Decoding Nth Atlantic weather patterns

[Normanby]

I've been obsessing about the weather looking for a chance to head from Plymouth to Camaret, France.
I'm finding a much greater discrepancy between forecasts and reality than I'm used to back home (Australia).
It seems to me that the weather forecasting models depend heavily on being able to predict the trajectory of the low pressure systems in the Atlantic. From what I can gather low pressure systems form somewhere near the equator and wend their way in a north easterly direction.
For example, today's southerly here in Plymouth is significantly stronger than the 14 knots forecast. There's a low to the west, and it seems to have moved closer than expected, hence the higher wind speed.
That same low is forecast to move north west and give us some westerlies for a few days, when another front comes over later in the week.
What I'm getting at here is that I'm trying to get my head around how the weather patterns here work. Rather than rely on forecasts that simply give the 'answers' I'd like to know the weather fundamentals at play in the area.
Back home it was easier. One look at the pressure map (synoptic chart) and one could tell what was coming for the next three days, the usual time it takes for weather systems to cross the Aussie continent, west to east.
Perhaps I need a quick idiots' guide to North Atlantic Gulf Stream Weather?

 

[capnsensible]

http://www.passageweather.com/

 

[Normanby]

http://www.passageweather.com/

Yep, I check that often, along with Windguru and the grib files that the Garmin app downloads and overlays on the charts.
The iPad has a few other weather apps too. Six in total.
My question remains.

 

[capnsensible]

Have a look at the synoptic chart page and 'animate' it. That helps. For fronts look at the precipitation page. Does it for me.

 

[macd]

From what I can gather low pressure systems form somewhere near the equator and wend their way in a north easterly direction.

For what it's worth, most Atlantic lows start life somewhere near the Great Lakes. During the hurricane season, recurving tropical revolving storms occasionally add to the mix.
Fair winds.

 

[Normanby]

Have a look at the synoptic chart page and 'animate' it. That helps. For fronts look at the precipitation page. Does it for me.

I do Cap'n, I do. I agree they're very useful.
What I have noticed though, observing the animated synoptic chart over a number of days, is that the forecast trajectory of a low changes sometimes, based, presumably, on some kind of forecast modelling software.
Perhaps a better question for me to ask is "what do I need to know about Nth Atlantic weather systems to be able to look at a synoptic chart and make my own predictions?"
Animating the PassageWeather chart shows me what their software says the systems will do, and how they'll travel, based on historical data. But experienced locals might have a more folkloric, intuitive feel for local conditions. Or like Neo in the Matrix, can see beyond the synoptic chart to 'read' the underlying data.
I just feel like there's something eluding me, something about life here in the Gulf Stream.
Am I overthinking it?
Probably. I should just pick a forecast that looks favourable, and leave, being also prepared for weather that bears little resemblance to any forecast.

 

[Neil_Y]

We live in a fairly difficult place to predict, three days if you are very lucky when there are lows passing by, and passing by can be North of Scotland or up the Bristol channel. You can usually manage 48hrs reliably but as you've seen sometimes that wind strength will be 10 knts over or under what was predicted. Heading across Biscay is the most daunting (especially in the winter) but off to Camaret just prepare for a bit worse than forecast and get the tides right avoiding wind against tide in that part of the world. Have a good trip, don't miss the Isle de Glenan as you head South.

 

[macd]

We live in a fairly difficult place to predict, three days if you are very lucky when there are lows passing by, and passing by can be North of Scotland or up the Bristol channel. You can usually manage 48hrs reliably but as you've seen sometimes that wind strength will be 10 knts over or under what was predicted. Heading across Biscay is the most daunting (especially in the winter) but off to Camaret just prepare for a bit worse than forecast and get the tides right avoiding wind against tide in that part of the world. Have a good trip, don't miss the Isle de Glenan as you head South.

Good advice, especially the bit I've put in bold, and that especially through the Ch du Four.

 

[KellysEye]

Another thing to look at is the Jetstream if it's over you or heading your way it will be bad weather, low after low. http://www.netweather.tv/index.cgi?action=jetstream;sess= Another good source we used for passages is www.grib.us

We also found Meteo France's forecasts are unusually accurate.

 

[Normanby]

Another thing to look at is the Jetstream if it's over you or heading your way it will be bad weather, low after low. http://www.netweather.tv/index.cgi?action=jetstream;sess= .

Ah ha!
That Jetstream tutorial explained what I wanted to know, thanks. That is, how the weather forecast models predict the trajectory of the low pressure systems.
Today the wind was stronger than the forecast, and high in the sky were "mares' tails and mackerel scales" as the old saying goes. That makes me suspicious of the mild wind forecasts for tomorrow and the next day, but does explain the nasty front that's due on Thursday.

 

[Mrnotming]

Thanks KellysEye and Normanby too!
That's a much easier website for the jetstream,I was using something like CRWS which is B&W graphics.I got caught out en-route Padstow to Brest end of April 2014 when an Atlantic low squeezed a high over the North Sea.got 40/50 knots instead of 20/25 knots.Interior of boat incl cooker,and chart table damaged by me being flung about!

 

[oldvarnish]

Perhaps I need a quick idiots' guide to North Atlantic Gulf Stream Weather?

You could start here: http://weather.mailasail.com/Franks-Weather/Home

 

[franksingleton]

Having had bandwidth problems lately, I have only just seen this thread. The following is largely a repeat of what I have said elsewhere.

Which GRIB or GRIB derived product you use depends on ease of access to you and ease of use – again to you. The basic data are the same. As ever with meteorology there are caveats. In this case, there can be short-lived differences due to precise update times.

The U.S. GFS, the basis for virtually all these services is remarkably good. We can see quite short weather windows several days, 5 or 6 ahead. Almost uncanny at times


We prefer to use zyGrib not because it is a better forecast but for several practical reasons.

• I can choose my own area of interest – currently Scillies across all of Biscay.

• I can select up to 10 days ahead, with intervals from 3 hours and grid spacing down to 0.25 degree. These are not interpolated but model grid point values. The GFS calculates on a 0.125 degree grid and provides data on a 0.25 degree grid.

• I can choose which elements I want to download including sea state and, even, high level winds although I never use thee.

• I can view on a grid or select any point and get a time sequence.

• I can download directly even with low bandwidth that will not support a browser – recently my situation in France.

• With no WiFi, I can use email to get the same data and view it in zyGrib

• The data are saved automatically. When SWMBO asks “What did you say for Camaret to Audierne tomorrow?” I can always tell or show her.

Sites such as XCWeather, WindGuru, Passageweather etc are equally as good but with much less flexibility. A plethora of tablet apps do much the same as zyGrib but, again, with a little less flexibility.


If limited area models are available I am happy to look at them – if they are free. I would not pay for any. I have a listing of many, certainly not all, GRIB or GRIB derived services at http://weather.mailasail.com/Franks-...casts-Reviewed.

Numerical models usually under-predict the strongest winds – typically by about one Beaufort force. That is why for immediate decision making, it is always sensible (essential) to know what the human forecaster thinks.

I am not sure which model the Australian Met services uses. I think it may well be the same as the UK Met Office. The GFS was upgraded early this year. It is likely to be as good, or better, than anything else that we can see on the Internet.

Synoptic charts give a good overall picture of the weather. However, I would back GRIB services to provide information useful to sailors over my interpretation of a conventional chart. Maybe use them in combination – but I must say that I rarely do. GRIBs + GMDSS texts give all I want.

Best advice to the original query is to look at forecasts each day or every 12 hours. Look for consistency between successive forecasts. That is a good guide of an approaching weather window. See Reeds Weather Handbook.