Extratropical Highlights

  Table of Indices  (Table 3)

  Global Surface Temperature  E1

  Temperature Anomalies (Land Only)  E2

  Global Precipitation  E3

  Regional Precip Estimates (a)  E4

  Regional Precip Estimates (b)  E5

  U.S. Precipitation  E6

  Northern Hemisphere

  Southern Hemisphere

  Stratosphere

  Appendix 2: Additional Figures

Extratropical Highlights

AUGUST 2008

1. Northern Hemisphere

The 500-hPa height pattern during August 2008 featured positive anomalies over northeastern Canada and the polar region, and negative anomalies in the Gulf of Alaska , the eastern U.S. , and across northern Europe and northwestern Russia (Fig. E9). Surface temperatures were above average in central and eastern Canada , and throughout the vicinity of the Caspian Sea and Black Sea , and below-average in eastern Alaska (Fig. E1).

Many areas recorded significantly anomalous precipitation during the August. Rainfall was above average in the Pacific Northwest , Gulf Coast , and Southeast regions of the U.S. (Fig. E5), as well as portions of northern Europe, southern Scandinavia, and eastern China, and below-average in Alaska, the Great Lakes region of the U.S., and southeastern Europe (Fig. E3).

a. North Pacific/ North America

The 500-hPa height pattern during August featured amplified troughs over the Gulf of Alaska and the eastern U.S. , and an amplified ridge over the central North Pacific. The circulation also featured positive height anomalies over north-central Canada .  This overall pattern was associated with enhanced jet stream winds (Fig. T21) and increased storminess extending from the central North Pacific to southwestern Canada, which contributed to above-average precipitation in the Pacific Northwestern U.S. (Fig. E3) for the first time since January (Fig. E5). It was also associated with below-average temperatures over eastern Alaska and above-average temperatures across central and eastern Canada .

For the first time since December 2007, precipitation was below average in the Great Lakes region of the U.S. , which was situated in the area of anomalous upper-level convergence and sinking motion upstream of the mean trough axis (Fig. T23). In contrast, rainfall was above-average across the Gulf Coast and Southeast regions of the U.S. for the second straight month, following eight consecutive months of below-average precipitation.

Two landfalling named storms struck the Gulf Coast and southeastern U.S. during August. The first was Tropical Storm Edouard, which made landfall in northeastern Texas on August 5^th. A very slow-moving Tropical Storm Fay then made landfall in Florida on August 19^th, bringing extremely heavy rainfall (150 mm to 350 mm) to much of the state.

b. Europe

The 500-hPa circulation featured negative height anomalies across northern Europe and northwestern Russia , with the mean trough axis situated just west of Europe (Fig. E9). This pattern was associated with an enhanced storm track and above-average precipitation across northern Europe , and with below-average precipitation over southern Europe .

  2. Southern Hemisphere

The 500-hPa height field during August featured an anomalous zonal wave-3 pattern, with large areas of above-average heights centered south of Australia and over the high latitudes of the central South Pacific Ocean , and below-average heights located over New Zealand , southern South America , and southern South Africa (Fig. E15). In Australia , an extensive area of anomalous northerly flow downstream of the mean ridge axis extended from the Great Australian Bight to northern Australia (Fig. T20). This circulation brought below-average surface temperatures to most of the continent, with many areas recording departures in the lowest 10^th percentile of occurrences (Fig. E1). In contrast, surface temperatures were again above average over central South America , and below average farther south. These conditions were associated with a deep upper-level trough and an associated strengthening of the mean cold frontal boundary (Fig. T20).