National Overview - April 2002
larger image The graph to the left shows monthly mean temperature averaged across the contiguous United States based on long-term data from the U.S. Historical Climatology Network (USHCN). The value for 2002 is estimated from preliminary Climate Division data using the first difference approach. April 2002 ranked as the 9th warmest April in the 1895 to present record. The preliminary nationally averaged temperature was 54.6° F (12.6° C) which was 2.6° F (1.4° C) above the long-term mean. The April temperature values from 1895 through 2002 are available.
Record warmth occurred in New Mexico and the Carolinas in April. The pattern of warmth and cold in the contiguous U.S. in April corresponds well with the mean 500mb height and anomalies chart. This shows that above normal 500mb heights existed across the entire southern tier of the U.S., with the largest anomalies occurring from Virginia to west Texas. Cooler than average surface temperatures were associated with lower than average 500mb heights in Canada and the extreme northern portion of the north central U.S. states. The pattern of state-averaged monthly temperature can be seen in the map to the right.
A more detailed analysis including how statewide and regionwide temperatures compare to other years is available.
larger image A map of temperatures for the last 6 months, shows that 16 states in the northeast quadrant of the country averaged record warmth. Very few arctic air masses penetrated this region during the 2001-2002 winter as compared to average, and this resulted in much above normal temperatures for much of the season.
The map to the right, based on over 500 airport stations, shows departures from the 1971-2000 normal temperatures for April 2002. Warmer than average temperatures extended from New England to the Southwest with the largest positive departures in the contiguous U.S. (greater than 5.4° F [3° C]) occurring in the Southeast, New Mexico, Arizona, Colorado and parts of Texas. Negative temperature anomalies were confined to the north central states and Alaska, with some cooler than average temperatures also occurring in coastal California. The most significant negative departures occurred in southeastern Alaska where temperatures were below average by more than 9.0°F (5.0°C).
larger image The day-to-day variability of daily mean temperature was up to 110-150% of normal over most Central Plains states in April. This was associated with a series of alternating cold and warm outbreaks throughout the month, as can be seen in an animated map of daily temperature anomalies. Below average temperature variability was notable along the Gulf coast where day-to-day variability was less than 70% of average.
larger image The graph to the left is a time series depicting precipitation averaged across the contiguous U.S. Based upon preliminary precipitation data, April 2002 was slightly drier than average, ranking 39th driest. April 2002 marks the 3rd consecutive drier than average April, though the last 10 years have generally been wetter than average for the month of April.
There was considerable regional variability in precipitation across the country. States in the southern U.S. continued to receive less rainfall than average in April, while most states in the East North Central, Central and Northeast regions were wetter or much wetter than average.
A more detailed analysis including how statewide and regionwide precipitation compares to other years is available.
The map to the right, based on more than 500 airport stations, shows April 2002 total precipitation as a percent of the 1971-2000 station normals. Above normal precipitation generally occurred from Oklahoma northeast through the Ohio Valley to northern New England, with more than 180% of normal precipitation falling in some locations. The Upper Peninsula of Michigan also received much above normal precipitation for the month, as did much of Alaska. The rest of the country was mostly dry, including, most notably, southern Californa, the Southeast and the plains just east of the Rockies. The islands of Hawaii were also drier than normal for April.
animation The map to the left shows national snow cover early in the month (April 3). Snow cover across the country generally decreased throughout the month as can be seen in an animation. However, there was significant snowfall in parts of the country, including Wisconsin and the Upper Peninsula of Michigan (see image below).
The map to the right shows snowfall totals in the East North Central region of the U.S. on April 28 after a storm dumped over a foot of snow on isolated portions of Wisconsin and Michigan's Upper Peninsula.