Maybe I have this wrong but TEMPO in itself is simple enough - something temporary that doesn't warrant inclusion in a more broad forecast. It's what follows the TEMPO that needs proper deciphering so awareness of the temporary conditions is understood. For example, there might be a temporary forecast of SHRA for an hour or two possibly accompanied by 21025KT G45 and further to that there may also be a probability estimate - PROB40 for 40%. All the while, the main forecast over (lets say) an 8 hour period might be 26015KT 9999 FEW015 SCT023. Can I assume that because the TEMPO is noted that the main forecast would no longer use NOSIG?
Have I missed something? I feel a school day in the making............
TEMPO in a TAF means occurring for less than half of the period specified after it, so, as you say, something like showers with conditions temporarily notably different than the general forecast for that period. Conditions are expected to revert back to those forecast unless a permanent change is expected, in which case BECMG is used.
PROB40 means there is a good chance but still not warranted in the main forecast. PROB30 means a remote chance worth mentioning but probability is low. Remember, a TAF only applies to within 10 miles of the airport.