/ MWIS versus the Met Office

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Flinticus - on 16 Oct 2013
MWIS gives 'little if any rain' for the West Highlands tomorrow.

The Met Office mountain summit forecast gives rain over the region (I've checked a wide spread of summits).

Which should be more accurate?
markh554 on 16 Oct 2013
In reply to Flinticus:
They are both guess-timates at best, so I would plan for armageddon and the three horsemen and anything better is a bonus
EddInaBox on 16 Oct 2013
In reply to Flinticus:

Sorry, I'm a bit too busy to type a proper answer right now, I'll have a bit of free time on Thursday evening so I'll let you know then.
Flinticus - on 16 Oct 2013
In reply to EddInaBox:
Cool. I'll check your reply if I can get a connection on the hill.
MG - on 16 Oct 2013
In reply to Flinticus: Anecdotally I find MWIS underestimates rain and over estimates wind.
Snoweider - on 16 Oct 2013
In reply to Flinticus:
The MWIS forecast for Thursday just looks plain wrong. There is an occluded front sat over the region at midday according to their own synoptic chart. Granted it is weakening, but I reckon its going to rain on the hills.
Flinticus - on 16 Oct 2013
In reply to Snoweider:
Cetainly it differs widely from the Met Office. I would expect a lot of cross over given that I am only looking to tomorrow.
Mark Bull - on 16 Oct 2013
In reply to Flinticus:

On the Met Office site, I would use the human-written text forecast here http://www.metoffice.gov.uk/public/weather/mountain-forecasts/west-highlands#?tab=mountainWeather rather than the entirely computer generated individual summit forecasts.

Remember that the MWIS West Highlands area only extends up as far as Lochaber, whereas the Met Office West Highlands includes the NW Highlands as well.

Tomorrow we will be under a pressure col, which does make forecasting particulary tricky. The latest BBC forecast doesn't have much rain in it after the early morning.

Snoweider says:
> The MWIS forecast for Thursday just looks plain wrong. There is an occluded front sat over the region at midday according to their own synoptic chart.

Confusingly, the synoptic charts on the MWIS site are the Met Office ones, but MWIS actually rely on data from the US-based GFS model (it's free and the Met Office data is not!)

Glyno - on 16 Oct 2013
In reply to Flinticus:

I used to follow forecasts religiously and often found MWIS wildly inaccurate, though their pdf print-outs are useful for giving an air of credibility to outdoor shops and YHAs.

Met Office and BBC are both better options.
Milesy - on 16 Oct 2013
I have always found MWIS pretty reliable. Remember Geoff Monk who runs MWIS *DID* work for the met office, and I believe he still advises them.

Met Office for thurs WH

Weather
Cloudy for much of the time with rain, occasionally heavy overnight then gradually easing and dying out during the course of the day. Remaining cloudy with showers through the day across the Northwest Highlands, but some brighter skies developing across Argyll in the afternoon although a few light showers are still possible. Further rain will spread from the south to southern ranges later in the evening.

MWIS says for rain in WH.

Today/Tonight

Increasingly wet from south afternoon

Dry until after midday. Rain then Arran, and extending north throughout the afternoon. May be nearly dusk before reaching N Lochaber/Creag Meagaidh.

Tomorrow:

How Wet?
Little if any rain

There may be a little light rain (or snow highest summits) here and there, but total rainfall small.

The forecasts look pretty similar to me, with rain moving in through this evening and through the night and then dying out as we move into tomorrow.
Milesy - on 16 Oct 2013
Also, BBC forecasts based on location are usually unreliable as well. BBC forecast for Fort William can be sunshine but we know that means feck all on the summit of the ben as cloud and precipitation can form on the hillside.
AG - on 16 Oct 2013
In reply to Milesy: I find the MWIS quite accurate (apart from the wind speeds). The west highlands forecast from the met office covers too large an area IMO. I usually go with the one i like best!.
Flinticus - on 16 Oct 2013
In reply to Milesy:

Its tommorw I have been looking at and comparing, not today (what's the point, I am at work and typing at a keyboard).

'Little if any rain' implies a good chance of no rain and, if any does fall, for it to be a minimum.

The Met Office summit forecast (not the regional one), for a range of peaks in the West Highland tomorrow, all indicated rain from heavy to light but predicted for most of the day (at least when I looked at them this morning.

Given that rain usually accompanies poor vis and clag, that changes my expectations dramatically. I'm not out to bag hills regardless of the weather. I've walked enough in the rain to know that it is largely crap, despite brave faces and manning up.
Milesy - on 16 Oct 2013
In reply to Flinticus:
> The Met Office summit forecast (not the regional one), for a range of peaks in the West Highland tomorrow, all indicated rain from heavy to light but predicted for most of the day (at least when I looked at them this morning.

Why are you looking at the summit forecast? That is pretty much all just model generated. Don't look at the regional areas either - look at the Met Office Mountain area Forecasts? MWIS takes a combination of model data but very importantly - local geopraphical and topographical knowledge and experience.

> Given that rain usually accompanies poor vis and clag, that changes my expectations dramatically. I'm not out to bag hills regardless of the weather. I've walked enough in the rain to know that it is largely crap, despite brave faces and manning up.

I wouldn't say that is true. I have been in the clag many times and the most wet you get is drizzle from the clag itself.
Milesy - on 16 Oct 2013
In reply to Flinticus:
> Its tommorw I have been looking at and comparing, not today (what's the point, I am at work and typing at a keyboard).

It was tomorrows west highlands mountain forecast for the met office I gave you. It says heavier rain during the night and then dying out through the night and tomorrow.
In reply to Flinticus:
The weather in NW Highlands has been beautiful for the last 6 days. Getting cloudier now but I don't think there will be much rain up here(Torridon northwards ) for the next couple of days.
999thAndy on 16 Oct 2013
In reply to Flinticus:

Have you tried yr.no? Seems to work well for Englandshire

this is what it turns up for Fort William http://www.yr.no/place/United_Kingdom/Scotland/Fort_William/
BnB - on 16 Oct 2013
In reply to 999thAndy: This forecast service is popular with the locals on Skye.
connor on 16 Oct 2013
In reply to Flinticus:
> MWIS gives 'little if any rain' for the West Highlands tomorrow.
>
> The Met Office mountain summit forecast gives rain over the region (I've checked a wide spread of summits).
>
> Which should be more accurate?

i've been using: http://beta-stream.com/ for a while, pretty accurate and you get loads of other info in winter which i find really useful, conditions reports ect.
dissonance - on 18 Oct 2013
In reply to Flinticus:

So who was right?
ads.ukclimbing.com
Flinticus - on 21 Oct 2013
Well, I went out Thursday, to Glen Coe.

All the way up to Glen Coe cloud was down real low, in places to near mountain base level, in others higher up. Generally no sun shine, no breaks. Cloud shrouded all mountains, most extensively, with the odd summit of the lower hills clearing for very short intervals before clouding up again. Rain was everything from heavy to drizzle, becoming more intermittent towards the latter half of the day (nearing 4pm).

This contrasts with the MWIS forecast, given on Wednesday, for that Thursday: Patchy cloud on higher areas will largely clear: leaving perhaps fragments on higher northern summits. Chance of cloud free munros: 70%.

From my experience of the day, I'd adjust that down to less than 5%.
Simon Caldwell - on 21 Oct 2013
In reply to Flinticus:
> Chance of cloud free munros: 70%.

> From my experience of the day, I'd adjust that down to less than 5%.

I don't think you understand how probability works...
Milesy - on 21 Oct 2013
I have met quite a few people (not saying you) who were under the impression that "Chance of cloud free munros" meant "cloud free" - when it means that just the summits will be clear. It could be all munro summits over 1000m are in cloud, but those under 1000m might be clear under the cloud base. On sunday the summit of stob choire odhair where I was, was pretty much clear, but stob ghabhar was shrouded the full day.
Flinticus - on 22 Oct 2013
In reply to Milesy: I think it should be clear that I know what cloud free means. On Thursday, none of the munros were out of the cloud. Only a few grahams were free for short periods.
Flinticus - on 22 Oct 2013
In reply to Toreador: I understand it to be a measure of the chance of something occuring. Whats yours?
Simon Caldwell - on 23 Oct 2013
In reply to Flinticus:
Correct, it doesn't mean that 70% of summits will be out of cloud, it means that there's a 70% chance that any summit will be out of cloud. So the conditions you observed fit in fine with this probability - most of the summits fell within the 30% - there's no need to adjust it to 5%.
Flinticus - on 23 Oct 2013
In reply to Toreador:
Looking at the conditions on the day, I'd say the chance of any munro summit being out of cloud (i.e the measurement of the likelihood of this event happening) was nowhere near 70%.

While the fact that that the forecast, in allowing for a 30% chance that the summits would not be out of cloud, doesn't make it right or wrong, it makes it a poor predictor of the probability.
Simon Caldwell - on 23 Oct 2013
In reply to Flinticus:
If you roll a dice, then the probability of getting a number greater than 2 is 66.67%.
If when you roll it it turns up with a 1, then that does not affect the probability. You appear to be saying that it should?
Flinticus - on 23 Oct 2013
In reply to Toreador:

The dice analogy is wrong, as the possible outcomes will always be between 1 to 6 as there will always be a facet with the numbers 1 to 6.

I'm saying that the probablity of rolling a 7 on a dice is Nil.
Flinticus - on 23 Oct 2013
Sorry, I'll amen that: the analogy can work.

Assume a 10 sided dice (like they used to use in RPGs).

MWIS say that the dice has 7 facets stating 'cloud free' and three 'clouded'. Rolling the dice gives a 7/10 chance of 'cloud free'.

Lets say instead the dice had 9 facets saying 'clouded' and one 'cloud free', then the chance is 1/10. That would be more accurate.
planetmarshall on 23 Oct 2013
In reply to Toreador:
> (In reply to Flinticus)
> If you roll a dice, then the probability of getting a number greater than 2 is 66.67%.
> If when you roll it it turns up with a 1, then that does not affect the probability. You appear to be saying that it should?

Depends on your interpretation of what a probability is. From a Bayesian perspective, rolling a 1 constitutes additional information that could well affect the posterior probability ( concrete example: it could affect the probability of the coin being biased ).

nufkin - on 23 Oct 2013
In reply to Toreador:

Another cheerfully obtuse interpretation might be that you could expect Munros to be free of cloud 70% of the time





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