September 2001

Report and Summary

Tyler, Texas
September 2001
by Robert K. Peters, Ph.D.
National Weather Service Cooperating Observer

The month of September 2001 was cooler and wetter than normal. The transitional pattern from the warm into the cool seasons occurred earlier than normal, which was responsible for the month's weather.

The 2001 warm season was the first since 1994 in which the temperature did not reach 100 in Tyler. The moderate temperatures were due to the wet Spring and early Summer, and the return of significant rainfall in late Summer and early Autumn.

The Sub-Tropical upper air high pressure ridge--a common feature in Texas weather during the warm season--was just as strong this year as last. However considerable soil moisture and lush vegetation held down temperatures.

September 2000 came at the end of the Summer 2000 heatwave and drought. Last month was 3.3 degrees cooler and 4.53 inches wetter than one year ago.

Year-to-date rainfall was 13.22 inches more through September 30, 2001 than through the same date one year ago. Precipitation over the past twelve months is 155.7 percent of normal.

The thirty-day outlook for September 2001 had called for above normal temperatures and near normal precipitation. The week August 26-September 1 saw temperatures three degrees cooler than normal, and rainfall 300 percent of normal. An unusual warm season rain event developed over the area. This was caused by an upper air trough over the area, ample tropical moisture, and dissipating cold fronts to the north of the region.

Rainfall amounts in the area during the week ranged between two and five inches, with the heaviest over the extreme north along Red River.

The week was six degrees cooler than the previous week. With an average temperature of 80.2 degrees, the week September 2-8 was two degrees warmer than normal, and 3.2 degrees warmer than the week previous.

Rainfall during the week was 2.95 inches, which was 300 percent of normal.

A closed low was over Central Texas for much of the week, until finally lifting out to the northeast on the 6th. Measurable rain was recorded in Tyler on twelve consecutive days, between August 27 and September 7.

By late-week, a cold front was approaching from the north. This caused warmer temperatures and an increasing south wind as the pressure gradient tightened.

The week September 9-15 saw temperatures about one degree below normal, and rainfall about one-eighth normal. During most of the week, weak surface and upper air high pressure were in control.

The upper air high was responsible for the dry weather. The surface high held temperatures near or slightly below normal. By late-week, high-level moisture from Eastern Pacific Hurricane Ivo and returning Gulf moisture raised humidities and nighttime temperatures.

Showers and isolated heavy rain occurred in the area on the 15th, with heaviest rains concentrated west of Tyler. The average temperature during the week was 75.7 deg., which was 4.5 deg. lower than the previous week.

The week of September 16-22 saw temperatures about two degrees warmer than normal and rainfall about 125 percent of normal. Tropical Maritime air was over the area for most of the week. Disturbances crossing in the Easterlies brought rain late in the week, while a system in a northwest flow aloft brought rain earlier in the week.

The presence of considerable cloudiness and rain held down daytime temperatures, but kept nighttime temperatures above normal. Frontal boundaries became stationary in or near the area early in the week, and again on the 20th. This further contributed to the precipitation.

Measurable rain occurred on six days during the week. The most intense activity was on the afternoon of the 20th, when strong thunderstorms brought power failures and gusty winds along with heavy rainfall. Some locations received as much as four inches of rain.

The mean temperature during the week was 76.9 deg., which was 1.2 deg. warmer than the previous week. The week September 23-29 saw s sharp change in temperatures, as readings averaged seven degrees cooler than normal, and there was but 0.01 inch of rainfall.

A Polar Continental air mass built into the region on the afternoon of the 23rd. Surface high pressure remained over the area through the end of the week. Light showers accompanied the cold front, and brought the only rain on the afternoon of the 23rd. Upper air high pressure to the west brought a northwesterly flow in the upper atmosphere. This continued the transport of mild and dry air into the region.

The week's average temperatures was 65.5 deg., which was 11.4 deg. cooler than the week before. A record low maximum temperature was established on the afternoon of the 24th.

A reinforcing surge of cool air arrived on the 29th, which kept the 30th mild.

• KEY TO SYMBOLS:
• A = HAIL
• B = BLOWING
• D = DUST
• F = FOG
• H = HAZE
• IP = SLEET
• K = SMOKE
• L = DRIZZLE
• R = RAIN
• S = SNOW
• T = THUNDERSTORM
• Z = GREENWICH MEAN TIME
• ZL = FREEZING DRIZZLE
• ZR = FREEZING RAIN
• - = LIGHT
• + = HEAVY

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 1961-1990.

SEPTEMBER 2001

• 01 84 72 75 0.40 PCPN 2300-2400
• 02 89 74 82 0.50 PCPN 0000-0200, 2200-2300
• 03 90 74 82 0.03 T, PCPN 2100-2200
• 04 82 71 76 0.03 T, PCPN 1200-2000 MIN 0000-1200 74, MAX 1200-2400 77 05 88
• 71 83 1.61 T, PCPN 2200-2300
• 06 86 71 79 0.01 T, PCPN 1700-2000
• 07 91 73 87 0.53
• 08 91 76 88
• 09 88 70 76 0.10 PCPN 0700-0800, 1500-1600 MAX 1200-2400 80
• 10 84 66 80 0.01
• 11 81 64 78
• 12 86 59 78
• 13 88 66 84
• 14 90 70 82
• 15 85 70 70 PCPN 2300-2400 MIN 0000-1200 73
• 16 85 69 76 0.08 T, PCPN 0000-0100, 1300-1500
• 17 87 69 82
• 18 88 70 83
• 19 83 68 79 0.06 PCPN 0500-0600, 1000-1100, 1300-1500 20
• 86 68 72 0.02 T, PCPN 1800-1900 MIN 0000-1200 70
• 21 83 67 77 0.75 T, PCPN 1400-1900
• 22 83 70 76 0.15 F,
• 23 83 69 76
• 24 76 64 66 0.01 PCPN 0600-0700 MIN 0000-1200 66, MAX 1200-2400 68
• 25 71 53 70
• 26 74 48 68
• 27 77 49 69
• 28 79 51 72
• 29 76 55 71
• 30 76 54 70

SEPTEMBER 2001, RECORDS AND SUMMARY:

• 8TH EQUALLED RECORD HIGH MINIMUM TEMPERATURE OF 76, FIRST SET IN
• 1925 AND EQUALLED IN 1930, 1970, AND 1990;
• 22ND ASTRONOMICAL ONSET OF AUTUMN 2305Z;
• 24TH RECORD LOW MAXIMUM 68, PREVIOUS RECORD 70 IN 1974;
• HIGHEST TEMPERATURE 91 ON THE 7TH AND 8TH;
• LOWEST TEMPERATURE 48 ON THE 26TH;
• AVERAGE MAXIMUM 83.0 F.,
• AVERAGE MINIMUM 65.3 F.,
• 1.8 DEG. COOLER THAN NORMAL.
• HEATING DEGREE DAYS 9,
• 225 PERCENT OF NORMAL HEATING DEGREE DAYS.
• COOLING DEGREE DAYS 271
• 70.2 PERCENT OF NORMAL COOLING DEGREE-DAYS;
• TOTAL PRECIPITATION 4.70 IN.,
• 0.67 IN. LESS THAN NORMAL.
• 116.6 PERCENT OF NORMAL PRECIPITATION.
• YEAR-TO-DATE PRECIPITATION 46.36 IN.,
• 7 DAYS ON WHICH THUNDERSTORMS OCCURRED;
• 1 DAYS ON WHICH THE VISIBILITY WAS RESTRICTED BY FOG, SMOKE, OR HAZE;
• 0 DAYS ON WHICH THE VISIBILITY WAS RESTRICTED BY DUST OR BLOWING DUST;
• 0 DAYS ON WHICH FROZEN PRECIPITATION WAS OBSERVED;
• 0 IN. TOTAL SNOWFALL.
• 15 DAYS ON WHICH MEASURABLE PRECIPITATION OCCURRED.
• 4 DAYS ON WHICH THE MAXIMUM TEMPERATURE REACHED OR EXCEEDED 90 DEG.;
• 0 DAYS ON WHICH THE MAXIMUM TEMPERATURE REACHED OR EXCEEDED 100 DEG.;
• 0 DAYS ON WHICH THE TEMPERATURE FAILED TO RISE ABOVE 32 DEG. 0 DAYS
• ON WHICH THE MINIMUM TEMPERATURE REACHED OR FELL BELOW 32 DEG.
• 9 SEASON-TO-DATE HEATING DEGREE DAYS,
• 2241 YEAR-TO-DATE COOLING DEGREE DAYS.

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