Weather Musing...Weekend Lowq

[simonjk]

Hi All,

Just a quick note to advise any of you sailing this weekend to check the latest forecast before you go out. I know this is teaching garnny to suck eggs, but it's a rare situation where you can't take any risks.

The low that is moving in has the potential to be very nasty with winds rapidly increasing from F3's to F7's in an hour or so.

Latest charts are causing concern so do please keep up to date with the forecast and don't be tempted to try and outrun it. Those sailing in the Channel, or western areas are especially advised to take note of this advice.

Keep safe, hope you don't mind me posting again.

Cheers,
Simon

 

[webcraft]

Simon,

This is all a little worrying - and not just from the point of view of people who are planning to sail in the channel or Western approaches this weekend - at least you will have managed to warn a lot of them.

What exactly caused this alarming type of behaviour in depressions??? Why do they just suddenly deepen like that?? Where does all the air go, straight up?? /forums/images/graemlins/confused.gif

Is this the sort of thing that caused the Fastnet disaster?

Is there any way lesser mortals can tell when this might happen by looking at the charts? Or are we just doomed if a depression takes it into its head to do this? How common is it?

- Nick

 

[Toutvabien]

I guess many of us have been keepin a weather eye open this week. I have already abandoned our planned trip to France this weekend, the latest forecasts for Monday certainly suggest that it would not be a comfy ride home.

 

[davel]

Curiously, the Sunday and Monday forecast from Metcheck looks relatively good. /forums/images/graemlins/confused.gif

 

[Toutvabien]

I think the point is that the forecasts seem to be changing quite rapidly, difficult to predict what might happen. I have stayed at home so the depression will probably track well North, and it will be force 4 and 25'C all weekend.

 

[jb2006]

We never mind you posting ... Thanks Simon

 

[BAtoo]

East Coast looks wet but not too windy.....http://www.metcheck.com/V40/UK/FREE/7days.asp

Port of Felixstowe Regatta weekend.

Working all weekend so not so worried!!

 

[claymore]

Thanks for that Simon - I'm now running a dual log to see where I would have been had I stuck to plan A.
By now I would have been somewhere beyond the Point of Ayre heading for the Mull of Galloway having motored into reasonable headwinds all night. This would have been OK!
Doh

 

[Whitelighter]

All the forcasts for the Solent - Brighton area look to be forcasting dropping wind speeds for sunday and Monday. F3/4.

The charts don't look to good with a low sitting on. Was planning Hamble to Eastbourne run tomorrow but don't understand where you comments on the wind speeds up to F7 come from????

 

[Nauti Fox]

Been following the forecasts for about a week and haven't seen the forecast change so frequently in a long time. I don't think the forecasters can pin down this nasty low.

 

[Whitelighter]

hmm. I guess prudence says there is always next WE. Not even like I have my usual crew, just me and the new owner who is green as anything.

 

[billmacfarlane]

I'm also sitting at home reading this when I should have been in Cherbourg. Better safe than sorry I suppose even though I was tempted to have a go for a VERY important reason. I drunk my last bottle of wine last night !!! I can feel the Channel Tunnel beckoning this week.
Spare a thought for the boats in the Royal Escape race who went across yesterday from Shoreham to Fecamp. Hope they have a reasonable journey home.

 

[simonjk]

This is exactly why forecasters get twitchy in this sort of situation. A ridge builds ahead of the low and the weather looks calm. People can't understand why forecasts have been warning of bad weather when it seems so good now.

Next, the computer models that you see on the web are at too coarse a resolution to pick up the true speeds of the winds most of the time, they smooth out the speeds and so a F8 can easily become a F5. This is where the skill (hopefully) of a forecaster comes in.

The thing to do is to watch the forecasts carefully. It will be as the low passes east late Sunday/early Monday that the strongest winds kick in. There's a hint on the latest Inshore Waters for Selsey Bill to Lyme Regis to 0100 Monday which puts the winds to F6-F8.

At the moment the forecast is absolutely spot-on. Saturday was always forecast to be a pleasent, benign day; Sunday being the wet one and becoing windy.

Watch this space and don't trust the model predicted winds!

Hope that clarifies things,
Simon

 

[webcraft]

[ QUOTE ]
the computer models that you see on the web are at too coarse a resolution to pick up the true speeds of the winds most of the time, they smooth out the speeds

[/ QUOTE ] This is interesting stuff SImon, because so many of us use GRIB files these days. Most of them have a resolution of 1 minute or 60nm, although you can get them with a res of 0.5' from some sources.

If I understand you, what you are saying is that if you have a small compact but deep low pressure with - say - wind F1 at the centre, F4 20 miles from the centre, F8 40 miles from the centre and F4 again 60 miles from the centre then the computer model is likely to just have one F6 arrow to cover the lot but anyone in the path of the depression is likely to experience gale force winds at some point as it passes over them.

Am I close, or am I speaking rubbish?

Are the predictions for bigger systems more reliable?

- Nick

 

[Niander]

Is this the sort of thing that caused the Fastnet disaster?

Yea that sort of thing
but the winds were alot stronger than F7!

 

[simonjk]

Yes, that's it. This is where GRIB is very useful as a guidance tool, but when it comes to situations like this it can be very misleading indeed as the resolution isn't there.

The resolution of the Met Office global model is +40km, however, most web sites don't use this. The vast majority run GFS which is at 100km resolution (1 deg) to 180hrs and then 250km resolution (2.5 deg) to 384hrs. Incidentally the model that you see on the BBC is (I believe) a forecaster modified version of the UK model run at 4km resolution (but don't quote me on this as it might still be the NAE model at 12km resolution).

You have it in one about the smoothing effect, you see how large that can be, especially on the GFS?

There is an interesting article at http://www.metoffice.gov.uk/research/nwp/publications/nwp_gazette/dec06/numerical.html

Simon

 

[Marsupial]

I agree

The weather pattern is too chaotic to be forecast accurately, better safe than sorry but Oh how I hate gardening!

 

[webcraft]

[ QUOTE ]
Yea that sort of thing
but the winds were alot stronger than F7!


[/ QUOTE ] erm . . . obviously . . .

That is why I was interested. We are planning to sail back from the Azores to Ireland in July and will be crossing the same sea area - as will a lot of other boats no doubt. It is also the area (SW approaches to Ireland) that Mike Richie in Jester encountered Jester's Ultimate Storm on 1st August 1986

It would be nice to think that there was some little hint one could look out for that an apparently normal low was going to turn vicious. Here's one theory for it:

[ QUOTE ]
Alan Watts has suggested that the particular violence of the storm was probably due to convection from unusually cold air at altitude, which would also account for the rapid deepening of the low. The low appears to have migrated through the axis of a more or less stationary cold trough at altitude. The measured gradient gave 80 knots of wind at 2000 ft which would give 60 knots at sea-level, though with convection currents the full 80 knots would at times be felt, creating conditions similar to those in the Fastnet gale of 1979, where episodic freak waves so devastated the ocean racing fleet.


[/ QUOTE ]
So there's one theory - perhaps Simon might like to comment. Bad weather we expect, but survival conditions ar something we would all like to avoid.