The month of September 1999 saw near normal temperatures and below normal rainfall. The upper air wind pattern over North America shifted from the warm-season to the cool-season pattern around mid-month. This allowed for two rather strong cold fronts to move through the region late in the month. The rainfall deficit, which commenced in early-July, persisted. Though there were two significant rains at the National Weather Service during September, those were not general. Parts of the area have not received substantial rainfall since the beginning of the Summer.
September 1999 was 5.2 deg. cooler than September 1998, and 3.03 inches drier. Year-to-date rainfall through 30 September 1999 was 4.44 inches greater than for the same period in 1998. Rainfall over the past twelve months is 56.17 inches. This is 124 percent of normal. The thirty-day outlook had called for near normal temperatures and near normal rainfall. The first week in September saw temperatures five degrees warmer than normal and rainfall about 200 percent of normal.
Upper atmospheric high pressure continued in control of the region's weather for most of the week. A quasi-stationary cold front coincided with an Easterly wave early in the week to cause scattered to numerous afternoon showers and thunderstorms. This brought the week's only rain to Tyler, though additional scattered showers fell in the area on the 4th and 5th. Air quality improved during most of the week as the grip of the upper air high was somewhat relaxed and there was increased cloud cover.
The second week in September saw a pronounced shift in temperatures. Though readings during the week averaged three degrees above normal, a cold front moved through on the 13th. This ushered in a period of dry and mild weather, which persisted well into the third week of the month. The front also brought the week's only rainfall. Surface high pressure was oriented from the Great Lakes to the Texas Coast between the 13th and 18th. Upper air high pressure was oriented east-west, with a center over the Southwestern United States. The dry air mass was reinforced periodically during this period. The result was a regime of cool to mild nights with mild to warm afternoons. Relative humidities were quite low during the period. After the 15th, air quality deteriorated, and ozone advisories were required.
The third week in Sep[tember saw temperatures near normal and no rainfall. A weak cold front passed through the region early in the week, followed by sharp warming, and a strong cold front late in the week. Air quality improved after the 20th, with another deterioration on the 23rd as bright sun and lihgt winds permitted the accumulation of pollutants. Only a few isolated showers occurred with frontal passage on the 21st, with no rain measured in Tyler. The final nine days of the month were two degrees cooler than normal, and saw near normal rainfall. Cold fronts on the 22nd and 28th lowered temperatures. The front of the 22nd returned northward as a warm front on the 25th. Interaction between this and a closed-off upper air low brought most of the rain. Both cold fronts were preceded by above normal temperatures.
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
September 1999, RECORDS AND SUMMARY: