Encouraged by a forecast of "cold and clear with wall to wall sunshine" in
the West of Scotland last Friday, I travelled from Perth (in the East of
Scotland) via Fort William to the Isle of Skye and finally up to Gairloch in
NW Scotland.



The only thing that was 'wall to wall' was the layer of gloomy grey cloud,
and with the exception of a tiny patch of sun at Kyle of Lochalsh, we never
saw the sun all day.



This reminded me of an incident several years ago when I vividly remember
leaving St Kilda in a hurry in the face of F6/7 winds forecast for the
following 24 hrs, only to have to motor for the next three days because
there was no wind at all.



Question: Is there an underlying reason why the current forecasting models
seem to struggle with the persistence of, and local effects associated
with, high pressure systems? (although I'm not sure that the journeys I
described can fairly be called local).



Many Thanks



Iain McKay

In article , Iain Mackay
writes
Encouraged by a forecast of "cold and clear with wall to wall
sunshine" in
the West of Scotland last Friday, I travelled from Perth (in the East
of
Scotland) via Fort William to the Isle of Skye and finally up to
Gairloch in
NW Scotland.
The only thing that was 'wall to wall' was the layer of gloomy grey
cloud,
and with the exception of a tiny patch of sun at Kyle of Lochalsh, we
never
saw the sun all day.

.... there are still serious problems with low-level cloud within the
atmospheric boundary layer - Stratocumulus is fiendishly difficult for
the models, even at high resolution, to capture (analyse) and forecast
properly. When you see the cloud we have outside now (Bracknell/southern
England), it's not particularly thick, and now and then a little gap
appears - how on earth a model is going to handle that is beyond belief.
Marine-based SC especially can behave differently from that formed (or
moving) overland - another variable in the mix.

Martin.


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Malcolm wrote in message ...

:Serves you right for going to the wrong part of the west of Scotland!
:We've been having wall to wall sunshine here and still have it today.

:Malcolm, Isle of Islay


Met Office website gave rainfall totals for Port Ellen of 1.0 mm on Sunday
and 0.7 mm today.

Are these figures likely to be correct ?

Colin Youngs
Brussels

Sorry to hear that . You picked the one day which was cloudy here on Skye.
Tuesday Wednesday and Thursday last week were cold and clear with "wall to
wall" sunshine. Most of Saturday all of Sunday and all of today was the same
and best of all, little or no wind over the whole of this period!! Usually
at this time of year there is a tendency for severe gales every other day.
Alan

"Martin Rowley" wrote in message
...

In article , Iain Mackay
writes
Encouraged by a forecast of "cold and clear with wall to wall sunshine"
in
the West of Scotland last Friday, I travelled from Perth (in the East of
Scotland) via Fort William to the Isle of Skye and finally up to Gairloch
in
NW Scotland.
The only thing that was 'wall to wall' was the layer of gloomy grey
cloud,
and with the exception of a tiny patch of sun at Kyle of Lochalsh, we
never
saw the sun all day

Martin
Thanks for your reply.

Do the difficulties you refer to also apply to predictions on a larger scale
(as in running from St Kilda in flat calm when we should have seen strong
winds?). What is the reason(s) for the model failure (If that is the right
word).

In my day job I have a fair bit to do with thermodynamics in process systems
and would have thought that the relatively stable conditions in a high
pressure system would be much easier to work with rather than the dynamic
conditions of a low pressure system?

Iain




"Martin Rowley" wrote in message
...

In article , Iain Mackay
writes
Encouraged by a forecast of "cold and clear with wall to wall sunshine"
in
the West of Scotland last Friday, I travelled from Perth (in the East of
Scotland) via Fort William to the Isle of Skye and finally up to Gairloch
in
NW Scotland.
The only thing that was 'wall to wall' was the layer of gloomy grey
cloud,
and with the exception of a tiny patch of sun at Kyle of Lochalsh, we
never
saw the sun all day.

... there are still serious problems with low-level cloud within the
atmospheric boundary layer - Stratocumulus is fiendishly difficult for the
models, even at high resolution, to capture (analyse) and forecast
properly. When you see the cloud we have outside now (Bracknell/southern
England), it's not particularly thick, and now and then a little gap
appears - how on earth a model is going to handle that is beyond belief.
Marine-based SC especially can behave differently from that formed (or
moving) overland - another variable in the mix.

Martin.


--
FAQ & Glossary for uk.sci.weather at:-
http://homepage.ntlworld.com/booty.weather/uswfaqfr.htm
and
http://booty.org.uk/booty.weather/metindex.htm

"Iain Mackay" no_one@here wrote in message
...
Martin
Thanks for your reply.

Do the difficulties you refer to also apply to predictions on a larger
scale (as in running from St Kilda in flat calm when we should have
seen strong winds?). What is the reason(s) for the model failure (If
that is the right word).

.... You would need to consider each situation individually, but the
current generation of mesoscale models (on which coastal forecasts would
be based), whilst not perfect, would not explain why you experienced
calm when strong winds are expected. Within the Minches for example, I
can think of examples where, with some wind directions, you might get
much lighter winds than 'generally' forecast, but not on a voyage
to/from St. Kilda which it much more 'open'.

Even several years ago, the model output was pretty good for such
situations - the only problem I can think of offhand is that St. Kilda
is part of 'Hebrides' sea area (in the Shipping Forecast), and a F6/7
*might* have been forecast for the far west and the peculiar spike
between Bailey and Faeroes, but would not be applicable to the rest of
Hebrides. This was/is a common problem with the larger sea areas -
sometimes the forecaster runs out of words to split the area
sufficiently and has to concentrate on the 'worst-case' scenario. When
writing the Shipping Forecast, by the time you get to Malin, Hebrides
etc., you find you're rapidly running out of words (strictly limited)
and have to lump areas together with no leeway to sub-divide. It's only
a guess, but I've known it happen.

Can you remember the dates? It is possible to look at archive mslp
charts now online and get an idea what sort of situation was in place.

Martin.


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In article ,
Martin Rowley m
writes:
When
writing the Shipping Forecast, by the time you get to Malin, Hebrides
etc., you find you're rapidly running out of words (strictly limited)
and have to lump areas together with no leeway to sub-divide. It's only
a guess, but I've known it happen.

Given that the worst weather in the seas round the UK tends to be to the
north-west - and it's therefore more likely to be important to put in
the detail there - and the limited time for the forecast, would there be
a case for dealing with the sea areas in anti-clockwise rather than the
current clockwise order? Or alternatively to stick to clockwise but to
change the starting point to, for example, Shannon?
--
John Hall
"Honest criticism is hard to take,
particularly from a relative, a friend,
an acquaintance, or a stranger." Franklin P Jones

"John Hall" wrote in message
...
In article ,
Martin Rowley m
writes:
When
writing the Shipping Forecast, by the time you get to Malin, Hebrides
etc., you find you're rapidly running out of words (strictly limited)
and have to lump areas together with no leeway to sub-divide. It's
only
a guess, but I've known it happen.

Given that the worst weather in the seas round the UK tends to be to
the
north-west - and it's therefore more likely to be important to put in
the detail there - and the limited time for the forecast, would there
be
a case for dealing with the sea areas in anti-clockwise rather than
the
current clockwise order? Or alternatively to stick to clockwise but to
change the starting point to, for example, Shannon?

.... The answer is more words! If you put the detail in there, then the
Channel fraternity will complain (more); if you put the detail in the
Channel, then the North Sea fishing fraternity will complain.

There was an article in 'Weather' a while back that pointed out that
there are many & better ways to convey forecast information to the
marine community rather than the text-based Shipping Forecast.

Martin.


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Martin
Sorry - can't remember the exact dates. It was probably mid June 1987. The
weather had been clear and settled for the previous week days before we
began thinking about the run home.

I am aware of the Met Office's tendency to forecast based on the poorest
conditions expected in a sea area, and given the lack of time, can see why.

However, in this case a dive boat went down to Rockall and found perfect
conditions for diving, while another yacht had 'enjoyed' (but not sailed)
fine weather in the North of the Minch and over to Orkney, so the settled
conditions seem to have been fairly widespread.
Typical weather conditions at that time of year around NW Scotland tend to
feature lots of (sometimes quite strong) Easterlies, but not that time.

As I mentioned earlier, predicting the behaviour of a stable high pressure
system seemed (on the face of it) to be a less daunting undertaking than
second guessing chaotic events along the polar front (at least to a user),
I was curious to to find out if the underlying modelling process for high
pressure systems had/has a known (or suspected) weakness under some
conditions and keep that in mind when considering how to respond to
forecasts in future.

Many Thanks

Iain



"Martin Rowley" m wrote in
message ...

"Iain Mackay" no_one@here wrote in message
...
Martin
Thanks for your reply.

Do the difficulties you refer to also apply to predictions on a larger
scale (as in running from St Kilda in flat calm when we should have seen
strong winds?). What is the reason(s) for the model failure (If that is
the right word).

... You would need to consider each situation individually, but the
current generation of mesoscale models (on which coastal forecasts would
be based), whilst not perfect, would not explain why you experienced calm
when strong winds are expected. Within the Minches for example, I can
think of examples where, with some wind directions, you might get much
lighter winds than 'generally' forecast, but not on a voyage to/from St.
Kilda which it much more 'open'.

Even several years ago, the model output was pretty good for such
situations - the only problem I can think of offhand is that St. Kilda is
part of 'Hebrides' sea area (in the Shipping Forecast), and a F6/7 *might*
have been forecast for the far west and the peculiar spike between Bailey
and Faeroes, but would not be applicable to the rest of Hebrides. This
was/is a common problem with the larger sea areas - sometimes the
forecaster runs out of words to split the area sufficiently and has to
concentrate on the 'worst-case' scenario. When writing the Shipping
Forecast, by the time you get to Malin, Hebrides etc., you find you're
rapidly running out of words (strictly limited) and have to lump areas
together with no leeway to sub-divide. It's only a guess, but I've known
it happen.

Can you remember the dates? It is possible to look at archive mslp charts
now online and get an idea what sort of situation was in place.

Martin.


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FAQ & Glossary for uk.sci.weather at:-
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and
http://booty.org.uk/booty.weather/metindex.htm

"Iain Mackay" no_one@here wrote in message
...
Martin
Sorry - can't remember the exact dates. It was probably mid June 1987.
The weather had been clear and settled for the previous week days
before we began thinking about the run home.

.... if it was 1987 (June), then it was certainly an unsettled start to
the month, with strong winds up until the start of the second week, but
gradients easing thereafter. I can't see anything in the synoptic
situation to indicate why a forecast of strong winds (across such a wide
area) would be issued when none occurred - it's usually the other way
about at that time of year! However, without knowing the dates etc., it
is pointless at this range to speculate further.

I was curious to to find out if the underlying modelling process for
high pressure systems had/has a known (or suspected) weakness under
some conditions and keep that in mind when considering how to respond
to forecasts in future.

.... the modelling has improved enormously since that time: I hadn't
realised you were thinking nearly 20 years ago ;-) As noted in my
earlier response, although not perfect, the modern generation of models
as regards *wind* forecasts are pretty good; it is usually the
interpretation and poor means of communication that are the problem,
rather than the raw model output. Other output (cloud, rain etc.) of
course *do* still cause problems, as at the start of this thread.

Martin.


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