This is another function I wrote to access the MET office API and obtain a 5-day ahead weather forecast:

METDataDownload <- function(stationID, product, key){
  library("RJSONIO") #Load Library
  library("plyr")
  library("dplyr")
  library("lubridate")
  connectStr <- paste0("http://datapoint.metoffice.gov.uk/public/data/val/wxfcs/all/json/",stationID,"?res=",product,"&key=",key)
  
  con <- url(connectStr)
  data.json <- fromJSON(paste(readLines(con), collapse=""))
  close(con)
  
  #Station
  LocID <- data.json$SiteRep$DV$Location$`i`
  LocName <- data.json$SiteRep$DV$Location$name
  Country <- data.json$SiteRep$DV$Location$country
  Lat <- data.json$SiteRep$DV$Location$lat
  Lon <- data.json$SiteRep$DV$Location$lon
  Elev <- data.json$SiteRep$DV$Location$elevation
  
  Details <- data.frame(LocationID = LocID,
                        LocationName = LocName,
                        Country = Country,
                        Lon = Lon,
                        Lat = Lat,
                        Elevation = Elev)
  #Parameters
  param <- do.call("rbind",data.json$SiteRep$Wx$Param)
  
  #Forecast
  if(product == "daily"){
    dates <- unlist(lapply(data.json$SiteRep$DV$Location$Period, function(x){x$value}))
    DayForecast <- do.call("rbind", lapply(data.json$SiteRep$DV$Location$Period, function(x){x$Rep[[1]]}))
    NightForecast <- do.call("rbind", lapply(data.json$SiteRep$DV$Location$Period, function(x){x$Rep[[2]]}))
    colnames(DayForecast)[ncol(DayForecast)] <- "Type"
    colnames(NightForecast)[ncol(NightForecast)] <- "Type"
    
    ForecastDF <- plyr::rbind.fill.matrix(DayForecast, NightForecast) %>%
      as_tibble() %>%
      mutate(Date = as.Date(rep(dates, 2))) %>%
      mutate(Gn = as.numeric(Gn),
             Hn = as.numeric(Hn),
             PPd = as.numeric(PPd),
             S = as.numeric(S),
             Dm = as.numeric(Dm),
             FDm = as.numeric(FDm),
             W = as.numeric(W),
             U = as.numeric(U),
             Gm = as.numeric(Gm),
             Hm = as.numeric(Hm),
             PPn = as.numeric(PPn),
             Nm = as.numeric(Nm),
             FNm = as.numeric(FNm))
    
    
  } else {
    dates <- unlist(lapply(data.json$SiteRep$DV$Location$Period, function(x){x$value}))
    Forecast <- do.call("rbind", lapply(lapply(data.json$SiteRep$DV$Location$Period, function(x){x$Rep}), function(x){do.call("rbind",x)}))
    colnames(Forecast)[ncol(Forecast)] <- "Hour"
    
    DateTimes <- seq(ymd_hms(paste0(as.Date(dates[1])," 00:00:00")),ymd_hms(paste0(as.Date(dates[length(dates)])," 21:00:00")), "3 hours")
    
    if(nrow(Forecast)<length(DateTimes)){
      extra_lines <- length(DateTimes)-nrow(Forecast)
      for(i in 1:extra_lines){
        Forecast <- rbind(rep("0", ncol(Forecast)), Forecast)
      }
    }
    
    ForecastDF <- Forecast %>%
      as_tibble() %>%
      mutate(Hour = DateTimes) %>%
      filter(D != "0") %>%
      mutate(F = as.numeric(F),
             G = as.numeric(G),
             H = as.numeric(H),
             Pp = as.numeric(Pp),
             S = as.numeric(S),
             T = as.numeric(T),
             U = as.numeric(U),
             W = as.numeric(W))
    
  }
  
  
  list(Details, param, ForecastDF)
  
}

The API key can be obtained for free at this link:
https://www.metoffice.gov.uk/datapoint/api

Once we have an API key we can simply insert the station ID and the type of product we want to obtain the forecast. We can select between two products: daily and 3hourly

To obtain the station ID we need to use another query and download an XML with all stations names and ID:

library(xml2)

url = paste0("http://datapoint.metoffice.gov.uk/public/data/val/wxfcs/all/daily/sitelist?key=",key)
XML_StationList <- read_xml(url)

write_xml(XML_StationList, "StationList.xml")

This will save an XML, which we can then open with a txt editor (e.g. Notepad++).

The function can be used as follows:

METDataDownload(stationID=3081, product="daily", key)

It will return a list with 3 elements:

1. Station info: Name, ID, Lon, Lat, Elevation
2. Parameter explanation
3. Weather forecast: tibble format