Synoptic Discussion - April 2018

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 transitioned from a weakening La Niña state to an ENSO-neutral state during April 2018. Like the last several months, the upper-level circulation pattern was quite active this month with ridge and trough patterns migrating through the jet stream flow over the contiguous United States (CONUS). The monthly averaged circulation pattern ended up consisting of a slight long-wave ridge in the West and trough in the East, with colder-than-normal temperatures across most of the CONUS east of the Rockies, warmer-than-normal temperatures temperatures in the Southwest, wetter-than-normal bands across the Far West, northern Rockies to northern High Plains, Lower Mississippi Valley, and along the East Coast, and drier-than-normal conditions across the Southwest to interior West, most of the Great Plains, and Mid- to Upper Mississippi Valley. The persistent dryness led to worsening drought conditions, especially from the Southwest and southern Plains to Mid-Mississippi Valley, while the wet conditions contracted drought and abnormal dryness in the Southeast and Far West. Drought contraction outpaced expansion, so the national drought footprint shrank a bit when compared to the beginning of April. The active fronts and low pressure systems generated severe weather — but mostly in the Lower Mississippi and Ohio Valleys to Southeast where Gulf of Mexico moisture and clashing air masses provided the energy and dynamics — and, in combination with the cold temperatures, kept a snow cover active across the northern Plains to Great Lakes. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the atmospheric drivers originating in the equatorial Pacific (i.e., La Niña and MJO) and North Pacific (WP) may have had a controlling 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 2018, however, was an aberration.

The upper-level circulation over the CONUS has been very active throughout the winter and so far this spring, with an almost never-ending parade of short-wave troughs and closed lows moving west-to-east in the jet stream flow. The air circulation around these weather systems forces warm air north toward the pole and cold polar air masses south. This back-and-forth movement of air masses helps balance out the Earth's temperature. Without it, the poles would get colder and colder and the tropics warmer and warmer; the redistribution of heat across the planet helps make Earth habitable. These patterns usually vary around the hemisphere, but sometimes they can set up camp in a region and stay awhile. This month, cold weather claimed squatters rights over much of the CONUS.

500-mb mean circulation for the CONUS for April 1-12, 2018, showing a generally westerly flow
500-mb mean circulation for the CONUS for April 1-12, 2018, showing a generally westerly flow.
500-mb mean circulation for the CONUS for April 13-22, 2018, showing an expanded circumpolar vortex
500-mb mean circulation for the CONUS for April 13-22, 2018, showing an expanded circumpolar vortex.
500-mb mean circulation for the CONUS for April 23-30, 2018, showing a long-wave ridge in the West and trough in the East
500-mb mean circulation for the CONUS for April 23-30, 2018, showing a long-wave ridge in the West and trough in the East.

During April 2018, the parade of weather systems continued, but their pattern of motion can be grouped into three distinct periods, each characterized by specific weather phenomena, temperature anomalies, precipitation anomalies, and impacts.


500-mb circulation anomalies for the CONUS for April 1-12, 2018
500-mb circulation anomalies for the CONUS for April 1-12, 2018.
Temperature anomalies (departure from normal) for the CONUS for April 1-12, 2018
Temperature anomalies (departure from normal) for the CONUS for April 1-12, 2018.
Precipitation anomalies (percent of normal) for the CONUS for April 1-12, 2018
Precipitation anomalies (percent of normal) for the CONUS for April 1-12, 2018.

During the first week (roughly April 1-8), the circulation pattern was generally westerly. Some slight ridging occurred briefly during April 9-12, but a westerly pattern generally dominated for the period April 1-12. The upper-level troughs dragged cold fronts all the way to the Gulf of Mexico, bringing below-normal temperatures to much of the country east of the Rockies. The Canadian air masses were below freezing as they crossed the northern Plains and Midwest, so surface lows along the fronts produced blankets of snow as they trekked across these regions. The air masses warmed above freezing as they plunged farther south, so precipitation in the South and along the East Coast fell mostly as rain. The troughs were laden with Pacific moisture as they hit the West Coast, with above-normal precipitation falling across northern California and Nevada to the Pacific Northwest and northern Rockies. With a more northerly storm track, the Southwest was left drier and warmer than normal during this period. The Pacific upper-level systems were generally dried out as they crossed the Rockies, leaving the Great Plains and Mid- to Upper Mississippi Valley dry. But they picked up Gulf of Mexico moisture to bring above-normal precipitation to the Lower Mississippi Valley, Ohio Valley, and parts of Florida. Some of the systems generated severe weather as they moved across the South and Midwest, with significant tornado outbreaks occurring on the 3rd and 6th.


500-mb circulation anomalies for the CONUS for April 13-22, 2018
500-mb circulation anomalies for the CONUS for April 13-22, 2018.
Temperature anomalies (departure from normal) for the CONUS for April 13-19, 2018
Temperature anomalies (departure from normal) for the CONUS for April 13-19, 2018.
Precipitation anomalies (percent of normal) for the CONUS for April 13-19, 2018
Precipitation anomalies (percent of normal) for the CONUS for April 13-19, 2018.

The circumpolar vortex expanded during April 13-22 with several large upper-level closed lows taking a more southerly track across the CONUS. Pacific cold fronts were dragged across the West, while Canadian air masses reinforced the fronts once they crossed the Rockies. The resulting below-normal temperatures spread across most of the CONUS, with the greatest cold temperature departures occurring from the northern Plains to Great Lakes (7-day temperature anomalies for April 13-19 and April 16-22). Above-normal precipitation fell across parts of the West, especially the far western and northern sections, but the Pacific air masses were dried out by the coastal mountain ranges, so the Southwest continued drier than normal (7-day precipitation anomalies for April 13-19 and April 16-22). The Gulf of Mexico provided an abundant moisture source for the fronts and low pressure systems as they trekked across the central and eastern U.S., bringing above-normal precipitation to the Plains to East Coast. A dynamic storm track and below-freezing air kept the snow cover area high, expanding the snow cover in the central Plains to Great Lakes where below-freezing air dominated. The fronts and lows also triggered severe weather outbreaks, especially on the 13th, 14th, 15th, and 22nd from the Lower Mississippi Valley to Mid-Atlantic states.


500-mb circulation anomalies for the CONUS for April 23-30, 2018
500-mb circulation anomalies for the CONUS for April 23-30, 2018.
Temperature anomalies (departure from normal) for the CONUS for April 24-30, 2018
Temperature anomalies (departure from normal) for the CONUS for April 24-30, 2018.
Precipitation anomalies (percent of normal) for the CONUS for April 24-30, 2018
Precipitation anomalies (percent of normal) for the CONUS for April 24-30, 2018.

During the last week (roughly April 23-30), a more seasonal circulation pattern became established, with an upper-level ridge dominating in the West and trough in the East. But even so, upper-level short-wave troughs and lows tried to traverse the western ridge, bringing a few areas of above-normal precipitation to the Pacific Northwest and northern Rockies. But the blocking ridge kept most of the West drier and warmer than normal. With a northwesterly upper-level flow over the central CONUS, cold Canadian air masses were directed into the South and East beneath the upper-level trough, leaving below-normal temperatures in their wake. The northwesterly flow also blocked Gulf of Mexico moisture, leaving much of the Plains, Midwest, and South drier than normal and starving the snowpack, resulting in a steady decline in snow cover area. Only the southern Appalachian states to New England received above-normal precipitation when the lows and fronts tapped Atlantic moisture.


500-mb mean circulation for the CONUS for April 2018
500-mb mean circulation for the CONUS for April 2018.
500-mb circulation anomalies for the CONUS for April 2018
500-mb circulation anomalies for the CONUS for April 2018.
Temperature anomalies (departure from normal) for the CONUS for April 2018
Temperature anomalies (departure from normal) for the CONUS for April 2018.
Precipitation anomalies (percent of normal) for the CONUS for April 2018
Precipitation anomalies (percent of normal) for the CONUS for April 2018.

When conditions are averaged across the entire month, a slightly distorted western ridge/eastern trough pattern can be seen, where the northern part of the western ridge is not quite as strong as usual, the southern part of the western ridge is stronger than usual, and the eastern trough is stronger than usual over the Great Lakes and Northeast, as indicated by the upper-level circulation anomalies. The monthly temperature anomaly pattern reflects the monthly circulation anomaly pattern, with above-normal temperatures in the Southwest and below-normal temperatures across the CONUS east of the Rockies. The coldest temperature anomalies were over the northern Plains to Great Lakes where the upper-level circulation anomalies were greatest (strongest trough) and the snow cover was most persistent. The precipitation anomaly pattern for the month represented an additive result of the anomaly patterns for the various weeks, with the areas that were much wetter than normal during one or more weeks showing up wet at the monthly time scale, and those areas (especially the Southwest, Great Plains, and Mid- to Upper Mississippi Valley) that were persistently dry every week showing up as quite dry at the monthly scale. The circulation during this month was also reflected in snow, drought, and regional records.

  • The persistent cold east of the Rockies was reflected in the statewide temperature ranks. April 2018 was the coldest April in the 124-year record for Iowa and Wisconsin, and second coldest for eight other states in the Great Plains to Great Lakes region. Altogether, 22 states ranked in the tenth coldest, or colder, category. Warmth in the Southwest gave Arizona its second warmest April. The dryness from the Southwest to Upper Mississippi Valley was reflected in the statewide precipitation ranks. Four states had the tenth driest, or drier, April, including New Mexico (eighth driest) and Arizona, Kansas, and Missouri (tenth driest each). Minnesota was close, ranking twelfth driest. It was wet along the coasts, with Washington having its third wettest April.
  • When daily temperature records are examined, the extreme cold overwhelmed the extreme warmth. When integrated across the month, there were 7,068 record cold daily high (3,778) and low (3,290) temperature records. This was about 2.7 times the 2,563 record warm daily high (972) and low (1,591) temperature records.
  • As noted earlier, April is in the middle of the Northern Hemisphere climatological spring, a period of transition from winter cold to the warmer temperatures of summer. Cold outbreaks will increase heating demand, while abnormally warm temperatures will reduce it. Temperatures averaged across the month and across the CONUS gave April 2018 a rank of 13th coldest. With most of the CONUS east of the Rockies colder than normal this month, the high population centers of the East, South, and Midwest were colder than normal, while the warmer-than-normal temperatures were focused mostly in the less populated Southwest. As a result, the REDTI (Residential Energy Demand Temperature Index) for April 2018 ranked somewhat more extreme — fifth coldest in the 124-year historical record for April.
  • Some of the precipitation fell on drought areas and contracted drought and abnormal dryness, while other drought areas continued quite dry. Drought and abnormal dryness developed, expanded, or intensified in the Southwest, eastern Oregon, and parts of the southern to central Plains, while improving in other parts of the West, parts of the Plains (especially northern Plains), western Great Lakes, Alaska panhandle, and much of the Southeast to Mid-Atlantic region. Contraction outweighed expansion, but just barely, so at the national level drought shrank from 29.4 percent of the CONUS at the end of March to 28.6 percent of the CONUS at the end of April (from 25.0 percent to 23.9 percent for all of the U.S.). Dry conditions during the last six months had set the stage for the widespread development of wildfires in the Southwest to southern Plains. April began with dozens of large wildfires burning across the southern Plains, southern to central Florida, and other parts of the Southeast. Beneficial rains at mid-month helped reduce the number of large wildfires, with most now located in the southern Plains to Southwest, and some in Kentucky. By month's end, a few were still burning in the southern Plains to Southwest (wildfire maps for April 1, 4, 8, 12, 18, 23, and May 1).
  • April began with 19.6 percent of the CONUS covered in snow. The weather was dominated by an active jet stream flow with several upper-level weather systems moving across the country throughout the month. A complicated synoptic weather pattern was associated with the upper-level systems. Warm southerly air was pushed up ahead of them, which melted the snow boundary and contracted the snow cover area. Cold (below-freezing) Canadian air was pulled in behind the systems, and strong surface low pressure centers helped lay down a new blanket of snow as the systems passed, expanding the snow cover. The region experiencing this contraction/expansion process was mainly from the central and northern Plains to Midwest and New England (snow cover maps for April 1, 3, 7, 10, 14, 16, 17, 23). Three cycles of this contraction/expansion process of snow cover area occurred through the 19th, with the snow cover reaching a peak area of 32.3 percent of the CONUS. But with increasing sun angle as the month progressed, temperatures followed the seasonal warming cycle, with the Canadian air masses becoming less and less cold, so that above-freezing temperatures occurred more often than below-freezing temperatures (temperature maps for April 1-7, 8-14, 15-21, 24-30). Precipitation fell more as rain than snow, so the snow cover area contracted to a monthly minimum of 4.7 percent by the 29th.
  • The atmospheric circulation needed to create the instability and dynamics favorable for severe weather consists largely of a southwesterly flow across the central part of the CONUS. This upper-level circulation pattern occurred periodically as short-wave troughs and closed lows moved across the region. Four significant tornado outbreaks occurred during the month — on April 3rd, 6th, 13th-15th, and 22nd. The specific synoptic details were discussed earlier. Based on preliminary data through the 26th, 144 tornadoes occurred in April 2018, which is a little less than the April average of 155.

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 transitioned from a La Niña state to an ENSO-neutral state during April 2018.

  • The Atlantic hurricane season runs from June 1st through November 30th and the Eastern North Pacific hurricane season runs from May 15th through November 30th. No tropical storms or hurricanes developed in these basins during April 2018. No tropical cyclones formed in, or moved into, the central North Pacific.
  • Two tropical cyclones and four tropical disturbances developed in the western North Pacific and South Pacific in or near the U.S.-Affiliated Pacific Islands (USAPI). Tropical Cyclone Josie, Severe Tropical Cyclone Keni, and a tropical disturbance (94P) were active in the South Pacific west of American Samoa during April. Josie, which formed in March, moved south of American Samoa at the beginning of the month before becoming extra-tropical. Keni formed west of American Samoa then moved south of the islands before going extra-tropical near the middle of the month. None of these systems posed a threat to American Samoa. Three disturbances (90W, 92W, and 93W) formed within the waters of Micronesia, but all quickly dissipated and did not pose a threat to the USAPI.
North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern during April, when averaged for the month, consisted of a ridge over the western CONUS and a trough over the east. The strongest positive height anomalies occurred over the Southwest. Below-normal height anomalies covered the Great Lakes, Northeast, and as far south as the Ohio Valley, and even stretched across the northern Plains to Pacific Northwest, but were strongest from the Great Lakes to northeast Canada.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

April was drier than normal across most of the Southwest, parts of the interior Pacific Northwest, most of the Great Plains and Mid- to Upper Mississippi Valley, south central Puerto Rico, and from northern Alaska to parts of the Alaska panhandle. The month was wetter than normal across southwest Alaska, most of Hawaii, the rest of Puerto Rico, the Lower Mississippi Valley, most of the Southeast to New England, northern California to the Washington coast, and Washington state to southeast Montana.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

Monthly temperatures were warmer than normal across the Southwest and Florida, but colder than normal across the rest of the CONUS, especially the northern Plains to Great Lakes. Alaska was near to colder than normal in the interior, but warmer than normal along the coasts.

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. Widespread north/south coupling of circulation anomalies is apparent. The above-normal (positive) height anomalies over the Arctic Ocean (East Siberian Sea to Beaufort Sea) coupled with the below-normal height anomalies further south over Canada-North Atlantic and northeastern Asia, then with above-normal heights further south over the North Atlantic and further south over eastern Asia to the adjacent Pacific Ocean (a "triple coupling"). East/west couplings of height anomalies are evident over the North Atlantic and Eurasia, as well as North America and the North Atlantic.

The upper-level circulation and its anomalies are associated with the Sea Level Pressure (SLP) pattern and its anomalies which reflect the semi-permanent centers of action of SLP. The above- and below-normal upper-level height anomalies over parts of the North Pacific and North Atlantic appear to be associated with above- and below-normal SLP, respectively. The locations of these anomalies with respect to climatology suggests that the North Pacific High was weaker in the eastern North Pacific and stronger in the western North Pacific. At the same time, the Aleutian Low seemed to be weaker in the east (Gulf of Alaska) and stronger in the west. With an increased pressure gradient, the jet stream and circulation would be stronger over the western North Pacific. Likewise, the North Atlantic High was weaker in the eastern North Atlantic and stronger in the western North Atlantic (along the U.S. coast) compared to climatology, suggesting a shift in its location. The Icelandic Low seemed to be shifted to the east a bit. Both the North Atlantic High and Icelandic Low seemed to be stronger, overall, compared to climatology, indicating an increased pressure gradient and stronger jet stream and circulation over the North Atlantic.

The above-normal 500-mb heights were associated with upper-level ridging, or with weakened troughs, at the mid-latitudes; below-normal precipitation (over the southwestern CONUS, Black Sea area, and northeastern Siberia); below-normal snow cover (over the western CONUS, southeast Europe, and northeast China); above-normal surface temperatures (over the southwestern CONUS, most of Europe, eastern Asia, and northern Argentina); and warm SST anomalies (in the western North Atlantic, southeastern North Pacific, and parts of the South Pacific and South Atlantic Oceans). The areas of below-normal 500-mb heights were associated with upper-level troughing, or with weakened ridges; near- to below-normal surface temperatures (over eastern North America and north central Asia); a cooling trend in SST anomalies (in the central North Pacific and parts of the South Atlantic, South Pacific, and southern Indian Ocean); above-normal precipitation (over the eastern CONUS and north central Asia); and above-normal snow cover (over central North America and western Russia). Parts of North America and Eurasia were near to cooler than normal, and parts of the equatorial Pacific, South Pacific, North Atlantic, and Indian Ocean had cooler-than-normal SST anomalies. But with parts of North America, and much of South America, Asia, Australia, and Africa having warmer- to much-warmer-than-normal temperatures, and large portions of the Atlantic, Pacific, and Indian Oceans having warmer-than-normal sea surface temperatures, the April 2018 global temperature was still well 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:


MJO agreement with the temperature and precipitation anomaly patterns, by week.
Week Temperature Precipitation
April 1-7
April 8-14 MJO
April 15-21 MJO
April 22-28

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed April 2018 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggest that the weather over the CONUS in April was influenced mostly by equatorial and mid-latitude atmospheric drivers originating in the Pacific. The PNA, AO, and NAO teleconnections do not match the observed anomaly patterns, so the Arctic and North Atlantic drivers were likely not players this month. The EP-NP was slightly negative, almost zero. If it had been positive, it would have had good agreement with the observed temperature and circulation anomalies. The MJO temperature teleconnections matched the monthly anomaly pattern and weeks 2 and 3, but the precipitation teleconnections mostly did not match the observed anomaly patterns. This suggests the MJO had some influence on April temperatures. Even though La Niña has technically ended, the La Niña precipitation teleconnections were a good match with observed conditions, suggesting that it still had some lingering effects on April precipitation. The WP showed the best agreement with the overall temperature and circulation anomaly patterns, which indicates that the North Pacific drivers were influential this month. But the teleconnections were not as extensive as the observed patterns. The MJO and WP may have had a synergistic influence.

This month illustrates how the atmospheric circulation for the month can reflect atmospheric drivers (or modes of atmospheric variability) originating in the Pacific Ocean.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for April 2018, published online May 2018, retrieved on August 17, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201804.

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