On Wednesday, August 30, 2017 at 10:51:01 AM UTC+1, Freddie wrote:
On Wednesday, 30 August 2017 09:29:47 UTC+1, wrote:
Only last night they were predicting heavy rain for kent this morning and
moderate rain for London and the rest of the SE. Actual weather? Nothing of
the sort. A few scattered patches of drizzle with the main rain band way off in
Belgium. How can they get it so wrong only 12 hours in advance? Did the wind
suddenly turn 90 degrees overnight?
Nothing to do with the wind direction (as you present it) and everything to do with the way small variations in shape and speed within the jet interact with the moisture in the lower and middle troposphere. Tiny errors in analysis have a massive impact on the end result - and the impacts change dramatically in a very short period of time. The forecasters know their stuff, it's just they sometimes can't keep up with the changes.

Never forget that the atmosphere is a complex three-dimensional beast that would be almost totally unpredictable without computer models.

Having said all that, I agree that it was a poor forecast. The only way handling of these situations can improve is through better modelling (more horizontal and vertical resolution, and more responsive assimilation of current data) which will enable a better response to when the forecast looks like going adrift.
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Forecasts go adrift for a variety of reasons.
Better resolution in the horizontal and vertical might help but will not solve the problem.
There needs to be a better understanding of the physics. The interactions are not well understood, especially the feedbacks. Parameterisation has to come into it somewhere. There is a chaotic aspect to it as well.

Len
Wembury
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On 30/08/17 16:16, Will Hand wrote:
On Wed, 30 Aug 2017 15:13:01 +0000 (UTC)
d wrote:

On Wed, 30 Aug 2017 15:57:58 +0100
Will Hand wrote:
On Wed, 30 Aug 2017 13:40:00 +0000 (UTC)
d wrote:

On Wed, 30 Aug 2017 05:56:05 -0700 (PDT)
wrote:
I usually don't rise to this kind of message (I usually post on another
new= sgroup) but today I will. Spud, I saw your message first earlier
this morni= ng - at that time I nearly replied with two words - 'look
west'. You clearl= y checked the radar imagery and correctly saw the
showers to the east of Lo= ndon. Just as forecast yesterday. To the
west, the reactivated front was a= lready spreading rain east across
southern England. As forecast yesterday. =
Careful viewing of yesterday's forecasts would have revealed that the
rain= in the SE was basically going to come from the front moving east
rather th=
an from the showers.=20

Yes, but it didn't. See below.

Here in north Surrey it has rained for the last 4 hours, quite heavily in
t= he last hour. It is soon to clear. You do not reveal where you
live.=20

London area. Yes, its raining now, but instead of a band of rain that came
in from france - which actually looked like it headed off to belgium
instead
-
on the rain radar it looks like the band of rain from the west which was
predicted to fade away instead intensified.


Sorry but that is simply not true. The MetO TV forecasts clearly indicated
rain coming in from the west today and persisting in the far SE. IIRC it was
also

Not the one I saw yesterday. The rain over the SE was clearly a front that
was going to come in from France and the front in the east of england would
fade.


OK fair enough. I didn't see all the forecasts.



I saw several and they all talked of the two rain areas merging, nothing
about any of them fading. In fact, that's been the story for a few days.
I agree with Spud that there was a band of thundery showers expected to
clip SE England overnight and early in the morning and that actually
missed but the rest of the forecast was spot on.


--
Graham P Davis, Bracknell, Berks. [Retd meteorologist/programmer]
Web-site: http://www.scarlet-jade.com/
“Like sewage, smartphones, and Donald Trump, some things are just
inevitable.� [The Doctor]

Freddie, I am surprised that you agree that it was a poor forecast. Maybe you have other reasons to say this.

Julian, I took Spud's transcription of the forecast for the morning at face value - I didn't actually see the forecast itself. I was focussing on what may go wrong with a forecast of an upper trough causing development of either the western polar front or the eastern elevated moist air. I apologise if I caused offence.

Not at all, I was genuinely interested in your reasoning. I think Graham summed it all up well in his contribution. I had 7mm in the end falling in 6 hours of fairly continuous rain. Clearly there was an issue of uncertainty on the track of the showers to the east and whether they would skim past over the sea and miss the South East. Ideally this should be communicated in forecasts of course.


Cheers Julian