The Heat and Drought of 2011

Above normal temperatures defined a majority of 2011 as numerous records fell at both Abilene and San Angelo.  Both the spring (March through May) and the summer (June through August) were among the hottest on record.  The primary player was the upper level ridge that was anchored over the region for much of the late spring and summer months. This provided a large scale sinking motion aloft which acted to inhibit cloud development and greatly reduces precipitation chances, in addition to providing warmer temperatures aloft.

We saw numerous temperature records fall across West Central Texas in 2011. We keep climatological records for both Abilene and San Angelo. The Abilene records date back to 1886 and are virtually complete while the San Angelo data begins in 1907 but contains sporadic missing data until 1947 when the official observing station was established. This article will highlight the new records, many of which shattered long standing benchmarks during the oppressive summer heat and drought of 2011.

San Angelo:

Records fell like flies in 2011 at Mathis Field in San Angelo.  After a slightly below normal January, average monthly temperatures (the average of the highs and lows for each day) were above the normal values for each month from February through November, despite one of the strongest arctic outbreaks in nearly 30 years arriving with the beginning of February.  The records began showing up frequently during the spring.  The average temperature during the meteorological spring months of March through May set a record for the warmest on record (since 1907).  San Angelo experienced its first 100 degree day of the year on April 9 (earliest on record is April 6); an omen of things to come!  Temperatures really began to heat up in May with a high temperature of 110 degrees reached on May 28.  This set a new record for the warmest May temperature but fell one degree shy of San Angelo's all-time warmest temperature of 111 degrees.

Maximum temperatures were 10-12 degrees above normal throughout the month of June, with highs topping 100 degrees on 26 of the 30 days. San Angelo recorded highs equal to or in excess of 105 degrees on 11 days throughout the month.  Overnight lows averaging nearly 75 degrees helped to boost the average monthly temperature to 88.6 degrees.  Not only did this set the record for the warmest June on record, but it also set the record for the warmest of ANY month on record at San Angelo.

The oppressive temperatures continued through July with an average temperature even warmer than that of June.  The average monthly temperature in San Angelo for July was 89.6 (a full degree warmer than June) degrees, thanks in large part to 29 days of triple digit heat.  Thus, the warmest month on record established in June 2011 stood for only one month before being surpassed by July 2011. The heat persisted through the month of August as well.  We saw high temperatures of 100 degrees or warmer on 28 days during the month of August with a brief respite from the heat during the middle of the month thanks to some very welcome rainfall.  However, this break did little to alleviate the long term heat wave as the average August temeprature ended at a stifling 89.7 degrees. Amazingly, this broke the record set in July 2011 for the warmest month on record (third month in a row!!!).  What this means is that in 104 years of record keeping, the record for warmest month in San Angelo was set three times in consecutive months.  The statistical significance cannot be overstated.

The meteorological summer (June through August) was obviously the warmest on record since we set all time records each month during that period.  The average temperature during this three month period was 89.3 degrees.  The previous record for the warmest meteorological summer was 85.8 degrees, set in 1910.  In other words, the San Angelo summer bested 100 year old seasonal record by 3.5 degrees.  The hundred degree temperatures continued into September with the final triple digit reading coming in on September 27 (104 degrees!).  The 100 degree day records are discussed in a separate section below.

October and November continued the trend of above normal temperatures with sporadic falling records.  However, a noticeable pattern change began to occur late in the year, resulting in increasing moisture and temperatures closer to normal.  The average temperature for the month of December was actually below normal, ending the year in a fashion similar to it's beginning.  However, I do not think that many will forget the months in between that resulted in the record heat and historic drought.

The average temperature in 2011 for San Angelo set a standard that may never be equalled.  The average temperature for the year was 69.3 degrees.  The previous mark for the warmest year on record was 67.7 degrees (dating back to 1943).  It is worth noting that the 10 warmest years on record were all bunched within 0.7 degrees of each other, but 2011 came in at 1.6 degrees warmer than the previous mark.  

Abilene:

Abilene was not quite as warm as San Angelo during the early spring months.  However, triple digit readings began to show up more regularly during the last week of May.  This became the norm in June as the average high temperature climbed to nearly 101 degrees (8-10 degrees above normal).  Triple digit temperatures were recorded on 19 days in June.  Numerous daily high temperature records were set during the middle portion of June with highs typically in the 105-107 range.  The average monthly temperature in June of 87.6 degrees fell just short of setting the record for the warmest June on record, winding up at #2 on the list.  The record remains 87.7 degrees set in 1953.

July was even warmer with the average high temperature approaching 102 degrees.  The overnight low temperatures averaged in the upper 70s, with a minimum temperature in the 80s on 9 July days.  The low temperature on July 9 was 84 degrees, setting a new mark for the warmest low temperature on record which dates back 125 years (1886).  Aided by these very warm low temperatures, and 100+ degree heat on 27 days of the month, the average monthly temperature for July finished at 90.1 degrees.  This set the record for the warmest July on record, surpassing the benchmark of 89.4 degrees set in 1980.  This also tied the record for the warmest of ANY month on record at Abilene, dating back to 1886.  The previous mark of 90.1 degrees was first set in August of 1952. 

The thermal onslaught continued through August with an additional 27 days at or above 100 degrees. Overnight low temperatures averaged 78.5 degrees during the month, remaining 80 degrees or wamer on 13 occasions. The low temperature was as warm as 83 degrees twice in August, falling just short of the all time record for the warmest low temperatures (84 degrees in July 2011).  The average monthly temperature for Abilene in August 2011 was 90.3 degrees.  This broke the previous record for the warmest month on record of 90.1 degrees which was orignally set in August 1952 and tied in July 2011. 

A few more 100 degree days were observed in September, with the last coming on September 29.  Again, see the next section for information on 100 degree day records.  The early fall months remained above normal but the pattern change resulted in an average December temperature more than 1 degree below normal.  It was a good way to end the year but it hardly made a dent in the overall statistics.

The average temperature for all of 2011 at Abilene as 67.5 degrees.  This fell just short of the all-time warmest year at Abilene (dating back to 1886), missing the mark by 0.1 degrees.  However, 2011 did finish at #2 on the list of warmest years on record. 
 

100 Degree Days:

The most visible records discussed in the summer of 2011 were the 100 degree day marks.  We saw several prominent records fall at both Abilene and San Angelo during this period that are very much worth talking about.  The first is the number of 100 degree days in a calander year.  On July 26, both Abilene and San Angelo set new records, reaching 47 and 61 days, respectively.  The heat continued through September, adding to the new records and putting them far beyond the pace set by previous years.  At years end, the tally of 100 degree days at San Angelo was 100 days, beating the previous mark of 60 days set in 1969!  At Abilene, triple digits were recorded 81 times, smashing the previous record of 46 set in 1934.  The Abilene records were complete and dated back to 1886.  Unfortunately, the records for San Angelo had to be pieced together for some years between 1907 and 1947 due to the missing data.

In addition to the total number of 100 degree days, we also saw some impressive streaks of consecutive 100 degree days.  San Angelo had a run of 28 consecutive days of high reaching 100 degrees or more snapped on July 30.  This broke record that was set just last year (2010) during the latter part of July and August of 26 consecutive days.  We had two more such streaks that made it into the top 5 at San Angelo as well.  We saw 17 consecutive days in August and recorded 16 consecutive days in June.  These rank fourth and fifth on the list, respectively.

At Abilene, the record for consecutive 100 degree days still stands at 30 days, set back in 1952.  The 2011 streak at Abilene does not compare with that record but remains impressive in its own regard.  A high temperature of 98 degrees on July 30 ended a run of 18 consecutive 100 degree days at Abilene, landing it in a tie for #5 on the list.  We saw several other streaks of greater than 10 days but these were not enough to make the list.

The records for consecutive 100 degree days begin in 1937 for Abilene and 1947 for San Angelo.
 

Precipitation:

While one could argue that the heat was the most unbearable aspect of the 2011 summer, probably the most pressing issue was the overall lack of rainfall for the southern Plains, including all of the San Angelo County Warning Area.  Spring thunderstorms were a bit more sparse than we are accustomed to, but little help arrived during the summer months thanks, in large part, to the large dome of high pressure sitting overhead.  Mathis Field in San Angelo recorded 2.94" of rain through July but did see some relief by mid August when another 1.64" fell. Through August, San Angelo had recorded 4.58" of precipitation, falling well below the normal of 14.07". This maintained the pace to best the mark for the driest year on record (since 1947).  Very dry conditions continued through September but we finally receive a bit more relief with the change to fall.  October precipitation was slightly above normal for San Angelo.  November reverted back to the dry regime but December was rather cool and damp, coming in above normal in precipitation.  This brought the year end total to 9.21".  The normal for San Angelo is 21.25", meaning San Angelo received less than half of its 30 year normal rainfall.  Precipitation records are rather sparse before 1943, so the comparision for driest years on record begins at that point.  The 9.21" of precipitation comes in as the 3rd driest year on record and is the first year of less than 10 inches since 1956!

Abilene finished the year well below normal as well, but did manage to record 16.83" of precipitation during the year.  The normal for Abilene is 24.82" so Abilene ended up with roughly two-thirds of the normal amount.  This allowed Abilene to finish well above the record driest year of 1956, when 9.78" of precipitation was observed.  The 16.83" of preciptiation in 2011 ranks #20 on the all-time driest list.

This lack of precipitation resulted in impressive soil moisture dry anomalies.  The dry soils play a significant role in perpetuating the heat and drought conditions.  Less soil moisture means that it takes less of the sun's energy to heat the air near the Earth's surface.  This also allows atypical vertical mixing to occur during the afternoon, resulting in less boundary layer moisture, which can decrease the chance for thunderstorm development.

It was a memorable and historic year to say the least.  This drought (not forgetting the heat) will have long lasting impacts on West Central Texas.  Many ranchers have sold portions or all of  their livestock due to a lack of food and/or water for their herds.  Farmers had just as difficult a time.  Those without irrigation have lost most, if not all, of their crops while some with the ability to provide water have seen wells run dry.  This spilled over into the cities and towns as well. Water supplies are limited in west central Texas anyway due to precipitation trends, but a particularly windy spring, combined with the hot and dry conditions have resulted in extreme evaporation rates.  Measurements taken at the National Weather Service office in San Angelo have shown evaporation rates averaging around 0.5" per day from April through September.  That adds up to nearly 90 inches (7 1/2 feet!) of evaporation during that 6 month period.

Several lakes in West Texas dried up recently and others are not far behind.  One of these lakes is right in San Angelo's backyard: O.C. Fisher (See image above).  Almost as bad is the state of Lake E.V. Spence near Robert Lee which is at less than 1% of capacity.  Most of the other reservoirs with the forecast area have fallen to less than 50% of capacity with O.H. Ivie, which serves numerous municipalities as a source of drinking water down to 18%.  The heat and drought of 2011 is definitely something many would like to forget but it will leave long standing memories for many residents of West-Central Texas that will no doubt be crystal clear until we see a prolonged wet period to recharge the water supply.