WASHINGTON — The tanks are staged and ready to roll. Fencing and barriers are up. Protective metal plating has been laid out on Washington’s streets.

And more than 6,000 troops are poised to march near the National Mall to honor the Army’s 250th anniversary today, which happens to be President Donald Trump’s 79th birthday.

With preparations well in hand, one big unknown is the weather. Rain is in the forecast, so there is a chance the parade could be interrupted by thunderstorms.

White House spokeswoman Anna Kelly said Thursday that rain or shine the parade will go on. But it could be delayed if there is lightning.

“No matter what, a historic celebration of our military service members will take place!” Kelly said in a statement.

Daylong festivities celebrating the Army are planned on the National Mall — featuring NFL players, fitness competitions and displays — culminating in the parade, which is estimated to cost $25 million to $45 million. The Army expects as many as 200,000 people to attend.

A special reviewing area is being set up for the president, where he will be watching as each formation passes the White House.

Here’s what to expect at the parade today:

A total of 6,169 soldiers as well as 128 Army tanks, armored personnel carriers and artillery will parade before the president and viewers, while 62 aircraft will pass overhead.

The parade will tell the Army’s story, starting with the Battle of Lexington — the first battle of the Revolutionary War — and move all the way to present day.

Each conflict will have 150 troops in period costume, followed by a section of hundreds of troops in modern-day dress. For the past several weeks, Army planners have been working out how to get it timed to exactly 90 minutes, Army spokesman Steve Warren said.

Planners first tried marching troops five across and 12 deep — but the parade ran long. To get it down to the exact time, each section will have soldiers marching seven across and 10 deep, Warren said. That means, for example, the Civil War gets exactly three minutes and 39 seconds and World War II gets 6 minutes and 22 seconds.

Then there are the tanks. For fans, 8 minutes and 23 seconds into the procession, the first World War I Renault tank will appear.

Compared with today’s tanks, the Renaults are tiny and almost look like a robotic weapon out of “The Terminator.” But they were groundbreaking for their time.

The first aircraft will fly over starting 13 minutes and 37 seconds into the parade, including two B-25 Mitchell bombers, four P-51 Mustang fighter aircraft and one C-47 Skytrain.