[South West England Cold Spell Discussion]

Where's your fighting spirit David? lol

[South West England Cold Spell Discussion]

Very much rain on the Western end of the band according to the radar here:

http://www.raintoday.co.uk/

but quite few a few snow showers following

[South West England Cold Spell Discussion]

I suppose "marginal" is the key word here. Depending on very local conditions cold be any of rain, sleet, freezing rain, snow, a mixture, sleet to snow, rain to snow etc. That's why BBC don't call it full-on - lets hope they get caught out and its a total whiteout for everbody. Be nice to have clear skies and really get that temp dropping as much as possible before the event. Should be a nice cold one tonight - mine's in the fridge!

Got little fluries of sleety snow here in Hayle

Hey Nice! I'd love to experience snow in that part of the country - one of my favourite areas! Welcome and lets hope you can get a good "dumping" tomorrow night.

[South West England Cold Spell Discussion]

I wonder if we could get the thoughts of somebody like Steve Murr in here to give the lowdown??

[South West England Cold Spell Discussion]

slightly worrying temperatures on the 12z when the band passes through of 4C max and 1C min for Bristol at midnight. Maybe why BBC are hedging their bets??

[South West England Cold Spell Discussion]

An all-nighter coming up tomoz I suspect.

[South West England Cold Spell Discussion]

Winds will graduall turn more easterly as the low moves south - you can see the orientation of the showers in the Thames Estruary region already point due West and the bands are orientating themselves more this way now. Run the Met office radar between 1am and 2am and watch the band in Est anglia start to orientate itself similarly. As is the one down near southampton which seems to be intensifying and starting to move further west.

[South West England Cold Spell Discussion]

I know what you mean but the whole system is also gradually moving west and the showers seem to be remaining strong, especially the pseudo-streamer coming out of the Wash. Just hope it can get far enough West. We could be in for a whole line of showers if that beauty gets across the country enough. Have to wait and see - its gonna take quite a few hours to get here if it does at all so I'm gonna hit the sack - good luck for the morning!

[South West England Cold Spell Discussion]

Bristol now has the snow symbol all morning on the Met Office SW Forecast.

[South West England Cold Spell Discussion]

I'm hoping the same thing Bristol Blizzard - there has been a wind change in that area from NE to ENE and is due to go more easterly as the morning goes on. If you look at this radar that wash streamer has a pretty long tail.

http://www.meteox.nl...&soort=loop1uur

If anythings gonna hit us it might be something from that.

[South West England Cold Spell Discussion]

The precipitation from the low in the SE is already around 40 miles west of where it was progged to be at 21:00 on the 12z GFS. I've got a feeling the 18z will be an upgrade for the West Country.

12z GFS Precip:

Compare this with the 20:00 met office radar.

If that low keeps developing on its north side we could be in luck.

[South West England Cold Spell Discussion]

Ah underutting low, yes that would make more sense for the crazy ramper, but looks later on 18z (more like Tuesday?) - haven't really looked at Met O 12z properly. Will go and check out the main model threads now. We've surely gotta see something decent at some stage outta this? Just gotta be patient.

[South West England Cold Spell Discussion]

I went straight to the charts after he said that about Monday (crazy man!). Must have a take on the low sinking south?

[South West England Cold Spell Discussion]

GFS (12z) shows current 850 hPA temps at between 0 and -5C for here probably - 2/-3? , and of course our surface air temps and DP are around 4 and 3.5 respectively. Hence marginal and just a bit of sleet tonight at the most untill the colder air arrives.

[South West England Cold Spell Discussion]

Is that a temperature thing?

It's the layer of air where pressure is 850 millibar and that air temperature will be below -5C (generally thought of as conducive to snow). As long as surface temps and dew points are low enough (say 0c) and there is precip around it should be snow.

[South West England Cold Spell Discussion]

Why's that, then?

We will have the -5 850 hPa layer well past Bristol from the easterly. Of course it depends if theres any precipitation about but if there is I'm sure it will fall as snow. Could even see some during the day tomorrow if theres any kind of precip about.

[South West England Cold Spell Discussion]

I'm confident we'll get snow from tomorrow night onwards.

[South West England Cold Spell Discussion]

Hi Noggin,

Yep here in Sleety Winterbourne Could get some precipatitive drag bringing the colder air down although the temp doesn't seem to be going down much. I use the filton weather station temps but they seem a tad high at the moment at 4.4 and 3.9DP

[South West England Cold Spell Discussion]

Rain in NE Bristol definitely got heavier sleet in it now.

[Model Output General Discussion]

With regards to the discussion re: low 2m temps being affected by snow cover etc - I found these new parameters that the para GFS now incorporates to be quite interesting and, one would hope, would aid it in giving more accurate outputs than ever - these are new parameters used by the surface files:

THE NEW PARAMETERS IN THE GLOBAL FORECAST MODEL SURFACE FILES

/SFLUXGRBFXX.GRIB2/ INCLUDE:

http://www.weather.gov/os/notification/tin09-32aabgfs_changes.txt

-MAXIMUM SPECIFIC HUMIDITY

-MINIMUM SPECIFIC HUMIDITY

-DOWNWARD SOLAR RADIATION FLUX AT TOP OF ATMOSPHERE

-CLEAR SKY /CS/ UPWARD LONG WAVE RADIATION FLUX AT TOP OF

ATMOSPHERE

-CS UPWARD SOLAR RADIATION FLUX AT TOP OF ATMOSPHERE

-CS DOWNWARD LONGWAVE RADIATION FLUX AT SURFACE

-CS UPWARD SOLAR RADIATION FLUX AT SURFACE

-CS DOWNWARD SOLAR RADIATION FLUX AT SURFACE

-CS UPWARD LONG WAVE RADIATION FLUX AT SURFACE

-SNOW PHASE-CHANGE HEAT FLUX AT LAND SURFACE

-WILTING POINT AT LAND SURFACE

-FIELD CAPACITY AT LAND SURFACE

-SUNSHINE DURATION

My emphasis added

[South West England Cold Spell Discussion]

hi kumquat , well from the netweather radar , theres a line from b,ham down to b,mouth , past that it looks dry hope that helps.

Hey - thanks for that - colder temps getting quite close then - should drop a further couple of degrees this afternoon hopefully.

[South West England Cold Spell Discussion]

Hi cityatnight, what sort of area east is getting sleet? Wiltshire or further than that?

Thanks!

[Model Output Discussion]

Babies are probably infinitely more interesting than the NS equations haha but I don't envy you one bit! BTW I hail from Wellingborough and know Rushden well. Worked there for a couple of years - Portland Road!

I'm very encouraged by the ECM 12z today. It seems to me that after the glitches of the last couple of days that its been tampered with by human hands - lol.

But that would not compute.

Anyway - I'm still betting on Snow in Bristol Thurs/Fr and the nice cold spell going to continue up until Christmas.

Unbelievable to see these synoptics getting into the reliable and just great to see the back of the wet and warmish mush.

[Model Output Discussion]

They do - the GFS Parallel run is evidence that these things get tweaked from time-to-time, but the modelling isn't the final part of the process, it doesn't churn out the final forecast, it churns out theoretical output based on the data input and the laws of thermodynamics and these are then taken and interpreted by a forecaster. We are dealing with systems of mind-boggling complexity, after all and the human factor is roundly ignored (at least on the evidence of the last 10 pages). There's more value in John's blog than there is studying every output from every run of every model, because it has the experienced interpretation of a meteorologist.

On general matters, I've walked into doorframes less painful than this thread is to read - even with heroic moderation. Want a silver lining? Even if this verifies as the UKMO output suggests (which is the least apocalyptic of the runs this evening) then at least it will help to cool the North Sea, one of the primary moderating influences of winter Easterlies. As a large, relatively shallow water body, it should cool fairly quickly, so if this does re-load at some future point, we wont have to worry so much about air mass temps being moderated by a warm North Sea as much.

Actually its the humans that came up with the laws of thermodynamics that totally own the model output.

To the OP, the equations do not change unless Euler or Newton or Einstein have recently been undone by some other genius. What does change and can only get better is the initial start data and the boundary conditions that are used for each run. And, of course, these could well use historical data and blend that into the programming, but at this point its getting very complex.

I would suggest looking up Navier-Stokes for some basic fundamentals on fluid flow analysis. Also check out multiphase flow. Honestly, we cant get this stuff right in a straight pipe never mind in the whole atmosphere, but its getting better all the time and the GFS Parallel has been kind of consistant.

edit - of course you're right and the computer programme can get more accurate and take on more data ad infinitum (on what its working with) and the grid can be smaller - like John says vertical and horizontal - but this does all come down to two words that hasn't been cracked yet. Chaos Theory.

[Model Output Discussion]

Might not get home from work Monday if the Netweather precipitation is anything to go by, its gonna snow all day as far West as Bristol and reaching Devon at times according to the GFS 00z. The -5 850 has cleared most of the UK and most of South East Ireland by 15:00 Monday 14th. Certainly doesn't look like any climbdown to the ECM position with my eyes!

I know I have to take these precip charts with a pinch of salt but here's midday Monday 14th according to the GFS 00z - sorry, couldn't resist:

And all hell breaks loose on Thurday as expected!!