The weather will be crucial to your success on Everest. You will find yourself frustrated by it's power to make you or break you. When you are ready, the wind might be yelling above you, when you decide to wait - the sun might shine from a clear blue sky.
You will lay at night in camp 2 and listen to the roar of a distant train -the Jet wind. You will turn your face countless times towards the ridges of the mountains, trying to make some sense and predictions of the dancing white tails of snow. To go or to stay - the decision will be like gambling red or black in Las Vegas. Your odds of the correct move just as poor.
Going too soon will mean wearing yourself down, having to climb back all the way to BC and your chances lost until you regain strength for a new attempt. Waiting too long means happy Summiteers greeting you on their way down while the weather is turning worse on you.
Expeditions sometimes share weather reports, subscribed to from various meteorologists. Daily weather reports are costly and therefore more frequent close to the time of the summit push. ExplorersWeb now provides daily customized weather reports free of charge. Expeditions download them on satellite phones and distribute them to those without computers.
Whilst weather reports can be quite accurate, there is a local weather system on the mountain that they can't foresee. Therefore, a day with reported mild wind conditions could very well turn into a blowing frenzy, or reported strong wind nowhere to be seen.
The best way to read weather reports has always been to look for extended changes in the weather pattern. 4-5 following days of high or low figures is often a good chance of good or bad weather. Use the information to rest or to climb.
The Jet stream is the main concern on Everest weather. This westerly wind will have a large impact on your choice for a summit day.
We have experienced C2 in total calm, while the jet wind roared at the summit with a force of more than 100 miles an hour (50 m/s). In these conditions, the sound is that of a jet-engine and gusts drop down at 50 mph (25 m/s).
Sometimes, the jet wind will rise and give a short period of calm and a summit attempt might be possible. A summit bid in these conditions is however hazardous. You will encounter people making the summit in perfect calm one day, while the next day others hardly make it above C4.
At times the jet wind is gone, but the weather is still unstable. At the time of the 1996 accidents there were just those unstable weather patterns. Dark blizzard clouds emerged from the valley. One of the strongest teams on the mountain (the IMAX) actually returned from C4 that day, only to meet a large group of people going up in the high winds and deteriorating weather. Well, the rest is history.
Don't climb if the weather forecast is jumping back and forth and/or there is snow and strange cloud formations. Use common sense. Don't climb if it is snowing. Sherpas go back to sleep if there is a heavy snowfall, and so should you.
What we primarily look for is a prolonged period (4-5 days) of stable weather with the jet far away. This is called "the window".
During our four Everest expeditions the window has come every year at about the same time, around the 23rd of May and has lasted for about a week. To find a window, it is valuable to look for the Monsoon starting to move north in the Bay of Bengal. The weather report will tell you when that occurs. This powerful weather system will pressure the jet wind to the north and create a period of perfect weather.
Don't wait too long though. As the Monsoon hits the Khumbu valley with heavy snowfalls, you should already be back in Kathmandu.
A problem with waiting for the window could be other climbers. Most expeditions schedule their attempt for the 10th of May, and by the 23rd at least half of the expeditions will have returned home. Most commercial expeditions have an end date around the 20th. When you plan for your expedition - make sure you have the resources to stay until the permit ends (1st of June).
Sometimes there is a period of weak winds and good weather in the beginning of May. You should be ready for summit attempt already from the 1st of May to take advantage of this.
If you don't summit on this first, early attempt, there will be plenty of time to go down the valley for a week's rest, and then head back up for a new attempt in the end of the month.
The wind force forecast should definitely not exceed 40 mph (20 m/s) when going for the summit. You could maybe make it in 50, but then you are extremely exposed for the wind-chill and exhaustion. In these conditions you will hardly make the summit without frostbites - or worse.
Look for less than 20 mph (10m/s). Even if that doubles, you will have a good chance of making it.