South West England & Channel Islands - Cold Spell Discussion Part 16

[MartUK]

Just started to rain very heavy here in North Cornwall.

[Bristawl Si]

After posting earlier this morning GP's little snippet of continued confidence in his Winter LRF i.e. less cold, average January, milder February -

BFTP is still v confident in his and Roger's Winter LRF in Winter Weather Discussion this eve - he remains bullish for eventual return of cold and snow later this month, into Feb.

This is basically the point in the Winter where their respective forecasts go in different directions - interesting to see who's proved correct or nearer to reality over the coming weeks - i would add to qualify that these guys have total respect for each other's different methods of forecasting.

[Wilxy]

I thought devon and cornwall would of had another warning by now lots of heavy rain at the moment maybe they need to start using different models cause the ones they have arnt proving to be very good.

NMM has this almost spot on so far.

[kevvo]

Can anyone explain why, when snow comes in, the Weymouth/Portland area gets very little. It seems to either evaporate over Lyme bay, when its atlantic frontal snow, or fails to reach us from the east when more convective. Yet when it rains, like right now, we get as much as anyone else?

[Dartmoor_Matt]

Absolutely pouring here again.

Could be some quite interesting amounts by morning. 29mm at the moment.

[MartUK]

I thought devon and cornwall would of had another warning by now lots of heavy rain at the moment maybe they need to start using different models cause the ones they have arnt proving to be very good.

NMM has this almost spot on so far.

Still very heavy rain here, just on my way to my mates and roads etc have a lot of running water surprised there are no flood warnings issued!!

[Cakie]

Absolutely pouring here again.

Could be some quite interesting amounts by morning. 29mm at the moment.

Agreed, it was bad this afternoon with some localised flooding on the sides of the road. Not too bad but in such a short time (30 mins) it was a bit worrying. The wind is very strong still and keep getting bands of rain coming through all the time

[Wilxy]

Still very heavy rain here, just on my way to my mates and roads etc have a lot of running water surprised there are no flood warnings issued!!

NMM has been very good at predicting this i only got it yesterday and has been very accurate if i was in charge of the metoffice warnings i would of put another out or extended it until midnight for devon and cornwall.

[Guest mycroft]

god it seems an age since the snow and ice were here,very heavy rain and a gusty wind here.Plus i am back to work tomorrow from the xmas break,was hoping that it would be cold and dry,all eyes to the E and NE now. thats where i think our next blast will come from.US looks set to take another pounding this coming week!

[Frosty_wsm]

Can anyone explain why, when snow comes in, the Weymouth/Portland area gets very little. It seems to either evaporate over Lyme bay, when its atlantic frontal snow, or fails to reach us from the east when more convective. Yet when it rains, like right now, we get as much as anyone else?

I'm glad you brought that up, I notice something similar here in weston that when it's atlantic frontal snow, the ppn has a habit of breaking up as it passes and then reforming, I've seen this process happen time and time again in winters past while watching the ppn meto radar, yet take today for instance and we've had pretty much heavy rain since late afternoon.

[Dartmoor_Matt]

Agreed, it was bad this afternoon with some localised flooding on the sides of the road. Not too bad but in such a short time (30 mins) it was a bit worrying. The wind is very strong still and keep getting bands of rain coming through all the time

Yup, we have exceeded the warnings amount here already (warning was for up to 35mm) its currently 38mm and still raining. Why could this not of been snow?!

Anyways. G'Nite all.

[Isolated Frost]

38mm in one day?! Good lord. Hope you guys are fine tomorrow Unfortunately, a lot of heavy stuff on the radar, should clear tonight though

[Phelpsy]

Very cold, realy windy and Raining

[egret]

Can anyone explain why, when snow comes in, the Weymouth/Portland area gets very little. It seems to either evaporate over Lyme bay, when its atlantic frontal snow, or fails to reach us from the east when more convective. Yet when it rains, like right now, we get as much as anyone else?

In the absence of other 'properly qualified' replies I will have a go at my understanding of this. Lets initially disregard snow and think about temperature and precipitation.

1. Temperature. Weymouth is sheltered from the mainland being at the foot of the Ridgeway. It therefore reflects sea temperatures more than temperatures over the land. It can therefore be significantly warmer than Dorchester (15 miles inland) in autumn and winter but conversely its air temperature scan be pegged back during spring and early summer due to cool breezes off the sea. This is most notable in May and June when warm Atlantic air chills over the sea and creates sea fog, but this evaporates by the time the air has passed over the Ridgeway and warmed up a bit. In Winter the air can be 3 or more degrees warmer in weymouth compared to Dorchester when wind is coming off the sea, but maybe only one to 2 degrees warner when wind comes rom the North, maybe less if the wind is strong.

2. Rainfall is often triggered as warm Atlantic air is forced up over higher land meaning that it can be sunny in Weymouth and cloudy and maybe raining in Dorchester. Conversely if we get a wet North westerly, it will rain on the North facing uphill side of the Ridgeway but the air will warm and become drier as it flows down the Weymouth side of the ridge and rainfall ends. (explore the Fohn effect?)

Now think about conditions for snow.

If strong wind comes from the North, we are usually marginal for any precipitation because of the point in 2 above as well as because this tends to be a relatively 'dry' wind direction anyway over England. So Weymouth will get light precipitation even though air temperatures might be low. If wind is light northerly you get the same effect but also the warm sea nearby has the effect of raising air temperatures and making temperatures more marginal for snow in Weymouth, therefore a tendency for light sleet.

With wind from the Southwest - there may be plenty of moisture in the air but air temperatures too high for snow to settle. Go up the Ridgeway and the sea effect is a little less and the extra height both reduces air temperature and also triggers 'convective' effect leading to heavier precipitation which in turn cools the air mass as it rises. Net effect - more snow.

In fact this effect becomes even more pronounced on the downland up towards Shaftesbury, which is why I head up there for snow. Typically it is 1 to 2 degrees cooler there than in Dorchester and therefore maybe 3 or 4 degrees cooler than Weymouth. The added height can give still more precipitation from a South Westerly with snow depths commonly twice those of Dorchester.

Very occasionally the conditions are just right and Weymouth can get a real dump of snow, but that is a very rare event as you are probably aware. I believe similar 'microclimate ' effects are found in a number of seaside locations, Poole is one and parts of Devon and Cornwall too- Torbay referred to as the English Riviera.

There is another factor that i do not fully understand that others may wish to comment upon (and maybe correct me if I have got any of this wrong!), that is the way that snow cloud apparently heading North towards us from France, tends to sheer away in a more Easterly direction up the English Channel to make landfall in Sussex and Kent. This effect seems less pronounced in summer when thunderstorms coming in from France over Portland seem happy to move straight inland rather than sheer off up the Channel. This may have to do with there being reversed air/sea temperature differential in summer to that in winter. (winter is cold land / warm sea, summer is cold sea / warm land) My personal assumption is that cooler wet air is less inclined to rise over higher cold land mass and therefore sheers sideways, whereas relatively cool moist summer air off the sea is warmed over land and will readily convect and lift over the higher ground and be displaced less sideways.

Edited January 10, 2011 by egret

[Rich_Clements]

Hammering down, mild though, very windy also, come back frosts all is forgiven

[kevvo]

In the absence of other 'properly qualified' replies I will have a go at my understanding of this. Lets initially disregard snow and think about temperature and precipitation.

1. Temperature. Weymouth is sheltered from the mainland being at the foot of the Ridgeway. It therefore reflects sea temperatures more than temperatures over the land. It can therefore be significantly warmer than Dorchester (15 miles inland) in autumn and winter but conversely its air temperature scan be pegged back during spring and early summer due to cool breezes off the sea. This is most notable in May and June when warm Atlantic air chills over the sea and creates sea fog, but this evaporates by the time the air has passed over the Ridgeway and warmed up a bit. In Winter the air can be 3 or more degrees warmer in weymouth compared to Dorchester when wind is coming off the sea, but maybe only one to 2 degrees warner when wind comes rom the North, maybe less if the wind is strong.

2. Rainfall is often triggered as warm Atlantic air is forced up over higher land meaning that it can be sunny in Weymouth and cloudy and maybe raining in Dorchester. Conversely if we get a wet North westerly, it will rain on the North facing uphill side of the Ridgeway but the air will warm and become drier as it flows down the Weymouth side of the ridge and rainfall ends. (explore the Fohn effect?)

Now think about conditions for snow.

If strong wind comes from the North, we are usually marginal for any precipitation because of the point in 2 above as well as because this tends to be a relatively 'dry' wind direction anyway over England. So Weymouth will get light precipitation even though air temperatures might be low. If wind is light northerly you get the same effect but also the warm sea nearby has the effect of raising air temperatures and making temperatures more marginal for snow in Weymouth, therefore a tendency for light sleet.

With wind from the Southwest - there may be plenty of moisture in the air but air temperatures too high for snow to settle. Go up the Ridgeway and the sea effect is a little less and the extra height both reduces air temperature and also triggers 'convective' effect leading to heavier precipitation which in turn cools the air mass as it rises. Net effect - more snow.

In fact this effect becomes even more pronounced on the downland up towards Shaftesbury, which is why I head up there for snow. Typically it is 1 to 2 degrees cooler there than in Dorchester and therefore maybe 3 or 4 degrees cooler than Weymouth. The added height can give still more precipitation from a South Westerly with snow depths commonly twice those of Dorchester.

Very occasionally the conditions are just right and Weymouth can get a real dump of snow, but that is a very rare event as you are probably aware. I believe similar 'microclimate ' effects are found in a number of seaside locations, Poole is one and parts of Devon and Cornwall too- Torbay referred to as the English Riviera.

There is another factor that i do not fully understand that others may wish to comment upon (and maybe correct me if I have got any of this wrong!), that is the way that snow cloud apparently heading North towards us from France, tends to sheer away in a more Easterly direction up the English Channel to make landfall in Sussex and Kent. This effect seems less pronounced in summer when thunderstorms coming in from France over Portland seem happy to move straight inland rather than sheer off up the Channel. This may have to do with there being reversed air/sea temperature differential in summer to that in winter. (winter is cold land / warm sea, summer is cold sea / warm land) My personal assumption is that cooler wet air is less inclined to rise over higher cold land mass and therefore sheers sideways, whereas relatively cool moist summer air off the sea is warmed over land and will readily convect and lift over the higher ground and be displaced less sideways.

Thanks egret a very informative reply! With current model outputs, I don't think we'll see snow either side of the ridgeway for a couple of weeks at least.

[Frosty_wsm]

I see the experienced posters in the model discussin thread are talking about the models showing a pattern change to cold being highlighted in the recent runs, that's exactly why you should never right off the rest of this month, let alone the rest of the winter! Will be interesting to see how this all pans out.

[Andy Bown]

Hope people are making the most of a glorious day, there won't be many of these in the next little while!

[MP-R]

Hope people are making the most of a glorious day, there won't be many of these in the next little while!

Quite right. I'm really not looking forward to the next few days. Bring on the next cold spell, sooner rather than later.

[Dartmoor_Matt]

Woo, lots of surface water around today and the rivers are very high on the way to work. But the sun is out. That said, still feels cold in the wind,

Least its not as bad as Qld, have friends in Brisbane and its getting worse all the time there.

Total for yesterday was 41mm of rain here... with another 3.5mm after midnight. Good job Dartmoor is a giant sponge!

[egret]

Thanks egret a very informative reply! With current model outputs, I don't think we'll see snow either side of the ridgeway for a couple of weeks at least.

Yes, I tend to agree, but don't give up yet.

I was expecting real cold to return around the 20th Jan but currently feel less confident that this will be either very cold or snowy.

Much of our best Dorset snow arrives in February or even early March - probably because the sea is at about its coldest then, offsetting or minimising the sea temperature effects described . I believe the sea temps have dropped 2 degrees since November but will probably have stabilised over the last two weeks.

Edited January 11, 2011 by egret

[Bristawl Si]

I see the experienced posters in the model discussin thread are talking about the models showing a pattern change to cold being highlighted in the recent runs, that's exactly why you should never right off the rest of this month, let alone the rest of the winter! Will be interesting to see how this all pans out.

Not after this evening's models they're not! Things looking bleak for cold and snow lovers - and that includes me. I'm not writing off winter, by any means, but my opinion made last weekend of 'no sign of cold and snow' for us sou westerners for 10 days' is looking v likely - and if i was gambling man i'd say we'll have to look well into the last week of Jan for first signs of return of cold.......and i'm not banking on that happening either!

[legritter]

just had a look at ,sat 24 site , havent looked for a couple of weeks . they have upgraded site . current radar looks threatening ,still looking fairly promising for around the 21st .but i wont get too excited yet . at least the reservoirs will fill up in time for bq summer ,

[Frosty_wsm]

And todays models ditch the pattern change they were showing yesterday lol, but that just goes to reinforce the fact that nobody should write off the rest of january in terms of seeing any cold and snow, the pattern change they picked up on yesterday could quite easily be back again tomorrow.

[❄️❄️❄️❄️❄️❄️❄️❄️❄️]

I think the title is a bit optimistic now...