National Climatic Data Center, 15 August 2001

National Highlights:

• Fourteenth warmest July in the 1895 to present record
• Third warmest May-July in the 1895 to present record
• Lower tropospheric temperatures near average
• July precipitation near normal

Contents of this Section:

• North American 500 Mb Maps
• National Temperatures
• Lower Tropospheric Temperatures
• Temperature Departures
• Standardized Temperature Anomalies
• National Precipitation
• Precipitation Anomalies

North American 500 Mb Maps

The mean upper-air pattern during July was generally characterized by positive height anomalies over the center of the nation which were associated with above normal surface temperatures. This persistent trough-ridge-trough pattern kept the east and west coasts cooler than normal. The flow around the ridge contributed to above normal wetness in the west, across Montana and the Dakotas and into portions of the mid-Atlantic. Additional information on hydrometeorological analysis and forecasting can be found at the Hydrometeorological Prediction Center's Web Page. The principles behind the 500 mb flow are briefly explained here.

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National Temperatures

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The graph to the left shows July monthly mean temperature averaged across the contiguous United States based on long-term data from the U.S. Historical Climatology Network (USHCN). The 2001 value is estimated from preliminary Climate Division data using the first difference approach. July 2001 ranked as the fourteenth warmest July in the 1895 to present record. The preliminary nationally averaged temperature was 75.5° F (24.2° C) which was 1.2° F (0.7° C) above the long-term mean. The actual July temperature values from 1895 through 2001 are available.

The period May-July 2001 was the third warmest such period in the 1895 to present record. View the time series to see how this period compares with other years.

Lower Tropospheric Temperatures

Data collected by NOAA's TIROS-N polar-orbiting satellites and adjusted for time-dependent biases by NASA and the Global Hydrology and Climate Center at the University of Alabama in Huntsville indicate that temperatures in the lower half of the atmosphere (lowest 8 km) were near average over the U.S. for July 2001. The average lower tropospheric temperature over the continental U.S. was 0.09° C (0.16° F) above the 1979-1998 mean. This was the seventh consecutive July where temperatures have been near to much above the mean.

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Temperature Departures

The map to the right, based on approximately 250 airport stations, shows departures from the 1961-1990 normal temperatures for July 2001. In general, the middle third of the country was warmer than normal. The east and west coasts were generally cooler than normal. Maps showing how the temperature anomalies varied day-to-day and how the day-to-day temperature variability compares to the long-term average are available. Most of Alaska reported below normal temperatures while Hawaii and Puerto Rico were near normal. For further regional analysis, view NCDC's regional page.

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Standardized Temperature Anomalies

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The largest postive standardized temperature anomalies for July 2001 were located in the middle third of the nation and in portions of Nevada and Utah. The largest negative anomalies were along the east coast. The adjacent animated map shows the geographical pattern of temperature anomalies for the last 12 months compared to a base period of 1931-1990. For further regional analysis, view NCDC's regional page.

National Precipitation

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The graph to the left is a time series depicting averaged precipitation across the nation. Based upon preliminary precipitation data, July 2001 ranked near the middle of the historical distribution. This marks a significant change from the unusual dryness of July 2000. The National Precipitation Index is a different view of the country's precipitation picture. An explanation of the Index, which ranked July 2001 as the twenty-fifth wettest July, and the time series is available. For further regional analysis, view NCDC's regional page.

Precipitation Anomalies

The map to the right, based on approximately 250 airport stations, shows July 2001 total precipitation as a percent of the 1961-1990 station normals. Below normal precipitation generally occurred in south, portions of the west and northeast. The rest of the country was generally normal to above normal, with areas of Montana, the Dakotas and portions of the mid-Atlantic showing the highest percent of normal precipitation. Hawaii and Puerto Rico were dry while Alaska was generally wetter than normal. For further regional analysis, view NCDC's regional page.

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References

The following NCDC datasets are used for the historical U.S. data: the climate division drought database (TD-9640), and the hurricane datasets (TD-9636 and TD-9697). It should be noted that the climate division drought database consists of monthly data for 344 climate divisions in the contiguous United States. These divisional values are calculated from the 6000+ station Cooperative Observer network.

Questions?

For all climate questions other than questions concerning this report, please contact the National Climatic Data Center's Climate Services Division:

For further information on the historical climate perspective presented in this report, contact:

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