National Overview - Spring 1999


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.

U.S. Spring Temp 1895-1999
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National Temperature - Spring

Preliminary data for Spring (March-May) 1999 indicate the mean temperature for the three-month period averaged across the contiguous United States was above the long-term mean. Nearly six percent of the country was much warmer than normal while less than one percent of the country averaged much cooler than normal.

The national temperature index expresses temperature departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the temperature for each of 344 climate divisions in the U.S. by using their 1931-90 mean and standard deviation, then weighting these divisional values by area.
These area-weighted values are then normalized over the period of record. Positive values are warmer and negative values are cooler than the mean. The preliminary national standardized temperature index ranked Spring 1999 also above the long term mean. USTZ Jan-May
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U.S. Temp. Dept. Dot Map
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The map to the left, based on approximately 250 airport stations, shows Spring (March-May) 1999 average temperatures as a departure from 1961-1990 station normals. Cooler than normal seasonal temperatures dominated much of the Pacific Northwest and California.

Warmer than normal seasonal temperatures dominated the northern tier states from the Great Plains to the Northeast, and the southern portions of the states from the Desert Southwest to the Florida panhandle. Spring temperatures averaged colder than normal along the southern and western coasts of Alaska, while the interior stations averaged near normal. The Hawaiian stations averaged predominantly near normal.
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National Precipitation - Spring

U.S. Spring Precipitation, 1895-1999
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Preliminary data for Spring (March-May) 1999 indicate the three-month precipitation averaged across the contiguous United States was below the long-term mean. Over ten percent of the country was much drier than normal while about five percent of the country was much wetter than normal. March-May 1999 was the first Spring season since 1994 to have below-normal precipitation for the contiguous United States.

The national precipitation index expresses precipitation departure from the 60-year mean in terms of standard deviations. Each year's value is computed by standardizing the annual precipitation in each of 344 climate divisions across the U.S. using the gamma distribution over the 1931-90 period. The gamma statistical distribution takes into account heavy precipitation years and extremely dry years in the historical record (in mathematical parlance, "a zero-bounded skewed distribution"). These gamma-standardized divisional values are then weighted by area and averaged to determine a national standardized value for each year.
These national values are normalized over the period of record. Negative values are drier and positive values are wetter than the mean. This index gives a more accurate indication of how precipitation across the country compares to the local normal (60-year average) climate.
The preliminary national standardized precipitation index ranked Spring 1999 also drier than the long term mean. 1895.
U.S. Spring Precipitation Index, 1895-96/1998-99
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U.S. Temp. Dept. Dot Map
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The map to the left, based on approximately 250 airport stations, shows Spring (March-May) 1999 precipitation as a percentage of 1961-1990 station normals. The spatial pattern for precipitation anomalies is less coherent than the spatial pattern for temperature anomalies. In general, spring precipitation was below normal across much of the eastern third of the country and from central California to the Pacific Northwest.
Above normal precipitation fell across parts of California, the Rocky Mountain states, and the central and northern Plains. A few stations in the panhandle and west coast of Alaska reported above normal spring precipitation, but most Alaskan stations had a near to below normal season. The stations in Hawaii had near normal spring precipitation.


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.

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National Tornadoes

Based on preliminary data from the National Weather Service Storm Prediction Center the count of 495 tornadoes for Spring (March-May) 1999 ranks as the 5th most active Spring season since 1953. The 1953-1998 average Spring tornado count is 347. The Spring season with the most tornadoes was 1991 with 700 while the least active Spring season for tornadoes was 1958 with 159 documented tornadoes.
U.S. Spring Tornadoes, 1953-1999
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It should be pointed out that the preliminary tornado count is traditionally higher than the final count and that the tornado observations have generally improved with time as better observing practices and instrumentation (especially weather radar and satellites) were utilized.

Citing This Report

NOAA National Climatic Data Center, State of the Climate: National Overview for Spring 1999, published online June 1999, retrieved on October 23, 2014 from http://www.ncdc.noaa.gov/sotc/national/1999/15.