National Temperature - April
Preliminary data for April 1999 indicate that temperature averaged across the contiguous United States was above the long-term mean, ranking as the 46th warmest April since 1895. Over 12% of the country was much warmer than normal while less than five percent of the country was 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 indicate warmer than the mean and negative values indicate cooler than the mean. The preliminary national standardized temperature index ranked April 1999 as the 36th warmest April on record.
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The April weather pattern in the United States consisted of a southerly storm track in the west and a ridge in the east. As a result, cold air masses dominated the country from the West Coast to the Rocky Mountains, while warm temperature anomalies prevailed from the South and Southeast to the Great Lakes and Upper Mississippi Valley. Colder than average temperatures were present in the adjoining Pacific ocean while above average temperatures were noted along in the Gulf of Mexico and off the U.S. east coast. The temperature anomaly map to the left is based on a blend of surface station data and satellite data.
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| The map to the right, based on approximately 250 First Order stations, shows April 1999 average temperatures as a departure from 1961-1990 station normals. Much of Alaska was warmer than normal. Stations in Hawaii reported near to below normal temperatures.
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The map animation to the left shows the geographical pattern of temperature anomalies for the last 12 months. On these standardized temperature anomaly maps, the color scheme ranges from blue (cold) to red (warm):
red = top 10 percentile
tan = 70-90 percentile
light blue = 10-30 percentile
dark blue = bottom 10 percentile.
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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 current month have been expressed as a ratio of the normal (1961-90) average daily difference. The magnitude of this ratio is expressed by the size 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.
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Daily temperature variability was near to above normal during April 1999 in three broad bands in the western, central and eastern United States, reflecting the frequent movement of air masses and storm systems across the country. In the western half of the country, the cold air masses lingered, resulting in below-average monthly temperatures. In the central portion of the country a dominant storm track was present while, in the east, the air masses quickly moved off the coast with a return flow of warm southerly air, which resulted in above-average monthly temperatures.
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National Precipitation - April
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April 1999 was the 43rd wettest such month since 1895. Nearly 21% of the country was much wetter than normal while about 11% percent of the country was much drier than normal. The last seven such months have been at- to above the long-term mean.
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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.
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The prelimininary national standardized precipitation index ranked April 1999 as the 37th wettest such month on record.
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Long-term drought coverage (as measured by the Palmer Drought Index) was virtually unchanged for the third consecutive month, with April 1999 having about four percent of the country in severe to extreme drought. The percent area of the country experiencing severe to extreme wetness increased slightly to about 13%. The core dry areas included portions of the Southwest, Southeast, mid-Atlantic, Gulf Coast, and New England. The core wet areas included portions of the northern and central Great Plains, the central California coast, southeastern and southwestern Oregon, and portions of the central and southern Rockies.
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| The Palmer Z Index shows how monthly moisture conditions depart from normal (short-term drought and wetness). The animated maps show the geographical pattern of the moisture anomalies for the last 12 months. On these maps, the red shading denotes dry conditions while the green shading indicates wet conditions.
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The Palmer Drought Index Maps show long-term (cumulative) drought and wet conditions. Long-term wetness decreased in the Pacific Northwest during April 1999 due to the displaced storm track, but wet spell conditions increased in the central U.S. The animated maps show how the geographical pattern of the long-term moisture conditions has changed over the last 12 months. On these maps, the red shading denotes drought conditions while the green shading indicates wet conditions.
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| The Palmer Crop Moisture Index is computed on a weekly basis by the National Weather Service's Climate Prediction Center and is useful for following the impact of precipitation anomalies on agriculture. The Animated Crop Moisture Index maps show the change in topsoil moisture conditions during the first 18 weeks of 1999. On these Crop Moisture Index maps, the red shading denotes dry conditions while the green shading indicates wet conditions.
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The April 1999 general circulation pattern consisted of a storm track in the western U.S. which was deflected south of its normal seasonal location. This resulted in drier-than-normal conditions in the northwestern U.S. The storm track turned north over the central part of the country. Above-normal precipitation associated with this weather pattern occurred in a band which stretched from southern California, across the central Plains, to the southern Great Lakes. Above-average April snow cover occurred in the Southwest, but the precipitation there was not enough to compensate for the below-average snowpack experienced during most of the snow season, nor did it end the long-term drought conditions there. Heavy precipitation in some areas, in combination with unusually cold temperatures, resulted in above-average snow cover anomalies across much of the West.
Above-normal precipitation occurred in the eastern and central Carolinas. Otherwise, the absence of the main storm track resulted in a dry April from the southern Plains, across the Southeast, and into the Northeast. Long-term drought conditions worsened in these areas, especially in the Southeast.
Storminess persisted for a good portion of the month across the central Plains into the mid-Mississippi River Valley. High surface wetness values are notable across portions of North Dakota, while large negative surface wetness values were observed in southern Canada near Lake Winnipeg. In the above satellite-derived surface wetness map, 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.
The dot map shows April 1999 precipitation as a percentage of the 1961-1990 station normals. Southwest and southeast Alaska had above-normal precipitation, but much of the rest of Alaska was dominated by dry conditions. Most of the stations in Hawaii reported below-normal precipitation.
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The snow cover anomaly map to the left reflects the overall pattern of the April storm track. Above-average anomalies occurred under the cold trough in the western U.S. where the number of days with snow cover was well above average. One snowstorm in central Wyoming dumped almost six feet of snow in a three day period. In contrast, negative anomalies extended from portions of the upper Great Lakes northwestward to the western shores of Hudson Bay. This area was drier and milder than normal during the month due to a persistant ridge of high pressure.
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According to the USDA's National Water and Climate Center, by the end of April 1999 the snowpack remained well below average in the southwestern U.S., and well above average in the Pacific Northwest due to heavy snows earlier in the season. Snowpacks were 285 percent of average in the Rogue and Umpqua basin in southwestern Oregon, and 253 percent of average in the Lewis and Cowlitz basin in southwestern Washington and in the Klamath basin of northern California.
National Temperature - January-April
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Preliminary temperature data indicate that the year-to-date, January-April 1999, was the 7th warmest January-April period since records began in 1895. About 40% of the country averaged much warmer than normal while near zero percent of the country averaged much cooler than normal. The last two such four-month periods have been much above the long term mean.
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| The preliminary national standardized temperature index ranked January-April 1999 as the 10th warmest January-April on record.
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National Precipitation - January-April
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Preliminary precipitation data indicate that the year-to-date, January-April 1999, was the 47th wettest January-April on record for the contiguous United States since records began in 1895. About two percent of the country was much wetter than normal for this period while about four percent of the country was much drier than normal. This contrasts with the wettest January-April on record which occurred just last year during the strong El Niño episode.
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| The prelimininary national standardized precipitation index ranked January-April 1999 as the 43rd driest January-April period on record.
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Tornadoes - April and Year-to-date
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During April 1999, 152 tornadoes were documented across the contiguous United States. The 47-year average is 111. The most tornadoes observed in the April record was 269 during the record outbreak month of April 1974, while the least amount was 20 in April 1987. More information regarding April tornado statistics is located at the NCDC April Climate-Watch Page.
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| For the year-to-date, January-April 1999, 358 tornadoes have been documented. This ranks as the third most active year-to-date, behind the count of 373 recorded just last year. The most tornadoes observed during this same four-month period is 405 in 1991 while the least amount was 83 in 1987.
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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.
NOAA's National Centers for Environmental Information
