London & South East Regional Discussion - January 14th 2013 12z Onwards

[Jasmine]

Any body know if it will it snow in East London all we got was light snow but it melted . â„â„⛄

[VillagePlank]

Why did we have so much disappointment today?

Let's look at GFS' Theta-W chart (my favourite snow forecast chart) This WBPT parameter encodes both temperature and humidity, so from that we can figure out - or at least have a good chance - of whether or not snow is going to fall.

Here are the charts for 12z, 15z, this afternoon.

As you can see the parameter goes up from lunchtime due to the 'airmass' change. A value of 3 pretty much means it's 1/2 chance (56%) 4 is about 1/3 (36%). The MetO went for snow, sleet, and later rain, because pretty much that's what the data told them; and it turned out just like that.

Now, what about tonight? Looking at 3am's chart,

It's obvious that the whole area is under a theta-w of 2 which is a 78% chance of precipitation falling as snow. Not a dead cert, but more people should see snow (orographic features, locally) than those who see rain. You can add 4% to each 50m ASL you are, so I am about 100m up, so I would see my chances as 86%. If it's still precipitating at 6am,

Which gives me about a 95% chance of precipitation falling as snow. Pretty much quids in, I reckon.

There are further corrections that can be made, but as a rule of thumb, I reckon, through experience, it's pretty good.

Edited January 14, 2013 by Boar Wrinklestorm

[abruzzi spur]

A pretty raw afternoon in Reigate with rain replacing the snow, with the odd burst of sleet, which has left things very wet, though it did clear up about 4:30pm.

Temperature is stubbornly refusing to fall at the moment and has been stuck at +2.7c for the last couple of hours. Dewpoint is -0.3c.

Not expecting much in the way of snow tonight in East Surrey but the next few days look increasingly colder and snow chances seem to increase later in the week.

AS

Edited January 14, 2013 by abruzzi spur

[steveinsussex]

Why did we have so much disappointment today?

Let's look at GFS' Theta-W chart (my favourite snow forecast chart) This WBPT parameter encodes both temperature and humidity, so from that we can figure out - or at least have a good chance - of whether or not snow is going to fall.

Here are the charts for 12z, 15z, this afternoon.

As you can see the parameter goes up from lunchtime due to the 'airmass' change. A value of 3 pretty much means it's 1/2 chance (56%) 4 is about 1/3 (36%). The MetO went for snow, sleet, and later rain, because pretty much that's what the data told them; and it turned out just like that.

Now, what about tonight? Looking at 3am's chart,

It's obvious that the whole area is under a theta-w of 2 which is a 78% chance of precipitation falling as snow. Not a dead cert, but more people should see snow (orographic features, locally) than those who see rain. You can add 4% to each 50m ASL you are, so I am about 100m up, so I would see my chances as 86%. If it's still precipitating at 6am,

Which gives about a 95% chance of precipitation falling as snow. Pretty much quids in, I reckon.

There are further corrections that can be made, but as a rule of thumb, I reckon, through experience, it's pretty good.

excellent post

[Sussex Jules]

Thanks for that explanation Steve M nice to actually hear why it went wrong and maybe we can learn from this. Let's all stay hopeful for that little white stuff falling from the sky.

[Lampostwatcher]

Is it going to snow here tonight?

[Snowman31]

PPN from the NW getting closer, hope it doesn't fade away before it arrives

[Sno' problem]

Looks like that the ppn heading towards us from the Midlands is still maintaining a fair amount of it's strength, so it does look like some of it will manage to make it into our region, (the next few runs will indicate this), while temperatures are plummeting in the clear skies.

Edited January 14, 2013 by Sno' problem

[Paul Sherman]

Why did we have so much disappointment today?

Let's look at GFS' Theta-W chart (my favourite snow forecast chart) This WBPT parameter encodes both temperature and humidity, so from that we can figure out - or at least have a good chance - of whether or not snow is going to fall.

Here are the charts for 12z, 15z, this afternoon.

As you can see the parameter goes up from lunchtime due to the 'airmass' change. A value of 3 pretty much means it's 1/2 chance (56%) 4 is about 1/3 (36%). The MetO went for snow, sleet, and later rain, because pretty much that's what the data told them; and it turned out just like that.

Now, what about tonight? Looking at 3am's chart,

It's obvious that the whole area is under a theta-w of 2 which is a 78% chance of precipitation falling as snow. Not a dead cert, but more people should see snow (orographic features, locally) than those who see rain. You can add 4% to each 50m ASL you are, so I am about 100m up, so I would see my chances as 86%. If it's still precipitating at 6am,

Which gives about a 95% chance of precipitation falling as snow. Pretty much quids in, I reckon.

There are further corrections that can be made, but as a rule of thumb, I reckon, through experience, it's pretty good.

Thanks BW

A well thought out and constructive post and easy for the newbies! Now these are the type of posts we want to see in here!

[snowfall09]

Hi all, it looks like we should see some persistant snow forming tonight/snow showers, just been looking through the charts, NAE and GFS both indicate snow across our region into the early hours of Tuesday, this time round it going to all be snow

Thickness 850/1000 hpa

the North Sea helping to brew up convective clouds enhancing heavy snowfall near the east coasts with some snow moving right across towards London for example.

Surface winds-GFS

Surface winds Nae

Surface winds-NAE

I expect Kent to be in the firing line overnight for the heavy snowfall and up into Suffolk/Norfolk/Essex, although some less significant snow could affect further west into areas as far as the far West Sussex/London west.

Staying cold with snow showers and some snow events at times in the days ahead

ESS

Chance of any heavy snow showers for wednesday in the South East?

[john pike]

i am just going to repost this from last night incase it was missed

tomorrow

cold

watch the radar

snow or rain

we will see

tomorrow night from midnight i feel is a good time for some snowfall

528 dam air over us

winds turning north east to east as the low goes through

precipitationn if rain or sleet earlier turning progressively to snow

cold temps around freezing

tuesday

cold north easterly wind

temps 1-2 degrees

528 dam air still over us

weather front over kent

so dry elsewhere snow showers in kent possible

these may move further west more runs needed

tuesday night

dry and very cold

easterly-southerly variable flow

hard frost

wednesday

again very cold

528 dam air still here

north-north easterly flow

looks dry unless the trough to our north east pays us a visit

0-2 degrees

thursday

still under 528 dam air

either a sw flow or a north easterly flow

need more to be specific

still looks very cold but dry

1-3 degrees

expect a hard frost overnight

tomorrows will be interesting

will they go with ukmo undercut?

just wondered how many people actually read these properly

back later

[BlizzardBelle]

I concur - as a newbie, SW's post up there ^^ was very helpful (and hopeful!)

[john pike]

Chance of any heavy snow showers for wednesday in the South East?

looks dry but cold

[Tom Jarvis]

Looks like I won't get any snow this week

[Sno' problem]

Is it going to snow here tonight?

I expect that you will get something from the ppn currently heading in through the Cheshire Gap from the NW, while the latest radar over the North East of England (Durham area right now) is beginning to show signs of the front starting to swing back South and build.

Just a case of radar watching at the moment.

[Snow Queen one]

Tom you are in good company with others on here. I saw some this morning and am hoping for some tonight but not holding my breath as i always seem to be just the wrong side of the PPN. Never mind there's always next week lol

[Met4Cast]

These showers will fall as sleet and rain I think even if they do reach us, Dew Points just aren't cold enough!

[john pike]

hi steve m if your here

check the raw ukmo charts above

the middle and bottom ones

undercut looks on with these

under 525 dam air then 520

very nice

[lawrenk]

Looks like I won't get any snow this week

I wouldn't rule out later tonight Tom based on JPs excellent Post.

[Downpour]

Its looking marginal in Middlesex tonight (have being waiting to use that one) Theres hope for us all yet !

Well Northholt is now 0.4c dewpoint -0c... going in the right direction...

[easton888]

I hope those showers from Wales dont reach us, they'll fall as rain and wet the ground again so the snow at midnight will melt again!

[VillagePlank]

Look at the theta-w charts and then read off from this,

Add 4% for each 50m ASL you are, and that's a pretty good indication of snow risk. Of course, you need it to actually precipitate, too ....

[Paul Sherman]

Temperature and Dewpoint holding steady down here

Temp 2.4c

Dewp 0.9c

Anything that would fall would be rain!

[Mark wheeler]

Cars freezing over in chessington. Grass is shining as ice begins to form. Glad we at least got something today. You never know fingers crossed for tonight.never say never

[Froze were the Days]

Poor here today - temps generally around 2.5c in the precipitation of rain and sleet although Maldon up the road had a spell of wet snow for about 35 minutes and the grass is partly covered over so I feel the further north (Essex) the more success in terms of snow.