A channel model for channels with unknown state and partial state information available causally at the decoder is proposed. The model is based on arbitrarily varying channels (AVCs), and the partial state information is characterized by allowing the decoder to know a subset in which the average empirical channel must lie. A randomized rateless coding scheme for these channels is presented that achieves rates near that if the channel quality were known in advance under the assumption that the average channel can be estimated accurately. The same coding scheme can use used to multicast a common message to several receivers with different state sequences.