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December 1998 - Report and Summary

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

The month of December 1998 was warmer and wetter than normal. It was a month of sharp temperature contrast, with very warm readings early in the month followed by two periods of below normal temperatures.

December 1998 was 2.0 degrees warmer and 1.24 inches wetter than December 1997.

Year-to-date rainfall was 0.11 inch greater in 1998 than in 1997. Indeed, total rainfall in 1998 was 116 percent of normal, despite the protracted dry period from mid-March through early August.

The year 1998 was quite warm. Eleven of the twelve months saw above normal temperatures. This, when combined with worldwide statistics on the extreme warmth of last year, affords substantial proof of the fact of global warming. There follows the divergence from normal for temperatures and precipitation for each month in 1998 for Tyler:

MONTH TEMP. PRECIPITATION

The year's hottest day was August 2 when the temperature reached 107. The year's coldest was December 25 when the mercury fell to 22. Further evidence of the warmth of the past year is the total accumulation of cooling degree days. 1998 saw 3390 cooling degree- days recorded. The average is 2562. The cooling degree-day value is calculated by subtracting 65 from the average of a day's maximum and minimum temperature.

The year saw a new record for consecutive days at or above 100 degrees: 20 between July 15 and August 3. It also saw the second greatest total of days with temperatures at or above 100 degrees: 46. The record year is still 1954, when there were 51 such days. The thirty-day outlook for December 1998 had called for above normal temperatures and below normal rainfall.

The first week in the month saw temperatures fifteen degrees above normal, and rainfall 175 percent of normal. Tropical Maritime air, which had moved over the region in mid- November, persisted until late on the 6th, when it was replaced by Polar Maritime air. The storm feature crossing on the 6th-7th brought widespread rainfall averaging an inch or more in the region. Earlier in the week, the warm and satchurated air mass had allowed for widespread light rain on some mornings, which brought a few hundredths of an inch to some locations. The southern branch of the Westerlies again became the focus for active weather over Central North America after the 7th, with migratory storms crossing during the following week.

The second week in December saw rainfall 300 percent of normal and temperatures five degrees below normal. The first freeze of the season was observed on the 31th, bringing to an end the unusually long 276-day growing season. Normally, the growing season is 245 days. A slow-moving storm crossed between the 9th and 12th. This brought considerable rainfall. With cool air trapped near the surface, there was little diurnal variation because of the presence of clouds and rain. The third week in December saw temperatures about a degree above normal, and rainfall about three-fourths normal.

For much of the week, a northwesterly flow aoft transported cool and dry air southeastward down the Plains and into East Texas. Temperatures warmed sharply from the 19th in advance of the major storm of the 22nd-25th. The week's only rainfall occurred on the 19th with the passage of a warm front and upper air feature.

The final ten days of the month saw temperatures nine degrees below normal and rainfall about one-third normal. A winter storm crossed between the 21st and 25th. This brought a mixture of rain, freezing rain, and sleet to the area on the 22nd, 23rd, and 24th. Arctic air accompanied the storm, with the storm departing on the 25th. Sixty-eight consecutive hours of sub-freezing temperatures were recorded between the 22nd and 25th. There was some ice left on vegetation on the 25th. Ice had coated exposed objects after the 22nd. Rapid warming occurred on the 26th and 27th, with a northwesterly flow aloft returning cool air between the 28th and 31st.

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.

DECEMBER 1998

MX MN OBS PCPN REMARKS

DECEMBER 1998, RECORDS AND SUMMARY:

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