extreme regional CEI in the 1910-2017 April record. The Southeast region had the second most extreme warm maximum and fifth most extreme warm minimum temperature components, second most extreme 1-day precipitation component, and tenth most extreme long-term drought component, contributing to the second most extreme regional CEI. The Central region had the most extreme warm minimum and tenth most extreme warm maximum temperature components, and eighth most extreme 1-day precipitation component, contributing to the ninth most extreme regional CEI. When aggregated across the nation, April 2017 had the second most extreme 1-day precipitation component, seventh most extreme days with precipitation component, and tenth most extreme warm minimum temperature component. But this combination was not enough to push the national CEI rank into the top ten category — the CONUS had only the 20th most extreme national April 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 northwestern CONUS, resulting from troughs frequently moving across the area, and above-normal height anomalies over the southwest and eastern CONUS.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

April 2017 was wetter than normal across much of the West, central to southern Plains, Mississippi Valley, Northeast, and Mid-Atlantic region; most of Puerto Rico; and parts of Hawaii. Drier-than-normal weather occurred across parts of the Southwest, northern Plains, and Southeast; parts of Hawaii; and most of Alaska.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

April 2017 was warmer than normal in most of Alaska and across the CONUS along and east of the Mississippi River, cooler than normal to near normal in the Pacific Northwest to Great Basin, and near to warmer than normal in the Plains to Southwest.

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. The usual east-west trough/ridge pairs (or couplets) and anomaly couplets were mostly absent this month. Instead, a band of negative 500-mb height anomalies could be seen across the Pacific and positive anomalies stretched across the Atlantic. Also, north-south anomaly couplets were more evident than east-west couplets. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (in western Europe), below-normal snow cover (over northern China), and above-normal surface temperatures over the eastern CONUS and southwestern Europe, and in parts of the North Atlantic and North Pacific. The areas of below-normal 500-mb heights were associated with upper-level troughing; near- to below-normal surface temperatures over the northern Europe and parts of the North Pacific and North Atlantic; above-normal precipitation over western North America and north central Asia; and above-normal snow cover over northwestern Russia. With large parts of the continents having warmer-than-normal temperatures, and large portions of the Atlantic and Pacific Oceans having warmer-than-normal sea surface temperatures, the April 2017 global temperature was 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 April 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in April was traced mostly to atmospheric drivers originating in the mid-latitudes, specifically the North Pacific and North Atlantic Oceans. But none of the indices were a perfect match. The tropical Pacific was in an ENSO-neutral state and the MJO was mostly incoherent. The PNA and AO teleconnections did not match the observed anomaly patterns. There were areas of agreement between the observed circulation anomaly pattern and the teleconnections for the NAO, WP, and EP-NP. None of them matched perfectly by themselves, but taken together they agreed across most of North America and the North Pacific. The observed temperature anomaly pattern might be a good match for the NAO teleconnection if it were shifted laterally a bit. The movement of large closed lows in the upper-level circulation, and the cooler-than-normal temperatures associated with their cloudiness and precipitation, could affect the monthly temperature anomaly pattern to disrupt a teleconnection match. The precipitation teleconnections for all of the circulation indices were either weak or not strongly correlated with the observed precipitation anomaly pattern.

This month illustrates how the weather and climate anomaly patterns can be influenced by atmospheric drivers (or modes of atmospheric variability) originating in the mid-latitudes. It also illustrates how high amplitude circulation anomalies (slow-moving closed lows) can add a random element which disrupts the influence of the main atmospheric drivers.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for April 2017, published online May 2017, retrieved on May 26, 2017 from https://www.ncdc.noaa.gov/sotc/synoptic/201704.

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