.... For the last two months, I've been verifying the GFES ensemble
extremes output available he-
http://www.met.rdg.ac.uk/~brugge/forecast/
[ I'll be doing a full analysis sometime late February and posting it
to Roger so he can review and publish as he sees fit.]

Today's issue has the following minimum & maximum values for Heathrow
for the 9th to 12th February inclusive:-

MIN(18-06Z): -4, -5, -5, -6

MAX(06-18Z): 0, 0, -1, -1

For Hurn, same period

MIN: -6, -6, -7, -6

MAX: 0, 0, -1, 0.

For such low daytime maxima, the model must think there is a bit of
snow on the ground!

These are based on the ensemble output, NOT the deterministic (or
operational) output.

It'll be interesting to see how they verify! One word of caution
though, my preliminary assessment of these data is that when a *major*
change of type takes place, the scheme has difficulty in settling down
to achieve the 'correct' solution, often not sorting itself out until
5 or 6 days before the event; this isn't a surprise to me, as
ensembles, though useful, shouldn't be regarded as the 'Holy Grail' of
forecasting - there's always that elusive Amazonian butterfly to play
its part ;-)

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

On Thursday 28 Jan 2010 12:20, Martin Rowley scribbled:

... For the last two months, I've been verifying the GFES ensemble
extremes output available he-
http://www.met.rdg.ac.uk/~brugge/forecast/
[ I'll be doing a full analysis sometime late February and posting it
to Roger so he can review and publish as he sees fit.]

Today's issue has the following minimum & maximum values for Heathrow
for the 9th to 12th February inclusive:-

MIN(18-06Z): -4, -5, -5, -6

MAX(06-18Z): 0, 0, -1, -1

For Hurn, same period

MIN: -6, -6, -7, -6

MAX: 0, 0, -1, 0.

For such low daytime maxima, the model must think there is a bit of
snow on the ground!

The thickness being below 510 over most of Britain on the 12th might help
also help a bit. ;-)

--
Graham P Davis, Bracknell, Berks., UK. E-mail: newsman not newsboy
"I wear the cheese. It does not wear me."

On Jan 28, 5:17*pm, Graham P Davis wrote:
On Thursday 28 Jan 2010 12:20, Martin Rowley scribbled:





... For the last two months, I've been verifying the GFES ensemble
extremes output available he-
http://www.met.rdg.ac.uk/~brugge/forecast/
[ I'll be doing a full analysis sometime late February and posting it
to Roger so he can review and publish as he sees fit.]

Today's issue has the following minimum & maximum values for Heathrow
for the 9th to 12th February inclusive:-

MIN(18-06Z): -4, -5, -5, -6

MAX(06-18Z): 0, 0, -1, -1

For Hurn, same period

MIN: -6, -6, -7, -6

MAX: 0, 0, -1, 0.

For such low daytime maxima, the model must think there is a bit of
snow on the ground!

The thickness being below 510 over most of Britain on the 12th might help
also help a bit. *;-)

--
Graham P Davis, Bracknell, Berks., UK. *E-mail: newsman not newsboy
"I wear the cheese. It does not wear me."- Hide quoted text -

- Show quoted text -

Is that the 12th of February, or the 12th of never Graham? It is *15*
days away!

On Thursday 28 Jan 2010 17:33, Dawlish scribbled:

On Jan 28, 5:17 pm, Graham P Davis wrote:
On Thursday 28 Jan 2010 12:20, Martin Rowley scribbled:





... For the last two months, I've been verifying the GFES ensemble
extremes output available he-
http://www.met.rdg.ac.uk/~brugge/forecast/
[ I'll be doing a full analysis sometime late February and posting it
to Roger so he can review and publish as he sees fit.]

Today's issue has the following minimum & maximum values for Heathrow
for the 9th to 12th February inclusive:-

MIN(18-06Z): -4, -5, -5, -6

MAX(06-18Z): 0, 0, -1, -1

For Hurn, same period

MIN: -6, -6, -7, -6

MAX: 0, 0, -1, 0.

For such low daytime maxima, the model must think there is a bit of
snow on the ground!

The thickness being below 510 over most of Britain on the 12th might help
also help a bit. ;-)


Is that the 12th of February, or the 12th of never Graham? It is *15*
days away!

Thank you, Johnny Mathis. ;-)

--
Graham P Davis, Bracknell, Berks., UK. E-mail: newsman not newsboy
"I wear the cheese. It does not wear me."

On Jan 28, 12:20*pm, "Martin Rowley"
wrote:
... For the last two months, I've been verifying the GFES ensemble
extremes output available he-http://www.met.rdg.ac.uk/~brugge/forecast/
[ I'll be doing a full analysis sometime late February and posting it
to Roger so he can review and publish as he sees fit.]

Today's issue has the following minimum & maximum values for Heathrow
for the 9th to 12th February inclusive:-

MIN(18-06Z): -4, -5, -5, -6

MAX(06-18Z): 0, 0, -1, -1

For Hurn, same period

MIN: -6, -6, -7, -6

MAX: 0, 0, -1, 0.

For such low daytime maxima, the model must think there is a bit of
snow on the ground!

These are based on the ensemble output, NOT the deterministic (or
operational) output.

It'll be interesting to see how they verify! One word of caution
though, my preliminary assessment of these data is that when a *major*
change of type takes place, the scheme has difficulty in settling down
to achieve the 'correct' solution, often not sorting itself out until
5 or 6 days before the event; this isn't a surprise to me, as
ensembles, though useful, shouldn't be regarded as the 'Holy Grail' of
forecasting - there's always that elusive Amazonian butterfly to play
its part ;-)

Martin.

--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N * Long: 01.88W
NGR: SU 082 023

I always thought Martin, that ensembles at least in part take care of
uncertainties in inputs, caused by for example butterflies and
elephants.
What they do not help with as regards NWP, are the errors involved in
finite differencing of the differential equations, and
parameterisations of all sorts of feedbacks at various scales, not
least the subgrid scales.
As you quite rightly say, beyond 5-6 days uncertainties grow
alarmingly, particularly in certain situations.
It also depends on how demanding the consumer is as regards the
forecast. A few 100 km can mean a lot to Jo public.

Len
Wembury, SW Devon

"Len Wood" wrote ...

I always thought Martin, that ensembles at least in part take care
of
uncertainties in inputs, caused by for example butterflies and
elephants.

.... I agree that up to (roughly) 6 days, the 'perturbation' method
should cope with extracurricular inputs; beyond that period, slight
variance of initial conditions can have dramatic downstream (in both
time and space) effect that, as we've seen, even 'tightly clustered'
plumes (or other types of ensemble output) can be significantly in
error.

What they do not help with as regards NWP, are the errors involved
in
finite differencing of the differential equations, and
parameterisations of all sorts of feedbacks at various scales, not
least the subgrid scales.

.... indeed, and I have a healthy skepticism of such beyond that period
(~6 days); I've seen too many 'certain' outcomes go awry to give too
much credence to any one particular run.

*If* successive runs (over, say, three days / open to debate) indicate
a similar output, then I might consider giving a %age probability for
6 to 9 days of 70%, and at 10 days, perhaps 60% (but I'd need to be
certain that the outcome was climatologically & dynamically sensible)
.... but even then, I seen several examples of 'rock solid' ensemble
output for the 6 - 10 day period that don't verify to be sucked into
any higher probs.

Beyond 10 days ... well, it's all good knockabout stuff I suppose :-)

Martin.



--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023