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.

Synoptic Discussion

Monthly upper-level circulation pattern and anomalies.

April is the middle month of the Northern Hemisphere's climatological spring (March-May), which puts it in the midst of the period of transition for the atmospheric circulation when the circumpolar vortex contracts to the north with warm southerly air masses beginning to dominate over cold northerly air masses. During April 2013, however, the polar jet stream (which marks the edge of the circumpolar vortex and the boundary between the cold polar air masses to the north and the warmer sub-tropical air masses to the south) lingered over the U.S., favoring an upper-level trough over the central part of North America. Upper-level systems moving in this very active flow intensified over the central U.S., sending a series of low pressure and frontal systems across the central and southeastern parts of the country. They triggered outbreaks of tornadoes, mostly in the South; but, for the most part, the circulation inhibited tornado formation with the monthly preliminary tornado count being about half of normal for April. The systems largely missed the Southwest and Northeast (weeks 1, 2, 3, 4), with these regions ending the month generally drier than normal and drought expanding in the West. But beneficial precipitation falling over the Plains, Midwest, and Southeast drought areas helped shrink the national drought footprint from 51.9 percent in moderate to exceptional drought last month to 46.9 percent at the end of April (according to U.S. Drought Monitor statistics). The dominant upper-level flow funneled cold air masses across the Plains and into the East and South (weeks 1, 2, 3, 4), giving seven Plains and north central states April temperatures ranking in the top ten coldest for April and leaving only the southwestern U.S. and parts of the eastern U.S. warmer than normal. With temperatures below freezing, these cold northerly air masses combined with humid southerly air masses to produce intense spring snowstorms across the Plains (snow maps for April 1^st, 10^th, 18^th, 21^st, 23^rd, 30^th), raising national snow coverage to a peak of 29 percent of the contiguous U.S. before the snow cover quickly melted.

Map of monthly temperature anomalies.

Map of monthly precipitation anomalies.

Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. The following describes several such large-scale atmospheric circulation drivers and their potential influence this month:

• El Niño Southern Oscillation (ENSO)
  • Description: Oceanic and atmospheric conditions in the tropical Pacific Ocean can influence weather across the globe. ENSO is characterized by two extreme modes: El Niño (warmer-than-normal sea surface temperature [SST] anomalies in the tropical Pacific) and La Niña (cooler-than-normal SST anomalies), with the absence of either of these modes termed "ENSO-neutral" conditions.
  • Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during April.
  • Teleconnections (influence on weather): To the extent teleconnections are known, while in a neutral state, ENSO normally is not a player in the month's weather. Historical data can be analyzed to show typical temperature and precipitation patterns associated with El Niño and La Niña ENSO episodes. Teleconnections are not available for ENSO-neutral conditions.
• Madden-Julian Oscillation (MJO)
  • Description: The MJO is a tropical disturbance or "wave" that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is characterized by regions of enhanced and suppressed tropical rainfall. One of its indices is a phase diagram which illustrates the phase (1-8) and amplitude of the MJO on a daily basis. The MJO is categorized into eight "phases" depending on the pattern of the location and intensity of the regions of enhanced and suppressed tropical rainfall. Overall, the MJO tends to be most active during ENSO-neutral years, and is often absent during moderate-to-strong El Niño and La Niña episodes.
  • Status:The MJO began the month active in phases 3 and 4, but then weakened and became less coherent during the last half of the month (indicated by the index moving into the inner circle on the phase diagram).
  • Teleconnections (influence on weather): The MJO's temperature and precipitation teleconnections to U.S. weather depend on time of year and MJO phase. To the extent teleconnections are known, the February-April teleconnections for precipitation are shown here and for temperature are shown here.
  • Observed: The MJO is transitory and can change phases (modes) within a month, so it is more closely related to weekly weather patterns than monthly. The April 2013 monthly precipitation pattern shows little correlation to the precipitation teleconnection patterns for the MJO this time of year. However, the weekly precipitation anomaly patterns correspond in some locations to the teleconnections for the MJO phase during these weeks — above-normal precipitation in the Pacific Northwest during week 1 is associated with phase 3, while above-normal precipitation in the South, Northeast, and Great Lakes during week 2 is associated with phase 4. The April 2013 monthly temperature pattern shows some similarity to the teleconnections for MJO phase 3 (colder than normal across much of the country). The temperature anomaly patterns for weeks 1 and 2 seem to follow the general trend for MJO phases 3 and 4, respectively. The temperature anomaly patterns for weeks 3 and 4 seem to reflect what would be expected with MJO phases 7 and 3 (respectively) when, in fact, the MJO phase was incoherent during those weeks.
• The Pacific/North American (PNA) pattern
  • Description: The PNA teleconnection pattern is associated with strong fluctuations in the strength and location of the East Asian jet stream. PNA-related blocking of the jet stream flow in the Pacific can affect weather downstream over North America, especially the West and especially in the winter half of the year.
  • Status: The PNA index was neutral (near zero) for the first half of April, then turned negative for the second half.
  • Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (April on the maps) shows that a negative PNA is correlated with warmer-than-normal temperatures in the Southeast and Southern Plains and cooler-than-normal temperatures in the Pacific Northwest and especially in western Canada and Alaska. The precipitation teleconnections are weak, but show some hint of dryness over Colorado and Kansas and wetness in the Upper Mississippi Valley. The upper-level circulation anomalies associated with a negative PNA are below-normal heights over much of Canada, especially western Canada, and above-normal heights over southeastern North America.
  • Observed: Alaska was cooler than normal during April (consistent with a negative PNA), but the Southern Plains and much of the southeastern U.S. were also cooler than normal which is consistent with a positive PNA. The April precipitation anomaly pattern is consistent with a negative PNA over parts of Colorado, Kansas, and Minnesota. The April 2013 upper-level circulation pattern had strong negative anomalies across much of Canada, which is consistent with a negative PNA, but the pattern showed little consistency across the United States.
• The Arctic Oscillation (AO) pattern
  • Description: The AO teleconnection pattern relates upper-level circulation over the Arctic to circulation features over the Northern Hemisphere mid-latitudes and is most active during the cold season.
  • Status: The AO index began April strongly negative, transitioned to strongly positive during the last half of the month, and ended the month neutral.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (February-April) is typically associated with cooler-than-normal temperatures across the contiguous U.S. from the Rockies to the east coast, dry conditions in the Lower Mississippi Valley to the Great Lakes, parts of the Northeast, and extreme Northwest, and wetter-than-normal conditions across Florida and southern Georgia, although the precipitation correlations are weak. A positive AO is typically associated with warmer-than-normal temperatures from the Great Plains to East Coast, wetter-than-normal from northeast Texas to Kentucky, and dry conditions across the Southeast Coast states, Kansas, and West Coast states. The February-April averaged upper-level circulation anomalies for a positive AO are above-normal 500-millibar (mb) geopotential heights (which translates to weajer trough or stronger ridge, depending on the circulation) over the southeastern U.S. and bands of the mid-latitude North Atlantic and North Pacific, and below-normal 500-mb heights (which translates to weaker ridge or stronger trough, depending on the circulation) over the Arctic and northern Canada.
  • Observed: The April monthly temperature anomaly pattern is strongly consistent with a negative AO. There is some agreement with the April precipitation pattern in the Far West. The April 2013 upper-level circulation is consistent over much of the North American sector with a positive AO.
• The North Atlantic Oscillation (NAO) pattern
  • Description: The NAO teleconnection pattern relates upper-level circulation over the North Atlantic Ocean to circulation features over the Northern Hemisphere mid-latitudes.
  • Status: Like the AO index, the NAO index began the month slightly negative, turned strongly positive during the last half, and ended the month neutral.
  • Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (April on the teleconnection maps) is typically associated with cooler-than-normal temperatures for much of the country except the South, wetter-than-normal conditions in the Southeast, Central Plains, and parts of the West (although the correlations are weak for precipitation), and negative upper-level circulation anomalies across most of the country, especially over the Central Rockies. A positive NAO is associated with the opposite conditions.
  • Observed: The April 2013 temperature anomaly pattern agrees with that expected for a negative NAO in the central U.S. but not the West. The precipitation and upper-level circulation anomaly patterns are not consistent with either a positive or negative NAO.
• The East Pacific-North Pacific (EP-NP) pattern
  • Description: The EP-NP teleconnection pattern relates SST and upper-level circulation patterns over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America. Its influence during the winter is not as strong as during the other three seasons.
  • Status: The SST pattern over the northeast Pacific during April 2013 became dominated by a large pool of warmer-than-normal SSTs in the central North Pacific while the cooler-than-normal SSTs along the North American coast shrank considerably. The EP-NP index (3-month running mean) surged further into positive territory.
  • Teleconnections (influence on weather): To the extent these teleconnections are known, a positive EP-NP index during this time of year (April on the maps) is typically associated with cooler-than-normal temperatures east of the Rockies, warmer-than-normal temperatures along the West Coast and Alaska, below-normal upper-level circulation anomalies (stronger upper-level trough) over eastern North America, and above-normal upper-level circulation anomalies (stronger upper-level ridge) over western North America (the correlations are weak for precipitation).
  • Observed: The April temperature anomalies are a good match for the central and eastern U.S., but not Alaska. The upper-level circulation anomaly patterns are consistent with those expected for a positive EP-NP over North America, although they seem shifted a few degrees longitude westward.

Map of three-month temperature anomalies.

Map of three-month precipitation anomalies.

Upper-level circulation pattern and anomalies averaged for the last three months.

Examination of these circulation indices and their teleconnection patterns, and comparison to observed April 2013 and February-April 2013 temperature, precipitation, and circulation patterns, suggests that the AO and EP-NP drivers shared dominance during April. ENSO was neutral, and thus not a player; the MJO weakened during the month and became less influential; and the PNA, NAO, and AO changed signs during the month, thus making their signals difficult to decipher. The AO started the month negative but shifted to a strongly positive value. The upper-level circulation pattern with a positive AO shares some similarities with that for a positive EP-NP. The dominance of below-normal temperatures for the first half of April could be traced to the negative AO, but also to the negative NAO, positive EP-NP, and MJO as well. When the AO (and NAO) shifted into positive territory, the increasingly positive EP-NP could have maintained dominance over the temperature anomaly pattern, persisting the cold anomalies. There is some indication that regional precipitation anomalies could be linked to the PNA, AO, and MJO at times, but there is no strong correspondence between the observed precipitation anomalies and any of these drivers. This month illustrates how competing atmospheric drivers can result in a complex weather pattern and how, when the atmospheric circulation drivers are neutral or in a state of transition, their influence can become difficult to trace and can be overwhelmed by other competing forces, including random fluctuations in the atmosphere.