Asking what’s normal weather in Austin is a loaded question, but meteorologists say if you take a long enough view, you can actually spot trends and a “normal” for temperature and rainfall emerges.
Meteorologists use observations from 30-year intervals to establish benchmarks for normal Austin temperature and rainfall. The current 30-year normal values, for instance, were derived from data collected from 1981 to 2010.
But record-setting heat since 2011 is likely to redefine what’s normal in the region. Four of Austin’s seven warmest years have occurred after 2010, which means the new normal temperatures could shift higher when they’re re-established two years from now, using data from 1991 to 2020.
Austin’s seven warmest years on record were:
• 2017: Average temperature of 72.1 degrees, included Austin’s warmest winter.
• 2011: Average temperature of 72 degrees, included a record 90 days of triple-digit temperatures.
• 1904: Average temperature of 71.7 degrees.
• 2012, 2006 and 1905: Average temperature of 71.6 degrees.
• 2016: Average temperature of 71.4 degrees.
To put these temperatures in perspective, the average temperature for the current 30-year normal period is only 69.3 degrees.
“There’s no doubt in my mind that when we roll over to the next ten years we’re going to see warmer temperatures,” said University of Texas meteorology lecturer and longtime forecaster Troy Kimmel.
To know why the 30-year normal values are important is to understand the difference between weather and climate.
“The definition of climate,” Kimmel said, “is the collective state of the Earth’s atmosphere over a long period of time.”
Lower Colorado River Authority meteorologist Bob Rose said the 30-year normal “helps in describing the climate and is used as a base to which current conditions can be compared.” A region’s climate can affect activities related to agriculture, commerce, industry and transportation, he said.
“One of the primary reasons for using a 30-year normal is to provide a metric that can be taken into account in long-term planning considerations,” said Rose, whose employer manages the Highland Lakes, a key water source for Central Texas.
For Rose, “knowing normal temperatures and rainfall for a particular time of the year gives forecasters information on what to expect and what would be considered an outlier,” he said.
He said knowing the normal high temperature for Feb. 5 is 64 degrees, for instance, gives meteorologists a starting point for the forecast.
“Events such as cold fronts or warm air spreading north from the Gulf will modify that temperature up or down depending on the weather situation,” Rose said.
The normal values can serve a practical purpose, too. Say you’ve got friends coming into Austin for the South by Southwest Interactive Festival this year and they want to know what the weather is going to be like.
The 30-year normal temperatures for March 9, the first day of the event this year, range from a nighttime low of 50 to a daytime high of 70 — not including a few hundredths of an inch of rain that day, according to the National Weather Service.
“The normals only tell you over the long term what you normally might expect. But it doesn’t mean that’s what you’re going to get,” Kimmel said — and that’s the difference between climate and weather.
Austin’s 30-year normal temperature and rainfall figures come from data collected at Camp Mabry, between 35th and 45th streets along MoPac Boulevard (Loop 1) in West Austin. The data are from one of the most complete sets of observations, dating back to the early 1890s, Kimmel said.
Data collection began in downtown Austin, then moved in the early 1940s to Robert Mueller Municipal Airport — now the Mueller neighborhood in East Austin — before finally settling on Camp Mabry. To maintain scientific integrity, the data-collection sites needed to be within 5 miles of each other and within 100 feet of the same elevation to capture data in Austin’s “urban heat island,” Kimmel said.
The current 30-year normal temperature and rainfall values for Camp Mabry are basically averages from measurements taken from 1981 to 2010, but also include the readings from such extreme weather events as the 1981 Memorial Day flooding in Austin or Tropical Storm Hermine in 2010.
But that’s why the 30-year normals are used as a benchmark, Kimmel said. From a statistical standpoint, he said, the 30-year normal “gets rid of some of that noise, some of those extremes.”