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December 1999 - Report and Summary

by Robert K. Peters, Ph.D. - National Weather Service Cooperating Observer

The month of December 1999 was warmer and slightly direr than normal. For a good part of the month, there was an upper air high over the Western United States. This blocked major cold air outbreaks from reaching East Texas. When that ridge backed westward to the Pacific Coast, as it did on a couple of occasions, storm features crossed bringing rain and colder air followed those storms.

December 1999 was 0.5 deg. cooler than December 1998, and 2.02 inches drier. Rainfall for 1999 was 41.70 inches. This was 3.53 inches less than normal, and 11.03 inches less than in 1998. Precipitation for 1999 was about 92 percent of normal. The first half of the year saw near to above normal precipitation. The last half was dry, with but 12.02 inches recorded from July 1 through December 31. This was 56 percent of normal.

1999 was not as hot as was 1998. The accumulation of cooling degree-days was 2681, compared with 3390 for 1998 and 2627 for the normal value. The coldest temperature in 1999 was on the 4th, when the mercury fell to 17 degrees. The hottest temperature was 103 on August 26th. The thirty-day outlook for December 1999 had called for above normal temperatures and below normal precipitation.

The week November 28-December 4 saw temperatures run about seven degrees abovenormal and rainfall about one-fourth normal. The upper air high pressure ridge over Western North America, after breaking down around the 24th, re-established itself on the 27th. This caused the high tenmperatures and low rainfall. A weak storm crossed on the 2nd, which brought light to moderate rain to the area. A stronger storm crossed on the 4th, which brought widespread rains, and some severe weather. The minimum temperature on the 4th equalled the record high minimum for that date.

The week December 5-11 saw temperatures near normal and rainfall about 200 percent of normal. Migratory storms crossed on the 9th and 11th. The storm of the 9th brought reports of severe weather from Kaufman, Van Zandt, and Harrison Counties. That of the 11th spread light rain into the area. Between the storms of the 4th and 9th, a sharp cool- down occurred on the 5th-7th, with a warm-up before the storm of the 9th. Only minor cooling followed the storm of the 9th.

The week December 12-18 saw temperatures near normal and rainfall about 250 percent of normal. Three storms crossed during the week--on the 12th, 15th, and 18th. The storm of the 12th brought most of the rainfall. That of the 15th crossed dry, and that of the 18th brought rainfall which varied from a few hundredths of an inch to over one inch. Minor cooling followed each of the storms, accounting for the seasonable temperatures.

The week December 19-25 saw temperatures three degrees below normal, and rainfall about one-tenth of normal. A storm crossed on the 20th, bringing the week's only rain. These were generally light, though there were a few heavier amounts along the Texas/Louisiana border. Modified Polar Continental air settled in after that storm departed. With a very dry air mass in place, nighttime temperatures were cold and daytime readings near seasonal normals. Upper air high pressure was in place over Western North America after the 22nd. This turned winds aloft into the northwest down the Plains. The result was a series of minor disturbances which crossed through the end of the Month. These brought reinforcing surges of cooler air, which kept nighttime readings near to slightly below seasonal normals after the 24th. With very little available moisture with which these disturbances could work, there was no rain with the weak fronts.

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 1971-2000.




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