Synoptic Discussion - May 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.

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Synoptic Discussion

Monthly upper-level circulation pattern and anomalies
Monthly upper-level circulation pattern and anomalies.

May marks the end of the Northern Hemisphere's climatological spring (March-May). As the sun's angle approaches its June maximum and solar heating continues to intensify in the Northern Hemisphere, the circumpolar vortex contracts further to the north with warm southerly air masses increasing their dominance over cold northerly air masses. The clash between the air masses can result in violent spring weather, with May having the most tornadoes, on average. At the same time, dry weather associated with the subtropical high pressure belt can become influential as this belt also migrates northward. During May 2013, 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) was weather patternvery active, with many upper-level troughs and ridges migrating across the country in the upper-level circulation. Some of these troughs moved very slowly and tapped Gulf of Mexico moisture, resulting in heavy rains and causing extensive flooding, especially in the Midwest, where Iowa had the wettest May on record. These upper-level troughs and their complex circulations also triggered widespread tornado outbreaks from the Plains to the Midwest, although the preliminary national monthly count of 242 tornadoes was below the average of 276 for May. Cold fronts and warm fronts moving with these upper-level systems brought migrating spells of cooler-than-normal and warmer-than-normal weather to parts of the country (weeks 1, 2, 3, 4, 5). About 2250 record warm daily highs and lows were recorded, but more than one and a half times as many record cool daily highs and lows (over 3600) were observed. When integrated across the month, the north central and southeastern parts of the country averaged cooler than normal while the Midwest to Northeast and parts of the West were warmer than normal. The intrusions of cold air masses produced heavy snow outbreaks when they mixed with the moist southerly flow, but the snow cover did not last long under the intensifying sunlight and subsequent warm fronts. Monthly snow cover averaged below normal for May due to the ephemeral nature of these snow storms and below-normal snow cover in the West. The rain-producing systems largely missed the West and Ohio Valley, and parts of the Mid-Atlantic coast and Gulf of Mexico coast were also drier than normal. Drought expanded in the West, where the circulation favored an upper-level ridge, but it contracted under beneficial rains in the Plains, Midwest, and Southeast. Overall, the national drought footprint shrank from 46.9 percent in moderate to exceptional drought last month to 44.1 percent at the end of May (according to U.S. Drought Monitor statistics). When the temperature and precipitation anomalies are integrated across the country and across the month, May 2013 ranked as the 40th warmest and 17th wettest May for the nation.

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 May 2013 and March-May 2013 temperature, precipitation, and circulation patterns, suggests that the MJO, PNA, AO, and EP-NP drivers each exerted some influence during May (the influence for temperature is stronger than for precipitation this time of year). ENSO was neutral, and thus not a player; the MJO weakened during the month and became less influential, but still was a player early in the month; and the PNA, NAO, and AO changed signs during the month, thus making their signals difficult to decipher. The MJO signal could be seen during the first 3 weeks in both the temperature and precipitation patterns, and the positive PNA likely reinforced the MJO for temperature during this period. The upper-level circulation pattern for May shows some hint of consistency with a positive PNA, AO, and NAO, but it is not a really good match for any of them. The influence of the AO and EP-NP drivers is more evident on a seasonal (last 2 to 3 months) basis than monthly (May) basis. 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 Centers for Environmental Information, State of the Climate: Synoptic Discussion for May 2013, published online June 2013, retrieved on November 27, 2015 from