Saturday, May 15, 2021

Meet the "new normal" in Memphis weather - NWS releases updated data

Most of you are familiar with the weather data referred to as "normals" - the normal high and low temperature for the date, normal precipitation for the month, etc. You see them every night on the TV meteorologist's weathercast and we post a climate summary graphic each evening on social media as well that shows the day's data and our normals for the day. 

[ Personally, though the term normal is generally accepted in the meteorological community, I actually prefer the term "average." However, the statistical calculations made are not precisely the simple mean, or average, of the previous 30 years of data, though they are very close. ]

So what defines "normal" in the meteorological context? The National Weather Service (NWS), as well as the World Meteorological Organization (WMO), use a 30-year average. Why 30 years? Here's how the NWS explains it:
Normals serve two purposes: a reference period for monitoring current weather and climate, and a good description of the expected climate at a location over the seasons. They provide a basis for determining whether today’s weather is warmer or colder, wetter or drier. They also can be used to plan for conditions beyond the time span of reliable weather forecasts. A 30-year time period was chosen by the governing body of international meteorology in the 1930s, so the first normals were for 1901-1930, the longest period for which most countries had reliable climate records. International normals were called for in 1931-1960 and 1961-1990, but many countries updated normals more frequently, every 10 years, so as to keep them up to date. In 2015 this was made the WMO standard, so all countries will be creating normals for 1991-2020. - "1991-2020 U.S. Climate Normals: An Update"
Up until just recently, our daily weather conditions have been compared to the climate normals from 1981-2010. However, as they do every 10 years, NOAA and the NWS has released updated climate normals for 1991-2020, which means we have a new set of normals to compare the day's weather to. It's always an interesting exercise to see how the normals change when a new set of data becomes available. Even though 20 years of data overlap between the two sets of data, in the case of the 1991-2020 data, the 1980s are dropped off and the 2010's are added. 

These two side-by-side maps of the contiguous United States depict the change in U.S. annual mean temperatures (in degrees; left map) and precipitation totals (% change; right map) between the new set of Climate Normals, 1991-2020 (most recent last 3 decades) and the previous set of Normals, 1981-2010. (NOAA NCEI)


So how do the 1991-2020 normals compare to the period from 1981-2010? Glad you asked! I've created a few charts to show temperature and precipitation comparisons. Let's start with monthly mean temperature and mean maximum and minimum temperatures (in other words, average highs and lows).

Comparison of new climate normals vs. previous normals for average, maximum and minimum temperatures at Memphis International Airport. Positive values indicate warmer temperatures in the most recent period.

The most obvious take from this chart is that, in general, temperatures are warmer in the 1991-2020 period versus the 1981-2010 period. The largest change in temperatures are in the cool season, while the summer did not warm as much. The notable values that are cooler are the overnight low temperatures in the heart of the summer (July and August) and in the month of November. The latter skewed the overall November temperature average cooler than the previous normals dataset as the average high temperature did not change. In fact, November was the only month that, on average, was cooler in the new dataset, while December had the largest positive (warmer) change. For the year, the average high temperature rose 0.4 degrees to 63.4 degrees. The average high temperature rose 0.6 degrees and the average low temperature rose 0.2 degrees.


So temperature has  generally warmed in the most recent data for Memphis. What about rainfall? Turning our attention to precipitation data, below are the changes in monthly precipitation from 1981-2010 to 1991-2020.

Comparison of new climate normals vs. previous normals for total precipitation by month at Memphis International Airport. Positive values indicate wetter conditions in the most recent period

The main takeaway of the new data from a precipitation perspective is we're generally receiving more water from the sky. In fact, for the year, average rainfall for the year increased 1.26" to just shy of 55" per year (or a bit more than 2%). The increase can be attributed to positive change from early spring through late summer. There was almost no change in September and October, while November and December saw rainfall totals fall, particularly November. This results in the wettest month of the year shifting from December to April. The driest month of the year moved later by a month, from August to September. 


Let's look into more precipitation data, specifically of the frozen variety. (Snow and sleet are lumped into "snowfall" data by the National Weather Service.)

Comparison of new climate normals vs. previous normals for monthly snowfall at Memphis International Airport. Negative values indicate less snow in the most recent period

The snow season for Memphis ranges from late November through early March with the highest totals typically in January and February. With the warmer temperatures discussed above (and to the surprise of no one who has lived in the Mid-South for an extended period of time!), average annual snowfall continues to decline. In the new data, the annual average has dropped from 3.8" to 2.7", or about 30%. The largest change has occurred in January, falling by an inch in the new normals. February's total also fell a bit. Interestingly enough, though the amounts are very small, average snow for November and March increased just slightly. The takeaway here seems to be that, although snowfall totals have fallen, the "winter season" seems to have expanded just a bit. In fact, the cooler month of November described above may have contributed to the slight increase in average snowfall in that month.

Recapping the 1991-2020 Normals

So, to wrap it up, our daily and monthly weather data will now be compared to a more recent set of average, or "normal" data. Overall, that data indicates a trend towards warmer and wetter conditions in the Memphis area, though there are exceptions such as a cooler and drier November. The largest positive change in temperatures occurred in the cool season, while precipitation increases were spread over many months, with the exception of late autumn to early winter. Snow lovers may want to look further north!

Erik Proseus
MWN Meteorologist

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