Skip Navigation Links 
NOAA logo - Click to go to the NOAA home page National Weather Service   NWS logo - Click to go to the NWS home page
Climate Prediction Center

CPC Search
About Us
   Our Mission
   Who We Are

Contact Us
   CPC Information
   CPC Web Team

HOME > Expert Assessments > Climate Diagnostics Bulletin > Extratropical Highlights
Extratropical Highlights - July 2004

1. Northern Hemisphere

The 500-hPa circulation during July featured above-average heights over Alaska, western Canada, and northwestern Russia, and below-average heights over the north-central U.S., northwestern Europe, and eastern China (Fig. E9). This circulation was associated with a continuation of above-average surface temperatures and below-average precipitation in Alaska, and with cooler than average conditions across eastern North America (Figs. E1, E3). It was also associated with above-average precipitation in eastern China.

a. North America

Over North America the circulation during July featured a persistent upper-level ridge over the Rocky Mountains and eastern Alaska, and an amplified trough over the Great Lakes (Fig. E9). For Alaska, the associated mean jet stream and storm track were located south of the state (Fig. E10), which led to a continuation of exceptionally warm and dry conditions throughout the state. Farther east anomalous northwesterly flow upstream of the amplified upper-level trough contributed to a continuation of below-average surface temperatures across eastern Canada and over most of the United States east of the Rocky Mountains. This anomalous circulation is similar to that observed during April-June. The persistence of these circulation features has led to four consecutive months of extremely warm and dry conditions in Alaska, and to a prolonged period of anomalously cold temperatures across central and eastern Canada and large portions of the central and eastern U.S. (Fig. E1).

Also in the United States, significantly above-average precipitation was observed over the central Plains States and in portions of the Mid-Atlantic and Northeast regions during July, with totals in most areas exceeding the 70th percentile of occurrences. This enhanced rainfall is related to a series of cold frontal passages into the central Plains States, and to increased storminess in the eastern U.S. downstream of the upper-level trough axis.

b. Eastern China

Eastern China experienced above-average rainfall during July, with totals in many areas exceeding the 70th percentile of occurrences (Fig. E3). This enhanced rainfall was linked to a persistent upper-level trough east of the Tibetan Plateau, which occurred in association with a below average strength of the Southeast Asian monsoon ridge (Fig. T22). There has been considerable month-to-month variability in the strength and eastward extent of the Asian monsoon ridge during the last few months. For example, during June the ridge was stronger than average and extended farther east than normal, which contributed to well below-average precipitation across eastern China.

Eastern China has experienced generally below-average precipitation since at least July 2003, with only two months (May and July 2004) recording above-average precipitation.

2. Southern Hemisphere

 The 500-hPa circulation during July featured above-average heights in the middle latitudes and below-average heights over Antarctica (Fig. E15). The associated circumpolar vortex was enhanced throughout the lower stratosphere (Fig. S1).

In Australia an east-west dipole of 200-hPa divergence anomalies spanned the continent during July (Fig. T23). The area of anomalous upper-level convergence and sinking motion was situated between mean upper-level ridge and trough axes, and was associated with below-average precipitation across the eastern one-third of the continent (Fig. E3).

NOAA/ National Weather Service
National Centers for Environmental Prediction
Climate Prediction Center
5200 Auth Road
Camp Springs, Maryland 20746
Climate Prediction Center Web Team
Page last modified: January 15, 2002
Disclaimer Privacy Notice