[R] Kalman Filter with dlm

[Pearl]

I have built a Kalman Filter model for flu forecasting as shown below. 

Y - Target Variable X1 - Predictor1 X2 - Predictor2 

While forecasting into the future, I will NOT have data for all three
variables. So, I am predicting X1 and X2 using two Kalman filters. The code
is below 

x1.model <-  dlmModSeas(52) + dlmModPoly(1, dV=5, dW=10) 
x2.model <-  dlmModSeas(52) + dlmModPoly(1, dV=10, dW=10) 

x1.filter <- dlmFilter(c(train$x1, rep(NA, noofsteps)), x1.model) 
x2.filter <- dlmFilter(c(train$x2, rep(NA, noofsteps)), x2.model) 

Now, I am forecasting Y using the predicted X1 and X2 as below 

pred  <- cbind(c(train$x1, x1.filter$f[260:312]),c(train$x2,
x2.filter$f[260:312])) 

Y.model   <- dlmModSeas(52) + dlmModReg(pred, dV=0.5, dW=c(10,0.05,0.1)) 
Y.filter  <- dlmFilter(c(train$Y, rep(NA, noofsteps)), Y.model)   

I have two questions 

1) The flu season usually starts in early September and it usually peaks
during the last week of December or the first week of January. The following
are the stages 

Stage1 - September, October 
Stage2 - November, December 
Stage3 - January, February 
Stage4 - March, April 

The dlmModSeas captures this seasonality very well. But what do I do to
forecast an off season wave that has the same stages as above. Say it starts
in May and it peaks at end of August or early September. I am able to get
the information of this off season wave from the user during the start, i.e.
in May. How do I introduce this change in seasonality, I think I have to
change $JGG and introduce $X but I am not sure how to do it. Please help. 

2) Is there a better way to do this forecasting? Please Advise

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