Weather forecasters need a ton of knowledge and a fair bit of experience with local weather patterns to do their job well. They also need a good forecast model. These computer models take in measurements from weather stations on the ground, satellites in orbit, and balloons in between and then simulate the physics of weather forward in time a few days.

For the first time in about 40 years, the guts of the US model got swapped out for something new today. The upgrade brings us a new “Finite-Volume Cubed-Sphere” (or FV3) dynamical core, which simulates the basic atmospheric physics at the heart of this endeavor, a change that has been in the works for a while.

The new core had its origins in simulating atmospheric chemistry but ended up being adapted into other models. A few years ago, it was selected to replace the old core in the US Global Forecast System model. And for more than a year now, the new version of the model has been running in parallel so its results could be compared to the operational model.

That evaluation also included retroactive forecasts (that is, forecast simulations with the same inputs that were available on each day in the past) of the past three years, with an additional focus on case studies of major hurricanes and common storm types.

The results have been a little mixed. The new core improves computational efficiency and allows some processes to be simulated at a higher resolution—unequivocal improvements. It also simulates the physics of water vapor more realistically. In a press conference today, NOAA scientists cited a number of areas where forecast improvements have been seen. Forecast tracks of hurricanes and the mid-latitude storms that frequently sweep across the US have both improved, they said, along with forecasts of hurricane strength. Forecast precipitation amounts were also cited as a key area of progress.

But there have also been grumblings in the weather community over the past year about results that didn’t seem so hot. For example, surface temperatures have been biased low in some situations, throwing off forecasts.

Of course, no model is a perfect replica of the planet, and part of using these models to forecast weather requires us to understand and account for their biases. NOAA says that the cold bias has been reduced by development over the past few months—and development is a continuous process that doesn’t stop when a model goes live. Still, there could be an adjustment period here as forecasters get used to their new souped-up model. At least through September of this year, the old model will still be running in parallel to aid in that transition.

Improvements to the forecast system can come from other fronts, too, from recent upgrades to NOAA’s supercomputers that drive the models to launches of the latest and greatest satellites. But switching out the core of the main forecast model is a big deal that will hopefully help it keep up with the progress being made by others. Right now, the European Centre for Medium-Range Weather Forecasts model is generally seen as the top dog.