You are likely experiencing an intermodulation mix, probably 3rd or 5th order like 2A-B = C or 3A-2B = C. You can find some IM caculators on line. A good calculator will consider the effects of modulation and bandwidth , some calculators simply do simple math, so if you get no direct hit , remember your receiver frequency is C, so if a product is C plus or minus 100 KHZ, it is probably indicitative of a "hit".
Ok how to fix? If the fire transmitter is on same site as the weather, The IM could be occurring in one or both those transmitters. However more than likely, it is occuring inside your receiver itself due to overload by the stronger of the weather or fire transmitter. Assuming you have no preamplifiers external to your receiver which can aggravate the IM, you will have to attenuate one of the two signals with a notch filter. The effectiveness of this filter is inversely proportional to the seperation in KHz between the undesired TX and the desired RX frequency.
If you can post the exact frequencies involved, I can help . Otherwise remove any preamplifiers and try using an attenuator if that option is acceptable.