Synoptic Discussion - October 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

Map of monthly temperature anomalies
Map of monthly temperature anomalies.
Map of monthly precipitation anomalies
Map of monthly precipitation anomalies.

In the Northern Hemisphere, October marks the middle of climatological fall (autumn) which is the time of year when the active jet stream and circumpolar vortex expand south, spreading polar and arctic air masses from the north across the United States. For October 2013, the jet stream was very active with several upper-level troughs and closed lows bringing cool and wet weather to parts of the country. The lows generated significant winter snowstorms in the north central U.S. near the beginning and end of the month, with snow covering 18 percent of the country on the 30th. These lows and their associated cold fronts also triggered severe weather outbreaks, with the tornado outbreaks on the 4th and the 31st accounting for the bulk of the (preliminary estimated count of) 72 tornadoes for October 2013 (this compares to a climatological average count of 61 for October).

The snowstorm track (weeks 1, 2, 3, 4, 5) resulted in above-normal precipitation from the Central Rockies to Northern Plains during October, and precipitation was above normal from the Southern Plains to Midwest where the cold fronts tapped Gulf of Mexico moisture; but the northerly flow and polar air masses funneled drier-than-normal air to much of the western and central parts of the country, and the circulation kept moisture systems away from much of the East. Rhode Island and Massachusetts had the fourth and ninth driest, respectively, October in the 1895-2013 record, with several other states in the Northeast, West, and Southeast not far behind. On the other hand, four North Central states (Nebraska, North and South Dakota, and Wyoming) ranked in the top ten wettest category. When integrated across the country, October ranked near the middle of the historical record at 50th driest. The wet conditions in the Rockies and Plains helped reduce drought coverage in those regions. Drought also shrank in the Midwest, but moderate drought developed in the Northeast and abnormally dry conditions expanded in the Southeast. The national drought footprint shrank to 31.8 percent of the U.S. as a whole (according to U.S. Drought Monitor statistics). Tropical cyclone activity in the North Atlantic basin during October was near normal in terms of the number of named storms, but below normal with respect to the number of hurricanes, with none of the cyclones making landfall on the U.S. mainland. The frequent troughs and frontal passages produced upper-level wind shear in the vicinity of the U.S. which inhibited tropical cyclone activity.

The troughs in the upper-level circulation favored the western and central U.S., with upper-level ridges dominating in the east for much of the month. The warm southerly flow associated with the ridge (and ahead of the cold fronts which eventually reached the region) resulted in above-normal temperatures most weeks (weeks 1, 2, 3, 4, 5). A shift in the circulation near the end of the month brought a warm ridge to the West and cool trough to the East, but for the month as a whole, temperatures averaged below normal in the West and Central regions and above normal in the East, especially the Northeast. Seventeen states in the West to Central U.S. had October temperatures in the coolest third of the historical record, with Oregon ranking eleventh coldest. Thirteen states in the East ranked in the warmest third of the historical record, with Delaware tenth warmest. There were slightly more (1.2 times as many) record cold daily highs (698) and lows (407, or a total of about 1100) as record warm daily highs (242) and lows (689, or a total of about 930). On balance for the nation as a whole, October ranked in the cool third of the historical record at 37th coolest.

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

The upper-level circulation, integrated across the month, produced a pattern of above-normal 500-mb heights (stronger-than-normal ridge) over the northwest Pacific, Alaska, and western Canada, and below-normal 500-mb heights (stronger-than-normal trough) from the Southwest U.S. to south central Canada. This monthly pattern reflects the frequent occurrence of troughs and upper-level low pressure systems moving out of the Southwest U.S. into the Central to Northern Plains and is somewhat indicative of the Pacific drivers of the atmospheric circulation, as noted below.

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 October 2013 temperature, precipitation, and circulation patterns, suggest that the extra-tropical Pacific atmospheric drivers shared some influence on October weather. ENSO was neutral, and thus not a player; the MJO was incoherent for much of the month; and the AO and NAO did not seem to be influential. Those drivers associated with the extra-tropical Pacific (PNA, EP-NP, and WP) appeared to exert some influence on the precipitation pattern in the Pacific Northwest, Alaska (EP-NP), and Southeast (WP), and on the temperature pattern (PNA and EP-NP). The EP-NP seemed to have the strongest influence on the upper-level circulation, but that influence appeared to be tempered by other forces. For example, Typhoon Wipha recurved into the North Pacific, creating a ridge in the circulation over the eastern Pacific and Alaska which forced a trough downstream over central North America. This ridge contributed to record warm temperatures over Alaska and likely shifted the below-normal 500-mb height anomalies (normally associated with a positive EP-NP) westward. Precipitation is not as strongly correlated to any of the atmospheric drivers during the early fall as it is during the winter, with random convective and synoptic processes playing a greater role in precipitation during October, but it should be noted that the NAO and PNA had opposite influences on precipitation in the Ohio Valley. A negative NAO is associated with drier-than-normal conditions while a negative PNA is associated with wetter-than-normal conditions. These two opposing drivers could have contributed to the mixed precipitation anomaly pattern in the Ohio Valley. This month illustrates how competing atmospheric drivers may work in phase in some regions, against each other in others, and be influenced by random atmospheric processes, to create a complex weather pattern.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: Synoptic Discussion for October 2013, published online November 2013, retrieved on August 22, 2014 from http://www.ncdc.noaa.gov/sotc/synoptic/2013/10.