Well after all the uncertainty of the past week over where the block will
sit, if indeed it wouldd. The models now seem to converge on the idea of a
Scandi block with a substantial deep vortex over eastern Canada. The die is
cast for a battle royal. Cold air will establish over the UK after today
forming a wedge. The deep vortex over Canada will help spawn deep lows in
western Atlantic with extremely cold air pushed SE into the Atlantic slowly
warming. As the lows approach the UK numerous trough disruptions look set to
take place with warm air occluding out and fronts edging into the west,
especially SW. These obviously bring the threat of widespread frontal snow
perhaps turning to rain before the cold comes back after each disruption.
Now, with phase 2 of the SSW expected soon and yet more uncertainty, it
would clearly be imprudenet to suggest how long that regime will continue.
All very interesting, and for meteorologists could be one of the most
fascinating winters for a while. Cold lovers will get satisfaction this
week, folk in the SW and Northern Ireland will be biting their nails "will
it, won't it" LOL.
COME ON ! :-)

Will
--
http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm
Will Hand (Haytor, Devon, 1017 feet asl)
---------------------------------------------

Will, wouldn’t cold air from the west have to be cold thru the depth, since there won’t be a surface inversion layer? I would guess an 850mb temperature of -6c would produce at best a sleety mix. You would need to go down to -8c before getting proper snow. Are we likely to see such cold air from the current set up?


On Monday, January 14, 2013 8:33:48 AM UTC, wrote:
Well after all the uncertainty of the past week over where the block will

sit, if indeed it wouldd. The models now seem to converge on the idea of a

Scandi block with a substantial deep vortex over eastern Canada. The die is

cast for a battle royal. Cold air will establish over the UK after today

forming a wedge. The deep vortex over Canada will help spawn deep lows in

western Atlantic with extremely cold air pushed SE into the Atlantic slowly

warming. As the lows approach the UK numerous trough disruptions look set to

take place with warm air occluding out and fronts edging into the west,

especially SW. These obviously bring the threat of widespread frontal snow

perhaps turning to rain before the cold comes back after each disruption.

Now, with phase 2 of the SSW expected soon and yet more uncertainty, it

would clearly be imprudenet to suggest how long that regime will continue..

All very interesting, and for meteorologists could be one of the most

fascinating winters for a while. Cold lovers will get satisfaction this

week, folk in the SW and Northern Ireland will be biting their nails "will

it, won't it" LOL.

COME ON ! :-)



Will

--

http://www.lyneside.demon.co.uk/Hayt...antage_Pro.htm

Will Hand (Haytor, Devon, 1017 feet asl)

---------------------------------------------

Will, wouldn’t cold air from the west have to be cold thru
the depth, since there won’t be a surface inversion layer? I would guess
an 850mb temperature of -6c would produce at best a sleety mix. You
would need to go down to -8c before getting proper snow. Are we likely
to see such cold air from the
current set up?

Don't forget that cooling from latent heat effects will be pronounced at
times in these frontal battlegrounds, producing deep isothermal layers
around melting point - so rules of thumb using temperatures 5000 feet up in
the atmosphere need to be used with care.
--
Freddie
Bayston Hill
Shropshire
102m AMSL
http://www.hosiene.co.uk/weather/
https://twitter.com/#!/BaystonHillWx for hourly reports

But wouldn't that depend on the precipitation being heavy, and the relative humidity being low enough to take on board some of the latent heat? I'm more thinking of the returning polar-maritime air after the pre-frontal continental air has been nudge out, i.e. the air mass behind the [warm] occlusion/sector.



On Monday, January 14, 2013 9:52:34 AM UTC, Freddie wrote:
Will, wouldn�t cold air from the west have to be cold thru

the depth, since there won�t be a surface inversion layer? I would guess

an 850mb temperature of -6c would produce at best a sleety mix. You

would need to go down to -8c before getting proper snow. Are we likely

to see such cold air from the

current set up?



Don't forget that cooling from latent heat effects will be pronounced at

times in these frontal battlegrounds, producing deep isothermal layers

around melting point - so rules of thumb using temperatures 5000 feet up in

the atmosphere need to be used with care.

--

Freddie

Bayston Hill

Shropshire

102m AMSL

http://www.hosiene.co.uk/weather/

https://twitter.com/#!/BaystonHillWx for hourly reports

"Alan (North West Surrey)" wrote in message
...
Will, wouldn’t cold air from the west have to be cold thru the depth, since
there won’t be a surface inversion layer? I would guess an 850mb temperature
of -6c would produce at best a sleety mix. You would need to go down to -8c
before getting proper snow. Are we likely to see such cold air from the
current set up?
====================

It's not cold air from the west per se that will produce snow. It's actually
the warm occluded Atlantic air rising over a cold wedge. 850s could well be
around 0C and it will snow heavily if the surface air wet-bulb is close to
or below zero. Generally the boundary layer temperature structure approaches
isothermal in these situations at around 0 to -1C. In those situations
snowflakes will be large, wet and sticky, giving rapid accumulation. Of
course, it all eases off and turns to rain quickly once the surface wet-bulb
rises above zero when the front comes through and surface Atlantic air
arrives. Forecasting that timing and location is the tricky bit not whether
it will rain or snow. Trough disruptions means that fronts will tend to
"slide away SE" as they approach the block.

PS as a general rule of thumb for lowland UK I take -5C at 850 as a good
snow line. -3C for my altitude at 1000 feet.
The best indicator though is wet-bulb freezing level. At sea-level you need
that at around 200 metres for frontal snow. Obviously showers are different
as they are normally falling in drier airmasses and the wet-bulb freezing
level can descend quickly in heavy precipitation and lightish winds.

HTH

Will
--

Thanks Will for that explanation. Apologizes I misunderstood, I thought you were also referring to the cold Canadian air pushing through with the rain perhaps turning back to snow in this air mass.

As a rule of thumb if there's a cold continental undercut I use a zero degrees 850mb temperature as guidance, anything higher and you may actually get freezing rain, if the undercut is cold enough. If there's ocean air at the surface, like we have today, I'm more conservative and use -9c as guidance as this should give a surface temperature of zero allowing 'proper' snow. I found anything higher usually ends in disappointment and a unpleasant slushy mess.

Oh well at least it isn't mild!


On Monday, January 14, 2013 10:01:43 AM UTC, wrote:
"Alan (North West Surrey)" wrote in message

...

Will, wouldn�t cold air from the west have to be cold thru the depth, since

there won�t be a surface inversion layer? I would guess an 850mb temperature

of -6c would produce at best a sleety mix. You would need to go down to -8c

before getting proper snow. Are we likely to see such cold air from the

current set up?

====================



It's not cold air from the west per se that will produce snow. It's actually

the warm occluded Atlantic air rising over a cold wedge. 850s could well be

around 0C and it will snow heavily if the surface air wet-bulb is close to

or below zero. Generally the boundary layer temperature structure approaches

isothermal in these situations at around 0 to -1C. In those situations

snowflakes will be large, wet and sticky, giving rapid accumulation. Of

course, it all eases off and turns to rain quickly once the surface wet-bulb

rises above zero when the front comes through and surface Atlantic air

arrives. Forecasting that timing and location is the tricky bit not whether

it will rain or snow. Trough disruptions means that fronts will tend to

"slide away SE" as they approach the block.



PS as a general rule of thumb for lowland UK I take -5C at 850 as a good

snow line. -3C for my altitude at 1000 feet.

The best indicator though is wet-bulb freezing level. At sea-level you need

that at around 200 metres for frontal snow. Obviously showers are different

as they are normally falling in drier airmasses and the wet-bulb freezing

level can descend quickly in heavy precipitation and lightish winds.



HTH



Will

--

But wouldn't that depend on the precipitation being heavy
Not necessarily heavy (moderate will do), but obviously the heavier the
better.

and the relative humidity being low enough to take on board some of the
latent heat?
Anything less than 100% will give you some cooling.

I'm more thinking of the returning polar-maritime air after the
pre-frontal continental air has been nudge out, i.e. the air mass behind
the [warm] occlusion/sector.
I think that any precipitation will be rain in the following air mass. Will
gives a better description in his reply.
--
Freddie
Bayston Hill
Shropshire
102m AMSL
http://www.hosiene.co.uk/weather/
https://twitter.com/#!/BaystonHillWx for hourly reports

In article ,
Eskimo Will writes:
huge snip
Now, with phase 2 of the SSW expected soon

Could you tell us who is expecting that and why, Will? Do we have any
way of telling in advance if a SSW is likely to continue or when it is
likely to end?
--
John Hall

"Whenever people agree with me I always feel I must be wrong."
Oscar Wilde

"John Hall" wrote in message
...
In article ,
Eskimo Will writes:
huge snip
Now, with phase 2 of the SSW expected soon

Could you tell us who is expecting that and why, Will? Do we have any
way of telling in advance if a SSW is likely to continue or when it is
likely to end?

John, phase one is the vortex splitting which is now complete. Phase two is
the descent of the zonal wind reversal down into the troposphere. I was
thinking of the latest ECM prediction
http://wekuw.met.fu-berlin.de/~Aktue...lert=1&lng=eng
looks like completing within a few days.
ECM forecasts have proved reliable.

Will
--

In article ,
Eskimo Will writes:

"John Hall" wrote in message
.. .
In article ,
Eskimo Will writes:
huge snip
Now, with phase 2 of the SSW expected soon

Could you tell us who is expecting that and why, Will? Do we have any
way of telling in advance if a SSW is likely to continue or when it is
likely to end?

John, phase one is the vortex splitting which is now complete.
Phase two is the descent of the zonal wind reversal down into the
troposphere. I was thinking of the latest ECM prediction
http://wekuw.met.fu-berlin.de/~Aktue...lert=1&lng=eng
looks like completing within a few days.
ECM forecasts have proved reliable.

Thanks, Will.
--
John Hall

"Whenever people agree with me I always feel I must be wrong."
Oscar Wilde