'self' captured by a closure before all members were initialized

You have two options, declare the properties as optionals, or initialize them with a default value (this means they will be non-optionals)

var temp: Float?
var condition: String?
var wind: Float?
var precip: Float?

or

var temp: Float=0
var condition: String=""
var wind: Float=0
var precip: Float=0

I'd hazard to guess that your are running into a concurrency problem. You are probably trying to access your object's properties before the asynchronous call to the DarkSkyClient returns (my apologies in advance if I got this wrong). i.e., the order of events is...

1. Weather object is initialized, setting temp to 0
2. Call to DarkSkyClient begins, runs in the background
3. Read temp variable - hey, it's 0!
4. Call to DarkSkyClient completes, sets the temp to the value you really wanted. Bad

So what you really need to do is switch to an inversion of control pattern:

class Weather {
    var temp: Float
    var condition: String
    var wind: Float
    var precip: Float

    init(forecast: Forecast) {
        temp = (forecast.currently?.temperature)!
        condition = (forecast.currently?.summary)!
        wind = (forecast.currently?.windSpeed)!
        precip = (forecast.currently?.precipitationProbability)!
    }

    static func getWeather() {
        DarkSkyClient(apiKey: "<api key>").getForecast(latitude: Utils().getLat(), longitude: Utils().getLong()) { result in

            switch result {
            case .success(let currentForecast, _):
                let weather = Weather(forecast: currentForecast)
                // Display the weather somewhere
                doSomethingWith(weather: weather)
            case .failure(let error):
                print(error)
            }
        }
    }    
}

If you're not familiar with developing with asynchronous APIs it's worth your while to read up on the subject; it can be very tricky (sadly, I don't have any recommendations for a good primer). Hope this helps!

For me, it's because I didn't call super.init() in the initializer.

class AnObject: NSObject {
    override init() {
//        super.init()

        let _: ()-> (Void) = {
            print(String(describing: self))
        }
    }
}