The emerging technology made these early forecasts possible, and its typographical constraints established capital lettering as the look of weather forecasting, said James R. Fleming, a historian at Colby College in Maine and the author of “Inventing Atmospheric Science.”

Because reports were shared only in closed circles in the years before the Internet, there was no real need to change the custom, even as the federal government’s weather forecasting services grew more sophisticated.

The Weather Service first proposed transitioning to mixed-case type in the 1990s, as the Internet was replacing old-fashioned Teletype, said Art Thomas, the meteorologist overseeing the project. But it took almost two decades for services relying on its reports to adopt technology that could handle the more modern type, and then more time to upgrade the Weather Service’s own systems.

Even so, the service is not rushing into anything. At first, only localized forecast discussions, public information statements and regional weather summaries will be in mixed case. Other widely circulated material, including severe weather warnings and other forecasts, will continue on in capitals until later in the year, Mr. Thomas said.

The new guidelines give forecasters the option to employ capitalization from time to time, as the rest of the world might, to add emphasis and urgency.

The transition to mixed-case type comes amid a series of changes, like dropping some abbreviations in published forecast discussions and clarifying warning categories, intended to make the Weather Service’s reports more digestible and effective for the Americans who come to them for everything from the afternoon forecast to extreme weather updates.

“There’s broader recognition at the Weather Service that we need to do more than just forecast the weather,” said Susan Buchanan, the agency’s acting director of public affairs. “Our warnings need to prompt the kind of decision that will save more lives and properties.”