National Overview - April 2001


NCDC transitioned to the nClimDiv dataset on Thursday, March 13, 2014. This was coincident with the release of the February 2014 monthly monitoring report. For details on this transition, please visit our public FTP site and our U.S. Climate Divisional Database site.

National Highlights:

  • Thirteenth warmest April in the 1895 to present record
  • Lower tropospheric temperatures third warmest April in the 1979 to present record
  • Twenty-seventh driest April in the 1895 to present record
  • Areas of the East North Central Region had record April precipitation totals

Contents of this Section:

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The North American 500 mb Maps for April


The mean upper-air pattern during April was generally characterized by negative height anomalies over the west coast states which were associated with below normal surface temperatures. High pressure was a controlling factor in the East and South most of the month causing above normal temperatures. Heavy rainfall combined with melting snow to cause severe flooding along the Upper Mississippi River. The general storm track kept the East North Central, West North Central and West regions wet and the rest of the country very dry.

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.

North American 500 millibar Map
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Top of Page National Temperature - April

U.S. April 2001 Temperature Time Series 1895-2001
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The graph to the left shows April 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. April 2001 ranked as the thirteenth warmest April in the 1895 to present record. The preliminary nationally averaged temperature was 54.2° F (12.3° C) which was 2.2° F (1.2° C) above the long-term mean. This was the fourth consecutive April where temperatures have averaged near to or above the long-term mean. The actual April temperature values from 1895 through 2001 are available.

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 the third warmest in the 1979 to present record over the U.S. for April 2001. The average lower tropospheric temperature over the continental U.S. was 0.8° C (1.4° F) above the 1979-1998 mean. This is the third consecutive April where temperatures have been above the mean. MSU Temperature
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The map to the right, based on approximately 250 airport stations, shows departures from the 1961-1990 normal temperatures for April 2001. In general, only the west coast states were cooler than normal. The rest of the contiguous United States was generally warmer than normal, especially in the Central region. 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, Hawaii and Puerto Rico reported above normal temperatures. For further regional analysis, view NCDC's regional page.

U.S. April 2001 Temperature Departures
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Animated Standardized Temp. Anomaly Map 05/2000 - 04/2001
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The standardized temperature anomalies for April 2001 were positive in a large portion of the eastern two-thirds of the nation, with the highest positive anomalies centered in the Central region. Below normal indices occurred in the west coast states. 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.

Top of Page National Precipitation - April

U.S. April Precipitation, 1895-2001
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Based upon preliminary precipitation data, April 2001 ranked as the twenty-seventh driest such month since records began in 1895. This was the second consecutive dry April, but seven of the last nine Aprils have had precipitation near to or above the long-term mean. The preliminary National Precipitation Index ranked April 2001 as the fiftieth driest April.

While most of the country was dry, areas in the East North Central Region received record precipitation. For further regional analysis, view NCDC's regional page.


The map to the right, based on approximately 250 airport stations, shows April 2001 total precipitation as a percent of the 1961-1990 station normals. Above normal precipitation generally occurred in the north central and western states. The western half of Alaska was also wetter than normal. Below normal precipitation occurred over much of the rest of the country including eastern Alaska, Hawaii and Puerto Rico. For further regional analysis, view NCDC's regional page. U.S. April Precipitation Departures
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Current data are based on preliminary reports from River Forecast Center stations and First and Second Order airport stations obtained from the National Weather Service (NWS) Climate Prediction Center and real time Global Telecommunications System (GTS) monthly CLIMAT summaries. THE CURRENT DATA SHOULD BE USED WITH CAUTION. These preliminary data are useful for estimating how current anomalies compare to the historical record, however the actual values and rankings for the current year may change as the final data arrive at NCDC and are processed.

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.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: National Overview for April 2001, published online May 2001, retrieved on September 30, 2014 from http://www.ncdc.noaa.gov/sotc/national/2001/4.