Weather Forecasts And Routing In The Real World: Good But Not Perfect

GW BArometric trace

We are looking at a barometric pressure trace from the FPB 64 Grey Wolf. This occurred at the edge of the tropics South of Rarotonga in the Cook Islands. As you close with the equator slight pressure gradients create big winds. A change of as little as two mb can indicate the onset of a hurricane. The weather models – all the majors – missed this event.

Bob McDavitt, one of the best routers around, and the resident expert on the S. Pacific, was handling this leg from New Zealand to Tahiti for Peter Watson and crew. After seeing the trace Bob had this to say:

“…good to see that the barometer is reading OK— this verifies that what was happening in the real world was indeed deviating radically from what the computer models were producing, and that the barometer was the FIRST to show this deviation. That’s why, in my voyage forecast, I mention that an update is required when the real barometer deviates more than 2 or 3 hPa from the forecast (even if the wind and waves at that time are still in agreement).
That was a real marine adventure, and I put a blog on my web site regarding that squall you experienced near Rarotonga.
Also the Low that formed those squally clouds got knocked to the west (by the strong High on its south side). This happens occasionally, but in this case it abnormally got knocked west-southwest all the way to NZ. Still raining from that Low over central and eastern NZ today – brought a southerly gale to Cook Strait forcing the Royals inside for the day.”

Grey Wolf and crew had no problems with the event. And this would have been the case for any well found and properly handled yacht. On the other hand, you’d not want to be unfamiliar with reefing, setting storm canvas, or be compromised by something going wrong. When the breeze is blowing 54 knots with steep seas, it is not the time to be worrying about deferred maintenance.

You will note that Bob McDavitt indicates the localized storm system grew a bit, and rotated backwards towards New Zealand, where it generated some breeze. This “unusual” event could have been much worse. When you get a warm, moist tropical system heading away from the equator, if it happens to come into contact with an upper level trough, which will be colder and drier, it is like adding gasoline to a fire. A weather explosion is possible. Examples of this are the Queen’s Birthday Storm and the 1998 Sydney Hobart race blow.

The odds are low, and there are warning signs at the 500mb level, and from the tropics.

The faster you can complete your passage the better your odds of avoiding this sort of thing.

The weather comments of the recent posts deal with the South Pacific, but the same issues exist in the North Pacific and the North Atlantic. Yes, the weather models have a better initialization, but on a long passage the pre-departure forecast only takes you so far. And the weather models are only marginally better at deciphering frontal boundaries, convergence zones, and low centers.

The purpose of these examples is to paint a realistic picture of what can happen. These are rare events, but we see them every cruising season, and people get themselves into trouble by ignoring them, or assuming the weather routing services or checking grib files can eliminate potential trouble. We feel strongly that it is better to acknowledge the risks, so that folks will take the proper steps to mitigate them.

In other words, be prepared.

A final word on risks in general. We live in a world filled with various levels of risk. From falling down and breaking a leg getting out of the bath, to having a drunk driver cross the center divider, to some major land based weather event, nothing in life is risk free. For all our commentary on weather risks we feel safer on a properly found yacht at sea than we do driving down the highway. We feel more in control of our destiny, and we take comfort in knowing that our maintenance procedures are up to snuff and we have at least a basic understanding of weather. We’d like everyone else to feel the same.

In a recent article, we posted a link to download two of our books for free. Simply click the links below to get your copy of Mariner’s Weather Handbook and Surviving the Storm:

Download Mariner’s Weather Handbook

Download Surviving the Storm

Posted by Steve Dashew  (April 11, 2014)

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