Precipitation for December varied from above to below normal across west-central Texas (Figure 1).
Figure 1: Percent of Normal Precipitation for December, 2013.
The monthly precipitation was above normal across the area southeast of a line from Mertzon to Abilene to Throckmorton, and northwest of a line from Sonora to Richland Springs. Across much of this area, the monthly amounts were between 1.5 and 3 inches. The highest amounts (over 3 inches) occurred in parts of Callahan, Brown and Coleman Counties. Across the rest of west-central Texas, precipitation was near to below normal for the month.
Temperatures averaged below normal for the month.
At Abilene Regional Airport, the average temperature for December was 42.3 degrees. This was 3.0 degrees below the normal average temperature of 45.3 degrees. Total precipitation for Abilene in December was 1.11 inches. This was 0.12 inches below the normal of 1.23 inches. Abilene recorded 0.8 inches of snow (sleet) in December.
At San Angelo Regional Airport, the average temperature for December was 44.7 degrees. This was 2.0 degrees below the normal average temperature of 46.7 degrees. Total precipitation for San Angelo was 1.15 inches. This was 0.30 inches above the normal of 0.85 inches. San Angelo recorded 1.5 inches of snow (sleet) in December.
Warm and dry conditions occurred on December 2-4, with daytime temperatures well-above normal. Highs were mostly in the 70s on the 2nd, and in the upper 70s to lower 80s on the 3rd. On the 4th, temperatures reached the upper 70s to mid-80s, ahead of an approaching strong, Canadian cold front.
A major change in the pattern brought an abrupt end to the above normal temperatures across the region, and resulted in record cold conditions with wintry precipitation. Following passage of the strong cold front, much colder air overspread West Central Texas on the 5th, with falling temperatures and brisk north winds on the 5th. On the afternoon of the 5th, temperatures were around 50 degrees colder than during the previous afternoon. The approach of a strong upper level disturbance brought mixed winter precipitation to West Central Texas, from the evening of the 5th into the early morning of the 6th. Freezing rain and sleet transitioned to all sleet across the Big Country, Concho Valley and Heartland, while the precipitation was mostly freezing in southern sections of West Central Texas. Some of the precipitation was accompanied by lightning and thunder to the southeast of a Sonora to Brownwood line. With temperatures well-below freezing, sleet accumulated to one-half inch, and ice from the freezing rain accumulated to between one tenth and one quarter of an inch. The most hazardous travel conditions occurred across the Big Country, where numerous traffic accidents were reported.
The very cold airmass was entrenched across the region through the early morning of the 8th. Record low maximum temperatures were set at San Angelo and Abilene on the 6th and 7th. In addition, record low temperatures were set at Abilene (14 degrees) and San Angelo (19 degrees) on the 7th. Across west-central Texas, low temperatures were mostly in the teens on the 7th and 8th. Temperatures across the area remained below freezing until the late morning and afternoon hours of the 8th, when a brief warmup occurred.
Cold frontal passages on the 9th and 11th brought reinforcements of cold air and helped to keep temperatures well-below normal. Areas of freezing fog and freezing drizzle occurred across most of west-central Texas on the 9th. A record low maximum temperature (32 degrees) was set at San Angelo on the 9th.
Scattered light rain showers occurred on the 13th, as an upper level disturbance moved into texas from the southwestern states. With temperatures well-above freezing, this was liquid precipitation. Where the showers occurred, rainfall amounts were mostly less than one tenth of an inch.
Following a dry cold frontal passage on the 14th, dry conditions with warmer daytime temperatures occurred on December 15-17. Increasing south winds brought an increase in low-level moisture on the 18th and 19th. With this setup and overnight low cloud development, nighttime temperatures were much warmer. New record high minimum temperatures were set at Abilene (53 degrees) and San Angelo (55 degrees) on the 19th. In addition, a peak wind gust of 45 mph (from the south-southwest) was recorded at the Abilene Regional Airport on the 19th.
The anomalous warmth was short-lived, as another strong cold front advanced south across West Central Texas on the 20th. Temperatures in the 50s and 60s fell into the 30s and 40s after frontal passage. With the approach of an upper level storm system from the southwestern states and northern mexico, rain and showers developed across West Central Texas on the 20th. Showers and a few thunderstorms continued into the morning of the 21st. This was a beneficial rain event for West Central Texas (Figure 2).
Figure 2: West Central Texas Rainfall for the seven day period, ending December 21.
The heaviest rainfall amounts of 1.5 to 2 inches (with a few locally higher amounts) occurred across parts of the eastern Big Country, Heartland, and eastern Concho Valley. Across the rest of West Central Texas, rainfall amounts varied between one-half inch and 1.5 inches.
Windy conditions occurred across much of West Central Texas (generally along and south of Interstate 20) on the 21st, as the storm system lifted out across Texas. Peak west-southwest winds reached 42 mph at an automated weather observation site 4 miles west-southwest of Sterling City, and a peak gust of 40 mph was recorded at the San Angelo Regional Airport.
Following a secondary cold frontal passage on the overnight of December 21-22, dry and colder conditions occurred. With a weak upper level disturbance, light rain occurred on December 27-29, mainly across the southern half of west-central Texas. With the exception of parts of Sutton and Kimble Counties, the rainfall amounts were mostly less than one quarter of an inch. Temperatures were mostly below normal late in the month. Cold frontal passages occurred on the 25th and 29th.