Ed Mahmoud wrote: I always thought the very late night/early morning showers and occasional t-storms that form just offshore the Texas coast in Summer had something to do with convergence offshore due to difference in frictional effects, and that this weakened during the day as the sea-breeze set up.
But I never actually heard that anywhere official.
That's also a contributing factor - an increase in speed convergence in onshore flow. Essentially, when you have moderate to strong onshore flow, air "piles up" along the coast - i.e. more air flows into the coastal zone than departs from it since (as you touched upon) shoreward flowing winds move faster than (although in the same direction as) the corresponding winds over land due to frictional effects.
But irrespective of whether there is onshore flow or not, a marine air mass is more unstable at night.