Final answer:
The lowest weather allowable to be forecast for the second alternate depends on specific guidelines or regulations, typically based on visibility and cloud cover conditions.
Step-by-step explanation:
The lowest weather allowable to be forecast for the second alternate under 17347 would depend on specific guidelines or regulations set by the organization or agency responsible for the forecasts. However, as a general rule, the lowest allowable weather would typically be determined by the visibility and cloud cover conditions. For example, some organizations may require a minimum visibility of at least 1 mile and a cloud base above a certain altitude for a forecast to be considered acceptable.
It's important to note that these guidelines may vary depending on the purpose of the forecast and the specific location. Different airports or airfields may have different minimum weather criteria for second alternates.