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.
The weather pattern over North America during September 2012 consisted of the seasonal battle between subtropical high pressure (High, or upper-level ridge) to the south and the polar jet stream and associated storm track to the north. The month began with the remnants of Hurricane Isaac entrained into a cold front draped across the Midwest to Northeast. After this system tracked out of the country, the jet stream frequently pushed upper-level troughs and cool fronts into the High over the eastern United States. When averaged over the month of September, this pattern resulted in warmer-than-average temperatures beneath a stronger-than-average upper-level ridge over the western U.S. and cooler-than-average temperatures beneath a stronger-than-average upper-level trough over the East. Showers and thunderstorms along the fronts and frontal lows dropped above-normal rainfall from the Southern Plains to the Midwest and parts of the Northeast, while summer monsoon showers brought above-normal precipitation to parts of the Southwest and frequent storms gave Alaska the fifth wettest September in the state's 1918-2012 record. But descending air ("subsidence") associated with the High dominated the West and Northern Plains, giving Montana, Minnesota, and the Dakotas the driest September in the 1895-2012 record. Numerous wildfires broke out in the hot, dry, windy weather across the West, especially in the Northwest, giving September 2012 near-record high acreage burned and near-record average size of fire. This weather pattern inhibited the formation of tornadoes, with the preliminary national count of 43 tornadoes being below the long-term average.
The movement of fronts across the South and East, and monsoon showers in the Southwest, can be seen in the weekly above-normal precipitation anomaly patterns (weeks 1, 2, 3, 4). The persistent dryness in the Northwest, Northern Plains, and Upper Midwest is also evident. The weather pattern during September shielded the U.S. mainland from most tropical activity. All of the four hurricanes and tropical storms active in the North Atlantic during September were steered away from the U.S. mainland. The remnants of Hurricane Isaac at the beginning of the month, and rains from subsequent frontal passages, helped shrink drought in the Ohio Valley where Kentucky and Ohio had the tenth wettest, or wetter, September in the 1895-2012 record. Ten states, from Wisconsin to the Pacific Northwest coast, had the tenth driest, or drier, September, with drought expanding in the Northern Plains and Upper Midwest. According to the end-of-September (October 2) U.S. Drought Monitor, 64.6% of the contiguous U.S. (54.0% of the U.S. including Alaska, Hawaii, and Puerto Rico) was affected by moderate to exceptional drought overall. These values are slightly higher than those at the end of August. According to the Palmer Drought Index, which goes back to the beginning of the 20th century, 51.9% of the contiguous U.S. was in moderate to extreme drought, a decrease of about 3 percent compared to last month. The 2012 Palmer Drought Index percent area values have been exceeded only by the droughts of the 1930s and 1950s.
The movement of the cool fronts can also be seen in the weekly temperature anomaly maps (weeks 1, 2, 3, 4). The persistence of warm anomalies in the West, and frequent excursions of cool air masses into the central and eastern U.S., gave four states in the West the tenth warmest, or warmer, September in the 1895-2012 record, while seven states in the Midwest and South had cooler-than-normal monthly temperatures. On a local basis, more than twice as many record warm highs and lows occurred than record cold highs and lows. Nearly 1050 daily high temperature records and 2150 record warm daily low temperatures were tied or broken. In comparison, about 750 record low temperatures and 630 record cool daily high temperatures were tied or broken. (These numbers are preliminary and are expected to increase as more data arrive.)
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 23rd warmest and 48th driest September in the 118-year record. Averaging extremes tends to cancel them out (as in the case for national precipitation this month). But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they may tell a different story. Nationally, the large spatial extent of very dry conditions ranked fourth largest for September 2012 (behind September 1934, 1954, and 1956). However, the CEI components for percent area with very warm maximum temperatures and days with no rain ranked only in the top 20, and the other components ranked even lower, giving the U.S. a September CEI that ranked only 39th largest. Regionally, the September 2012 CEI for the West North Central and West regions ranked sixth and seventh largest, respectively. The preponderance of unusual warmth and dryness for the last several months has ranked the national CEI largest for the warm season (April-September) and year-to-date (January-September), and second largest for the last twelve months (October-September).
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 September:
El Niño Southern Oscillation (ENSO)
- Status: Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in an ENSO-neutral state during September, although equatorial sea surface temperatures were close to weak El Niño conditions.
- 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 July-September temperature anomaly pattern is below-normal temperatures for most of the West and northern states with a hint of above-normal temperatures in the Southern Plains. The typical El Niño July-September precipitation anomaly pattern consists of drier-than-normal conditions across the Southern Plains to Southeast, Ohio Valley, and much of the East Coast, and areas of wetter-than-average conditions scattered across the Northern Plains to western Great Lakes. For a La Niña, the typical July-September temperature and precipitation anomaly patterns are warmer than normal in the central third of the country, cooler-than-normal in the West, drier-than-normal across much of the Plains, central Rockies, and Midwest, and wetter than normal in the Southeast to Mid-Atlantic.
- Observed: The September and July-September 2012 precipitation patterns are a closer match to the La Niña teleconnections than El Niño's, although there are significant differences. The September and July-September 2012 temperature patterns match La Niña in the central U.S. during the three-month period, but not otherwise, and show little similarity to the El Niño teleconnections.
- The Pacific/North American (PNA) pattern
- Status: The PNA index fluctuated around zero (near neutral) for most of the month then trended negative near the end of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, the temperature teleconnection map for this time of year (between July and October on the maps) is trending toward a pattern of opposite extremes at opposite corners of the country. A negative PNA is correlated with cooler-than-normal conditions in the Northwest and warmer-than-normal conditions in the Southeast. The precipitation patterns are weakly correlated this time of year, but show some hint of wetter-than-normal conditions from the Ohio Valley to Great Lakes with a negative PNA, and vice versa for a positive PNA.
- Observed: With weak teleconnections and a mostly neutral value, the PNA played a minimal role in U.S. weather during this month, although September was wetter than average in the Ohio Valley, but the September temperature pattern hinted at a positive PNA.
- The Arctic Oscillation (AO) pattern
- Status: The AO index was positive during the first half of the month and near neutral to slightly negative during the last half, then positive again near the end of the month.
- Teleconnections (influence on weather): To the extent teleconnections are known, a positive AO this time of year (July-September) is typically associated with warmer-than-normal temperatures across the Northern Plains, wet conditions from the Southeast to Mid-Atlantic coast, and dry conditions in parts of the Plains, Midwest, and Northeast. A negative AO this time of year is typically associated with cooler-than-normal temperatures in the Northern Plains, warmer-than-normal conditions in parts of the Southern Plains, dry conditions in parts of the Southern Plains and Northeast, and wet conditions in the Central Plains and a few areas of the Southeast. A neutral AO has little relationship to temperature or precipitation patterns. The July-September averaged upper-level circulation anomalies for a positive AO are below normal 500-millibar (mb) geopotential heights (which translates to stronger trough or weaker ridge, depending on the circulation) over the Arctic and above-normal 500-mb heights (which translates to stronger ridge or weaker trough, depending on the circulation) over southeast Canada into the north central to northeast United States. A negative AO has the reverse pattern (above normal anomalies over the Arctic and below normal over southeast Canada into the north central to northeastern U.S.).
- Observed: The July-September 2012 temperature pattern matches the positive AO where there is a correlation, but the precipitation patterns show little similarity. The July-September 2012 upper-level circulation had an above-normal pattern over the northern Atlantic, Greenland, and adjacent Arctic, with above-normal pattern across western North America and a trough over eastern North America. This has some resemblance to a negative AO, but not a positive AO.
- The North Atlantic Oscillation (NAO) pattern
- Status: The NAO index was near neutral to slightly positive during the first half of the month but turned sharply negative during the last half.
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative NAO during this time of year (July to October on the teleconnection maps) typically transitions to drier-than-normal conditions in the Midwest, while a positive NAO has the opposite association. Temperatures show little correlation to the NAO as one approaches October.
- Observed: The September and July-September 2012 precipitation anomaly patterns bear more resemblance to a positive NAO than a negative NAO.
- 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 EP-NP index (3-month running mean) transitioned from negative to positive during the last few months. The SST pattern over the northeast Pacific has experienced a warming trend over the last four months with SST anomalies becoming less cool (June, July, August, September).
- Teleconnections (influence on weather): To the extent teleconnections are known, a negative EP-NP index during this time of year (between July and October on the teleconnection maps) is typically associated with warmer-than-normal temperatures east of the Rockies, cooler-than-normal temperatures along the West Coast, wetter-than-normal conditions over the Northwest (although the correlations are weak for precipitation), above-normal upper-level circulation anomalies (weaker upper-level trough) over eastern North America, and below-normal upper-level circulation anomalies (weaker upper-level ridge) over western Canada and Alaska. A positive EP-NP index is typically associated with the opposite pattern — 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.
- Observed: The July-September 2012 temperature pattern bears some resemblance to the negative EP-NP teleconnection, while the monthly patterns (September and August) more readily reflect a positive EP-NP. The precipitation correlations are less obvious, although the July-September 2012 precipitation pattern (dry in the Northwest) bears some resemblance to the positive EP-NP teleconnections for later in the season (closer to the October teleconnection map). The pattern of above-normal upper-level heights (stronger ridge) over western North America, and trough over the east, in the July-September 2012 upper-level circulation matches the pattern associated with a positive EP-NP. This upper-level circulation pattern (stronger-than-normal ridge in the west and stronger-than-normal trough in the east) is even more evident on the September 2012 map.
Examination of these circulation indices and their teleconnection patterns, and comparison to observed September and July-September 2012 temperature, precipitation, and circulation patterns, suggests that ENSO, PNA, and NAO had little influence on the observed weather patterns. The AO and EP-NP may have exerted some influence on the weather this past month and season. The temperature patterns over the last three months, especially the frequent movement of cold fronts across the U.S. east of the Rockies, and upper-level circulation patterns reasonably reflect the transition from a negative to a positive EP-NP. As noted above, some of the indices were near neutral values for part or 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.