By Robert K. Peters, Ph.D. - National Weather Service Cooperating Observer
The month of April 2000 saw temperatures slightly below normal and near normal rainfall.
The month saw the expected transition from the cool season to the warm season pattern. The southern branch of the jet was most active, with disturbances frequently crossing in this flow. These often pulled cold fronts in their wake. Thus, the expected precipitation and fluctuation in temperatures occurred. A couple of the air masses were modified Polar Continental in nature, which allowed for sharp cooling.
April 2000 was 4.7 degrees cooler than April 1999, and 1.05 inches drier. Year-to-date rainfall through April 30, 2000 was 2.30 inches less than that through April 30, 1999. Rainfall over the past twelve months is 44.78 inches. This is 99.0 percent of normal. The rainfall deficit of July 1999-February 2000 is beginning to be made up by the thusfar rather moist Spring of 2000.
The 30-day outlook for April 2000 had called for above normal temperatures and below normal precipitation. The week March 26-April 1 saw temperatures three degrees warmer than normal, and near normal rainfall. Upper air storms crossed early in the week and again at mid- week. One of these brought a major severe weather outbreak to East Texas on the night of the 29th, and a small outbreak on the afternoon of the 30th. Temperatures were warm for most of the week, cooling late in the week as a cool and humid air mass was trapped beneath a thick cloud layer.
Temperatures during the week April 2-8 averaged three degrees below normal, and rainfall was about 175 percent of normal. A wet storm system crossed during the 1st/3rd. Another storm crossed on the 7th. The result was widespread rains averagin between one and three inches in the first, and rains of about one-half inch on the 7th.
Severe weather accompanied the storm of the 7th. A modified Polar Continental air mass moved in behind each storm, with a sharp warm-up in the two days preceding the second.
The week April 9-15 saw temperatures six degrees cooler than normal and rainfall about 175 percent of normal. A slow-moving storm crossed between the 10th and 12th. This brought heavy rainfall on the 11th. There were reports of severe weather in the area on the 10th and 11th. The storm was preceded by a Polar Continental air mass, and followed by a Polar Maritime air mass. These two air masses along with the clouds and rain were responsible for holding down temperatures.
By late in the week, upper air high pressure was beginning to re-assert itself. This caused a general warming trend which continued into the following week, and also kep conditions dry. The storms which crossed on the 16th and 20th brought little or no rainfall to East Texas.
The week April 16-22 saw temperatures four degrees above normal and no rainfall. Migratory storms crossed on the 16th and 20th. There was only widely scattered rainfall with the second storm, and none with the first. Both storms crossed through Central North America well to the north of East Texas.
Tropical Maritime air was over the region for about half of the week, with either modified Polar Continental or Polar Maritime during the remainder. Upper atmospheric high pressure was over Western North America during the week. This warmed the lower atmosphere, and kept temperatures warm.
The week April 23-29 saw temperatures near normal and rainfall about ten percent of normal. Upper atmospheric high pressure was over Central North America. Weak disturbances crossing through this ridge were responsible for the rainfall.
Weak cold fronts on the 23rd and 27th brought cooling, with returning Tropical Maritime air warming readings on the 25th and 29th.
There was severe weather over the northeastern counties on the afternoons of the 23rd and 27th.
The reporting period for temperatures and phenomenon on each day is for the twenty-four hours ending at midnight hours GMT--6 p.m. CST and 7 p.m. CDT. The reporting period for precipitation is for the twenty-four hours ending at noon GMT--6 a.m. CST and 7 a.m. CDT. All times are given using the twenty-four hour clock, and are expressed in Greenwich Mean Time.
Observations are from NWS Station 41/9207/4 in Tyler, Texas.
The term "normal" refers to averages from the standard climatic period 1961-1990.
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APRIL 2000, RECORDS AND SUMMARY: