It’s hard to imagine anything that could spoil the Washington Nationals’ two-games-to-none lead in the franchise’s first World Series, but here’s one possibility: Nationals Park does not have a roof.

Planet Earth does not care that the Nationals are coming home on the verge of capturing the District’s third professional sports championship in three years, following the Capitals and Mystics. It will rain whenever it pleases. Right now, there’s a possibility of that on Saturday night, in the middle of a potentially decisive Game 4.

The forecasters at the Capital Weather Gang are predicting a 30 percent chance of rain that night, with temperatures in the mid-50s and cloudy skies. Bringing a sweater and raincoat would be worth your while. Noncollapsible umbrellas too big for 16-inch-by-16-inch-by-8-inch bags are not allowed in the stadium.

But if things go haywire and Washington gets more severe weather than anticipated, Major League Baseball has a protocol for a World Series rain delay, postponement or even a suspended game.

During the regular season, staffers inside the commissioner’s office work with each ballclub to plan ahead for bad weather and schedule makeup dates when games get washed out.

The home team, according to the MLB rule book, is the “sole judge” of whether a game should be started because of “unsuitable weather conditions or the unfit condition of the playing field.” That means the home team can decide to delay a game, too, and for how long. Once teams exchange lineup cards before the first pitch, only the umpire-in-chief, or home plate umpire, can suspend, call off or resume a game, and he can hold out that suspension indefinitely “so long as he believes there is any chance to resume play.”

In the postseason, though, that all goes out the window. MLB Commissioner Rob Manfred, not the home team, has the ultimate authority to delay, postpone or suspend games.

“Our staff monitors the conditions in consultation with weather services to determine whether there is a window in which to play a nine-inning game, while maintaining competitive integrity on the field,” an MLB spokesman told The Post. “We track the playability of the field and make our best judgment regarding the safety of players and the comfort of fans.”

That philosophy had long been major league custom but was only codified in 2009 after Commissioner Bud Selig suspended Game 5 of the 2008 World Series between the Phillies and Rays in the sixth inning.

It was the first suspended game in World Series history. Selig told both teams ahead of the first pitch that he would not allow a World Series game to be decided by rain, and sure enough a storm settled over Citizens Bank Park in Philadelphia and sunk the infield “basically underwater,” Phillies second baseman Chase Utley said at the time.

After pulling both teams off the field, Selig said the game was on hold indefinitely until the rain lifted. The game resumed two days later with the Phillies up to bat in the bottom of the sixth. They won the game, 4-3, to clinch the series, four games to one.

That offseason, MLB franchise owners voted to install Selig’s policy permanently and decided that all postseason games, including tiebreaker games added to the end of the regular season, could not end because of rain. Those games must be resumed and played to completion at the same site, no matter how many innings have been played or what the score is at the time.

So what does that mean for the Nationals and Astros?

Well, if it rains more than expected Saturday and the game is delayed, it could be a long night. MLB officials are disinclined to suspend any game, let alone a World Series game. The Capital Weather Gang puts the odds of a stoppage at 15 percent.

A postponement is the final option, but again, that’s unlikely based on both MLB precedent and the forecast. Sure, playoff and World Series games have been postponed before, but officials in recent years have been known to push back or move up start times — sometimes by even more than an hour — to find a dry (enough) window for a full nine innings. In 2016, for example, MLB started Game 2 of the World Series between the Cubs and Indians an hour earlier than planned to avoid projected overnight showers. (Game 7 that year was delayed for 17 minutes because of rain, at the end of nine innings and with the score tied at 6.)

The Capital Weather Gang puts the odds of a Game 4 Nats-Astros postponement at just five percent. Still, if that were to happen, Game 4 would probably get pushed back to Sunday and Game 5 (if necessary) to Monday, eliminating the teams’ day off to travel back to Houston, if necessary.

The most likely bad-weather scenario at this point: It will rain a little, fans and players will get wet, and the game will go on.

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