Predicting a time-series based on level, season, and cycle

I have time-series data for stock price. A sample of dataset is given below:

DATE    STOCK_PRICE
1/7/2012    26
1/8/2012    31
1/9/2012    28
1/10/2012   38
1/11/2012   36
1/12/2012   20
1/13/2012   23
1/14/2012   23
1/15/2012   1.1
1/16/2012   1.4
1/17/2012   1.9
1/18/2012   2.3
1/19/2012   5.8
1/20/2012   39
1/21/2012   20
1/22/2012   25
1/23/2012   31
1/24/2012   40
1/25/2012   29
1/26/2012   20
1/27/2012   24


Now, I'm trying to get UCM (using PROC UCM), such that it fully captures the pattern of the series. PROC UCM in SAS breaks the series into LEVEL, SEASON & CYCLE, then tries to predict the series based on these 3 factors. From the data & graph you can see at some point the data pattern breaks down completely. So UCM can't able to capture the sudden drop in this series. I can use DUMMY variables to do that. But don't want to use it as it is very difficult to explain the nature of dummies.

Can anybody can help me to find how I can predict the series using LEVEL, SEASONAL & CYCLE (or some other parameters)? I'm using PROC UCM to get it, but some R equivalent would also be helpful. If there are other methods to capture this pattern, those would be helpful as well.

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Stock prices simply do not behave like this. It might help if you revealed what these numbers really represent. – whuber Oct 13 '12 at 21:19
These are not stock prices & the drops are also not so frequent. These numbers are just created for an example. I just wanted to know how to capture sudden drop in time series (as shown here)without using dummy. The series which I actually using is having exactly similar pattern I showed in the picture. The problem is I can't share the data here. – Ari Oct 14 '12 at 5:19
Your time series looks seasonal, you may use a square wave ( $t \mapsto\alpha \left| cos(2\pi w t + \phi) + \beta \right|$, where $\alpha$ is the amplitude, $\phi$ the phase ..etc). You can try and remove this seasonal effect and analyse the erros (gaussian or not?) – D.Khireche Mar 6 at 9:31
Thanks DaK! In my data the drops are at some specific dates. So, I used dummies to capture those drops. The results came ok. I'll try your method & see if things improve. – Ari Mar 12 at 16:35