Is there a Democratic wave building for 2018?

American big wave surfer Nic Lamb in competition in December of 2016.
"Obviously no one knows what is going to happen in next year's midterm elections, but analysts who have watched congressional elections for a long time are seeing signs that 2018 could be a wave election that flips control of the House to Democrats."
Even with the caveat that the 2018 election is 545 days away and lots can and will change, that's a bold statement.
So, is it born out? Let's go through the indicators that have foretold past wave elections in the House.

1. History

With the notable exception of 2002, the first midterm election of a president's term is very bad news for his party. Democrats lost 63 seats in the 2010 election. They lost 54 House seats in Bill Clinton's first midterm in 1994. Republicans lost 26 House seats in the first midterm election of Ronald Reagan's presidency in 1982. (In 2002, Republicans actually gained 8 House seats as President George W. Bush's approval rating soared in the wake of the September 11, 2001 terrorist attacks and as the country looked toward war in Iraq.) The average loss for a president's party in midterm elections since 1862 is more than 30 seats, according to PolitFact. Democrats need only 24 House seats to retake the majority.

2. Presidential approval

There's no question that how popular -- or unpopular -- the president is can either mitigate or exacerbate the historical trends working against his party in midterm elections. This chart via Gallup is instructive:
"Since 1946, when presidents are above 50% approval, their party loses an average of 14 seats in the US House in the midterm elections, compared with an average loss of 36 seats when presidents are below that mark."
At the moment, Trump's approval rating sits at 40% in the latest Gallup poll. The highest Trump's approval ratings has been in Gallup polling since the start of his presidency is 45% in the three-day rolling average on March 11.

3. Generic ballot

One of the most common questions pollsters have been asking for years is some version of this: "If the election was held today, which party's candidate would you vote for in your Congressional district?" (That exact wording is how CNN asks the question.) It's a broad question that shouldn't be used to determine whether one specific member of Congress will win or lose. What the generic ballot has proven useful for over the years is as a sort of national weather vane -- suggesting which way the political winds are blowing, and how hard. (Alternative take: The generic ballot matters less than we think.)
Democrats, traditionally, have a slight edge on the generic ballot. So, if Republicans have a big margin in it or Democrats have a large -- double digit -- lead, it's usually interpreted to mean the wind is blowing strongly in one direction.
In the April CNN-Opinion Research Corporation poll, 50% chose the generic Democratic candidate while 41% chose the generic GOP candidate. That compares to a 46% Republican/45% Democratic generic ballot in the final CNN-ORC poll before the 2010 election and a 54% Democratic/39% Republican generic ballot in the final CNN/ORC poll before the 2006 election. (Democrats netted 30 seats in that election.)

4. Retirements/Open seats

It's long been true that it's harder to beat an incumbent -- unless that incumbent is horribly damaged by self-inflicted scandal -- than it is to win a seat where no incumbent is running. The more retirements -- or members leaving their seats for other gigs -- the more volatility in the House playing field and the better chance a wave takes out a whole lot of seats.
There are currently four open Democratic seats and six open Republican seats. The Cook Political Report, Cook's handicapping tip sheet, lists 9 more potentially open Democratic seats and 25 potentially open Republican seats. By comparison, there were 19 Democratic open seats and 22 Republican open seats in the 2010 election.

The takeaway?

Several of the pieces -- history, low presidential approval ratings, a generic ballot edge -- are in place for a Democratic wave election. What we don't know: What will the poll numbers for Trump and on the generic ballot look like in October 2018, and just how many Republican House members will head for the exits or run for higher office?
In short: The wave, if it's forming, is still way out in the ocean. But the conditions are right -- right now -- for it to get bigger and bigger.