...in about a 2-3 week period.  I am looking to go fishing in Northeastern MO and the reports are showing the lake temps at about 50-55 as of April 7th.  I'm not sure if it's raised any since then but the forecast for the next week has a good 4-6 days in the low to mid 80's and another day or two in the upper 70's.  How much could this affect surface temps and the chance of better fishing?

It depends on the size, depth, layout, water clarity, and bottom composition of the lake.  If it is a small lake and the weather stays warm for the next 2-3 weeks I think (not saying that I know for sure) that it could warm up as much as 10 degrees of a little more.

This lake is pretty large...18,000 acres

ok.. I just saw how big your lake was... in my 10acre pond which is 15' deep... it went from 60 to 71(checked last night)  in 2 weeks.. .we've just had a spell of high days ranging from mid 70s to low 80s... so it could go up considerably, then again i have no idea on an 18000 acre lake

From the last week of warm weather I would say you will be in the 60's.

I would locate warm water in the backs of coves and secondary points.  This is where you should locate most of your fish.

Once it hits 62 the males will be farthest up cove fanning beds. Until then the bass will be feeding increasingly aggressively together, but preferring slow easy to catch baits. I'd guess top average temp on areas most exposed to sun to hit 58, shallow open sunny flats maybe reaching 62-65. If so a suspending jerkbait like an X-Rap ought to produce, paused a lot. That bait and others like it should be good now at present temps.

Jim