First of all I would use cell2mat() to concatenate your cell array tempTOT_hot into a 261x285 array (double)
Then everything is much easier. You can make "column-wise" subtraction of arrays with size 1xn and a mxn.
T1 = rand(1,285).*100;
T2 = rand(261,285).*100;
T_diff = abs(T2-T1);
out = T2;
out(T_diff<20) = NaN;
Not sure if it is the output you're looking for.