I've done some research to try to pinpoint the conditions I'll need to prepare for psychologically, logistically, and gear-wise. Obviously, gear can be received or sent home (or further ahead to another trail town) at any resupply stop, so gear decisions do not have to be final.
As I understand, one needs to be prepared for nighttime temps down to as low as -7 C, or 20 F. These can occur in the mountains of southern and central California in late April to early June, or in the Cascades in September (assuming you're hiking north, as I plan to do). In July and August I'm expecting night temps to be around 8-10 C (45-50 F) most of the time. Daytime temperatures will be high most of the time, maybe even soaring as high as 40 C (104 F). Even on hot days, temperatures will quickly drop after dark, usually reaching the 40s at night. So one must be prepared for both heat and cold. One must have shoes that allow the feet to ventilate as much as possible during hot sections, or face blistered feet.
Rain is rare on the PCT, mainly striking in the Cascades in early autumn. That said, thunderstorms and downpours can happen nearly anytime. One must have a shelter of some kind. Wind appears to be common, too, making wise campsite selection important.
The sun will be shining most of the time, and it will be a killer. Much of the PCT is not in the shade. This dictates either reflective umbrellas (my personal choice) or clothing and a hat that covers all skin. Hikers who leave skin exposed and rely on sunscreen report that it does not work. I prefer staying cool by going shirtless in running shorts, protected from the scorching sun by my Golite Chrome Dome umbrella. This way I sweat less, need less water, and have fewer clothes to wash.
Water, or the lack thereof, is a serious issue for some parts of the PCT, notably the arid portions in southern California and some of Oregon's cattle country. One simply must know in advance where water will be and prepare accordingly. There will be waterless stretches of up to 30 miles. That could mean carrying 2 or more gallons of water. This requires advance planning and sufficient water bottles for these waterless sections. In other areas there is plenty of water, and one will hardly need to carry more than a liter of water at any time.
Some sources of water may dry up as the summer wears on. One may need to follow water availability reports in order to not receive a bad surprise. I'll post a link to these in a subsequent post.
Most sources say to treat all your water. Most thru-hikers report not treating much of their water. It takes a bit of experience to learn to discern a good water source from a potentially tainted one. Some form of water treatment (mechanical filter, electrical treatment, or chemical drops/tablets) will be needed. I'll be going with Aquamira drops and tablets, personally.
One will almost certainly encounter snow on the trail in southern and central California, and probably even into Oregon and Washington. There is snow even in low snow years, but the amount can vary widely. This necessitates following snow reports to know what amount of snow to expect. If there's an average amount or a lot, an ice axe is almost certainly necessary for safety on snowy slopes (as a self-arrest tool) in the mountains of southern and central California. Some people take crampons, but most never use them. Some people claim to self-arrest just fine with trekking poles. I personally will be taking an ice axe and Kahtoola microspikes (a traction device with spikes to put over one's shoes) for California's higher mountain areas.
There is no special gear you can have to make river crossings easier. Rope does not help and even paves the way for very dangerous situations. The best you can probably do is find a thick stick to help you through the water, and use a plastic pack liner to avoid getting important food and gear wet. Most or all difficult river crossings will be in the Sierra Nevada.
Most hikers report quite a bit of mosquitos along many parts of the trail. Their amount depends on seasonal factors. This necessitates the use of DEET spray, headnets, and some kind of netted sleeping enclosure.
Much of the PCT passes through black bear country. Bears are most active in stealing hikers' food in the Sierra Nevada, where bear canisters are now required by law. You'll apparently be fined by a ranger or escorted out if you don't have one. I plan to get the Bear Vault 500, possibly borrowing one from previous PCT thru-hikers. I'll pick it up at Kennedy Meadows and send it home when I get out of the Sierras. Other sections of the trail aren't as dangerous, but the best strategy is to avoid established campgrounds and not cook food in camp, but before you stop for the night. Hanging food apparently is not very effective against experienced bears. Most thru-hikers keep it at their side and report no problems, though mice can occasionally gnaw at your food bags.
Rattlesnakes, ants, and yellowjackets may also occasionally pose problems, but they will not affect one's choice of gear.