National Overview - March 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. March Temp 1895-1999
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National Temperature - March

Preliminary data for March 1999 indicate that temperature averaged across the contiguous United States was above the long-term mean, ranking as the 37th warmest March since 1895. Over 12% of the country was much warmer than normal while less than one percent of the country was much cooler than normal. Twelve of the last fifteen months of March have been above- to much-above the long term mean.

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 indicate warmer than the mean and negative values indicate cooler than the mean. The preliminary national standardized temperature index ranked March 1999 as the 38th warmest March on record. U.S. March Temperature Index
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NA Tmp, 1895-1999
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Frequent storms on both the east and west coasts of the U.S. corresponded with below average temperatures. In contrast, a mean ridge was maintained across the northern plains where above average temperatures were observed. Mild temperatures occurred across the Big Bend area of Texas and portions of northern New England. The temperature anomaly map to the left is based on a blend of surface station data and satellite data.
The map to the right shows March 1999 average temperatures as a departure from 1961-1990 station normals. Much of Alaska was dominated by below normal temperatures. Stations in Hawaii reported near to below normal temperatures. U.S. March Temperature Departures
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Temperature Variability

Monthly mean temperature maps show the average conditions during a month, but give no information about changes that occurred within the month. A measure of the day-to-day variability of temperature provides some insight into how temperatures changed during the month. Daily temperature variability is highly dependent on the weather systems and air masses that affect a region.

The daily difference in temperature may be lower in areas where a single air mass remains dominant. This can happen under a stable circulation pattern (at the jet stream level) that locks an air mass in place--for example, a strong zonal flow, or a stable ridge/trough pattern. The daily difference in temperature will be higher in areas that experience a greater frequency of frontal passages as cold arctic air moves southward and warmer, maritime air moves northward. This will happen under a variable circulation pattern, or along a stable storm track.

To quantify the variability in daily temperature, the average daily differences in temperature for the month of March 1999 have been expressed as a ratio of the normal (1961-90) average daily difference. The magnitude of this ratio is expressed by the size and color of the dot representing the value at each of approximately 250 First Order airport stations across the United States. Small dots indicate that daily variability in temperature was less than normal, and may be a consequence of a dominant air mass. Large dots indicate that daily temperature variability was greater than normal reflecting a more frequent passage of differing air masses.

US Daily Temperature Variability
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Daily temperature variability was near to below normal during March 1999 for much of the United States east of the Rocky Mountains, extending across Montana and into the Pacific Northwest. Variability was greater than normal from the central Rockies into the intermountain basin.

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National Precipitation - March

U.S. March Precipitation, 1895-1999
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March 1999 was the 16th driest such month since 1895. Nearly 16% of the country was much drier than normal while about four percent of the country was much wetter than normal.
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.
U.S. March Precipitation Index, 1895-1999
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The prelimininary national standardized precipitation index ranked March 1999 as the 15th driest such month on record. This standardized Z-score is estimated to be accurate to within 0.222 index units.
U.S. March Percent Area Dry and Wet
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Long-term drought coverage (as measured by the Palmer Drought Index) was virtually unchanged during the month, with March 1999 having about three percent of the country in severe to extreme drought. The percent area of the country experiencing severe to extreme wetness dropped significantly to less than ten percent. The core dry areas included portions of the Southeast, Southwest, mid-Atlantic, and central and northern Great Lakes. The core wet areas included portions of New England, northern and central Plains, and much of the Pacific Coast.
Storminess produced wetter than average conditions from northern Texas into southern Kansas for the second month in a row. The negative wetness values over the midwestern states are likely a consequence of below normal precipitation for the month. In addition, surface moisture in a completely frozen state will not be detected as surface wetness. High surface wetness values are notable in the Red River Valley region.
NA March Wetness Anomaly
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U.S. March Precipitation Anomaly
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The Southeast was drier than normal, but the Mid-Atlantic states to New England were wetter than normal. Dense vegetation obscures the surface water from the satellite observation, which makes the wetness index less accurate over areas like the eastern U.S. and the Pacific Northwest. Above normal temperatures in the far northern Plains states melted the snow pack earlier than usual, which correspond to the positive wetness values there. The southwestern quarter of the country was drier than normal, which has been the pattern throughout the winter season. Most of the stations in Alaska and Hawaii reported below normal precipitation for March 1999.
March Snow Cover Anomaly
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The snow cover anomaly map to the left is directly related to the general storm track across the U.S during the month of March and the persistent ridge across the northern Plains which resulted in above average temperatures there. Storms typical of a La Niņa pattern continued to slam into the Pacific Northwest coast, where many locations were approaching all-time records of snowpack.
According to the USDA's National Water and Climate Center, nearly every river basin in Washington reported 150% or greater snow water equivalent in their snowpack. Oregon's Rogue, Umpqua Basin reported 206% of average. In contrast, the lack of snowpack in the mountains of the Southwest has contributed to below average seasonal water supplies and a concern over increased fire potential. In the eastern half of the country, the northern sections were persistently snow covered.
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National Temperature - January-March

U.S. YTD Temperature, 1895-1999
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Preliminary temperature data indicate that the year-to-date, January-March 1999, was the 6th warmest January-March period since records began in 1895. About 44% of the country averaged much warmer than normal while about zero percent of the country averaged much cooler than normal. Nine of the last fourteen January-March periods have been much above the long term mean.
The preliminary national standardized temperature index also ranked January-March 1999 as the 6th warmest January-March on record.
U.S. YTD Temp Index, 1895-1999
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National Precipitation - January-March

U.S. YTD Precipitation, 1895-1999
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Preliminary precipitation data indicate that the year-to-date, January-March 1999, was the 49th wettest January-March on record for the contiguous United States since records began in 1895. About three percent of the country was much wetter than normal for this period while about seven percent of the country was much drier than normal. This contrasts with the wettest January-March on record which occurred just last year during the strong El Niño episode.
The prelimininary national standardized precipitation index ranked January-March 1999 as the 39th driest January-March period on record. U.S. YTD Pcp Index, 1895-1999
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Tornadoes - March

U.S. March Tornadoes, 1953-1999
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During March 1999, 19 tornadoes were documented across the contiguous United States. The 47-year average is 55. The most tornadoes observed in the March record was 180 in March 1976, while the least amount was 8 in March 1969.
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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 March 1999, published online April 1999, retrieved on July 26, 2014 from http://www.ncdc.noaa.gov/sotc/national/1999/3.