Hi there,

Yesterday's band of snow was pretty unusual for a weather front, what with
the sudden temp drop, heavy snow, thunder & lightning and all. What would
you say it was we experienced? The closest thing I can think of is a squall
line. That's the great thing about the weather - just when you think you've
got it sussed nature throws something unexpected at you.

Regards.

David


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In article ,
David Gartrell writes:
Yesterday's band of snow was pretty unusual for a weather front, what with
the sudden temp drop, heavy snow, thunder & lightning and all. What would
you say it was we experienced? The closest thing I can think of is a squall
line.

I'd call it an active cold front (but a "squall line" may just be
another way of saying the same thing0.

That's the great thing about the weather - just when you think you've
got it sussed nature throws something unexpected at you.

Yep.
--
John Hall
"Three o'clock is always too late or too early
for anything you want to do."
Jean-Paul Sartre (1905-1980)

"David Gartrell" wrote in
:

Yesterday's band of snow was pretty unusual for a weather front, what
with the sudden temp drop, heavy snow, thunder & lightning and all.
What would you say it was we experienced? The closest thing I can
think of is a squall line. That's the great thing about the weather -
just when you think you've got it sussed nature throws something
unexpected at you.

It looked as though we had some sort of hybrid squall line/line convection
event - it reminded me, in sort, of a squall line type event described by
Browning and Reynolds in the Quarterly Journal of the Royal Met Soc in 1993.

Diagnostic study of a narrow cold-frontal rainband and severe winds
associated with a stratospheric intrusion
235 - 257
KA BROWNING; R REYNOLDS

Cheers
Richard

As a new poster to this group have found the last two days exiting and
informative. To be able to take my partner out in the road with the first
rain falling from the leading edge of the front saying "Watch this!" and
then the rain / snow hitting accompanied by the temperature fall monitored
on a temp probe borrowed from our Australian Bearded Dragon's reptarium was
a splendid climax to the tracking of the front down the country.
I've been called a geek a few times this past few days... but it sure paid
off. Even my partner was running from probe position calling out the temp
readings and back again as we both marvelled at the drama unfolding.
Thanks all for the hints, tips, obs, photos and humour. It's been a
memorabe few days.

Les



Richard Dixon wrote:
"David Gartrell" wrote in
:

Yesterday's band of snow was pretty unusual for a weather front,
what with the sudden temp drop, heavy snow, thunder & lightning and
all. What would you say it was we experienced? The closest thing I
can think of is a squall line. That's the great thing about the
weather - just when you think you've got it sussed nature throws
something unexpected at you.

It looked as though we had some sort of hybrid squall line/line
convection event - it reminded me, in sort, of a squall line type
event described by Browning and Reynolds in the Quarterly Journal of
the Royal Met Soc in 1993.

Diagnostic study of a narrow cold-frontal rainband and severe winds
associated with a stratospheric intrusion
235 - 257
KA BROWNING; R REYNOLDS

Cheers
Richard

This event was very similar to last year almost to the day on the 30th. A
trough line moved south with very strong winds, imbedded thunderstorms, snow
squalls with the tragedy of a child being killed by a falling tree in the
playground of a Surrey school. In fact this situation could become more
common. The high arctic is still very cold for instance around Svalbard it
has been minus 30C or so over the past week or so. As this cold air moves
south when there is low pressure over Scandinavia it comes across warmer
seas and then it becomes unstable. As it moves south to our latitude where
the sea temperature is relatively high at 9C- 10C but upper air is still
very cold this is highly unstable. This effect could increase as the seas
tend to become that little bit warmer in our latitude especially as there is
no prolonged winter cold these days. When the air moves south it behaves
very much like water down a spillway of a dam or when you see water run down
a road on a hill. It takes a pulse form occurring in waves i.e. organized
into bands. This bands are areas of uplift where the clouds had sufficient
depth and upthrust to create enough electric charge to produce lightning.
These days the mild air is quickly restored but the cloud depth could be
enhanced as there is more of a temperature contrast from surface to cloud
top.
Active thunderstorms preceded the great burst of cold in 1895 when on the
23rd January vicious thunderstorms accompanied by snow occurred on a very
active cold front and there was a lot of damage in southern England.
Thereafter there was a very severe cold spell which lasted for much of
February.

Ian Currie--Coulsdon
www.frostedearth.com







"David Gartrell" wrote in message
...
Hi there,

Yesterday's band of snow was pretty unusual for a weather front, what
with
the sudden temp drop, heavy snow, thunder & lightning and all. What would
you say it was we experienced? The closest thing I can think of is a
squall
line. That's the great thing about the weather - just when you think
you've
got it sussed nature throws something unexpected at you.

Regards.

David


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"Waghorn" wrote in
:

Hi Rich,
my view on this is that this was a squall line event forced by a low
level cold front propagating as a gravity current.Frictional effects,as
the cold air headed south,wld've sharpened the frontal boundary.I'm not
sure about the role of the upper air,though mode of propagation of the
GC is sensitive to enviromental shear,and p'haps some tropopause
folding occured as well.It's possible the Chilbolton image I pointed
out shows this GC head.

Hi Dave

I imagine the colder denser air behind the cold front could well have acted
as a density current, especially when augmented by sublimation of the snow
falling from the cloud above - I think the mechanism for this sort of front
was still in question when I left it, one author tried to rubbish the
density current idea, others still held on to it.

Plenty of upper level support for this one, look how the dry slot
dramatically appears in the imagery between 1130 and 1730. It may be that
the dry air overran the surface front as well as dug in behind, causing some
convective instability that led to the storms? Plenty of food for thought.

http://meteosat.e-technik.uni-ulm.de...V/E2/20040128-
1130-WV-E2.gif
http://meteosat.e-technik.uni-ulm.de...V/E2/20040128-
1730-WV-E2.gif

Cheers
Rich

It looked as though we had some sort of hybrid squall line/line convection
event - it reminded me, in sort, of a squall line type event described by
Browning and Reynolds in the Quarterly Journal of the Royal Met Soc in 1993.

Diagnostic study of a narrow cold-frontal rainband and severe winds
associated with a stratospheric intrusion
235 - 257
KA BROWNING; R REYNOLDS

Cheers
Richard
Hi Rich,
my view on this is that this was a squall line event forced by a low level cold front propagating as
a gravity current.Frictional effects,as the cold air headed south,wld've sharpened the frontal
boundary.I'm not sure about the role of the upper air,though mode of propagation of the GC is
sensitive to enviromental shear,and p'haps some tropopause folding occured as well.It's possible the
Chilbolton image I pointed out shows this GC head.
Classic papers on this are-
Carbone, RE, 1982: A severe frontal rainband.Part I: Stormwide hydrodynamic structure. J. Atmos.
Sci., 39, 258-279
Shapiro, M.A., T. Hampel, D. Rotzoll, and F. Mosher, 1984: The frontal hydraulic head: A micro-alpha
triggering mechanism for mesoconvective weather systems. Mon. Wea. Rev., 113, 1166-1183.
Anyone not familiar with GCs see-
http://www.damtp.cam.ac.uk/user/fdl/...14/jeshome.htm
'cleft and lobe' instabilty at the head forces convergence and updrafts which form Cbs.(cf the sea
breeze or US dry line),

--
regards,
david
(add 17 to waghorne to reply)

Ooops.. sorry, "Thanks All" should have been a new post. Interesting
thread.. distracted me.

Les

Plenty of upper level support for this one, look how the dry slot
dramatically appears in the imagery between 1130 and 1730. It may be that
the dry air overran the surface front as well as dug in behind, causing some
convective instability that led to the storms? Plenty of food for thought.
Yes,there does seem to be a dry area in WV that moves SW then SE.A few signs of overrunning in this
image-
http://www.sat.dundee.ac.uk/abin/pro...roject.ch4.jpg

The kinematics of gravity current are fairly simple,at least in theory.I wonder if wld be possible
to figure the rate of motion of the cold front and compare it with GC theory,allowing for frictional
effects,

--
regards,
david
(add 17 to waghorne to reply)

Yesterday's band of snow was pretty unusual for a weather front, what
with
the sudden temp drop, heavy snow, thunder & lightning and all. What would
you say it was we experienced? The closest thing I can think of is a
squall
line. That's the great thing about the weather - just when you think
you've
got it sussed nature throws something unexpected at you.

Early April 1968 is indelibly etched in my mind when Scotland experienced
electrical snowstorms and lunchtime temps. below freezing courtesy of the
after-effects of a well-defined cold front.I am reliably told Reykjavik's
midday temp. may have been around -16c then indicating the nature and fetch
of the Arctic blast.In those days the shipping reports frequently warned of
severe or very severe icing in the Icelandic seas,something that would be a
rarity now methinks.