Synoptic Discussion - December 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.
With low sun angles in the Northern Hemisphere at the time of the winter solstice (December), solar heating is minimal and cold polar air masses expand southward. The polar jet stream marks the boundary between the polar air masses and the warmer subtropical air masses. Winter storm systems moving in the storm track with the jet stream flow normally lay a blanket of snow in the cold air following in their wake. As winter progresses, the polar jet and polar air masses continue their inexorable expansion to the south — unless other atmospheric drivers act in opposition. This was the case in December 2012. The weather pattern for the first part of the month consisted of a fairly flat westerly flow with weak storm systems and a contracted polar jet. The result was unusually warm temperatures across the contiguous United States, a mixed precipitation pattern with large dry areas, and well below-normal snow (on December 6th, snow covered only 6 percent of the country). But the circulation pattern shifted at mid-month to a more vigorous flow with stronger storm systems which brought colder air masses further south (weeks 1, 2, 3, 4), laid down an extensive blanket of snow, and triggered outbreaks of severe weather in the Deep South and Southeast. Snow coverage expanded to 31.9 percent of the country by the 10th, then to 49.3 percent by the 22nd, then again to 65.0 percent by the 27th. Beneficial precipitation helped alleviate drought conditions a bit where it fell (weeks 1, 2, 3, 4), yet many of the drought areas still missed out on the rain and snow. A tornado outbreak associated with a strong storm system on Christmas Day brought the monthly total to 51 preliminary reports of tornadoes, which is about twice the average for December. The number of wildfires for this month was the most for December in the 13-year record, but the amount of acres burned was low so the average fire size was the smallest on record for December.
The movement of the weather systems can be seen in the weekly precipitation anomaly patterns (weeks 1, 2, 3, 4). Beneficial rain and snow helped improve drought conditions over parts of the West, especially the Sierra Nevada and Intermountain Basin (69.3 percent of the West was affected by moderate to exceptional drought at the end of December compared to 72.7 percent at the end of November), and precipitation from frequent frontal systems and low pressure centers chipped away at the drought areas in the Southeast (45.7 percent compared to 46.8 percent) before moving northeast. But with the storm track bringing systems to the West and Southeast, they largely missed much of the Plains, where drought intensified (extreme to exceptional drought expanded in the Southern Plains from 26.8 percent at the end of November to 32.8 percent at the end of December and in the Central to Northern Plains from 57.9 percent to 60.3 percent). Six states (two in the West, three in the Northeast, and Ohio) had the tenth wettest, or wetter, December, while three states (Texas, Oklahoma, and Missouri) ranked in the driest third of the historical record for December. Overall, the precipitation helped reduce the national moderate to exceptional drought footprint from 62.7 percent at the end of November to 61.1 percent at the end of December. According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 51.8 percent of the contiguous U.S. was in moderate to extreme drought at the end of December, a decrease of about 8 percent compared to last month. The 2012 Palmer Drought Index percent area values have been exceeded only by the drought of the 1930s.
The weekly temperature anomaly maps (weeks 1, 2, 3, 4) show how December began on a very warm note and how warmer-than-normal temperatures dominated until the last week, when dips (troughs) in the jet stream allowed cold polar air to pour into the middle of the country and parts of the West. Twenty states, from the Mississippi River to the East Coast, had the tenth warmest, or warmer, December, with Delaware ranking warmest on record. On a local basis, six times as many record warm highs and lows occurred than record cold highs and lows. About 1800 daily high temperature records and 2100 record warm daily low temperatures were tied or broken. In comparison, about 300 record low temperatures and 340 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.) The unusual warmth contributed to a national Residential Energy Demand Temperature Index (REDTI) for December 2012 that was sixth lowest for December in the 1895-2012 record.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the tenth warmest and 20th wettest December 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 (USCEI), they may tell a different story. Nationally, the large spatial extent of drought conditions (fourth largest PDSI component for December 2012), the frequent heavy precipitation in the West and South (14th largest days with precipitation component), and the large spatial extent of very warm minimum (14th largest) and maximum (22nd largest) temperatures combined to give the U.S. a December USCEI that was 29th largest (out of 103 years). The preponderance of unusual warmth and dryness for much of 2012 has ranked the national USCEI for the year (January-December) as second largest and for the last six months (July-December) as 19th largest (both of these ranks include the warm season tropical cyclone component).
Subtropical highs, and cold fronts and low pressure systems moving in the storm track flow, are influenced by the broadscale atmospheric circulation. Five such large-scale atmospheric circulation drivers were potentially influential during December:
El Niño Southern Oscillation (ENSO)
- Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during December.
- 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 the ENSO episodes. For an El Niño, the typical October-December temperature anomaly pattern is below-normal temperatures for the southern states and above-normal temperatures for the Northwest to Northern Plains. The typical El Niño October-December precipitation anomaly pattern consists of drier-than-normal conditions for much of the country except patchy wetter-than-normal conditions for the Southwest, Southern Plains, and parts of the Southeast to Mid-Atlantic coast. For a La Niña, the typical October-December temperature anomaly pattern consists of warmer-than-normal conditions from the Great Basin to the Mississippi River, and precipitation anomaly pattern is drier than normal across much of the country from the Southwest to Northeast and Great Lakes to Gulf of Mexico coast, with above-normal precipitation mostly in the Pacific Northwest and Northern California.
- Observed: The October-December 2012 precipitation pattern matches the La Niña teleconnections for much of the country, and the December 2012 precipitation pattern matches for the Southern Plains and Northwest, but not the East. The October-December 2012 temperature pattern agrees with La Niña, especially for the central part of the country. The December 2012 temperature pattern is not a good match to either phase but agrees more with La Niña than El Niño.
- The Pacific/North American (PNA) pattern
- Status: The PNA index was negative for the first two-thirds of December then positive for the last third.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (between October and January on the maps) shows that a negative PNA is correlated with warmer-than-normal temperatures in the Southeast and cooler-than-normal temperatures along the west coast of North America, especially in western Canada and Alaska. The precipitation teleconnections become stronger as the season progresses, and show wetter-than-normal conditions for the Northwest and for much of the country east of the Plains with a negative PNA. The upper-level circulation anomalies associated with a negative PNA are below-normal heights over western North America and above-normal heights over southeastern North America. The opposite conditions are associated with a positive PNA.
- Observed: December 2012 was wetter than normal in the Northwest and much of the country from the Southeast to Northeast, which agrees with a negative PNA. The December 2012 temperature pattern agrees with a negative PNA in the South and East, but more with a positive PNA in the Northwest. Alaska was colder than normal in December, which is consistent with a negative PNA, and generally drier than normal, but PNA teleconnections are weak for Alaska precipitation. The December 2012 upper-level circulation pattern had strong negative anomalies over western North America, which is consistent with a negative PNA, but the neutral anomalies over southeastern North America and strong positive anomalies over northeastern North America are not consistent with a negative PNA.
- The Arctic Oscillation (AO) pattern
- Status: The AO index oscillated from a slightly negative to very negative phase throughout the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative AO this time of year (October-December) is typically associated with cooler-than-normal temperatures from Montana to Florida and eastward to the Canadian border and the Atlantic Coast, dry conditions from the Southeast to portions of the Central and Southern Plains, and patchy wetter-than-normal conditions in Northern California and New England. The October-December averaged upper-level circulation anomalies for a negative AO are below normal 500-millibar (mb) geopotential heights (which translates to stronger trough or weaker ridge, depending on the circulation) over the eastern half of the U.S. and over the North Pacific Ocean, and above-normal 500-mb heights (which translates to stronger ridge or weaker trough, depending on the circulation) over the Arctic. A positive AO is typically associated with the opposite circulation and temperature patterns.
- Observed: The December 2012 temperature pattern is not characteristic of any AO phase. While the October-December 2012 temperature pattern has the least positive anomalies in the Northern Plains and Southeast, which hints at a negative AO, the temperature anomalies are mostly positive which do not match what is expected with a negative AO. The October-December 2012 precipitation pattern matches the negative AO teleconnection for Northern California and the Southern and Central Plains to Southeast, but the December 2012 precipitation pattern does not match in the Southeast. The October-December 2012 upper-level circulation had an above-normal pattern over the northwestern Atlantic and Bering Sea area with below-normal geopotential heights over the southeastern U.S. and eastern North Pacific into southwestern Canada. This has some resemblance to a negative AO, but the patterns are significantly shifted and the negative heights are not as strong as in the teleconnection maps.
- The North Atlantic Oscillation (NAO) pattern
- Status: The NAO index fluctuated near neutral to negative the first half of December, then near neutral to positive the last half of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (January on the teleconnection maps) is typically associated with wetter-than-normal conditions in Northern California and the Great Basin, while a positive NAO is typically drier than normal there. Temperatures this time of year are trending toward cooler than normal for much of the country east of the Rockies for a negative NAO (warmer than normal for a positive NAO). The upper-level circulation anomalies for a negative NAO are below-normal 500-mb geopotential heights over the eastern two-thirds of the contiguous U.S. (above-normal heights for a positive NAO).
- Observed: The December and October-December 2012 precipitation anomaly patterns match that expected with a negative NAO, while the December temperature pattern matches that expected with a positive NAO. The October-December 2012 temperature pattern and December and October-December 2012 upper-level circulation patterns do not match any NAO teleconnections.
- The East Pacific-North Pacific (EP-NP) pattern relates sea surface temperature (SST) and upper-level circulation patterns over the eastern and northern Pacific to temperature, precipitation, and circulation anomalies downstream over North America.
- Status: The SST pattern over the northeast Pacific experienced a warming trend from June to October with SST anomalies becoming less cool (June, July, August) to warmer than normal in places (September, October). The trend reversed itself during November, with the warm SST anomalies becoming less warm and the cool SST anomalies expanding. The November transition continued in December, with the below-normal SST pool expanding to cover much of the northeast Pacific. The EP-NP index (3-month running mean) had been positive for the last several months, but its current value is unknown since the Climate Prediction Center (CPC) does not compute the index for December. However, for the seasons during which the EP-NP is calculated, the positive phase of the EP-NP pattern is associated with above-average surface temperatures over the northeastern Pacific (thus, the negative phase is associated with below-normal SSTs). The negative December SST anomalies hint at a possible transition to a negative EP-NP.
- Teleconnections (influence on weather): Teleconnection maps are not available for the winter (January), but they are for the bookend seasons autumn and spring. To the extent these teleconnections are known, a positive EP-NP index during these seasons 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 Canada and Alaska (the correlations are weak for precipitation). A negative EP-NP index is typically associated with the opposite patterns (especially warmer-than-normal temperatures east of the Rockies).
- Observed: The December 2012 temperature and upper-level circulation anomaly patterns readily reflect a negative (spring/autumn) EP-NP teleconnection. Above-normal precipitation in the Northwest shows up in the December 2012 anomaly pattern and in the autumn teleconnection pattern for a negative EP-NP.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed December and October-December 2012 temperature, precipitation, and circulation patterns, suggests that no single atmospheric driver dominated the weather during December, but the weather was influenced in part by several of the drivers. ENSO was neutral and, thus, not a player. The AO was negative, which is normally associated with colder-than-normal temperatures over much of the United States. The dominance of December warmth suggests that the negative PNA and positive NAO countered the negative AO for much of the month. While the EP-NP is not considered by CPC to normally be a leading mode of variability in the winter, if the EP-NP were negative then it, too, would have countered a negative AO. A pattern shift occurred during the last third of the month, with the PNA and NAO changing modes and colder-than-normal air pouring into the country. 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.