by Robert K. Peters, Ph.D. - National Weather Service Cooperating Observer
The month of February 2007 was cooler and drier than normal. Compared with February 2006, the month was 0.4 deg. Cooler and 3.79 inches drier. Year-to-date rainfall through February 28 was 1.33 inches less in 2006 than this year. The thirty-day outlook for February 2007 had called for near normal temperatures and above normal precipitation.
Through February 18, very cold air was in place on most days a cold outbreak which began on January 14. At mid-February, a pattern change began which moderated temperatures through the end of the month. Two related features were responsible for the cold air. First, the Polar Vortex a large upper air low pressure feature was somewhere over Central Canada. Disturbances rotating about the center brought cold air southeastward into the Central and Eastern United States.
Second, the Pacific-North American flow pattern was positively tilted meaning a northwest to southeast orientation. This meant that very cold air from Northeastern Asia and the Canadian Northwest Territories was brought southeastward, and picked up by disturbances rotating around the Polar Vortex. The pattern change was an eastward shift in the Polar Vortex, and its replacement by upper air high pressure over Central and Eastern North America, and the negative tilting (northeast to southwest orientation) of the PNA (Pacific-North American)pattern.
The week January 28-February 3 saw temperatures about 12 degrees colder than normal, and rainfall about 75 percent of normal. Arctic air was present throughout the week, with reinforcing fronts on the 28th, 30th, 1st, and 2nd. A storm crossed to the south of the region on the 30th, bringing light rain to the southern counties. A stronger storm crossed between the 31st and 2nd, bringing widespread rains of around one- half inch, and about an inch of snow north of the IH-30 Corridor on the morning of the 2nd. Light sleet fell in Tyler on the evening of the 1st with no ice accumulation. The week's average temperature was 37.4 deg., which was 8.5 deg. Colder than the previous week. Precipitation was 0.61 inch. Compared with the same week in 2006, the week was 17.4 deg. Colder, and 0.42 inch drier.
The week February 4-10 saw temperatures about 3 degrees colder than normal, and precipitation about 10 percent of normal. A modified Polar Continental air mass was present early in the week, with a brief warming at mid-week, followed by a strong Polar Continental air mass at late-week. The week's only rain came with the arrival of the new cold front on the 8th. The week's average temperature was 48.4 deg., and precipitation was 0.07 inch. The week was 11.0 deg. Warmer than the previous week. Compared with the same week in 2006, the week was 0.9 deg. Cooler, and 0.46 inch drier.
The week February 11-17 saw temperatures about 11 degrees colder than normal, and precipitation about one-half normal. Polar air was present each day except on the 12th when a storm system crossed ahead of the first of the week's three cold fronts. The other two crossed on the 15th with light snow, and the second on the 17th with strong winds. There was no accumulation of snow. The 17-degree minimum on the morning of the 16th was the coldest temperature in Tyler since January 3, 2002. The week's average temperature was 40.6 deg., and precipitation was 0.48 inch. The week was 7.8 deg. Colder than the previous week. Compared with the same week in 2006, the week was 9.9 deg. Colder, and 0.42 inch wetter.
The week February 18-24 was in sharp contrast to the previous week, running 4 degrees warmer than normal, and rainfall continuing below normal. The Polar Continental air mass gave way to Tropical Maritime air early in the week, which persisted until a cold front with thunderstorms arrived on the 24th. There were reports of severe weather east of Tyler that morning, and very strong winds blew that afternoon with gusts to 46 mph. Blowing dust lowered Tyler's visibility to under 2 miles on the evening of the 24th. A weaker storm crossed on the 20th bringing small rainfall amounts. The week's average temperature was 58.4 deg., which was 17.8 deg. Warmer than the previous week. Rainfall was 0.32 inch. Compared with the same week in 2006, the week was 14.5 deg. Warmer, and 2.34 inches drier. The final four days of the month began cool, with a reinforcing front on the 26th. Tropical maritime air returned on the night of the 27th ahead of a storm which was approaching from the west.
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.
MX MN OBS PCPN REMARKS
February 2007, RECORDS AND SUMMARY: