FrenchScotPilot1
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Posts posted by FrenchScotPilot1
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Based purely on Skew-T (for Aberdeen) I see good potential convection this evening and overnight with thicker clouds and a higher inversion.
Winds are also fovourable with lack of shear, leading to organised bands of showers off the E'lies.
Granted, N'Sea SSTs had been warmer it would have been much more subtantial. But with ifs...
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I know it is a national passtime in this country (and probably other ones too) but there is nothing people can do.
Yes it's been long, yes last year was different (was it seasonal though, probably not).
Wait and ride it out
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The following chart is the only thing close to decent convection I can see for Aberdeen at the moment. It is for tomorrow 12.00.
But even there, it shows conditionnally unstable conditions (unstable for moist air), we'd need absolute instability.
Cloud thickness is not that great either (around 1,500 feet) and the inversion is quite low (800 hPa).
Winds are light for efficient moisture pick up. Winds ate 850 hPa are light too, therefore penetration inland will be limited.
The more I write about it the more I realise it's not great
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And in Dyce
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Who's for battleground situation for Easter week end as the Atlantic weather system makes its way in?
White Easter anyone? I know it is a long way away but it is interesting to speculate...
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The following is theoretical knowledge gathered from various sources. I don't have any operational forecasting experience and I'm very keen to learn more. So, here goes my humble take.
To answer the query above, clouds occur when dew point line and environmental line are close to each other, i.e. high relative humidity. Where the 2 lines separate, air is dry and cloud free.
The inversion (cap) is where the temperature increases with increasing height and marks the top of the convective layer, this is where air gets warmer.
So you want the 2 lines to stay close to each other for a bit to ensure sufficient cloud thickness.
For organised heavy showers, you need instability, wind/fetch, favourable wind shear, moisture.
Instability: For the vertical transport of moisture and heat, an absolute instability is required. As a measure for this instability in the boundary layer, the temperature difference between sea surface and 850 hPa is used. The temperature difference needs to exceed 13 °C. Affected indirectly by this, is the inversion. If it is found below 1000 meters, the boundary layer is considered to be too shallow for the development of convective showers. If the inversion is below 2500 meters (higher than 850 hPa) usually (heavy) snowfall may be taken into account.
Wind/Fetch:
Another factor playing a role in the formation of the cloud streets is the wind. Sufficient wind must be available allowing the polar air to flow over sea and affecting enough moisture in the direction of the coastal regions. More wind means a better exchange of moisture between the "warm" seawater and the cold air above. Too much wind however is not ideal as it would mean less exchange of moisture. The wind direction at 850 hPa helps determine the areas which are likely to be affected by snow. The direction also helps to determine the fetch. The fetch is the distance the showers move over relatively warm water; the longer the fetch the more active the convection will also will be. For serious convection the fetch needs to be at least 80 km.
Wind shear:
Directional shear is one of the most important factors governing the development of the cloud streets; environments with weak directional shear typically produce more intense convection than those with higher shear levels. If directional shear between the surface and 700 hPa level is greater than 60°, hardly any convection and showers will be experienced. If the directional shear between the surface and 700 hPa level is between 30 and 60°, weak lake-effect bands are possible. In environments where the shear is less than 30° strong, well organized bands can be expected.
Moisture:
High relative humidity upstream moisture is needed.
Synoptic conditions: Cold air advection at mid levels will also help by increasing the instability (parcel temperature warmer than environment). Maximum vorticity advection aloft will help by increasing the the large scale upward motion (positive vorticity dissipation out of a trough leads to upper level divergence which draws surface air upwards to replace the upper mass depletion).
I hope it helps a little.
A little addition...
When the 2 lines are close together but only for a short depth of the atmosphere, the clouds are shallow.
This is what is happening right now: light snow showers from stratocumulus/cumulus clouds with very limited vertical extent, 2000-3000 feet at most.
When the right conditions are present, deep convection create towering cumulus and cumulonimbus clouds with 8000-10000 feet extent, able to support large updraughts and creating heavy snow showers.
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i think if i remember what LS told me before the cap on convection on them is pretty low and look more like snizzle than anything decent
i think its where the two vertical lines seperate is where the cap is on the cloud level as where they seperate is where there is a dry level and convection cant build past there so that where the cap is so think u have it right lorenzo
The following is theoretical knowledge gathered from various sources. I don't have any operational forecasting experience and I'm very keen to learn more. So, here goes my humble take.
To answer the query above, clouds occur when dew point line and environmental line are close to each other, i.e. high relative humidity. Where the 2 lines separate, air is dry and cloud free.
The inversion (cap) is where the temperature increases with increasing height and marks the top of the convective layer, this is where air gets warmer.
So you want the 2 lines to stay close to each other for a bit to ensure sufficient cloud thickness.
For organised heavy showers, you need instability, wind/fetch, favourable wind shear, moisture.
Instability: For the vertical transport of moisture and heat, an absolute instability is required. As a measure for this instability in the boundary layer, the temperature difference between sea surface and 850 hPa is used. The temperature difference needs to exceed 13 °C. Affected indirectly by this, is the inversion. If it is found below 1000 meters, the boundary layer is considered to be too shallow for the development of convective showers. If the inversion is below 2500 meters (higher than 850 hPa) usually (heavy) snowfall may be taken into account.
Wind/Fetch:
Another factor playing a role in the formation of the cloud streets is the wind. Sufficient wind must be available allowing the polar air to flow over sea and affecting enough moisture in the direction of the coastal regions. More wind means a better exchange of moisture between the "warm" seawater and the cold air above. Too much wind however is not ideal as it would mean less exchange of moisture. The wind direction at 850 hPa helps determine the areas which are likely to be affected by snow. The direction also helps to determine the fetch. The fetch is the distance the showers move over relatively warm water; the longer the fetch the more active the convection will also will be. For serious convection the fetch needs to be at least 80 km.
Wind shear:
Directional shear is one of the most important factors governing the development of the cloud streets; environments with weak directional shear typically produce more intense convection than those with higher shear levels. If directional shear between the surface and 700 hPa level is greater than 60°, hardly any convection and showers will be experienced. If the directional shear between the surface and 700 hPa level is between 30 and 60°, weak lake-effect bands are possible. In environments where the shear is less than 30° strong, well organized bands can be expected.
Moisture:
High relative humidity upstream moisture is needed.
Synoptic conditions: Cold air advection at mid levels will also help by increasing the instability (parcel temperature warmer than environment). Maximum vorticity advection aloft will help by increasing the the large scale upward motion (positive vorticity dissipation out of a trough leads to upper level divergence which draws surface air upwards to replace the upper mass depletion).
I hope it helps a little.
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Trend through tomorrow will be for the medium- high level cloud to move southwards leaving better convective opportunities for the east, though still with potential frontal snow tonight for central areas courtesy of a pulse of heavier frontal precipitation moving north.
Is that what is missing today? Too many clouds ahead of the fronts hamper convection? Otherwise it would be good for more abondant snow showers up the East coast?
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Stunningly cold again this morning with strong to gale force SE wind, very dull with flurries and currently 1c.One positive is that the ground is slowly drying up with the wind and some sublimation with the frosty wind overnight.
Can see snow plumes several hundred feet high swirling off the top of the Knock of Braemoray about 8 miles away. Hill conditions must be atrocious with Cairngorm staying stormbound until perhaps Monday(some kind of record 6 days?) Probably enough snow now to see skiing till mid summers day now once they get everything dug out.
Latest report at Cairgorms was: current winds of 84mph, temp -9C (16F) with wind chill of -26C (-15F)!
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Nice streamers running WN-SE offshore NE English coast. Showers are getting organised for hundred of miles there.
But as said above, not all conditions are fulfilled.
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Is that near englandshire?
I've been told it's south of Stonehaven somewhere
Completely off topic but nothing is going on anyway.
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You should be ok, it looks pretty much clear for both airports, although I'm not 100% sure where Gatwick is
Halfway between London and the S coast.
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Watching, waiting...
Observation round...
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Very very quiet in here, calm before the storm?
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LS,
Referring the the skew T diagram below for Aboyne on Friday at 12.00, I see:
- T850 of-11, with N Sea temps are around 5 derees we should achieve absolute instability
- little directional wind shear between the surface and 700 hPa should allow banded showers
- there is an inversion between 840 hPa and 730 hPa, it is shallow but present
- convective layer depth is about 850 hPa
It looks like good ingredients for moderate snow showers. What's your take?
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Simon Cardy tweeted: potential major snowfall parts UK Fri & weekend. map gives early indication of possible snow to 0900 Sat
https://twitter.com/weather_king/status/314640181712023552/photo/1
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Chaos into Dyce this morning, showers just as people head off into work, roads not cleared/gritted. Massive jams.
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It's only going to settle on any vertical surfaces BUS:
Especially the east coast based on this. Great time to go to the beach Friday lunchtime then
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heres dewpoints, uppers and thickness.
i am not sure what parameters we need in frontal snow for it to stay snow maybe LS can help me here
Fair question, I bow to LS's expertise here and await with great anticipation
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heres the NAE 2m temp chart from the end of the 12z run so assuming this is right and the front does make it in with 2m temps that low it shouldnt have any problem settling
What are the DP forecasted at?
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The explanation I saw said it was because the nth sea is several degrees cooler in March than December.
It depends what snow it's been talked about: convectinve or frontal.
A cooler sea means very cold uppers are needed to trigger convective showers (absolute instability).
For frontal activity, I don't think it makes a difference. Not sure why the N Sea is mentioned.
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Just requoting what I read, but it Might be because easter is 3 days rather than 1 also.
Or 4 if you count the full bank holiday weekend. Also they don't say where, could be true for the UK as a whole, but not for Sth England.
I would think more along the lines of Easter is at the end of the winter: the atmosphere had the whole winter to cool down, just before the Sun starts to get strong again.
December is the start. I'll stand corrected though
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Sure, the gist of it is that you need -8C 850hpa temperatures in an easterly flow for lying snow to occur in lowland eastern Scotland. It's not based on anything other than past experience of these events but so far the rule has held up pretty well. For anything other than easterlies or southeasterlies -5C upper air temperatures are usually fine and higher than that can still sustain snow under some circumstances (up to 0C even in a southerly flow with cold embedded at the surface) but without -8C or lower uppers the coastal marginality tends to lead to slushy coverings at best.
Great thanks! I knew abut the -5 or lower but it indeed depends on specific location.
Scotland - Alba Regional Discussion 20/03/13 21z ------>
in Regional
Posted
Yes, the Edinburgh sounding looks similar too.
Interpolating between Edinburgh and Aberdeen, I'd expect the whole east coast to experience the same conditions (plus or minus very localised effects).