The Federal Aviation Administration is clearing the way for Verizon and AT&T to turn on more 5G wireless network towers as the regulator's concerns about 5G signals interfering with critical safety equipment on commercial jetliners at most airports has eased.

But there still could be significant flight delays, diversions and cancellations at some airports as the FAA has still not verified that certain airplanes would be free of 5G interference when taking off and landing in bad weather there.

Those planes that remain subject to flight restrictions are disproportionately smaller jets flown by regional airlines, which work in partnership with the larger mainline carriers such as American, Delta and United. And the 5G problem could add to the large number of flight disruptions already happening this weekend because of severe winter storms in parts of the Midwest, the Mid-Atlantic and Northeast regions of the country.

The National Weather Service said in an advisory that high winds and heavy snowfall from this weekend's Nor'easter will create blizzard-like conditions, making travel in some places "nearly impossible."

But at some airports where flights can still take off and land amid reduced visibility, some kinds of regional jets are nonetheless prohibited from flying because of concerns that radio signals from nearby 5G cellphone towers will interfere with radio altimeters on the airplanes.

Altimeters measure precisely how high the plane is above the ground and are a critical tool for pilots making instrument landings in bad weather when visibility is reduced.

Verizon and AT&T are using a segment of the "C" band of the radio spectrum for their 5G service that is very close to the radio frequencies used by altimeters. The FAA says interference from wireless broadband operations can make altimeter readings inaccurate and unreliable and, as a result, certain critical airplane systems may not function properly during takeoffs and landings.

As a precaution, the FAA has issued dozens of Notices to Air Missions (NOTAMs) that prohibit certain operations involving certain planes at certain airports. One of those airports is Paine Field in Everett, Wash., north of Seattle, where every single commercial flight on Monday and Tuesday this week was cancelled.

"Oh, it's always sunny in Seattle," joked Brett Smith, CEO of Propeller Airports, the private company that runs Paine Field, before admitting the reality that "unfortunately, this time of year, it rains a lot, and we've had our fair share of rain over the past few months."

Rain and dense fog were the problems earlier this week. Smith said that in the past such conditions could delay a few flights and might even cancel one or two, "but not our entire schedule. That's never happened," until this week when all 24 scheduled commercial arrivals and departures on both Monday and Tuesday were scrapped because the FAA won't allow Embraer E175s, the only commercial jets currently using Paine Field, to operate there in low visibility.

"It's massively frustrating," Smith said, noting that hundreds of travelers and all of his airport's venders suffered as a result.

"This should not have happened," he said.

All of the flights cancelled at Paine Field are operated by Horizon Air, which is a regional carrier owned by and flying exclusively for Alaska Airlines. Of the 300 flights Horizon operates on average each day, about 135, or 45% of them, are on the 76-seat Embraer E175.

"Really, it's a disproportionate impact on this E175 aircraft, which is common not only in our fleet but in a number of other regional airlines around the country, as well," Horizon CEO Joe Sprague said.

And Sprague fears even more smaller airports served by regional airlines like his will be affected as the telecom companies activate more high-speed 5G wireless service, as Friday's announcement by the FAA allows them to do.

"The activation of 5G towers near airports that took place last week was just the initial wave that AT&T and Verizon are planning and that there are multiple subsequent waves of activations, with each one likely to include smaller and smaller communities," affecting even more airports, airlines and would be air travelers, Sprague said.

The rollout of new 5G service near U.S. airports this month by Verizon and AT&T has been plagued by delays, confusion and controversy. After delaying the start of the high-speed wireless service late last year, the telecom giants were to switch on their 5G towers Jan. 5. But they agreed to another two-week delay as the FAA and airlines raised concerns about altimeter interference.

On Jan. 18th, after the FAA and the major airlines warned of potentially catastrophic flight disruptions, they reached an agreement with Verizon and AT&T to further postpone powering on some 5G network towers near some airports while the FAA worked to determine which aircraft altimeters are free of interference and would be reliable and accurate in 5G areas.

The FAA has now determined that 20 types of radio altimeters are safe and reliable in 5G environments and has thus approved 90% of commercial aircraft for takeoffs and landings in low visibility at most of the nation's airports.

In a statement Friday, the FAA said the wireless companies "have provided more precise data about the exact location of wireless transmitters and supported more thorough analysis of how 5G C-band signals interact with sensitive aircraft instruments. The FAA used this data to determine that it is possible to safely and more precisely map the size and shape of the areas around airports where 5G signals are mitigated, shrinking the areas where wireless operators are deferring their antenna activations. This will enable the wireless providers to safely turn on more towers as they deploy new 5G service in major markets across the United States."

But that doesn't provide the all-clear for many regional jets, including the E175 and its smaller sibling, the Embraer E145.

And these and other smaller, regional jets actually make up a significant portion of the commercial aviation network in the U.S.

Often times, when you're flying on a regional airline, you might not even know it. You likely booked the flight on a carrier like American, Delta or United. The pilots and flight attendants are dressed in those airlines' uniforms and the small planes are called United Express, American Eagle or Delta Connection.

But they're actually 17 separate airlines, including SkyWest, Horizon, Endeavor, and Republic, among others.

"The major airlines and their equipment, they're all too large to serve smaller airports that have fewer passengers traveling each day," Regional Airline Association President and CEO Faye Malarkey Black said. "So they partner with regional airlines and regionals specialize in operating smaller regional aircraft that's the right size to reach those customers that are traveling from smaller and often rural towns."

Malarkey Black says though they're not as well known, regional airlines fly 43% of the nation's departures, reaching 94% of the country. In fact, regionals fly the only commercial air service to 66% of U.S. airports.

But they were largely left out of the recent deal between the big airlines, the FAA and Verizon and AT&T over how to minimize possible 5G interference with airplane navigation systems.

"We do have a feeling and a sense that when this deal was cut, it was cut in consultation with the bigger users of the system," Malarkey Black said. "And for that reason, it didn't meet our needs."

The problems with 5G come at a very difficult time for the regional airlines, as they struggle to recover from the huge slump in air travel demand caused by the pandemic and try to cope with an acute shortage of pilots, flight attendants and aviation mechanics, many of whom are leaving the regionals to move up the ranks to the better-paying bigger airlines.

Malarkey Black said that's leading some of the major airlines, including American and United, to cut back on regional flights to smaller cities.

"When we're dealing with a scarcity in workforce that is forcing a capacity retraction, and history tells us that anytime the major airlines are forced to retract capacity that the smallest communities get hit first and worst and we're seeing that now with the pilot shortage."

The pandemic already forced four regional airlines to go out of business. To avoid further disruptions, the regional carriers are asking the FAA and the telecom companies to find a way to work through the 5G issues and approve more regional jets to fly into and out of smaller airports in bad weather.