Better Know a City Climatology: Nashville, TN

In this series we will explore the weather characteristics of a particular city in the United States. Using weather data and analytics, we will explore what makes this area unique in its climatology. For each city, an analysis of a big event will be performed, and there might be other surprises along the way.

Today, we will explore the Music City of Nashville, Tennessee. This state capital is home to the Grand Ole Opry, Vanderbilt University, and its famous style of hot chicken.


Temperature

Speaking of hot, we are currently finishing up the dog days of summer at the time of writing. The official weather station is at the airport, east of the city, and has been in operation since 1928. These airport stations are typically used for climatological analysis, and according to the data, daily maximum temperatures typically peak in early August, at 89.5F. Temperatures in the afternoon are typically between 88.4 and 89.5 for most of July and early August. Just because these are the normal temperatures, it does not mean that is what is currently happening.

The figure below depicts current temperatures for the year 2018. Daily maximum temperatures (red line) and minimum temperatures (blue line) are compared against its 30 year climatology (smoothed dark bars) and its extreme (jagged light bars) values, indicating the hottest / coldest it ever recorded on that day. For the most part, 2018 maximum and minimum temperatures are above their climatological means. This indicates above normal (or positive) conditions for the area.

USW00013897_temperature.png

While most people enjoy the warmer temperatures, it can cause issues for energy companies. A good measure of energy demand is known as a Cooling Degree Day (CDD). It may sound complicated, but the mathematical formula is quite simple. Take the days average temperature (typically [max+min] / 2) and subtract it from 65 degrees F. If the difference is above zero, then that difference is known as the number of cooling degree days. So an average of 78F would be 13 cooling degree days. If the difference is negative (i.e, below 65F), then it is indicated as zero cooling degree days. The daily value is then accumulated each day, to give an indication of demand for a particular month or year. The higher CDD increases, the more demand is typically needed. Energy companies are very cognizant of CDD (as well as heating degree days, HDD, in the winter). They will typically use these numbers to determine if rates need to go higher, or if they need to allocate resources to a particular area. The graphic below shows the number of cooling degree days Nashville typically sees each month. The highest demand is in July and August, while it tapers off in September and October. However, it has been so warm this year, there were much higher Cooling Degree Days for the months of July, August, and September (523, 476, 381, respectively).

Source:  xmACIS

Source: xmACIS


Precipitation and Snowfall

Nashville typically sees 45 to 50 inches of rainfall per year. Most of that precipitation occurs in May, although can vary year to year. The graph above attempts to place the current year (2018) against its other years. At the time of writing, there has been 47 inches of rain, which is 10 inches above what is typically seen at this time (37 inches). This can be depicted as either a precipitation anomaly (+10) or a percent of normal (127%). The dark blue line shows the wettest year on record, which occurred in 1979. Part of this was due to 6.6 inches of rain falling on September 13th, around the time Hurricane Frederic was weakening over the area.

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Surprisingly enough, Nashville does see snowfall. It typically sees about 6 inches each year. The most it has ever snowed in 1 day was about 8.2 inches, occurring in March of 1968. In 2018, it has observed 3 inches of snow, most of it falling in January.


Weather Event: May 2010

The 6.6 inches that fell in 1979 was only the second wettest day on record. The wettest day on record occurred in May 2010, as 7.25 inches fell on the 2nd. On the prior day, 6.32” fell, which gave a total of 13.57,” which is the most it has ever rained in this two day period. During this time, there was also a 24 hour record of 9.09” that fell over both days. Because of the unprecedented rainfall, there was substantial flooding in the area, leading to the deaths of 18 people and over a billion dollars in estimated damage.

Accumulated rainfall between May 1st and 2nd, 2010. Star indicates Nashville location, and red triangles indicate the location of tornado reports. Rainfall data provided by  PRISM Climate Group , Oregon State University. Tornado reports provided by NOAA’s  Storm Prediction Center  (SPC).

Accumulated rainfall between May 1st and 2nd, 2010. Star indicates Nashville location, and red triangles indicate the location of tornado reports. Rainfall data provided by PRISM Climate Group, Oregon State University. Tornado reports provided by NOAA’s Storm Prediction Center (SPC).

Most of the rainfall occurred due to a stalling frontal system in the western part of the state. This helped transport lots of moisture from the Gulf of Mexico up into the area. Not only did this create a lot of rainfall, the dynamics were also set up for the system to spawn numerous tornadoes in the area. The graphic above indicates the amount of rainfall during the first two days of the month, along with the location of tornadoes, noted as red triangles. Although Nashville (indicated with the black star) received historic rainfall, a good portion of western Tennessee saw over 15” of rain in a two day period.


Notable Weather Statistics for Nashville

  • Highest Max Temp Ever Recorded: 109 F (June 29th, 2012)

  • Lowest Min Temp Ever Recorded: -17 F (January 21st, 1985)

  • Most Precipitation in One Day: 7.25” (May 2nd 2010)

  • Most Snowfall in One Day: 8.2” (March 22nd, 1968)

If you would like to see your city highlighted in the next segment, please let me know.