Winter Wave at Hollister
December 15, 2002    - by Harry Fox

With the winter storms upon us, the winds pick up, and wave days become possible at Hollister.  Hollister does not have consistently outstanding wave conditions like Minden, but you might be surprised at the number of days with usable wave.  Hollister has two kinds of wave days: pre-frontal wave, with winds out of the south to southwest, and post-frontal wave, with winds out of the north to northeast.

As a cold front approaches Hollister, there are usually strong winds aloft coming out of the south to southwest.  Winds on the surface often come from the southeast due to terrain funneling of the local mountains.  These pre-frontal winds may generate wave off of Fremont Peak and the Gavilan Range, with the best lift found south to southwest of Hollister or sometimes right over the town.  The wave may also set up over Gilroy, generated by the Santa Cruz Mountains, particularly when the winds are more westerly.

The challenge with pre-frontal wave is usually the accompanying storm.  The period during which the wave can be flown may be very short – the winds pick up, and soon thereafter it’s overcast and raining.  The ideal forecast to look for is a windy but relatively dry front, with rain forecast no further south than San Francisco.  This type of storm may carry enough moisture to form lennies or roll clouds over Hollister, without clouding over completely as the front passes.

The strongest example of pre-frontal wave I’ve seen at Hollister had the wind blowing straight up Runway 13 at 15 knots.  Gliders and towplanes would take off, and appear to climb away at a 30 degree slope because the wind only a few hundred feet up was at 20 or 30 knots.  Miguel Flores went to 14,000 feet right over the town of Hollister in the 1-34, and could have gone higher if he had oxygen on board.  The good conditions lasted a few hours, and then the cloud cover went from 50% to 100% in about an hour.  Soon after we got everyone on the ground and the gliders tied down, the rain started.  So, the key with these conditions is DON’T GET CAUGHT ON TOP OF THE CLOUDS.

Post-frontal wave days occur a day or two after the storm has moved through, when we find ourselves between the low to the east and a high to the west.  Strong north to northeast winds will come down the back side of the low, and generate wave in the north end of the Quien Sabe Valley, or off the ridges between Henrietta Peak and Pacheco Pass, or over Highway 152 near Casa de Fruta.  These type of winds are more likely to occur when the low pressure center passes to the south of Hollister, rather than passing right through Hollister or to the north of us.

I have had a three-hour flight in these conditions, with peak altitude of 7,000 feet, and others flying out of Hollister have gone to 12,000 feet over Casa de Fruta.  Tales are told of epic north-wind wave days, such as a flight from Calistoga to Monterey and back via Mount Diablo and Mount Hamilton, including a climb to 18,000 feet over Watsonville.  Post-frontal wave days are  usually cloudless, although I have seen at least one day with a classic stack of lennies right over Watsonville (too bad I couldn’t fly that day).  I think post-frontal wave days tend to be weaker than pre-frontal days, but it may just be that post-frontal wave is harder to work because of the lack of lennies or roll clouds to mark the wave.

For forecasting wave conditions, look first at the Forecast Discussion on the National Weather Service forecast page, at www.wrh.noaa.gov/Monterey/forecast.html .  If strong winds are projected a few days out, you will usually find out about it there.  If you want to see the isobars and wind vectors as forecast by the various numerical models, go to http://weather.unisys.com/index.html .  The one to focus on is the 850mb chart (which corresponds roughly to ridgetop level), but you should also look at some of the higher altitude charts to confirm that the wind increases with altitude.  There are also direct links to these charts on the BASA Weather webpage.  Another place to look is the jetstream forecasts at http://squall.sfsu.edu/crws/jetstream.html  (the forecasts are in the lower left part of the page).  A good pre-frontal wave day often coincides with a jetstream overhead out of the west to south.

When you get within 24 hours of the day in question, links to winds aloft forecasts can be found on the National Weather Service aviation page, or directly at http://aviationweather.gov/fdwinds/ .  Hollister is located about midway between San Francisco (SFO) and Fresno (FAT), but my experience is that our wind conditions are usually closer to the SFO forecast than the FAT forecast.  Since the ridges that generate our wave are only about 3000 to 4000 feet MSL, you should look mostly at the winds forecast for the 3000, 6000 and 9000 foot levels.  When the winds at ridgetop get in the 20 knot range, weak to moderate wave is possible.  Higher winds at 3000 feet are uncommon, except in the midst of a storm, but when they occur strong wave is more likely.  When the BLIPs become available for the day you plan to fly, you can also look at these for a more detailed winds-aloft forecast.

For both wave and thermal forecasting, there are lots of good links on the Hollister Gliding Club weather page, including surface forecast maps going out five days into the future.  Also, on the main Hollister Gliding Club webpage you can sign up for weather alert e-mails from Russell Holtz.  Russell usually sends out an e-mail alert a few days before good conditions are forecast.  If you don’t have the time or inclination to read the Forecast Discussions and other long-term forecast information, you can let Russell pay attention to the forecast trends for you.  When you get an alert from Russell, then you can dig deeper into the forecast details yourself, and try to pinpoint the best day of the period.

Harry Fox

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