Extratropical Highlights –December 2018

 

1. Northern Hemisphere

The 500-hPa circulation during December featured above-average heights over the central North Pacific, western and central Canada, western Europe, and along an axis extending from Greenland to northwestern Russia. Below-average heights were present over the high latitudes of the central North Pacific, the central North Atlantic, and central Asia (Fig. E9).

The main land-surface temperature signals during December included above-average temperatures across most of Alaska and Canada, the north-central U.S., and western Europe, and below-average temperatures in central Asia (Fig. E1). The main precipitation signals included above-average totals in much of the eastern half of the U.S., and below-average totals in southern Europe (Fig. E3).

 

a. North America

The 500-hPa circulation during December featured above-average heights across western and central Canada, and the north-central U.S., and below-average heights over the high latitudes of the central North Pacific (Fig. E9). This anomaly pattern reflected a stronger than average jet stream over the central and eastern North Pacific (Fig. T21), and a weaker than average strength of the Hudson Bay Low. The pattern was associated with anomalous southerly flow into central and eastern Alaska, and with a reduced northwesterly flow across the western half of Canada (Fig. E10). The result was exceptionally warm surface temperatures (above the 70^th percentile of occurrences) in central and eastern Alaska, western and central Canada, and the north-central U.S. (Fig. E1).

Also during December the extended North Pacific jet stream, combined with a mean upper-level trough over the south-central U.S., contributed to increased storminess and above-average precipitation across most of the eastern half of the U.S. (Fig. E3). Area-averaged precipitation totals were above the 90^th percentile of occurrences in the Great Plains, Gulf Coast, Southeast, and Ohio Valley regions, and above the 70^th percentile of occurrences in the Midwest and Mid-Atlantic regions (Fig. E5).

According to the U.S. Drought Monitor, severe-to-exceptional drought continued in the four-corner states of Utah, Colorado, Arizona, and New Mexico. Farther north in Oregon, extreme drought persisted across the central part of the state, and severe drought persisted elsewhere.

 

b. Europe and Asia

The 500-hPa height pattern featured above-average heights over southwestern Europe and along an axis extending from Greenland to northwestern Russia, along with below-average heights over central Asia (Fig. E9). In Europe, the amplified ridge contributed to a continuation of exceptionally warm surface temperatures across western Europe and Scandinavia (Fig. E1). It also resulted in below-average precipitation in southern Europe (Fig. E3), where area-averaged totals were in the lowest 20^th percentile of occurrences (Fig. E4).

In Russia and central Asia, the 500-hPa height anomaly pattern reflected an amplified ridge-trough configuration, and was associated with an extensive anomalous northerly flow of arctic air into central Asia (Fig. E10). These conditions produced well below-average surface temperatures (3ºC-5ºC below average) in central Asia (Fig. E1).

 

2. Southern Hemisphere

The mean 500-hPa circulation during December featured above-average heights centered to the southeast of both Australia and southern Africa, and generally below-average heights at higher latitudes (Fig. E15). At 200-hPa, the circulation featured an amplified ridge over central Australia, and also in the area upstream of southern Africa (Fig. T22). The result was exceptionally warm (Fig. E1) and dry (Fig. E3) conditions in much of Australia, and also across the heart of the South African monsoon region.

The South African monsoon season runs from October to April. This area has recorded well below-average precipitation during each of the first three months (October-December) of the 2018 monsoon season, with area-averaged totals in all three months being in the lowest 20^th percentile of occurrences.