When I make a reference to the spread I'm usually talking about ground level highs for the day and the temp at 6k.
The last couple times I’ve been at launch I saw a lot of new faces. For the benefit of the newer pilots, I’ll do a quick break down of what I look for in SB as to temp spreads.
I normally don't put down my ice cream and make the drive out to SB for anything under a 20 degree difference between the forecasted highs for the day at ground level and the temp at 6k. That doesn't mean that SB won't work, it just means it's most likely going to be marginal with a little scratch and sniff thrown in. 20 - 25 degree spread is pretty good, and I'll put down the ice cream (but save it for later), call my fellow delinquents, and boogie to launch. 25-30 degree spread is awesome, as long as the humidity is in check, and I'll toss the ice cream, trip over my laundry on the way out the door, call all my delinquent friends and tell them their freakin’nuts if they don’t take the day off, and act like a 5 year old all the way to launch

Sound familiar…
Of course there are other factors; wind, etc, but that’s generally what I look for as to temp spreads. Tomorrow is a good example being that there’s almost a 30 degree spread in SB, however, the window of opportunity looks tight with high humidity and another weak front coming in tomorrow night – but I’m still watching it (ice cream in hand).
Bob, maybe you and a few others could pitch in for the noobs – what do you look for?