"I need winds under 30 miles per hour for a 12-hour period" is not an unusual request. However, the request usually continues, "Oh, by the way, I would like to have this information five to six days in advance." And this forecast is to determine winds at 29,035 feet, the top of Mount Everest. Now it is time for what I call the "heartburn forecast."
Finding when the winds atop Everest will be less than 30mph is a difficult task when you consider winds at 29,035' are in the jet stream. And this is a region where the strongest winds in the atmosphere are found, with common speeds ranging from 50mph to more than 150mph. How cold does this feel on your bare skin? The average air temperature right now on the Everest summit is -22°F and if you have a 60-mph wind, the wind chill factor is -62°F. This is one good reason why one can't climb in these winds. If the winds are at 5mph, the wind chill goes up to a balmy -34°F.
This climbing season has brought the usual strong winds which have been blasting the mountain for most of this month, so finding the weather window (the opportune time to make a summit attempt) is not an easy task. However, usually for a brief period during the month of May, the winds may blow under 30mph on the summit of Everest.
The million-dollar question is: How does one forecast this? What we need to have happen is a big ridge of high pressure that builds in the upper atmosphere over the Everest region. This high pressure is sometimes called a "blocking high" and it is found on weather maps at 500 millibars, which is usually found at about 18,000 feet. In the example below, the best spot (lowest winds) is usually found right where you see the letter H. In this example, the exact location is over the Texas Panhandle region. Under a blocking high, any weather systems that try to move in are usually diverted away.
The final difficulty with this forecast is this blocking high usually only lasts for several days and can quite often be followed by high winds and snow. Now you know why this is a heartburn forecast.
Michael Fagin, owner Washington Online Weather.