Synoptic Discussion - April 2013


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.


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.


Synoptic Discussion

Monthly upper-level circulation pattern and anomalies
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 weather patternvery 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 1st, 10th, 18th, 21st, 23rd, 30th), 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 temperature anomalies.
Map of monthly precipitation 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:

Map of three-month temperature anomalies
Map of three-month temperature anomalies.
Map of three-month precipitation anomalies
Map of three-month precipitation anomalies.

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

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for April 2013, published online May 2013, retrieved on December 21, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/4.