The big problem with respect to multipath is that trees leaf out in the spring then drop their leaves in the fall, so the relative strength of direct and reflected signals can change with the seasons, depending on how many trees are in your neighborhood and where they are.
Also, multipath is more of a problem when it's windy, even if your antenna is indoors. When the wind isn't blowing, modern tuners can often adapt to whatever multipath they're receiving. But when the wind blows, those tree limbs bend and sway, changing the signals - often too rapidly for tuners to keep up. I usually get my worst reception on windy summer days. (My situation is worse than yours, though, because my antenna is outdoors. So on windy days, both the tree limbs and my antenna move around.)
Temperature variations don't have as much effect unless a signal isn't line-of-sight. In that case, you can get multipath interference from ground reflections, causing the dreaded "fading" as the ground warms or cools. If that happens, sometimes angling the mast to point an antenna upward a few degrees will help.
It's very hard to predict in advance how you'll fare. Your current antenna may be just fine year-round. But if you do run into problems with multipath, a higher-gain antenna will often help since it's less sensitive to off-axis reflections. For cramped attic installations, I usually prefer panel-style antennas to airplane-style ones.