New climate normals, describing the average weather over the past 30 years, were released by the National Oceanic and Atmospheric Administration on Tuesday. Around the country, the increase seen in temperatures and precipitation compared with earlier 30-year periods is palpable. Around D.C., it’s a similar tale.
Precipitation is up. Especially in summer. Despite that, snow is declining, a downer for fans of winter.
Temperatures are up across the board
All months of the year now have a higher temperature average than they used to in Washington. Let’s compare the new normals, based on the period 1991-2020, with the old normals, based on the period 1981-2010.
December has seen the biggest gain, warming 2.0 degrees. Oddly enough, November is the smallest, with a bump of just 0.3 degrees.
Annually, Washington’s average temperature is up from 58.2 to 59.3, a gain of 1.1 degrees. In general, average low temperatures are rising faster than highs. Annual lows are up 1.4 degrees and average highs are up 1.1 degrees.
The increase in temperature over the past two 30-year periods continues a long-term upward trend. During the last update in 30-year normals a decade ago, every month gained warmth as well. The gains were even larger with the current update.
More hot days and fewer freezes
Perhaps the most significant change in the new normals is that our hottest summer days now have an average high of 90 degrees.
There’s now a 22-day streak from July 6 to July 27 where the typical high is indeed 90.
This is a breathtaking increase, given that in the old normals our summer peak temperature was 89 degrees for 16 days in a row. The new normals have a remarkable 45 days at or above 89 degrees, running from June 28 to Aug. 11.
But perhaps this change shouldn’t come as a surprise. Recall that last summer we hit 90 degrees on 20 straight days, the second-longest such streak on record. And, in July, we posted 28 90-degree days, the most in any month on record.
Cold is less cold, as well. Before the new normals were published, the city had 37 days with lows at or below 29 degrees from Dec. 31 to Feb. 5.
Now? We have zero such days.
The period during which we experience freezing lows has shrunk from Dec. 16-Feb. 23 to Dec. 27-Feb. 18. Essentially, we’ve lost a week of freezing lows at both the beginning and end of the coldest part of winter.
It’s wetter. Especially in summer and early fall.
Precipitation averages have also ticked up, in no small part thanks to a year such as 2018, the wettest on record across the region.
Examining the new normals, the largest increase in rainfall is during July, when the average jumped 0.60 inches to 4.20 inches. Both June and July top the four-inch mark now, which is something no month did amid the old normals.
Every summer (June-August) month gained precipitation, and so did the first two months of fall.
There was a somewhat similar pattern of increasing precipitation during the core of the warm months seen the last time the normals were updated in 2011. Over these past two cycles, June has now added more than an inch of rain to its average.
There’s not a strong explanation for the decrease in November precipitation, but it may be related to the fact that November has tended to be a bit chilly in recent times. Colder air masses are often drier than warm ones.
The D.C. snow hole is becoming deeper
Avert your eyes, snow lovers.
Washington’s snowfall average is down 1.7 inches, falling to 13.7 inches. We had already crunched these numbers in March, but NOAA’s publication of the new normals makes it official.
The average snowfall dropped in every wintry month except March. The core of winter in January and February saw the biggest decreases in snowfall, or almost 1.5 inches combined.
And while downtown Washington and its surroundings are something of a snow hole because of milder temperatures, similar losses are seen at the other nearby locations. Dulles International Airport’s normal snow average dropped from 22 to 21 inches, while Baltimore-Washington International Marshall Airport saw its average drop 0.8 inches to 19.3 inches.
Contextualizing the new normals
These new normals probably won’t be normal for long, given the continuing increase in temperatures in the region due to increasing greenhouse gas emissions and urbanization.
We should be particularly cautious about how we describe below-normal temperatures in the current climate. The baseline or what’s now considered normal is much higher than it used to be.
As an example, the summer average temperature of 78.8 degrees in 2017 was 1.1 degrees above normal based on the period 1981 to 2010. But using the new normals, based on 1991 to 2020, it’s 0.1 degrees below normal.
To assess months and seasons in a longer-term context, they might be framed using historical data. That 78.8-degree summer reading from 2017, while 0.1 degrees cooler than the new normal, is also the 22nd warmest out of 149 years in the record.
