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Old 15-05-2022, 06:45   #1
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PredictWind CAPE forecast

I'm preparing for a passage from Charleston, SC to Chesapeake Bay. I usually use PredictWind for most of my forecasting. I'm tracking a weather window starting May 19 that looks quite good, with winds 12-20 on mostly a broad reach. However, PredictWind also provides a CAPE forecast, which does NOT look good.

PredictWind describes CAPE as a measure of atmospheric instability, and they say it can be used to forecast the potential for lightning and severe storms. During my weather window on the 19th, the CAPE forecast in the area is in the "Extremely Unstable" range. This naturally is a pretty big concern, as I wouldn't want to go out when there will be a ton of severe storms.

However, when I look at other indicators of severe weather, I'm not really seeing anything concerning. No significant rain in the forecast. No rotational systems in my path. Gust forecasts don't predict any big gusts more than ~5kts over the base wind speed.

So my questions are:

Has anyone used the CAPE forecast to good effect?

Would the forecast I've described be concerning enough to delay your passage and miss the nice wind window?
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Old 15-05-2022, 07:17   #2
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pirate Re: PredictWind CAPE forecast

I think Oz has the best outlook when forecasting weather with the advisory.. "Winds and sea's can exceed by 40% the forecast speeds and heights.."
Prepare for the worst, hope for the best..
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Old 15-05-2022, 07:19   #3
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Re: PredictWind CAPE forecast

CAPE is one ingredient, albeit an important one. I usually look at the Euro rain forecast along with it. Recently I have started relying mor and more on Windy. The “thunderstorm” prediction seems to combine rain and CAPE, though I don’t know that for a fact. For a largely coastal run, Wunderground for the various areas can also be effective.

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Old 15-05-2022, 08:23   #4
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Re: PredictWind CAPE forecast

For those like myself on limited budgets and so totally reliant on the free version, CAPE isn't included in the Predict Wind software bundle. Sorry doesn't help you OP, I realise.

But perhaps I can offer a thought. For me the more forecasting models available the more difficult the task of making a decision to sail, to go or to wait. And I'm not the only one. I struggle with cognitive bias, that I think is put down to my desire or reluctance to do the trip. If I'm really keen and fizzing at the cork to go then I'll find a forecast that affirms that desire and so my take on the weather data presented will be favourableto going or if I'm not especially keen on the trip, then I conclude the opposite.

And I see others doing the same. Particularly when the the trip is a group thing. There's a cognitive dissonance going on. Because at the end of the day it's only a forecast, and, I'm also backing the boat and crew to handle a bit of rough stuff, or I'm thinking nah, that's looking too tough, let's plan for a couple of weeks whilst this forecasted system passes over (and that will better suit my work commitments e or whatever that I have concerns about being away on the trip)

So I guess what I am suggesting is that there is so much weather information we're sort of overloaded. So OP perhaps you're over thinking?

Predict Wind itself states "CAPE is used by weather officials to understand what the potential might be for thunderstorms and how powerful those storms might become if they do materialize." So is CAPE a model that the amateur can use to advantage without relevant training? For myself nope.

But at the end of the day the weather itself doesn't read any forecast, it does what it does. And for the sailor, really, all you can do is scratch your head, ponder the forecast, and in reality take a chance. And if you experience a bit of bad weather use it as a learning experience. At the end of the day your odds of getting killed are far higher travelling by car to the boat than on the boat trip itself.
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Old 15-05-2022, 19:11   #5
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Re: PredictWind CAPE forecast

I use CAPE as a regular part of my offshore planning, along with wind and waves. If the CAPE forecast is extremely unstable, I interpret that as a certainty I will face rain and squalls. If the sea state is forecast as low, and winds as low, I might go, but I would prefer not to. If the passage requires an overnight, things get more complicated. If there is heavy cloud cover, and little to no moon, I can't see squalls at night, and I must rely on radar. Radar can reliably paint squalls (rain actually), but ypu need to know how to use your radar and how to interpret the results.

In my early days of cruising, I didn't know this stuff, and lost a sail to a squall that came up to 20 seconds.

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Old 15-05-2022, 19:24   #6
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Re: PredictWind CAPE forecast

I use cape index for all crossings. However I use the windy app, not predict wind.
Cape index is available on the free version of windy
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