Synoptic Discussion - June 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.

Synoptic Discussion

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

June marks the beginning of the Northern Hemisphere's climatological summer (June-August). With the sun's angle at its maximum inclination in the Northern Hemisphere and, thus, solar heating at its greatest intensity, the circumpolar vortex is normally weak and contracted far to the north with warm southerly air masses associated with the subtropical high pressure belt influencing the weather over the United States. During June 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) continued in an weather patternactive phase, with many troughs and ridges migrating across the country in the upper-level circulation.

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). Some of these troughs moved very slowly and, in the east, tapped Gulf of Mexico and Atlantic moisture, resulting in heavy rains and causing extensive flooding, especially in the Upper Midwest and along the East Coast, where Delaware and New Jersey had their wettest June on record and New York and North Carolina ranked second wettest. These upper-level troughs and their complex circulations also triggered tornado outbreaks and widespread wind damage from the Plains to the East Coast, although the preliminary national monthly count of 145 tornadoes was below the June average of 243.

Although weather patterncool troughs and their associated weather patternfronts traversed the West during the month, high pressure ridging dominated — bringing hot and dry weather to the southwestern third of the country, where June ranked in the top ten warmest category for six states and top ten driest category for four states, including Utah which had its driest June on record. Dozens of large wildfires flared up, especially in the Southwest but also in Alaska (where the statewide average temperature ranked third warmest for June in the 1918-2013 record). The total number of fires in the country was below the June average, but the acreage burned was above average. About 2800 record warm daily highs and lows were recorded across the nation, almost four times as many record cool daily highs and lows. When integrated across the month, the Southwest averaged warmer than normal and the north central parts of the country averaged cooler than normal with a mixture of anomalies elsewhere.

June marks the beginning of the Atlantic hurricane season. Two tropical storms formed this month in the North Atlantic basin, one of which (Tropical Storm Andrea) merged with a cold front and contributed to heavy rain along the East Coast during the first week of June. Based on a 30-year (1981-2010) average, a tropical storm forms in June in the basin about once every other year.

The rain-producing systems shrank drought in the East and Midwest, but hot and dry weather expanded drought in the West and Alaska. Overall, the national (all of the U.S.) drought footprint expanded from 37.2 percent in moderate to exceptional drought last month to 39.3 percent at the end of June, but remained about the same at 44.1 percent for the contiguous U.S., according to U.S. Drought Monitor statistics. When the temperature and precipitation anomalies are integrated across the country and across the month, June 2013 ranked as the 15th warmest and 13th wettest June 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 June 2013 and April-June 2013 temperature, precipitation, and circulation patterns, suggests that the MJO and AO drivers each exerted some influence during June (the influence for temperature is stronger than for precipitation this time of year) but the EP-NP driver was most dominant. ENSO was neutral, and thus not a player; the PNA changed signs during the month, thus making its signal difficult to decipher; and the NAO showed no agreement to the observed patterns. The MJO signal could be seen during the second and third weeks in both the temperature and precipitation patterns. The EP-NP driver seemed to be the most influential for June temperature in the north central U.S. while the MJO driver seemed influential for June temperature in the West. This month illustrates how dominant signals can be extracted from the data, even though competing atmospheric drivers may result in a complex weather pattern.

Citing This Report

NOAA National Centers for Environmental Information, State of the Climate: Synoptic Discussion for June 2013, published online July 2013, retrieved on January 18, 2018 from