.... and another routine question that pops up about this time of year
(and in this situation) relates to thickness (500 - 1000 hPa variety)
*extremes*.

This is covered in the FAQ he-

http://weatherfaqs.org.uk/node/67

and by following the link at the bottom, you find a diagram giving an
'idea' of the extremes at a selection of locations for each month.
Note the caveats at the bottom.

The latest GFS run (12Z as I write this) out to T+180 has a large area
sub-512 dam across Wales, most of Scotland and large area of north &
central England. This would appear to be nudging the lower boundary of
those figures for November ... and the series would have included the
cold spells during November 1985 which, in some respects, is an
approximate analogue to the current set-up, though note that in that
month the *whole* month was cold, not just the latter third.

However, this is a long way off! Experience would suggest that models
tend to 'over-egg the pudding' particularly by 'over-advecting' model
cold air downwind - and of course we *are* dealing with the 'model'
atmosphere not the real one!

Tricky thing - reality.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

On 21/11/10 21:40, Martin Rowley wrote:
... and another routine question that pops up about this time of year
(and in this situation) relates to thickness (500 - 1000 hPa variety)
*extremes*.

This is covered in the FAQ he-

http://weatherfaqs.org.uk/node/67

and by following the link at the bottom, you find a diagram giving an
'idea' of the extremes at a selection of locations for each month.
Note the caveats at the bottom.

The latest GFS run (12Z as I write this) out to T+180 has a large area
sub-512 dam across Wales, most of Scotland and large area of north&
central England. This would appear to be nudging the lower boundary of
those figures for November ... and the series would have included the
cold spells during November 1985 which, in some respects, is an
approximate analogue to the current set-up, though note that in that
month the *whole* month was cold, not just the latter third.

However, this is a long way off! Experience would suggest that models
tend to 'over-egg the pudding' particularly by 'over-advecting' model
cold air downwind - and of course we *are* dealing with the 'model'
atmosphere not the real one!

Tricky thing - reality.

Martin.



Out of interest, typically what surface temperatures, daytime and
nighttime would you expect for, say, a 510dam thickness assuming clear
skies? Can't remember experiencing a thickness that low before.

"Martin Rowley" wrote in
:

The latest GFS run (12Z as I write this) out to T+180 has a large area
sub-512 dam across Wales, most of Scotland and large area of north &
central England. This would appear to be nudging the lower boundary of
those figures for November ... and the series would have included the
cold spells during November 1985 which, in some respects, is an
approximate analogue to the current set-up, though note that in that
month the *whole* month was cold, not just the latter third.

Martin

I've gone back to my thickness study I stuck on here last month - top 10
lowest thicknesses I could find from the NCEP reanalysis (which I realise
will have its errors, and I've only looked from 1960-2000) is - and these
are daily mean thickness for a gridpoint in west/central England:

Date TT500
22/11/1965 517.2
19/11/1971 519.2
19/11/1962 519.4
21/11/1993 519.4
29/11/1969 519.7
23/11/1993 520.7
28/11/1985 520.8
22/11/1993 520.8
28/11/1980 521.2
25/11/1978 521.3

Richard

"Adam Lea" wrote ...
snip
Out of interest, typically what surface temperatures, daytime and
nighttime would you expect for, say, a 510dam thickness assuming
clear skies? Can't remember experiencing a thickness that low
before.


.... In the January 1987 severe cold spell, when thickness values
dropped below 500 dam (probably, briefly, around 495dam), then
widespread day maxima below -6degC were recorded across southern
Britain .. (12th).

I developed a diagram he-
http://weatherfaqs.org.uk/node/155

which attempted to relate theoretical maxima to total thickness; this
was based on various papers written back in the 1950s & 1960s when
this method of forecasting extremes was all that was used ... things
have moved on, and in particular, we now use algorithms based on
vertical slices of the atmosphere below 1500 m, and in these extreme
cases (i.e. very low thickness values), the lowest few tens of metres
of troposphere often becomes 'uncoupled' from the general air mass and
both maxima and minima can depart wildly from any relationship
developed on a layer meaned over 5km. See also the notes at the bottom
of the referenced/linked page.

On the continent last winter (09/10), a cold pool [defined by the
500-1000 hPa thickness] of ~510dam drifted across southern Poland and
parts of Germany (Saxony, Thuringia etc.) 18th/19th December 2009:
remembering that this area is in the middle of a continent, and was
already well-chilled/snow-covered, the typical day maxima were -14 and
night minima -17 (over the lower ground). In our part of the world,
given the more 'maritime' character of our weather, I wouldn't expect
anything like such low values, at least not for the maxima; minima, as
pointed out above, behave completely differently, especially over
snow-covered surfaces.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

"Richard Dixon" wrote ...
I've gone back to my thickness study I stuck on here last month -
top 10
lowest thicknesses I could find from the NCEP reanalysis (which I
realise
will have its errors, and I've only looked from 1960-2000) is - and
these
are daily mean thickness for a gridpoint in west/central England:

Date TT500
22/11/1965 517.2
19/11/1971 519.2
19/11/1962 519.4
21/11/1993 519.4
29/11/1969 519.7
23/11/1993 520.7
28/11/1985 520.8
22/11/1993 520.8
28/11/1980 521.2
25/11/1978 521.3

Richard


.... interesting, thanks Richard: *if* the GFS output is correct (still
indicating sub-512 dam), then we'll be testing all of those values
come this weekend!

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023

?"Dave Cornwell" wrote in message
...
I can vouch for the havoc caused by the 520 thickness at exactly this time
of year in 1993. I was due to fly from Heathrow
I hadn't realised that the thickness was so high then!

I remember the snow well - it started bucketing down at school and of course
any hope of us lot concentrating was dashed - especially as the French
teacher kept looking out of the window too! Eventually a Tannoy announcement
was made that all pupils who live on the Isle of Sheppey were to go home
immediately and buses were on their way. As I waited for the bus the snow
was dripping from the rose bushes planted at the front of the school and one
of my friends reckoned it was because they were photosynthesising and
warming up! (He went on to get 4 As at A-level, must have been a blip that
day).

Back home there was around 3 inches of slushy snow and by the end of it we
had just under 6 inches - which froze solid overnight. The snow stayed in
the ditches by the fields for a good week or so before finally melting.

850s seem to have been around -10C (briefly) and my end of the (9 mile by 5)
island got far more snow than the other end simply because of the onshore
wind... SSTs were around 13C but all it did was add fuel to the clouds.

This time around? Who can say, but I'm expecting to see at least some
snowfall despite slightly higher 850s and a slightly cooler sea.

"Richard Dixon" wrote ...
I've gone back to my thickness study I stuck on here last month -
top 10
lowest thicknesses I could find from the NCEP reanalysis (which I
realise
will have its errors, and I've only looked from 1960-2000) is - and
these
are daily mean thickness for a gridpoint in west/central England:
Date TT500
22/11/1965 517.2
19/11/1971 519.2
19/11/1962 519.4
21/11/1993 519.4
29/11/1969 519.7
23/11/1993 520.7
28/11/1985 520.8
22/11/1993 520.8
28/11/1980 521.2
25/11/1978 521.3
Richard


.... have just looked at the soundings from yesterday (27th November,
nominal time 12Z), and at Lerwick [Shetland] the total thickness
(500-1000 hPa) was 507 dam, and at Albermarle [Northumberland] it was
511 dam. Using the data published in the FAQ (reference elsewhere in
the thread) then these would appear to fall *outside* the minimum
values implied from that series for November (but note caveats
regarding period used etc.).

Following Richard's post (above), although we don't have an explicit
value for 'west/central' England (somewhere like the old Aughton), I
reckon we've gone below even the lowest value he quoted (517 dam
November 1965), with Watnall/Nottingham (east Midlands) 517 dam at
27/00Z and 513 dam at 28/00Z; the coldest air would have passed over
them late yesterday.

I made a comment that reality doesn't always accord with model at the
range of the forecast ( roughly a week ahead ). I am very happy to
note that the model (GFS in this case) played a blinder & I should
have had more faith!

A superb indication on the 'depth' of the cold air (in terms of how
cold) and where the cold air would be.

Martin.


--
Martin Rowley
West Moors, East Dorset (UK): 17m (56ft) amsl
Lat: 50.82N Long: 01.88W
NGR: SU 082 023