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December 2001 - Report and Summary

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

The month of December 2001 was warmer and wetter than normal. The month began very warm, with significant cooling after the 9th. A series of strong storm systems crossed in the southern branch of the jet stream, which caused the heavy rain events. Persistent upper air high pressure over the Southeastern United States was responsible for the very warm temperatures of early in the month. A Polar Vortex set up over Northeastern North America at mid-month, which combined with upper air high pressure over Western North America to bring the cold weather of the mid- and late-month.

December 2001 was 9.7 degrees warmer than December 2000, and 0.63 inch drier. The year 2001 was 1.86 inches wetter than the year 2000. Total precipitation during the past twelve months is 131 percent of normal. January 3 was the coldest day of 2001, when the temperature fell to 18 degrees. The year's warmest day was August 5, when the temperature rose to 98. The cool season into early-Summer was wet, as was the early- Autumn and early-Winter. Mid-Summer and mid-Autumn were both dry. The excessive precipitation was one factor responsible for the year's moderate temperatures. 2001 was the first year since 1994 when the temperature in Tyler did not reach 100 degrees. The growing season in 2001 was 274 days--29 days longer than normal. The last frost was on February 18, and the first on November 21.

The thirty-day outlook for December 2001 had called for near normal temperatures and above normal precipitation.

The week November 25-December 1 saw temperatures four degrees below normal and precipitation about 300 percent of normal. The week began warm, but an Arctic air mass moved into the region on the 26th, and persisted through the 1st. Moist air overrode this shallow cold air layer, and an upper air trough was crossing the Southern Plains. The result was a period of rain, which was heaviest on the 28th. On that day, amounts averaged between two and four inches. On the 29th, mixed wintry precipitation occurred, but no ice accumulated in the immediate Tyler area. The week's average temperature was 48.6 deg., which was 9.7 deg. cooler than the previous week.

The week December 2-8 was 13 degrees warmer than normal, and had near normal rainfall. The week began with a Polar Maritime air mass over the area, which was replaced between the 3rd and 7th by a Tropical Maritime air mass. The weather map looked more like mid-Summer than early- Winter, with a sub-tropical upper air high pressure ridge building into the area, and a Bermuda High building in from the east at the surface. The result was a warm and moist air flow, which held nighttime temperatures about 20 degrees warmer than normal, and caused daytime readings to warm into the 70s. On the night of the 7th, a cold front and storm system crossed the region. This brought the week's only rainfall, and lowered temperatures significantly with another Polar Maritime air mass on the 8th. The week's average temperature was 63.4, which was 14.8 degrees warmer than the previous week.

The week December 9-15 saw temperatures about one degree cooler than normal, and rainfall about 225 percent of normal. The week began with a Polar Maritime air mass over the region. On the 11th, a powerful upper air storm moved into the Inter- Mountain West, crossing the Plains over the next three days. This brought widespread and heavy rainfall on the 11th, 12th, and 13th. Tropical Maritime air was over the region on the 12th and 13th, followed by a Polar Maritime air mass on the 14th. On the 15th, another strong upper air storm was over Western North America. This brought additional rainfall on the 15th. The week saw an average temperature of 47.2 deg., which was 16.2 deg. cooler than the previous week.

The week December 16-22 saw temperatures three degrees warmer than normal, and rainfall about 350 percent of normal. Heavy rains fell in the area on the 15th and 16th, causing major rises on area rivers. The Sabine went into major flood between Hawkins and Gladewater, with the river reaching a crest of eleven feed above flood stage at Gladewater late in the week. The rains, which ranged between two and ten inches in the area, were caused by a slow-moving upper air low and a strong inflow of moisture from the Gulf of Mexico. Polar maritime air moved into the region on the 17th, which persisted through the 21st. It was replaced by Tropical Maritime air on the morning of the 22nd, which produced additional light rainfall from a crossing upper air disturbance on that day. The week's average temperature was 51.0 deg., which was 3.8 deg. warmer than the previous week.

The week December 23-29 saw temperatures 3.5 degrees cooler than normal, and precipitation about one-fourth normal. Modified Polar Continental air was over the region for most of the week, though there was warming on the 27th and 28th because of a downslope southwesterly flow and nearly full sunshine. A blocking upper air high extending from Greenland into the Northwest Territories, and from there southward into the Southwestern United States allowed for a northerly to northwesterly flow aloft which transported cold air southward for much of the week. Helping this was the positioning of the Polar Vortex, which wobbled from the Great Lakes into Southeastern Canada. Crossing disturbances on the 24th and 25th brought some scattered snow flurries late each evening. An Arctic air mass moved into the area on the morning of the 29th, again lowering temperatures.

The week's average temperature was 43.5 deg., which was 7.5 degrees cooler than the previous week, and 19.9 cooler than the first week of the month. The final two days of the month were also cold. A shortwave crossed early on the morning of the 31st, bringing a few light snow flurries. There was no measurable precipitation in Tyler.

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 2001

MX MN OBS PCPN REMARKS

DECEMBER 2001, RECORDS AND SUMMARY:

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