Synoptic Discussion - May 2012
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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The weather pattern over North America in May 2012 consisted of the seasonal battle between subtropical high pressure (Bermuda High) in the south and the jet stream (polar front) in the north. The Bermuda High brought warmer-than-normal air to much of the United States and drier-than-normal weather to western and central regions of the country. Cold fronts and low pressure systems scraped against the fringes of the Bermuda High, bringing bouts of cooler air, areas of rain, and occasional severe weather. Only the northwest corner of the country had widespread cooler-than-normal monthly averaged temperatures, courtesy of frequent upper-level troughs which, near the end of the month, brought areas of snow to parts of the Northern Rockies and Montana High Plains. But on the whole, May clocked in with below-average snow cover and tornado activity.
Did You Know?
The sun is the ultimate source of energy that drives the earth's weather. Most of the energy reaches the equatorial regions and the least energy reaches the poles, causing the tropics to warm and the poles to cool. The earth's atmosphere redistributes this heat imbalance through a complex set of atmospheric circulation patterns. The warm air at the low latitudes rises and moves toward the poles. The rising air, and the subsequent clouds and precipitation, cause the tropics to be very wet. As the air moves towards the subtropics, it descends over the oceans and creates semi-permanent circulation features called subtropical highs. In the Northern Hemisphere, these high pressure systems are located over the North Pacific and North Atlantic oceans. The North Atlantic High is generally centered over Bermuda, so it is also known as the Bermuda High. The descending air under subtropical highs warms and dries as it descends, resulting in generally sunny skies and dry weather. Cold air from the poles flows toward lower latitudes in order to complete the redistribution of the heat imbalance in the atmosphere. This cold polar air collides with warmer subtropical air in the mid-latitudes, resulting in frontal precipitation and low pressure cyclonic storm systems.
This entire system of fronts, subtropical highs, and tropical rain migrates with the seasons, moving northward during the Northern Hemisphere summer and southward during the Northern Hemisphere winter. Sometimes during the summer, the Bermuda High will extend further to the west than usual, encompassing a significant part of the southern and eastern United States. Its descending air inhibits precipitation and its anticyclonic circulation pattern deflects tropical storms and hurricanes to the south and weakens cold fronts to its north, resulting in heat waves and droughts.
As seen in weekly precipitation anomaly maps (weeks 1, 2, 3, 4, 5), the migrating fronts and low pressure systems generated precipitation along two storm tracks — one in the northern tier states and one in the southern tier states along the Gulf of Mexico. Rain from Tropical Storm Beryl near the end of May brought relief from the drought in northern Florida to the coastal Carolinas. But much of the interior U.S. had below-normal monthly precipitation, with two states in the Great Basin and three in the Central Plains to Mid-Mississippi Valley ranking in the top ten driest category for the month. Five states (Minnesota, the Carolinas, New Hampshire, and Vermont) ranked in the top ten wettest category. May 2012 was the 27th driest May nationwide in the 1895-2012 record. This rainfall pattern, in combination with increased evaporation caused by above-normal temperatures, expanded drought conditions in the West, Great Plains, and Mid-Mississippi Valley. The month had significant wildfire activity in the Southwest and western Great Lakes, but total acreage burned nationally was near average. Rainfall along the Atlantic coast shrank drought areas in the Southeast and Northeast. By May 29, moderate to exceptional drought expanded to cover 53 percent of the West, 43 percent of the South, and 32 percent of the Central and Northern Plains, according to the U.S. Drought Monitor. On a national scale, however, drought coverage shrank slightly, to 37 percent of the contiguous United States, due in large part to contraction in the Northeast and Southeast.
With the Bermuda High extending its seasonal reach into the U.S., and with warm southerly winds blowing northward ahead of the cool fronts, above-normal temperatures dominated much of the country for much of the month (weeks 1, 2, 3, 4, 5), resulting in the second warmest May in the 1895-2012 record. The unusual warmth is reflected in the daily temperature records, with more than five times as many record warm highs and lows occurring than record cold highs and lows. About 3200 daily high temperature records and over 3400 record warm daily low temperatures were tied or broken. In comparison, beneath the migrating upper-level troughs and associated cold fronts, only about 430 record low temperatures and about 700 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) Twenty-six states, from Colorado to New England, ranked in the top ten warmest category for May. The national Residential Energy Demand Temperature Index (REDTI) for May 2012 was the 47th lowest May value in the 1895-2012 record, indicating that the anomalous warmth reduced heating demands in the typically-cool (this time of year) North and introduced or increased demand for cooling in the South. When the above-normal temperatures of the past several months are factored in, the national REDTI value was lowest on record for the last three months (March-May 2012), year-to-date (January-May 2012), and last six months (December 2011-May 2012).
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. (as noted above) the second warmest and 27th driest May in the 118-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they may tell a different story. The large spatial extent of unusually warm highs (second largest), unusually warm lows (fourth largest), and very dry conditions (tenth largest) gave the U.S. its eighth largest CEI for May. The combination of persistent and unusual warmth, drought, and extreme daily precipitation events over the last several months, however, contributed to the highest national CEI for the last three months (March-May), year-to-date (January-May), and last 12 months (June-May), and the second highest CEI for the last six months (December-May). Several regions also had the highest regional CEI for the last 3, 6, and 12 months and year-to-date. Concurrent with these high CEI values are the record warm temperature ranks for the nation for March-May 2012, January-May 2012, December 2011-May 2012, and June 2011-May 2012.
Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Four such large-scale atmospheric circulation drivers were potentially influential during May:
- Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during May. 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 the ENSO episodes. For an El Niño, the typical March-May temperature and precipitation anomaly patterns do not match the temperature and precipitation anomaly patterns for March-May 2012. The typical March-May temperature and precipitation anomaly patterns for La Niña have a hint of a resemblance (cold Northwest, warm South Central and Southeast) (dry Southwest to Central Plains and coastal Northeast) to the March-May 2012 temperature and precipitation anomaly patterns, possibly reflecting the last vestiges of La Niña early in the period.
- The Pacific/North American (PNA) pattern was briefly negative at the beginning of the month but hovered around the neutral point for most of the month. A negative PNA this time of year (April on the teleconnection maps) typically is associated with warmer-than-normal temperatures from the Southern Plains to Northeast and cooler-than-normal temperatures in the Northwest. The precipitation patterns are weakly correlated this time of year, but show some hint of drier-than-normal conditions from the Great Basin to Central Plains and wetter-than-normal conditions in the Upper Mississippi Valley.
- The Arctic Oscillation (AO) pattern was neutral (zero) for most of the month with occasional slight bumps into positive index territory. A positive AO this time of year (March-May) is typically associated with warmer-than-normal temperatures across most of the country (especially the central half) and dryness from the central Rockies to the Great Plains and in parts of the Southeast, Mid-Atlantic, and Northeast. The March-May averaged upper-level circulation anomalies for a positive AO consist of an above-normal pattern over the eastern U.S. into the North Atlantic and over the North Pacific, and below-normal pattern over the Arctic.
- The North Atlantic Oscillation (NAO) pattern was neutral for most of the month but had dips into negative index territory at the beginning and end of the month. A negative NAO during this time of year (April on the teleconnection maps) is typically associated with wetter-than-normal conditions in the Southeast and Central Plains, while temperatures trend cooler than normal across most of the country.
The boundaries between positive and negative anomalies are shifted slightly, but the March-May 2012 averaged upper-level circulation anomalies are generally consistent with a positive AO pattern across the Lower 48 States, North Atlantic, and North Pacific. The above-normal heights over the eastern U.S. are shifted north reflecting a stronger Bermuda High, with below-normal heights over western North America (May 2012 and March-May 2012) coinciding with below-normal sea surface temperatures in the northeastern North Pacific. The pattern of observed temperature anomalies for May 2012 and the last three months (March-May) corresponds to the positive phase of the AO and negative phase of the PNA across most of the country. The May 2012 and March-May precipitation patterns also correspond to a positive AO and a negative PNA where the correlations exist. As noted above, most of the indices were near neutral values for much of the month. When the atmospheric circulation drivers are neutral or in a state of transition, their influence becomes difficult to trace and can be overwhelmed by other competing forces, including random fluctuations in the atmosphere.