Winter weather, combined with the grounding of Boeing 737 Max 9 aircraft, is causing major disruptions for air travel.

There have been more than more than 2000 flight cancellations Friday, the highest number since July 2023, data from the tracking site FlightAware show.

Only a handful of days in all of 2023 had more cancellations than Friday’s total. Last year, there were only three days when more than 2000 flights were cancelled, FlightAware data show, making today the fourth worst day for flight cancellations in the last 12 months. It is the most flights cancelled since June 2023.

Most of the cancellations are due to a winter storm that is pounding the Midwest. Chicago’s two major airports are seeing the bulk of the cancellations, with nearly 40% of departing flights at O’Hare, and more than 60% of departing flights at Midway are cancelled, according to FlightAware.

O’Hare airport posted on social media that more than 650 flights have been “proactively canceled” by the airlines. Denver and Milwaukee flights are also being hard hit by the storm. Nearly 40% of Milwaukee’s flights are also cancelled.

Cancellations due to the grounding of the 737 Max 9 planes are also contributing to the totals. More than 200 United and Alaska Airlines flights have been cancelled each day this week due to the FAA-mandated grounding. The FAA and Boeing are still trying to settle on an inspection protocol that would allow those planes to resume flying.

FlightAware shows Southwest, which doesn’t fly the 737 Max 9, cancelling nearly 400 flights, the most of any airline.