By Matt Frei
BBC News, St Paul
At one end of the Mississippi in New Orleans, journalists have been holding on to lamp posts and palm trees in defiance of Hurricane Gustav.
Mrs Palin may present some worrying unknowns for Mr McCain's campaign
At the other, quieter end of the same mighty river in the Twin Cities, journalists have been holding on to their notebooks in defiance of all expectations about this unconventional convention.
First we had the unscheduled storm in the Gulf of Mexico. At the same time, the country was digesting the news of the unexpected vice-presidential candidate, Alaska Governor Sarah Palin, and then came the news of her unscheduled grandchild.
How many surprises can you fit into a news-cycle? As Donald Rumsfeld might have put it: there are the known unknowns and the unknown unknowns. The worry amongst the delegates here is that Mrs Palin might actually belong to the latter category.
What we know already about Sarah Palin is extraordinary enough. It's difficult to know where to start. The moose stew. The former Wasilla beauty queen and Miss Congeniality who is a better shot than current Vice-President Dick Cheney.
Her five children? Unusual names: Track - apparently named because his mother is a keen runner - Willow, Bristol, Piper and Trig - the Nordic word for strength.
Or should we dwell on the fact that the woman who might one day command the most powerful army in the world, run three wars and fix the economic engine of the planet has only been abroad twice in her 44 years of life? Once to neighbouring Canada and last year to Kuwait and Germany for 36 hours to visit American troops.
Given the fact that John McCain, a cancer survivor, is already 72, the US needs to figure out how comfortable it is with Mrs Palin being a heartbeat away from the Oval Office.
In Berlin and Beijing, they are scratching their heads but in much of Republican heartland America, Mrs Palin is seen as Mr McCain's best hope for winning the White House.
James Dobson, head of the conservative group Focus on the Family and perhaps the most influential evangelical in the nation, has led a chorus of conservative Christians in congratulating Mr McCain on his choice.
Mrs Palin was the missing ingredient from the McCain dish, the evangelical spice needed to reassure sceptical fundamentalists that the candidate is indeed wedded to their causes.
In that sense, it was a masterful move that rallied the crestfallen base of the Republican Party.
Far from being an embarrassment, Bristol Palin's unscheduled child has been heralded as fresh blood for the family cause. Abortion was never an option. Bristol will marry her boyfriend Levi - his MySpace page, littered with expletives, has been removed from the website - and the crowded Palin family will soon have another addition.
Then there is the fact that Sarah has been hunting, fishing and shooting ever since she could walk. Add the fact that her husband Todd - known in Alaska as the "First Dude" - was arrested for driving under the influence of alcohol in 1986 and you get the picture
This family could not be more different from stuffy Washington. They are real, unvarnished Americans.
They make Cindy McCain, with her $100m (£56m) fortune and husky eyes rimmed by gothic mascara, look positively extra-terrestrial.
The Democrats' senatorial duo of Barack Obama and Joe Biden suddenly begin to appear quite staid.
Suspension of disbelief
John "Wayne" McCain, as he was known at the Annapolis Naval Academy, has rolled the dice with Sarah "Barracuda", as she was known on the basketball court. The movie rights are already being fought over. I'm thinking Juno-meets-Fargo-meets-Mr and Mrs Smith, topped off with Patton.
The media and political operatives are now busy vetting the Palin family
Whether the gamble pays off depends for now on the small armies of private detectives, campaign snoops and hacks descending on Alaska to hunt for more Palin facts.
From the New York Times to the McCain campaign, everyone is vetting Sarah Palin in full public view. The fortunes of the Republican ticket rest on what they might find. Then, it depends on how Mrs Palin will perform in the vice-presidential debates.
Once again, the Republicans are asking the world to suspend disbelief.
They did it with Ronald Reagan and George W Bush. Why not do it with a 44-year-old governor of America's second most distant state, a very modern woman with very conservative values? Does America want to go along for the ride?
The world is watching, reaching for the Prozac and wondering what other historic surprises are in store for the 2008 election.
Matt Frei is the presenter of BBC World News Americawhich airs every weekday at 0030 BST on BBC News and at 0000 BST (1900 ET / 1600 PT) on BBC World News and BBC America (for viewers outside the UK only).
Send us your comments in reaction to Matt Frei's Washington diary using the link below: