How coronavirus is making weather forecasting more difficult

It’s not like forecasting Denver’s weather is particularly easy to begin with. But a loss of weather observations from commercial aircraft, due to a sharp decrease in flights related to coronavirus lockdowns, may make local weather forecasting even more difficult.

On Thursday, the European Centre for Medium-Range Weather Forecasts (ECMWF), which operates the European model, said that a big loss of aircraft observations was likely going to have direct implications on computer forecast models’ skill levels.

— ECMWF (@ECMWF) April 2, 2020

The European computer forecast model is generally regarded as the world’s most accurate computer forecast model, and in particular, it excels in its handling of medium-to-long-term weather forecasts. A decrease in the European model’s skill alone could have a direct impact on the accuracy of weather forecasts, including Denver’s.

Airplanes take continuous weather observations when they’re flying, and the data from those observations are directly fed into computer forecast models. Because the cruising altitude for most commercial jetliners is around 30,000-40,000 feet high, that data, in particular, is critical in helping a computer forecast resolve information about the jet stream. The jet stream is the narrow ribbon of strong upper-level winds that often has a major influence on the weather at the surface, particularly in mid-latitude locations with volatile weather like Denver.

Scientists at the ECMWF said that the loss of aircraft observations would likely have the highest impacts on their computers’ ability to accurately depict the jet stream’s current location and strength.

“At short-range (one-day forecast), most (airplane observation data loss) impact is on ‘jet level’ (10-12 km) winds and temperatures, partly because many of the aircraft reports are at this level, but also because this is an active part of the atmosphere,” said Bruce Ingleby, a weather data expert at the ECMWF, in an email to The Denver Post.

According to the ECMWF, “aircraft reports are second only to satellite data in their impact on forecasts.” That could lead to a potentially substantial drop in forecast accuracy as a result, though the exact impacts are still in question.

“Because of day-to-day variability in forecast skill (all the time), it is not easy to attribute changes in skill to specific sets of observations, except in an average sense (over a month or more),” Ingleby said. “We are about to start looking at this aspect.”

It’s not just the European model that relies, in part, on aircraft-based weather observations. The U.S. government’s Global Forecast System (GFS) model takes those into account as well, as do most other weather models.

While the National Weather Service office in Boulder said Friday that it hadn’t noticed any immediate differences in the skill level of computer forecast models, it’s quite possible that the loss of valuable airplane data could lead to substantial future impacts on forecasts.

“It’s too soon to quantify the exact impact because the decrease is only occurring for certain flights and routes, and while there is a reduction of commercial passenger flights, we still receive valuable aircraft data from overnight cargo and package carriers,” the National Oceanic and Atmospheric Administration (NOAA) said in a statement to The Denver Post. “Even though a decrease in this critical data will possibly negatively impact forecast model skill, it does not necessarily translate into a reduction in forecast accuracy since National Weather Service meteorologists use an entire suite of observations and guidance to produce an actual forecast.”

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