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The North Pacific High is clearly the dominant synoptic feature in this composite image. Here the University of Hawaii's Meteorology department has laid a 12 hour surface pressure chart - produced with the US Aviation Model (AVN) - over the top of an infra-red satellite image to show the relationship between cloud and pressure over the ocean between Los Angeles and Hawaii. From July 1, a fleet of swift and narrow yachts will pay homage to this natural wonder by sliding to Honolulu from Los Angeles, celebrating the 41st Transpac. Dodging
high pressure Californian
navigator Stan Honey routed Roy Disney’s June 24, 2001 Look
at any synoptic chart of the North East Pacific and one feature dominates: A big
circle of high pressure. Imagine
a great swirling of winds clockwise around this bubble of heavy, dry air, and
you can begin to understand why it’s all downwind to Hawaii. But do be careful! Images of trade winds, puffy clouds, flying fish and dorado flood the mind of those blissfully unaware of the subtle nature of an ocean yacht race. Subtlety is key in this race, and those who ignore the delicate balance between trade winds and calm, short cuts and long routes, will pay dearly at the finish. For starters, westerly winds are encountered at the start, more often than not. As Stan Honey explains, the California desert sucks and creates a powerful seabreeze come afternoon - a heat low in weather terminology - which helps to twist winds flowing over the top of the North Pacific High around to the west. Yachts leaving Los Angeles typically bash to windward until they round the Catalina Islands - the first waypoint in the race 25 miles from the start. As evening progresses, and yachts move further offshore, synoptic winds take over, veering to the north and the real reaching race begins in earnest. Cutting the corner with care Playing the trade winds, as they veer from west off California to east off Hawaii is what the Transpac is all about. At the heart of this phenomenon is the North Pacific High, a bubble of heavy, dry air which Honey describes as semi-permanent, but liable to bounce around. Typically, over the course of a week, this high pressure bubble will move to the north and strengthen before a low pressure zone moving across the North Pacific pushes the high south and east, weakening it. Then, following this typical scenario, the frontal cloud band of the low will split the high into two zones. Finally, the front moves over the US west coast and the Pacific High re-combines, migrates northward and builds again. It is very important, says Honey, to figure out what the high will be doing when your yacht is half way to Honolulu. The great circle route from California to Hawaii typically cuts the corner of the North Pacific High, so if navigators stick to the shortest route - i.e. the great circle - they run the risk of sailing into calms underneath high pressure. So the first trick is to sail south of the rhum line to stay within the wind band - but not so far south that miles are sacrificed to competitors. Complicating matters is the fact that a ridge of high pressure usually extends to the south-east. It is not an obvious ridge, like those projecting off the South Atlantic high, or the Azores’ High, but it is a ridge none the less. According to Honey, choosing where to cross this ridge is critical, because once a yacht is 'slotted in', there is no way skipper and crew can change position north or south without losing ground to other boats. Once the ridge of high pressure is crossed, winds veer aft, flowing from the east, north east, dramatically favouring 'starboard pole'. This wind angle is so heavily favoured, that to jibe and sail south would mean a yacht would have to travel south-east, on a heading away from the Hawaiian islands. Remember, modern yachts refuse to sail directly downwind - the slowest point of sail - preferring to jibe from broad reach to broad reach and travel at maximum velocity. For this reason, Honey describes yachts beyond the high pressure ridge as being caught in a 'slot'. "You pick your slot, and once you cross the ridge you are stuck with it," he says. And it typically takes a couple days from the time of crossing the high pressure ridge until a navigator can confirm the success, or otherwise, of a decision made only hours after the start. Unless a sub tropical squall crosses your path and winds back to the left, or veers sufficiently to allow the yacht to jibe, a yacht will be unable to get to leeward - towards stronger winds - without losing valuable time. If your are too far south of the rhumb line, it is unlikely that you will sail fast enough - despite stronger winds - to make up the extra miles required. In addition, boats too far south will eventually find themselves on a dead down-wind course for Hawaii, unable to take real advantage of reaching wind angles. Although, in the latter stages of the race, as winds veer to the east, yachts can jibe port or starboard and begin to shift north or south without losing speed and too much distance. Approaching Hawaii During the last third of the race, Honey reckons yachts should make their final approach to the islands from the north on port pole because the wind at this point is as far to the right as it will ever be, and therefore port pole is favoured. At the same time, be prepared to play wind shifts associated with tropical squalls blooming north from the equatorial trough. Creating bigger shifts are tropical depressions and inverted troughs sneaking east to west along the inter-tropical convergence zone. During last year’s Kenwood Cup tropical depression Daniel drifted north of the island chain disrupting already weak trades to bring a surprisingly calm start to the international yachting series - and this despite 70 knot winds close to its centre. Depressions and tropical storms are obvious players, but small inverted troughs tend to complicate matters equally well. Winds ahead of these tropical low pressure 'bulges' tend to back to the left and strengthen, while winds behind them tend to veer right and lighten. Honey says keep a sharp eye for inverted troughs - tropical bulges in isobar maps which tend to bend to the north, with low pressure south of the bulge. If the trough is approaching from behind, plan to get down in front of the trough, take advantage of strengthening left-shifting winds, and approach these islands from the south on starboard pole. If the trough migrates north and crosses the race course at right angles, stay well away to the north, try to avoid the light winds immediately behind and approach the islands on port pole. Local
knowledge Once yachts enter Molakai Channel, local knowledge comes to the fore. Many serious competitors in the Transpac have raced in Kenwood Cups over the years and understand how these volcanic islands influence trade winds, and how local currents create favoured routes to Honolulu. After all, little things can make a big difference in the event of a tight finish. Honey says try and hit the right hand corner of the channel and be warned: You will be headed, so you want to hit it perfectly. Honey says jibe to starboard near Ilio, on Molakai's north-west corner, and try to make it across the channel in one jibe. If it is windy you will likely succeed. But you will get headed as you approach Oahu. Honey notes that there is typically accelerated wind on north shore of Molakai,generally no further east than Molakai Light. Get close sail through the accelerated winds, then jibe back down and get close to Ilio - a point on the north-west corner of Molakai. Wiley skippers can typically make it to the finish on starboard pole. Next, Honey says, get close to Koco Head. But beware, near that head will be a big gust that will head yachts and force them to bear away. That gust can be nasty, but the finish looms near. Get through that last bit of choppy water and not far away lies a cozy bar with lots of beer. Just the thing to shake the salt crust out of your eyes! Stan Honey has navigated 17 Transpac races, winning eight of them. As navigator, Stan holds both the single-handed, double handed Pacific Cup and Transpac passage records for monohulls to Hawaii. He has also held the 24 hour distance record for multihulls while navigating aboard Playstation soon after its launch. © Jamie Brown/weather-wise.com 2001 More great reading Description of an inverted trough: In
the mid latitudes of the Northern Hemisphere a trough is usually seen as a
southerly bulge in the height contours. The lowest heights are generally located
to the north of the trough. In an inverted trough situation, the height contours
bulge to the north. This is more common in the tropical regions where regions of
low pressure ride south of a mid-latitude high pressure but can happen in the
mid-latitudes when low pressure is south of high pressure. An inverted trough bulges to the north. At first it may look like a ridge, but on further inspection it is a trough. Both a trough and an inverted trough have a cyclonic (counterclockwise) flow pattern. A trough will tend to have more westerly winds associated with it while an inverted trough will tend to have more easterly winds associated with it. Tropical waves will show up as inverted troughs because they are generally south of mid-latitude high pressure and have easterly wind associated with them. Tropical wave affects Hawaiian trades Radar-derived winds (Quickscat) Low level cumulus activity, left over from Tropical storm Barbara, is seen here disrupting trade wind flow north of the Hawaiian Island chain. Prevailing winds can be seen backing to the left and increasing in strength from 15-20 knots (green arrows) to 20-25 knots (yellow) up to 35 knots (red). The black arrows indicate that rain has contaminated the readings. Infra-red animation Image © National Environmental Satellite, Data, and Information Service In the above image, remnants of Barbara are seen here moving east to west over the top of Hawaii. The yellow shade indicates heavy rain and tall cloud tops. Gradient winds
circulating around North East Pacific winds derived
by Quickscat While the detail has been reduced to make this image fit the screen, you can still see different strength winds circulating around the North Pacific High. The lightest winds (less than five knots) exist near the centre of the high - north, north-east of Hawaii - and are indicated by black arrows. Winds in the 30 knot range are blowing from the north along the California Coast, thanks to the high being squeezed against low pressure to the north and east. This image clearly shows the wind band - green arrows indicating winds in the 15-20 knot range - blowing in an arc from the US west coast to Hawaii Tropical low pressure west of Mexico also shows up as an area of light winds Hot links for the central and east North Pacific Composite surface pressure and infra-red images courtesy of the University of Hawaii. Animated satellite images of the NE Pacific: Surface pressure for the eastern North Pacific thanks to Ocean Weather. Pressure data taken from the National Center of Environmental Prediction's Aviation Model. These maps are plotted every 3 hours, and are available approximately 1 hour after valid time. Surface winds in the North East Pacific using 'quickSCAT' satellite technology. Surface winds in the vicinity of Hawaii Wind analysis from the University of Wisconsin showing surface, upper and jet stream - or water vapor - winds. As well, there are charts showing wind shear and upper level divergence. For the arm-chair sailor, the low-mid level infra-red winds tell an awful lot about the sea state where the boats are. Colour-coded arrows report wind strengths and direction at different levels. Remember: Friction at the surface slows the wind down a bit, so it will never be as strong at wave height as it shows on the 900hPa chart. The Aviation Weather Centre offers these terrific flight level charts SIGWX) which show the position of jet streams over much of the globe (mostly Northern Hemisphere). Because upper level winds, like the jet stream, steer our weather systems, knowing where these winds are and how they are behaving helps us to understand whether systems are moving east to west quickly or whether they will stalling because of a kink in the pipe-line. Intellicast (a division of Weather Services International) offers this infra-red image of the eastern North Pacific, showing Hawaii and the Gulf of Alaska. Cold cloud tops - indicating heavy rain or snow - are coloured dark red, warmer cloud tops - showing less precipitation - are coloured light yellow. Intellicast also offers an animated loop of the eastern Pacific. The National Oceanographic Data Center in Hawaii issues this great surf and wind forecast for Oahu, thanks to Pat Caldwell. Scripps University offers this animated 72 hour forecast of significant wave heights for the entire Pacific rim. For tropical storm information around the Hawaiian Islands, the Central Pacific Hurricane Center is a good place to start. Hawaiian weather information thanks to NOAA. Animated loop of the Central Pacific - specifically Hawaii, thanks to Weather Services International and Yahoo. This image shows high cloud tops better than surface clouds, but is good for tracking major weather events. More great reading: Follow the Challenge Mondial Assistance through the official web site and get a preview of weather conditions on the route. Re-live the solo trans-Atlantic race of June 2000 and the Quebec to St Malo of last August. Catch up on all the past action of the recent Vendee Globe in our race archive. Re-live the action between the world's fastest multihulls in The Race Re-live the 2000 Sydney to Hobart yacht race! Break ice in the Ross Sea! |
