The month of July 2009 saw near normal temperatures and much above normal precipitation. The month neatly divided meteorologically into its two halves and on the 16th. The first half of the month saw much above normal temperatures, and below normal rainfall. The last half saw much below normal temperatures and much above normal rainfall. The reason was the development of an upper air trough over Eastern and Central North America, and the retreat of the upper air Sub-Tropical High Pressure Ridge into the Western United States. Compared with July 2008, the month was 1.0 deg. Cooler and 6.58 inches drier.
Year-to-date rainfall through months' end was 3.83 inches less in 2009 than in 2008. The thirty-day outlook had called for near normal temperatures and near normal precipitation.
The week June 28-July 4 saw temperatures about 4 degrees above normal, and no rainfall. A weak cold front moved into the region on the 28th, lowering temperatures slightly for early in the week. At mid-week, the upper air high pressure ridge returned, again sending temperatures to above normal levels, and requiring a heat advisory for the 3rd and 4th. There were a few isolated afternoon showers on the 3rd and 4th. The week's average temperature was 86.0 deg., which was 0.9 deg. Cooler than the previous week. Compared with the same week in 2008, the week was 6.5 deg. Warmer, and 2.21 inches drier.
The week July 5-11 saw temperatures about 2 degrees above normal, and rainfall nearly 200 percent of normal. A cold front moved into the area on the morning of the 5th, which lowered temperatures for a few days as the upper air high pressure ridge retreated westward. The front, and its accompanying upper air disturbances, brought scattered showers and thunderstorms on the 5th and 6th. The upper air ridge returned on the 7th, again warming temperatures and ending showers. The week's average temperature was 84.5 deg., which was 1.5 deg. Cooler than the previous week. Rainfall was 1.39 inches. Compared with the same week in 2008, the week was 0.1 deg. Warmer, and there was no rain in 2008.
The week July 12-18 saw temperatures about 2 degrees above normal, and rainfall about one- third normal. The upper air ridge persisted until the 17th, when it was shunted westward by an upper air trough and its associated cold front. As a result, temperatures were well above normal through the 16th, but lowered to below normal and with scattered showers on the 17th and 18th. The week's average temperature was 85.9 deg., which was 1.4 deg. Warmer than the previous week. Rainfall was 0.24 inch. Compared with the same week in 2008, the week was 2.3 deg. Warmer, and 0.22 inch wetter. The week July 19-25 was dramatically different: temperatures about 3 degrees below normal and rainfall about 300 percent of normal.
The northwest flow event which set up on the 17th, continued through the 24th. Upper air high pressure made a brief re-appearance late in the week. Two cold fronts on the 20th and 23rd set off showers and held down temperatures. Disturbances in the northwest flow contributed to the shower formation. Severe thunderstorms developed on the night of the 21st, with power outages and wind damage in Tyler. Measurable rain fell on four days during the week, and normal temperatures were only observed on one day the 25th. The week's average temperature was 80.9 deg., which was 5.0 deg. Cooler than the previous week. Rainfall was 2.07 inches. Compared with the same week in 2008, the week was 5.3 deg. Cooler, and 1.97 inches wetter.
The final six days of the month saw two occurrences of heavy rain in the area: the first on the 28th-29th, and the second on the 30th-31st. Cold fronts moved into the area on the first day of each event, along with severe thunderstorms. In the first event, heavier rains were over the north; in the second, over the central and south. Temperatures ran below normal for the six days.
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
July 2009, RECORDS AND SUMMARY: