Default weather / Modified weather can be used in cases where the reporting area is relatively small, meaning that most of the weather phenomenae are about identical. When entering NBC1's or 4's, you can press either "Default Weather" which will copy the left part of this form to the Y / Z fields, or "Modified Weather, which will present this form. You can then change the relevant details in the right hand part of the form, and press Exit, which will copy the data into the Y / Z fields.