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.


The Earth's ocean-atmosphere system was technically in an ENSO-neutral state during May 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 eastern half of the country, especially from the Lower Mississippi Valley to Ohio Valley and central Appalachians. The heavy rains reduced drought from the Lower Mississippi Valley to Northeast, but persistent dryness expanded drought in parts of the Plains and Southwest. The fronts and upper-level lows triggered bouts of severe weather and kept temperatures cooler than normal across much of the central CONUS. The upper-level circulation, temperature, and precipitation anomaly patterns suggested the atmospheric drivers originating in the mid-latitudes, specifically the North Atlantic, 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, May marks the end 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.

Like April, May 2017, however, did not follow the usual climatological script. And like the previous month, a highly meridional and 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.

While deep closed lows and upper-level troughs moved across the CONUS throughout the month, sharp upper-level ridges popped up in between the troughs. The ridging drew warmer-than-normal air far to the north, with warm and cold anomalies migrating across the country as the month progressed (temperature anomaly maps for weeks 1, 2, 3, 4, 5). Averaged over the month, the lows and troughs (below-normal 500-mb heights) dominated east of the Rockies, with above-normal heights over the Northwest. A weakened North Atlantic (Bermuda) subtropical high went along with the dominance of troughs and upper-level lows in the eastern CONUS, while a slightly stronger North Pacific High was associated with the upper-level ridging in the Northwest. The migratory nature of the upper-level systems tended to average out the monthly temperatures. May temperatures averaged cooler than normal from the Southwest to Northeast in association with the below-normal upper-level heights, but only four states were cooler than normal on a statewide basis. Likewise, temperatures averaged warmer than normal in the West and Southeast, but only nine states were warmer than normal on a statewide basis. No state was much warmer or cooler than average. When averaged across the CONUS, May ranked only 55th warmest.

  • Some of the potent cold fronts brought much cooler-than-normal air behind them (temperature anomaly maps for weeks 1, 2, 3, 4, 5). 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 3,381 record warm daily high (1,187) and low (2,194) temperature records. This was just a little more than the 3,007 record cold daily high (1,973) and low (1,034) temperature records.
  • The REDTI (Residential Energy Demand Temperature Index) for May 2017 ranked near the middle of the 123-year historical record for May at 58th lowest. This was due to the mixed pattern of above- and below-normal temperatures and the time of year. Below-normal monthly temperatures in the Northeast increased heating demand. Below-normal monthly temperatures in the southern Plains reduced cooling demand, but above-normal temperatures in parts of the Southeast increased cooling demand. And these heating and cooling demand anomalies varied by region and week with each passing cold and warm front.
  • Some of the upper-level lows and their associated cold fronts had air that was cold enough for snow to form. About 10 percent of the CONUS was snow-covered as the month began. With the seasonal increase of temperatures, that percentage rapidly dropped to about 3 to 4 percent. At mid-month, a deep and slow-moving upper-level low pressure system, with its attendant cold front, pulled in below-freezing air which transformed the precipitation into snow across the central Rockies (snow cover maps for May 18, 19, 20). The national snow cover increased to about 8.5 percent on the 19th, but it rapidly fell as the system moved to the east and sunshine ate away at the snow cover. By the end of the month, the snow cover hovered around seasonal norms of about 2 percent.

The Climate Extremes Index (CEI) aggregates temperature and precipitation extremes across space and time. The occurrence of slow-moving deep closed lows resulted in extremes of precipitation. However, the frequent passage of cold fronts followed by warm fronts and ridges in the variable circulation evened out the temperature extremes. Consequently, while some components of the CEI ranked high, when the components are aggregated together the regional and national CEI values for May 2017 did not rank in the top ten category. Specifically, the East North Central region had the most extreme wet spell component and the Northwest region had the sixth most extreme wet spell component. The South region had the fourth most extreme one-day precipitation component, as did the Southeast region, and the Central region had the tenth most extreme one-day precipitation component. But the other components were not extreme enough to give any of these regions a top ten regional CEI. When aggregated across the nation, none of the national components ranked in the top ten category.

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 much of the CONUS east of the Rockies, resulting from troughs and closed lows frequently moving across the area, and above-normal height anomalies over the Northwest.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

There was a pronounced pattern of precipitation anomalies across the CONUS during May 2017, with wetter-than-average conditions dominating in southwest-to-northeast streaks from the central Plains to western Great Lakes, mid-Mississippi Valley to Northeast, and Lower Mississippi Valley to Southeast. The month averaged drier than normal across much of the West, northern Plains, southern Plains, and southern Florida. There was a mixed precipitation anomaly pattern across Alaska, especially for the high elevation (SNOTEL) stations; the low elevation stations were mostly wetter than normal. Hawaii had a mixed precipitation anomaly pattern, while Puerto Rico was mostly drier than normal.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The dynamic and meridional upper-level circulation pattern brought a mixture of cold and warm air masses migrating across the CONUS throughout the month. Their cold and warm anomalies balanced each other out so that the monthly temperature averaged near the middle of the historical record. Warmer anomalies tended to dominate the colder anomalies over the northwest and southeast portions of the CONUS, while the month averaged colder than normal across the central to northeast portions. Alaska averaged warmer than normal in the west and north, and colder 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. The usual east-west trough/ridge pairs (or couplets) and anomaly couplets were evident from the eastern Pacific, across North America and the North Atlantic, to western Europe. But north-south coupling was also apparent in latitudinal bands across Asia. The above-normal 500-mb heights were associated with upper-level ridging at the mid-latitudes, below-normal precipitation (over northern Europe), below-normal snow cover (over parts of western North America), and above-normal surface temperatures over parts of North America and Eurasia, 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 the eastern CONUS and northern Asia and parts of the North Pacific and North Atlantic; above-normal precipitation over the eastern CONUS and northern Asia; and above-normal snow cover over northern Asia. Much of northern Asia was 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 May 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 May 2017 temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in May was traced mostly to atmospheric drivers originating in the North Atlantic Ocean and possibly the equatorial Pacific. The PNA index was near zero, indicating little influence, and the patterns did not match the teleconnections for the AO, WP, and EP-NP, suggesting that these Arctic and North Pacific drivers also had little influence. There was good agreement between the anomaly patterns and NAO teleconnections, which indicated that the North Atlantic drivers had the strongest influence. But the anomaly patterns also suggested the equatorial Pacific might have had some influence. Even though the tropical Pacific was technically in an ENSO-neutral state, the SST anomaly pattern reflected what might be expected with an El Niño and the observed temperature anomaly pattern matches that expected with an El Niño. The monthly temperature and precipitation anomaly patterns also match those expected with a phase 3 MJO.

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 (North Atlantic) and possibly equatorial Pacific.

Citing This Report

NOAA National Centers for Environmental Information, Monthly Synoptic Discussion for May 2017, published online June 2017, retrieved on April 21, 2024 from