Synoptic Discussion - April 2019

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 was experiencing a weak El Niño during April 2019. The upper-level circulation was quite active this month, with the long-wave pattern shifting from a long-wave trough to a long-wave ridge back and forth. Two storm tracks were evident — short-wave troughs migrated through the jet stream flow along the U.S.-Canadian border, while troughs and upper-level closed lows tracked across the Southwest and southern Plains then northeast toward the Great Lakes. The upper-level troughs and their associated fronts and surface lows brought rain and snow along their paths, with above-normal precipitation especially notable in the Pacific Northwest and Deep South. In between these storm tracks, precipitation was below normal across much of California to Arizona and in the central Plains. The troughs and closed lows, and their surface fronts and lows, triggered outbreaks of severe weather from the Plains to East Coast, with the preliminary tornado count for April well above average. Some of the air masses were below freezing in the northern CONUS, which helped create an expansion of snow cover there at mid-month, but seasonal increases in sunshine and temperatures contributed to an otherwise steadily decreasing snow cover area as the month progressed. The frequent long-wave ridging, and protrusion of the North Atlantic High into the eastern CONUS, brought warmer-than-normal temperatures. But troughing in the north, and cloudiness and rain associated with the southern closed lows and their surface systems, squeezed the warm anomalies in the north and south, resulting in a monthly temperature anomaly pattern that was warmer than normal in the east and west, and near to colder than normal in the north and south. The precipitation this month gave the CONUS the twelfth wettest April in the 125-year historical record, and much of the precipitation fell on drought areas, contracting the national drought footprint to about 2.6 percent of the U.S. The upper-level circulation, temperature, and precipitation anomaly patterns suggest that the weather during March reflected the combined influence of atmospheric drivers originating in the North Pacific Ocean, Arctic, and North Atlantic Ocean. 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, the mid-latitude circulation in the Northern Hemisphere begins to slow down, 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.

500-mb mean circulation for North America for April 1-3, 2019
500-mb mean circulation for North America for April 1-3, 2019.
500-mb mean circulation for North America for April 4-10, 2019
500-mb mean circulation for North America for April 4-10, 2019.
500-mb mean circulation for North America for April 11-17, 2019
500-mb mean circulation for North America for April 11-17, 2019.
500-mb mean circulation for North America for April 18-24, 2019
500-mb mean circulation for North America for April 18-24, 2019.
500-mb mean circulation for North America for April 25-30, 2019
500-mb mean circulation for North America for April 25-30, 2019.

500-mb circulation anomalies for North America for April 1-3, 2019
500-mb circulation anomalies for North America for April 1-3, 2019.
500-mb circulation anomalies for North America for April 4-10, 2019
500-mb circulation anomalies for North America for April 4-10, 2019.
500-mb circulation anomalies for North America for April 11-17, 2019
500-mb circulation anomalies for North America for April 11-17, 2019.
500-mb circulation anomalies for North America for April 18-24, 2019
500-mb circulation anomalies for North America for April 18-24, 2019.
500-mb circulation anomalies for North America for April 25-30, 2019
500-mb circulation anomalies for North America for April 25-30, 2019.

During April 2019, the circumpolar vortex and main storm track did contract northward, but the circulation remained vigorous with a strong meridional component. The meridional component was characterized by several deep troughs and closed lows that moved across the CONUS, especially across the southern states. These two features — the seasonal contraction and continued vigorous meridional component — opposed each other in the monthly aggregate. The long-wave pattern shifted back and forth in a complex interaction between the broad atmospheric drivers and the local influence of short-term weather systems. There are many ways the long-wave pattern can be sectionalized throughout the month. The following discussion breaks the pattern into five sections or time periods:

500-mb mean circulation for North America for April 1-3, 2019
500-mb mean circulation for North America for April 1-3, 2019.
500-mb circulation anomalies for North America for April 1-3, 2019
500-mb circulation anomalies for North America for April 1-3, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 1-3, 2019
Temperature anomalies (departure from normal) for the CONUS for April 1-3, 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 1-3, 2019
Precipitation anomalies (percent of normal) for the CONUS for April 1-3, 2019.

April 1-3: The month began with a long-wave trough over eastern North America and ridge over western North America. A storm system was moving up the U.S. East Coast with another one from the Pacific undercutting the western ridge.

500-mb mean circulation for North America for April 4-10
500-mb mean circulation for North America for April 4-10, 2019.
500-mb circulation anomalies for North America for April 4-10, 2019
500-mb circulation anomalies for North America for April 4-10, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 4-10, 2019
Temperature anomalies (departure from normal) for the CONUS for April 4-10, 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 4-10, 2019
Precipitation anomalies (percent of normal) for the CONUS for April 4-10, 2019.

April 4-10: A generally westerly flow characterized the following week. Short-wave troughs migrated through the flow with their accompanying surface lows and fronts. Some brought Pacific moisture to the Pacific Northwest and northern Rockies, while others pulled in Gulf of Mexico moisture to give the southern Plains to Southeast a wetter-than-normal week. One of the systems spawned a tornado outbreak in the South on April 7th. The contracted circumpolar vortex and ridging across the central CONUS resulted in above-normal temperatures across most of the country during this period. A deep trough began moving into the West at the end of this period, with the southerly flow of warm air ahead of it contributing to the above-normal temperatures in the Plains.

500-mb mean circulation for North America for April 11-17, 2019
500-mb mean circulation for North America for April 11-17, 2019.
500-mb circulation anomalies for North America for April 11-17, 2019
500-mb circulation anomalies for North America for April 11-17, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 11-17, 2019
Temperature anomalies (departure from normal) for the CONUS for April 11-17, 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 11-17, 2019
Precipitation anomalies (percent of normal) for the CONUS for April 11-17, 2019.

April 11-17: The circulation pattern became extremely meridional during April 11-17 with several strong deep troughs and closed lows moving across the CONUS. They plunged into the Southwest, then tracked across the Plains toward the Great Lakes and eastern Canada. These were powerful storm systems, with their fronts and northerly flow pulling in Canadian air, their surface lows drawing Gulf of Mexico moisture northward, and their dynamics generating rounds of severe weather in the South and East (severe weather maps for April 13th, 14th, 17th). Pacific moisture contributed to above-normal precipitation across parts of the West, while Gulf of Mexico and Atlantic moisture fed above-normal precipitation in many areas from the Plains to East Coast. The combination of moisture and below-freezing Canadian air resulted in a deep snow cover across the central Plains to western Great Lakes. The Canadian air masses, clouds, and snow kept temperatures below normal for this period across much of the country. Upper-level ridging and a southerly flow ahead of the strong surface lows kept the East warmer than normal for the week.


500-mb mean circulation for North America for April 18-24, 2019
500-mb mean circulation for North America for April 18-24, 2019.
500-mb circulation anomalies for North America for April 18-24, 2019
500-mb circulation anomalies for North America for April 18-24, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 18-24, 2019
Temperature anomalies (departure from normal) for the CONUS for April 18-24, 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 18-24, 2019
Precipitation anomalies (percent of normal) for the CONUS for April 18-24, 2019.

April 18-24: April 18-24 was dominated by two slow-moving closed lows, one which lingered over the eastern CONUS and another which moseyed onto the Southwest. These systems and their surface lows and fronts spread Gulf of Mexico moisture across the southern Plains to Midwest and parts of the East Coast. Their dynamics also triggered rounds of severe weather, with about a hundred tornadoes spawned from the Plains to East Coast at the beginning of the period (severe weather maps for April 17th, 18th, 19th, 24th). Some of the Pacific fronts gave parts of the West above-normal precipitation for the week, but with most of the action in the South and East, much of the West to central and northern Plains was drier than normal. The clouds and rain associated with the eastern low kept temperatures below normal along the Gulf Coast states to Southeast, but ridging in the West and Northeast, along with the seasonal contraction of the circumpolar vortex, gave much of the rest of the country warmer-than-normal temperatures for this period.


500-mb mean circulation for North America for April 25-30, 2019
500-mb mean circulation for North America for April 25-30, 2019.
500-mb circulation anomalies for North America for April 25-30, 2019
500-mb circulation anomalies for North America for April 25-30, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 23-29, 2019
Temperature anomalies (departure from normal) for the CONUS for April 23-29, 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 23-29, 2019
Precipitation anomalies (percent of normal) for the CONUS for April 23-29, 2019.

April 25-30: In the waning days of the month, closed lows and troughs, with their surface lows and fronts, traversed the CONUS in the westerly flow, spreading above-normal precipitation across parts of the South, northern Plains, Midwest, and Northeast. Some triggered severe weather, especially on the 30th in the southern Plains. By the end of the month, the circulation pattern shifted into a long-wave trough which deepened over Canada and sagged a bit into the northern states. This contributed to below-normal temperatures across the northern CONUS. It also forced an upper-level ridge to develop over the Pacific Ocean which created a northerly flow across western North America, and this contributed to a drier-than-normal period over the West. The lack of deep troughing over California to the Four Corners States during much of this period helped keep temperatures warmer than normal in this region.


500-mb mean circulation for North America for April 6-19, 2019
500-mb mean circulation for North America for April 6-19, 2019.
500-mb circulation anomalies for North America for April 6-19, 2019
500-mb circulation anomalies for North America for April 6-19, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 6-19, 2019
Temperature anomalies (departure from normal) for the CONUS for April 6-19, 2019.

500-mb mean circulation for North America for April 16-29, 2019
500-mb mean circulation for North America for April 16-29, 2019.
500-mb circulation anomalies for North America for April 16-29, 2019
500-mb circulation anomalies for North America for April 16-29, 2019.
Temperature anomalies (departure from normal) for the CONUS for April 16-29, 2019
Temperature anomalies (departure from normal) for the CONUS for April 16-29, 2019.

Two-week slices: If the atmospheric circulation and temperature anomaly fields are sliced into 2-week periods, an interesting east-west pattern can be seen. The preference of short-wave troughs and closed lows to deepen over the Southwest then move across the Plains towards the Great Lakes, beginning on April 10 and lasting through about April 27, resulted in a circulation anomaly field with above-normal heights (or ridging) along or just off the West and East Coasts with near-normal heights over the interior CONUS, for both April 6-19 and April 16-29. Ridging with above-normal temperatures dominated the East Coast during April 6-19, while they dominated the West Coast during April 16-29. This shows up in the monthly field as well.


500-mb mean circulation for North America for April 2019
500-mb mean circulation for North America for April 2019.
500-mb circulation anomalies for North America for April 2019
500-mb circulation anomalies for North America for April 2019.
Temperature anomalies (departure from normal) for the CONUS for April 2019
Temperature anomalies (departure from normal) for the CONUS for April 2019.
Precipitation anomalies (percent of normal) for the CONUS for April 2019
Precipitation anomalies (percent of normal) for the CONUS for April 2019.

The full monthly circulation pattern reflected the combined effect of ridging along and off the West and East Coasts, the undercutting of the western ridge by strong troughs and closed lows which tracked across the Southwest, Plains, and Great Lakes, and the dominance of a long-wave trough with its below-normal height anomalies across Canada and extending a bit into the central CONUS. The surface temperature anomaly pattern matched the circulation anomaly pattern quite well, with above-normal temperatures in the West and along the East Coast, and near- to below-normal temperatures in the southern Plains to Lower Mississippi Valley and along the northern tier states. The precipitation anomaly pattern for the month (the wet areas) represented an additive result of precipitation from the individual frontal passages and low pressure systems. The dry areas resulted from the areas being missed by the storm track, which were mainly parts of California-Nevada, the Southwest, and the central Plains. The circulation during this month was also reflected in severe weather, drought, and regional records.

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. Warm sea surface temperatures (SSTs) fuel tropical cyclones while vertical wind shear tears them apart. The tropical Pacific Ocean was in a weak El Niño state during April 2019.

  • The Atlantic hurricane season runs from June 1st through November 30th and the Eastern North Pacific (ENP) hurricane season runs from May 15th through November 30th.
  • No tropical systems developed in the North Atlantic, Eastern North Pacific, or Central North Pacific during April 2019.
  • In the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia) portion of the Western North Pacific, an area of convection developed to the point of being noticed (Invest 99W) early in the month. It did not develop further, lasting only a couple days before dissipating by April 3. Post El Niño-like weather dominated this region of the Pacific during April. Dry trade winds blowing from the North Pacific subtropical ridge (North Pacific High) kept northern portions of the USAPI in drought. When these faster-moving trade winds encountered slower winds near the equator, the resulting convergence generated rainfall throughout the month in the southern portions of Micronesia. The convergence was frequently accompanied by near-equatorial troughs and weak tropical disturbances, and the combination of these features comprised the Inter-Tropical Convergence Zone.
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 ridges, with above-normal height anomalies, along the West Coast into the eastern Pacific, and in the western Atlantic off the East Coast. A slight trough was evident over the Southwest, with the Hudson Bay Low extending a trough across Canada and into the U.S. Midwest.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

April 2019 ended up drier than normal across the northern portions of the USAPI, much of Hawaii, and parts of California to Nevada, Arizona to Colorado, the central Plains, Alaska, and Puerto Rico and the U.S. Virgin Islands. The month was wetter than normal in the Pacific Northwest to interior West, the southern Plains to Southeast, the southern portions of the USAPI, eastern to southeastern Alaska, and parts of Puerto Rico and the northern Plains to Northeast.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The northern tier states had a near- to cooler-than-normal April, as did the southern Plains to Lower Mississippi Valley. April was warmer than normal in much of the West and along parts of the East. Alaska was mostly warmer than normal.

Northern Hemisphere monthly upper-level circulation pattern and anomalies (cylindrical projection)
Northern Hemisphere monthly upper-level circulation pattern and anomalies (cylindrical projection).
Northern Hemisphere monthly upper-level circulation anomalies (polar stereographic projection)
Northern Hemisphere monthly upper-level circulation anomalies (polar stereographic projection).

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 sinusoidal nature of the jet stream was evident in east to west couplings of ridges and troughs. There seemed to be two sets of couplings — a high latitude set and a mid-latitude set — as well as north-south couplings. A two-wave pattern at the high latitudes was comprised of a trough over Canada with ridge over northern Europe to Greenland, and a trough over north central Russia coupled with a ridge over eastern Siberia. A two-wave mid-latitude pattern could be said to consist of a broken ridge over the CONUS and adjacent Pacific and Atlantic Oceans coupled with a trough over the Mediterranean and northern Africa, and a ridge over China coupled with a trough over the western North Pacific. Geographically, these two sets of anomalies might be coupled north-south — ridge over CONUS with trough over Canada, ridge over China with trough over north central Russia, etc.

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 appear to be associated with above- and below-normal SLP anomalies. During this time of year, storm systems usually congregate in certain favored areas, creating (on average) low pressure centers over the North Pacific (Aleutian Low) and North Atlantic (Icelandic Low). The North Pacific High and North Atlantic (Bermuda) High, which are normally weak and shifted further south during winter, are beginning to strengthen during spring. The location of the April 2019 SLP anomalies with respect to the long-term climatology suggests that the North Atlantic High was stronger and shifted further west (toward the U.S. East Coast) compared to climatology. (This could account for the warm temperature anomalies across the East Coast.) At the same time, both the Icelandic and Aleutian Lows appeared to be stronger compared to climatology (which could be associated with the more active storm pattern).

During April 2019, above-average sea surface temperature (SST) anomalies continued to dominate the North Pacific, much of the Atlantic, and most of the Indian Ocean. The map of the change in SST anomalies from the end of March to the end of April showed cooling off the coast of California, off the coast of Japan, in the Gulf of Mexico, and in parts of the North Atlantic.

The above-normal 500-mb heights were associated with upper-level ridging, or with weakened troughs, at the mid- and high latitudes; below-normal precipitation (over northern Europe); below-normal snow cover (over northern China and Mongolia); above-normal surface temperatures (over the eastern and western CONUS, Greenland to northern Europe, eastern Siberia, and China); and warm SST anomalies (over the eastern North Pacific and western North Atlantic). 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 Canada and north central Russia); cool SST anomalies (in the northern North Atlantic south of Greenland); above-normal precipitation (over north central Russia); and above-normal snow cover (over southeast Canada to north central CONUS, and along the southern periphery of the north central Russia below-normal heights region). Parts of North America and Asia were near to cooler than normal, and southern parts of the Southern Hemisphere oceans had cooler-than-normal SST anomalies. But with all of South America, most of Australia and Africa, and much of Eurasia 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 2019 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:


Indices and their agreement with the temperature, precipitation, and upper-level circulation anomaly patterns, by time period (month, week, or other sub-monthly period).
Time Period Circulation Temperature Precipitation
Month NAO, WP,
EP-NP
NAO, WP,
EP-NP
WP, EP-NP
April 1-3 NAO,
EP-NP
April 8-17 AO
April 18-24 NAO AO
April 16-29 WP, NAO NAO
April 25-30 EP-NP

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed April 2019 weekly and monthly temperature, precipitation, and circulation anomaly patterns, suggests that the weather during this month was influenced, in whole or in part, by atmospheric drivers originating over the North Pacific, North Atlantic, and Arctic Oceans. The equatorial Pacific was in a weak El Niño state. The MJO was incoherent for much of the month, emerging late in the month. The observed anomaly patterns were not consistent with what would be expected if the MJO or El Niño were controlling factors for CONUS weather (except the El Niño may have contributed to the southern Plains and Southeast wet conditions). The circulation and temperature anomaly patterns appeared to be an amalgamation of the teleconnections associated with the NAO, WP, EP-NP, and AO. None of them matched perfectly, but elements of each could be seen in the observed patterns. A complicating factor was the frequent passage of closed upper-level lows across the Southwest to southern Plains, whose influence were not obvious from the teleconnection patterns and which tended to detract from correlations. Precipitation is the most difficult climate variable to match to atmospheric drivers, as indicated by the frequent weak to no teleconnections; this is likely caused by the many ways precipitation can be triggered (frontal systems, short-wave troughs, etc.) and by a random factor (e.g., convective instability). But the drivers originating in the North Pacific Ocean seemed to have the greatest influence, which one would expect from the frequent passage of closed upper-level lows originating from the Pacific.

This month illustrates how the atmospheric circulation for the month can reflect the combined influence of the mid-latitude atmospheric drivers (or modes of atmospheric variability) as opposed to those originating in the equatorial Pacific.


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for April 2019, published online May 2019, retrieved on August 20, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201904.

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