Synoptic Discussion - March 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 March 2019. The upper-level circulation was quite active this month, with the long-wave pattern shifting from a long-wave trough in the western contiguous United States (CONUS) and a long-wave ridge in the East, to a long-wave ridge in the West and trough in the East. Short-wave troughs migrating through the jet stream flow complicated the overall pattern and the transition between long-wave patterns. The upper-level troughs and their associated fronts and surface lows brought rain and snow along their paths, but the dominant storm track left a ribbon of above-normal precipitation across the CONUS from California to the central Plains, into the Midwest, and ending along the Mid-Atlantic Coast, with below-normal precipitation to the north and south. For some of the weather systems, the surface lows and fronts, and dynamics of the associated upper-level troughs, were strong enough to generate outbreaks of severe weather, with the preliminary tornado count for March well above average. The air masses were below freezing in the northern CONUS, which helped maintain an extensive snow cover there early in the month, but seasonal increases in sunshine and temperatures, along with lack of precipitation in some areas due to the storm track, contributed to a gradually decreasing snow cover area as the month progressed. Cloudiness associated with the fronts and lows, along with frequent incursions of cold and dry Canadian air masses directed southward beneath a northerly flow at the upper levels, resulted in monthly temperatures that were colder than normal for most of the CONUS. In Alaska, however, where an anomalously strong upper-level ridge dominated, temperatures were record warm for the month. Continued drier-than-normal conditions in the South and parts of the Northwest increased drought and abnormal dryness in those areas, but with above-normal precipitation falling across many of the western drought areas, the national drought footprint contracted to about 5.4 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 equatorial Pacific Ocean, 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, March is the first month of climatological spring, marking the early transition from winter conditions to the warm season. Spring 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. The result is transient weather patterns are common during March, with (typically) increasingly warm (subtropical) air masses building from the south, typically interrupted by incursions of the cold, and sometimes frigid, polar air.

500-mb mean circulation for North America for March 1-6, 2019
500-mb mean circulation for North America for March 1-6, 2019.
500-mb mean circulation for North America for March 7-13, 2019
500-mb mean circulation for North America for March 7-13, 2019.
500-mb mean circulation for North America for March 14-31, 2019
500-mb mean circulation for North America for March 14-31, 2019.

500-mb circulation anomalies for North America for March 1-6, 2019
500-mb circulation anomalies for North America for March 1-6, 2019.
500-mb circulation anomalies for North America for March 7-13, 2019
500-mb circulation anomalies for North America for March 7-13, 2019.
500-mb circulation anomalies for North America for March 14-31, 2019
500-mb circulation anomalies for North America for March 14-31, 2019.

March 2019 was typical in some ways yet atypical in others. The upper-level circulation was very active, with several short-wave troughs and closed lows moving through the jet stream flow (especially at lower latitudes — over the CONUS), but the pattern was highly meridional (especially at higher latitudes — over Canada). This allowed Pacific weather systems to traverse the CONUS, dropping their rain and snow, yet also enabled Canadian air masses to surge southward across the country east of the Rockies and keep monthly temperatures colder than normal. Snow cover was above average yet steadily declined in area as the month progressed. 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 three sections: a long-wave trough across central to eastern North America with a ridge over the southwestern CONUS and northwest Canada to Alaska (March 1-6); a trough in the west and ridge in the east (March 7-13); and the trough shifting back to the east with a strong ridge building in the west (March 14-31).

500-mb mean circulation for North America for March 1-6, 2019
500-mb mean circulation for North America for March 1-6, 2019.
500-mb circulation anomalies for North America for March 1-6, 2019
500-mb circulation anomalies for North America for March 1-6, 2019.
Temperature anomalies (departure from normal) for the CONUS for March 1-6, 2019
Temperature anomalies (departure from normal) for the CONUS for March 1-6, 2019.
Precipitation anomalies (percent of normal) for the CONUS for March 1-6, 2019
Precipitation anomalies (percent of normal) for the CONUS for March 1-6, 2019.

March 1-6: A long-wave trough was centered over eastern North America and extended into central North America (south central Canada and north central CONUS), with a long-wave ridge across western North America (Alaska and northwestern Canada, plus southwestern CONUS). Pacific weather systems and their associated upper-level short-wave troughs moved through the western ridge then across the CONUS. These systems brought above-normal rain and snow to California, the Great Basin, and central Rockies, bringing the CONUS snow cover area to the monthly maximum of 57.1 percent on March 5th. A large upper-level closed low moved into the Pacific Northwest and southwest Canada near the end of this period, splitting the western ridge in two. The month began with a cold front across the eastern CONUS. Weather systems tracked along the front, bringing above-normal precipitation to the Southeast, Mid-Atlantic States, and southern coastal New England, as well as severe weather to the Southeast. With the long-wave trough over eastern North America, a northerly flow at the upper levels directed cold and dry Canadian air masses into the Plains, Midwest, and Great Lakes. The western ridge helped keep the Pacific Northwest mostly drier than normal.

500-mb mean circulation for North America for March 7-13
500-mb mean circulation for North America for March 7-13, 2019.
500-mb circulation anomalies for North America for March 7-13, 2019
500-mb circulation anomalies for North America for March 7-13, 2019.
Temperature anomalies (departure from normal) for the CONUS for March 7-13, 2019
Temperature anomalies (departure from normal) for the CONUS for March 7-13, 2019.
Precipitation anomalies (percent of normal) for the CONUS for March 7-13, 2019
Precipitation anomalies (percent of normal) for the CONUS for March 7-13, 2019.

March 7-13: The upper-level pattern shifted, with a long-wave trough becoming anchored over the western CONUS; the trough kicked up a long-wave ridge over the eastern CONUS, and this eastern ridge shifted the downstream trough further east. In the meantime, a long-wave ridge continued over western Canada, resulting in what is known as a "split flow". Pacific weather systems and their short-waves weakened as they moved into the area of the split between the northern ridge and southern trough. But they intensified as they moved into the western CONUS long-wave trough, producing strong surface lows and fronts which tracked to the Great Lakes. These lows tapped Gulf of Mexico moisture as they migrated eastward, spreading above-normal precipitation across the Southwest to Ohio Valley. One particularly powerful low pressure/frontal system, which moved across the West to Great Lakes March 11-14, triggered severe weather in the Southeast and Ohio Valley on the 14th. The long-wave trough kept temperatures colder than normal in the West, while lingering Canadian air (from the previous week), and clouds and precipitation from the migrating lows, kept temperatures below normal across the northern and central Plains to East Coast. The building eastern ridge and a southerly flow brought near- to above-normal temperatures to the southern Plains and Gulf Coast states. But the track of the low pressure systems left the Gulf Coast and Atlantic Coast drier than normal.

500-mb mean circulation for North America for March 14-31, 2019
500-mb mean circulation for North America for March 14-31, 2019.
500-mb circulation anomalies for North America for March 14-31, 2019
500-mb circulation anomalies for North America for March 14-31, 2019.
Temperature anomalies (departure from normal) for the CONUS for March 18-31, 2019
Temperature anomalies (departure from normal) for the CONUS for March 18-31, 2019.
Precipitation anomalies (percent of normal) for the CONUS for March 18-31, 2019
Precipitation anomalies (percent of normal) for the CONUS for March 18-31, 2019.

March 14-31: After the strong low pressure system of March 13-14 tracked out of the CONUS, a persistent pattern developed in the upper atmosphere, with a large ridge over western North America and trough over the East. The ridge allowed temperatures to warm over the West. At first, Pacific weather systems were mostly deflected to the north into Canada by the ridge, and an upper-level northwesterly flow over central North America directed dry and cold Canadian air into the eastern CONUS, so most of the country was drier than normal. A few Pacific weather systems and short-waves managed to wiggle beneath the western ridge later in the period, but they were weakened and produced a ribbon of above-normal precipitation across just the West, central Plains to Midwest, and Mid-Atlantic region. However, a low pressure and frontal system intensified in the East at the end of the month.


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

The full monthly mean circulation pattern reflected a combination of the ridge west/trough east pattern, with short-wave troughs undercutting the western ridge over the CONUS. The monthly temperature anomaly pattern reflected the frequency and magnitude of the troughing — long-wave troughing in the east; the resultant strong upper-level northerly flow over central Canada which directed particularly cold air masses into the CONUS, especially during the first two weeks of the month; and Pacific weather systems moving through the ridge across the West. 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, reflected as a ribbon of above-normal precipitation from California to the Ohio Valley. The dry areas resulted from the areas being missed by the storm track, the dominance of the western ridge (Pacific Northwest), and the northerly flow over south-central Canada and the north-central U.S. (northern High Plains, South, and East Coast). 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 March 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 March 2019.
  • Two tropical systems developed in the Western North Pacific and affected the U.S.-Affiliated Pacific Islands (USAPI) (Micronesia). An area of convection, that developed in central Micronesia at mid-month, intensified into Tropical Depression 03W and moved across western Micronesia before dissipating after a few days. Another area of convection (Invest 97W) formed near the end of the month in central Micronesia (near Chuuk and Pohnpei) before dissipating after a couple days. Both systems mainly brought rain to the parts of Micronesia which they passed over, bringing relief from a building drought in the region.
North America monthly upper-level circulation pattern and anomalies
North America monthly upper-level circulation pattern and anomalies.

The upper-level circulation pattern during March, when averaged for the month, consisted of a ridge over the western CONUS extending across western Canada into Alaska, with above-normal height anomalies over the southern Plains to Southeast and from Montana to Alaska, and a trough across eastern Canada extending into the Ohio Valley, with below-normal height anomalies over eastern Canada into the Great Lakes, as well as over the eastern North Pacific off the coast of California.

Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

March 2019 ended up drier than normal in the Pacific Northwest to northern High Plains, southern Plains to Southeast, central Appalachians to New England, parts of the Great Lakes, and across Hawaii. The month was wetter than normal from California to the central Plains, across much of the Ohio Valley, and parts of western Texas and the Mid-Atlantic Coast. Alaska was wetter than normal except in the southeast and panhandle. Puerto Rico was wetter than normal in the west and drier than normal in the east.

Map of monthly temperature anomalies
Map of monthly temperature anomalies.

The ridge over western North American gave Alaska its warmest March on record, but the frequent troughing over the CONUS resulted in colder-than-average temperatures across most of the country. Monthly temperatures averaged near to above normal only in the Southwest and parts of western Washington and the Southeast.

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, and the height anomaly pattern indicated a highly meridional circulation, especially over North America and Asia. The mean and anomaly patterns suggested that the jet stream undulations took the form of three waves, or trough/ridge couplets, across the Northern Hemisphere — one couplet over North America (ridge west/trough east), another over the North Atlantic (ridge) and western Eurasia (trough), and the third over central Asia (ridge) and eastern Siberia to the North Pacific (trough). Below-normal heights across the Arctic joined the below-normal anomaly centers over the Bering Sea, western Eurasia, and eastern Canada. There was another interesting anomaly couplet at the mid-latitudes — above-normal height anomalies over the central North Pacific and below-normal height anomalies over the eastern North Pacific (off the coast of California).

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 March 2019 SLP anomalies with respect to the long-term climatology suggests that the North Atlantic High was stronger and shifted further north and east compared to climatology, and the Icelandic Low correspondingly shifted further north. In the Pacific, both the Aleutian Low and the North Pacific High appeared to be shifted further west compared to climatology.

During March 2019, above-average sea surface temperature (SST) anomalies continued to dominate the North Pacific, most of the Atlantic, and much of the Indian Ocean. The map of the change in SST anomalies from the end of February to the end of March showed warming off the coast of California associated with mixing of the surface ocean layers due to frequent passage of Pacific storms (and due to reduced coastal upwelling).

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 western Canada and southern Europe); below-normal snow cover (over Europe and Mongolia); above-normal surface temperatures (over Alaska, northwestern Canada, and much of Eurasia); and warm SST anomalies (over parts of the North Atlantic and North Pacific, especially off the coast of Alaska and western Canada). 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 southeast Canada into the central CONUS); cool SST anomalies (in the northern North Atlantic near Iceland and in the eastern North Pacific near the California coast); above-normal precipitation (over California, Scandinavia into northwestern Asia, and eastern Siberia); and above-normal snow cover (from southeast Canada into the north central CONUS). Parts of North and South America were cooler than normal, and some parts of the Atlantic, Pacific, and Indian Oceans had cooler-than-normal SST anomalies. But with all of Australia, most of Eurasia, and much of Africa and South America 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 March 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 AO, NAO, WP, EP-NP EP-NP NAO, WP,
EP-NP
March 1-6 MJO MJO
March 7-13 MJO
March 10-16 AO
March 14-23 PNA PNA, AO
March 14-31 NAO, WP

Examination of the available circulation indices and their teleconnection patterns, and comparison to observed March 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 all of the main atmospheric drivers. The equatorial Pacific was in a weak El Niño state. The precipitation anomalies agreed with the teleconnections expected with an El Niño in some areas but not others, and the temperature anomalies were inconsistent with the teleconnections. The MJO came on strong early in the month, but then became incoherent. The CPC concluded that mid-latitude drivers, acting through Rossby wave activity over the East Pacific, overwhelmed the influence of both the El Niño and MJO. The circulation pattern during March was very complicated and didn't fit any driver's teleconnections perfectly. The AO and NAO were a good match in most areas of the Northern Hemisphere, while the WP seemed to be driving the below-normal anomalies over the eastern North Pacific into the adjacent western CONUS, and the EP-NP appeared to drive the ridging over Alaska and western Canada and troughing over eastern Canada. The PNA may have helped with ridging in western Canada at mid-month, but otherwise the PNA was a non-player. For temperature, the MJO seemed to be influential during the first half of the month, but when it became incoherent other forces dominated during the last half of the month. The dominance of cold temperature anomalies east of the Rockies, and warmth in Alaska, seemed to be driven by the EP-NP. 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). The month began with the MJO appearing to have a strong influence on the precipitation anomaly pattern. But after the first week, competition from the various forces resulted in little bits of influence here and there at different times of the month.

This month illustrates how the atmospheric circulation for the month can reflect the combined influence of all or most of the atmospheric drivers (or modes of atmospheric variability).


Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for March 2019, published online April 2019, retrieved on August 20, 2019 from https://www.ncdc.noaa.gov/sotc/synoptic/201903.

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