# Updating meteorological forecasts, part 1

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As Mark Twain said “the

art of prophecy is very difficult, especially about the future” (well, actually I am not sure Mark Twain was the first one to say so,

but if you’re interested by that sentence, you can look here). I have been rather surprised to see how Canadians can

be interested in weather, and weather forecasts (see e.g. here for a

first study). For instance, on that website (here),

we can have forecasts of the temperature (but also rain and wind, which

is interesting when you ride a bike) over the next week, almost on an

hourly basis (I show here only GFS forecasts (*Global Forecast System*,here), which is quite standard, see here or there, but other meteorological models are considered on the website)

As a statistician (or maybe more as a bike rider) I was wondering if I

should trust those forecasts… and if a mistake today means an error

tomorrow, too…

So, to look at this problem, Vincent (alias @Vicnent)

helped me (again, since he helped me already to get backups of betting

websites during the soccer world cup, in June) to make backups of those

tables…

- A descriptive study

For instance, if we look at forecasts made on the 21st of October (00:00) we have the following for the temperature in °C, for different time horizon,

6 hours latter (21st of October (06:00)), we have the following

On the other hand, consider forecasts made for the 25th of October (11:00) we have

while for 3 hours later (25th of October (14:00)) we have

and again for 3 hours later (25th of October (17:00)) we have

So obviously, 5 days earlier, there has been a bump of optimism, or

perhaps simply a typing mistake since we jump from 4 before and after

to 14 during 6 hours^{*}.

If we can expect to have different predictions for different days (even

within the day since it is usually cooler during the night), we can try

to see if bumps, or forecast errors can be understood.

Here, it is difficult to observe the scatterplot of temperature,

That graph was based on the following points, with here the scatterplot of prediction made on the 21st of October (00:00)

with a smoothed version below (using standard splines),

or, if we consider forecasts made for the 25th of October (11:00) we have

with again, a smoothed version,

The code is the following,

donnees=read.table("http://perso.univ-rennes1.fr/ arthur.charpentier/pred-meteo2.txt") x=donnees$DATE1+donnees$HEURE1/24 x[x<15]=x[x<15]+31 y=donnees$DATE2+donnees$HEURE2/24 y[y<15]=y[y<15]+31 z=y-x h=donnees$TEMPERATURE D=data.frame(x,y,z,h) X0=sort(unique(x)) Y0=sort(unique(y)) Z0=sort(unique(z)) matY.X=matrix(NA,length(X0),length(Y0)) library(splines) for(i in 1:length(X0)){ dd=D[D$x==X0[i],] ax=dd$y ay=dd$h a=data.frame(ax,ay) ra=lm(ay~bs(ax,df=5),data=ba) X=Y0[(Y0>min(ax))&(Y0min(ax))&(Y0 So, let us look at levels of iso-temperature, as a function of the date the prediction is made (the

x-axis) and the horizon (they-axis). First, we we fix the date of the prediction, we get (from the code above)and then, when we fix the horizon of the prediction

It looks like the small

heat wave

we experienced at the end of October has been perfectly predicted...

And here, it looks like the only pattern we can observe is thestandardevolution of temperature: there are no bumps due to forecasting problems..

- Modeling predicted temperatures
Let us use some additive or multiplicative models, i.e. either

where denotes the date the prediction was made and the date the horizon, or

This

was easy with the gam framework... For instance, consider the following

code (some regressions mentioned below will be used later on)library(mgcv) library(splines) REG1=gam(h~s(x,y),data=D,family=gaussian) REG2=lm(h~bs(x)+bs(y),data=D) REG3=gam(h~s(x)+s(y),data=D,family=gaussian) REG4=gam(h~s(x)+s(y),data=D,family=gaussian(link="log")) REG5=gam(h~s(y),data=D,family=gaussian) D$e=residuals(REG5) REG6=gam(e~s(x,y),data=D,family=gaussian)Here is the prediction we had with the additive model

and for the multiplicative model

(colors are different because of the log scaling but the shape is similar).

For the component on (in blue), we have, with the additive model

and for the multiplicative model

On the other hand, for the component on (in red) we have, with the additive model

and for the multiplicative model

Note that we observe something which looks like the

truetemperature (we actually had)So

finally, on average, those predictions are consistent with the true

temperature we experienced. But so far, we cannot say anything about prediction

errors. If we consider a bivariate spline regression, i.e.for some smoothed function, we have, again, the same kind of predictions,

It

looks like what we've seen before. And because we have those horizontal

lines, only the horizon should be considered.... So let us study the

following modelwhere we obtain the following prediction

(which is similar with previous graph) but if we fit a spline on

residualsobtained from that regression, i.e.where

we have the following distribution for

This

is where we can see prediction errors. We can observe that meteorologists

have been a bit surprised by the heat-wave we observed just after the

25th. But let us wait a few more days to get more data, and perhaps

we'll have a look at something thatIdo care about: prediction of rainfall....

^{*}

perhaps I should mention that we do not have backups of tables, but backups of

html pages... then I use simple linguistic tools available on R to

extract the information where it should be....Toleave a commentfor the author, please follow the link and comment on their blog:Freakonometrics - Tag - R-english.

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