Checking It Yourself
Close the door at night and run a flashlight along the bottom edge from outside while someone watches for light leaking through from inside - any gap wider than a sheet of paper means the seal has given up. Do the same along both side jambs and across the top; older torsion setups on pre-1990s doors in this area often have a top seal that was never upgraded past a single flap of foam weatherstrip, which does almost nothing in a cold-severe winter. Feel for cold air with your hand rather than just looking, since a seal can look intact and still have lost its compression against the slab. Check the corner welds on the bottom astragal retainer too - on aluminum track from the 1980s-90s era common here, that retainer corrodes and lets the seal twist out of its channel. If your garage shares a wall with living space, put your hand on that wall on a cold morning; if it's noticeably colder than the rest of the house, the door seal is very likely part of the problem, not just the wall insulation.
Season by Season
Fall is the window to deal with this before the hard freezes hit - in a cold-severe climate band, rubber that's already brittle in October will crack outright the first week it sits at single digits, so replacing a worn bottom seal in September or October costs you nothing in comfort now and saves a dead-of-winter scramble. Winter is when you'll feel every gap: a flattened seal on a north-facing door in this valley can drop the garage temperature enough to freeze anything stored against that wall. Spring is worth a second look because freeze-thaw cycling is what actually tears vinyl seals apart - the material contracts hard overnight and expands in the afternoon sun, and after a winter of that the seal often needs replacing even if it looked fine in November. Summer is the easy season here; it's the one stretch where a worn seal mostly just lets in dust and the occasional draft rather than real cold, so it's a reasonable time to order parts and schedule the swap without urgency.
