As Internet-of-Things (IoT) and its applications are increasingly popular, where diverse multi-scale sensors and devices are seamlessly blended for ubiquitous communication infrastructure, broadcast operation still plays an essential role in scalable information dissemination to enhance information accessibility and availability. A unit-disk signal propagation model has been implicitly assumed and extensively applied to prior broadcast protocols, but we need to relax this assumption in reality. In this paper, we propose a transitional region aware broadcast protocol, called TCast, in variable wireless link qualities due to the signal propagation effects and non-uniform radiation pattern from the omni-directional antenna. The TCast is a stateless protocol and consists of two major operations, forwarder search and probabilistic rebroadcast. A sender neither maintains any neighbor information nor searches for a set of forwarders, but broadcasts a set of Beacon packets followed by a single Data packet. The sender repeatedly conducts the broadcast operations depending on the number of rebroadcasted packets overheard. Each receiver independently makes its own rebroadcast decision based on the number of received Beacon packets. A network-level random backoff mechanism is also proposed to avoid any packet contentions and collisions. The transitional region and its corresponding probability of packet reception are further investigated through a simple mathematical analysis. Extensive simulation experiments are also conducted using the OMNeT++, and simulation results indicate that the TCast shows competitive and scalable performance and is deployable in time-varying packet reception rates at receivers.