06z model discussion - rain, snow, rain, or rain, snow, snow?

[smhouston]

Those T850 charts indicate that it will be rain from the word go here, and probably over most of the South.

Looks like turning gradually more zonal if the 06h GFS is to be believed?

Even if it does go more zonal for a bit, it'll still be fairly cool according to the ensembles

[Thundery wintry showers]

I still believe that there's a lot of uncertainty over this; the UKMO and ECM seem to keep the cold air battling away over northern areas during the weekend (I checked out the ECM for Sunday- the day I said it would almost certainly turn milder everywhere- and yes, the cold easterlies are still there over Scotland)

[SMU]

Good afternoon folks-

An interesting thread with hows & wherefors etc... Although not a great deal of reference to the models-

Anyway- For the few of us that have been in these types of events before ( an almost rarity) the solutions offered up by the GFS have to my opinion & some others been 'generous' in the migration Northwards of the Front-

Whether this is a model bias or just a poor anaylsis of the mesoscale situation its difficult to understand, but my expectancy of the 'actual' situation V Modelled being further south holds firm in my eyes-

We saw the 12z, 18z & the 00z move the frontal zone ever further North & to the excitment of some this looked to have been the current trend- although the UKMet model stood fast consistently ejecting the Shortwave further East-

We are now post mortem 06Z GFS & a sizeable shift 'South' has been recorded- again not unexpected, but never the less that is a large step back away from the last 3 solutions-

Of course there isnt consensus yet & for those living North of the current boundry I offer some form of hope that there is STILL scope for movement back- however Im sincere when I say I THINK the Northern cut off point has already been reached & for reference this is where I see its maximum northern reaches- Just use this ONE chart from the unibas site for reference.....

http://pages.unibas.ch/geo/mcr/3d/meteo/nmm22/PCP0330.PNG

Also worthy of note here is out of the 6 pages of discussion No-one has added the updated fax charts for tomorrow- So here it is-

http://www.wetterzentrale.de/pics/brack0.gif

For those that have studied these close over the last few days will note the occlusion have been pegged back south as well & the forecasters at the Met office have responded to that with the updated 24 & 36 hour charts-

I will also add in ( Courtesy of Retron from TWO the early morning Mesoscale rainfall plots from the Met)

http://www.metoffice.gov.uk/weather/images...0070207_024.gif

http://www.metoffice.gov.uk/weather/images...0070207_027.gif

http://www.metoffice.gov.uk/weather/images...0070207_030.gif

http://www.metoffice.gov.uk/weather/images...0070207_033.gif

http://www.metoffice.gov.uk/weather/images...0070207_036.gif

Again the northern extent is highlighted by the 30 hour Chart-

A few people have mention the front will stall out- & yes in a manner of speaking it will- the continual migration East across the UK will probably roll on throughout Thursday, however there is going to be a sharp cut off zone where the frontal boundry ends, & along that edge this where the significant snow Could fall & this zone looks pretty static for quite a portion of time-

( This is still subject to change of course ) but my estimate would be along this line-

Starting from the NW Wales through SE-

Bangor > Across to Wrexham & Shrewsbury > Across the heart of the midlands up to Around Stafford> out East to Leicester & Maybe to Peterborough> then across to Norfolk-

This is the frontal Boundry with somewhere within say 50 miles Southside of this getting the Maximum potential from the Front-

Im reluctant to include the SE in this analysis- I dont want people assuming that Im just doing this from a NIMBY perspective-

But if pushed for the SE- I would say yes to the snow potential although the banding will noticably thinner in the SE & 'PERHAPS' not as intense-

With regards to temperaures Upper Temperatures & Surface Temps - well- I actually think the demarkation zone runs nicely along -5C line on this Chart-

http://www.wetterzentrale.de/pics/Rmgfs242.gif

Correlating that up with the Snow zone you will see that there is a nice band in the middle thats in the sweet zone-

Surface temps concur with this- but remember this chart isnt that well gridded so

http://www.wetterzentrale.de/pics/Rmgfs274.gif

Although you may be under say a 3 or 4c this by no means set in Stone...

Snow to Rain????????

This has to be discussed because its a reality in this type of set ups-

Whenever a low tracking in from the SW arrives the transition to milder air & steepness of the gradient is within 30 or so miles- which is why forecasting specific events for any one locale becomes a tightrope-

My feelings since last nights 'depth' of cold & this mornings models will be that the zone of rain/sleet will be limited to the SW-

Let me explain, The system that is approaching has always been quite shallow & part of the trough extension - its shape has been getting flatter & flatter with time, which from an advection point of view means its increasingly difficult to mix the warmer air in-

Notice on the 24 chart -

http://www.wetterzentrale.de/pics/Rmgfs242.gif

how the WAA ( Warm air) goes along the nose of the low pressure but because of the steep gradient cannot filter properly back North-

If the system has less lateral sheer & was the usual 'Circular' shape this warm air would move northwards in abundance-

These is however going to be the zone that I think sees quick snow to rain- & I guess my southern line would be coastal South wales > Bristol > Towards andover & Out SE towards Portsmouth-

This is NOT set in stone- Places like the Salisbury plain, Dartmoor could still do well & indeed if the PPN comes in with less of a Northerly vector then these regions could so easily stay 'White' rather than 'green'-

But that is my guide anyway-

As for developments further down the week- Again more emphasis in the North at the moment , although the shortwaves could filter further south like the GEM indicates- But at least there is the Onshore SE winds to blow some convective Activity in- as well as possible trough formation-

Well- that should wrap things up until the 12's....

Best regards

Steve

Edited February 7, 2007 by Steve Murr

[Thundery wintry showers]

I'd also like to add that for cold/snow lovers in general, in the long term it probably doesn't pay for the front to get too far north. Yes, it would mean a significant snow event for the north, but then the cold easterlies would be shunted too far north and east for Friday, allowing Atlantic zonality to get a grip through Saturday and Sunday with mostly rain at low levels.

If the Thursday feature doesn't get too far north- as has consistently been reflected by UKMO outputs- the weekend holds potential for even bigger snow events as cold air returns from the east on Friday, and a larger depression sweeps northwards on Saturday. Also, beyond the weekend, it's worth noting that the further south the Atlantic systems are, the more chance there is of an easterly further down the line whenever we have a large gap between Atlantic systems.

[Upgrade]

thanks Steve for that very comprehensive and well explained report.

Posters like you really increase people like myself's understanding of these events.

thank you.

[phil nw.]

Thanks to Steve M for a clear and informative post.

Of course I hope you have got it spot on ,considering my location,lol.

[Iceberg]

06Z raw meto output has shifted everything further south by around 30miles from the 00z Run.

Bringing more snow and no return to rain into central southern england and even south of the M4.

[BLAST FROM THE PAST]

06Z raw meto output has shifted everything further south by around 30miles from the 00z Run.

Bringing more snow and no return to rain into central southern england and even south of the M4.

Aye, and as Big Steve M says the fact the Lp is sausage like will make the difference and prevent a northward march and mixing in of warm temps.

BFTP

[Iceberg]

Indeed a spot on assessment by Steve.

Winds are forecasted to be around 15mph, not much you think but enough to quickly make 30-50cm drifts.

The sausage elongation will encourage areas such as Oxford, Andover and even London to really get a pasting with 10-20cm quite possible. Grit won't have much effect either with the modelled precip rates.

[mathall1]

Yeah all of us west country folk aint gunna get snow turn to rain now.Few hrs of good steady snow forecast on local .Then clearing and he even said showers of rain/snow during Thursday

[stuski]

Good afternoon folks-

<snip>

Top info Steve, much appreciated .Really interested in the signifcance on shape the LP.

Edited February 7, 2007 by shuggee
Removed quote to save space!

[The Penguin]

I still believe that there's a lot of uncertainty over this; the UKMO and ECM seem to keep the cold air battling away over northern areas during the weekend (I checked out the ECM for Sunday- the day I said it would almost certainly turn milder everywhere- and yes, the cold easterlies are still there over Scotland)

Yes, TWS, the MetO are forecasting snow showers for Glasgow on Saturday, warming slightly on Sunday with rain showers.

[Frosty.]

I still think it will turn less cold / milder across all except northern scotland by the end of the weekend. Cold air may then return south as the complex low get's away to the east.

Edited February 7, 2007 by Frosty039

[coxxy22]

so is this gonna put us in line for a decent bit of snow then if it all moves south slightly ???

[Stratos Ferric]

Models arent very good at deciding how much slowing takes place on a front as it encounters colder air and hence this is why the fax charts at very close range are the best you can do as the human input allows for this. Its beyond me why people would follow the gfs over the ukmo at this closer range.

I suspect there's a lot of whisful thinking in that post Nick. The French site has the precipitation really scurrying through on Thursday. In these situations it can speed up, slow down, stall, or even reverse; however, recent history favours the former and none of the latter I'm afraid. All the models are agreed on progression, speed is the issue. AT present it's looking more progressive that previously forecast, meaning less snow, and what falls occurring mainly further north. There's always likely to be snow on the leading edge, particularly with an overnight incursion into cold air, but I wouldn't be hanging my hat just now on anything as significant as looked possible a couple of days ago: I knew I should have kept schtum.

[shawng]

well ive just been put on snow alert one i work for network rail and now have to sit in the middle of no where in case the snow causes disruption to trains unfortunately i live 80 miles from were im going to have to be so hopefuly i will be able to get home in the morning.

[Iceberg]

Are we talking about the french arpage model or the European GFS based models ?, if it's the GFS ones then they will likely prog the same progressiveness of the GFS ops run, which has very little support from GEFS, or the METO 00Z or 06Z runs.

[guitarnutter]

mwa

is the 850temps dewpoint?

[grab my graupel]

I'd also like to add that for cold/snow lovers in general, in the long term it probably doesn't pay for the front to get too far north. Yes, it would mean a significant snow event for the north, but then the cold easterlies would be shunted too far north and east for Friday, allowing Atlantic zonality to get a grip through Saturday and Sunday with mostly rain at low levels.

If the Thursday feature doesn't get too far north- as has consistently been reflected by UKMO outputs- the weekend holds potential for even bigger snow events as cold air returns from the east on Friday, and a larger depression sweeps northwards on Saturday. Also, beyond the weekend, it's worth noting that the further south the Atlantic systems are, the more chance there is of an easterly further down the line whenever we have a large gap between Atlantic systems.

Yes that is exactly what I am wishing for as you explain very nicely there.

The sitution would be better alround, and for the longer term if things were to pan out as you describe. Being one of the people likely to do badly from Thursdays event , like you I will be willing the fronts to dive further South with future runs.

In the meantime I'll share the joy with those who have had to wait a conciderable amount

of time to see anything noteworthy, for their area's.

I't will be good radar watching times tonight.

Brian.

[MKN]

people keep talking about the models for this thursdays even which commences in 12 hours, in my opinion the most accurate way to see whats going on will be to keep check of the bbc weather forecasts, along with the flash warnings from the metoffice and of course the radar

The precip for tonight/tommorow is currently starting to show on the radar

Edited February 7, 2007 by SnowTornado

[One way or another]

people keep talking about the models for this thursdays even which commences in 12 hours, in my opinion the most accurate way to see whats going on will be to keep check of the bbc weather forecasts, along with the flash warnings from the metoffice and of course the radar

The precip for tonight/tommorow is currently starting to show on the radar

Do people think that the BBC and Meto have to be more cautious than most to try and get people to understand the worst case scenario. I remember a few years ago gettin off the M1 prior to a deluge of snow and people being stuck there for 16 hours+.

There were little to no warnings and since then it seems that they always tend to go over the top when forecasting snow. I could be wrong but I've tended to assume this to be the case.

[Stratos Ferric]

Good afternoon folks-

....

This has to be discussed because its a reality in this type of set ups-

Whenever a low tracking in from the SW arrives the transition to milder air & steepness of the gradient is within 30 or so miles- which is why forecasting specific events for any one locale becomes a tightrope-

My feelings since last nights 'depth' of cold & this mornings models will be that the zone of rain/sleet will be limited to the SW-

Let me explain, The system that is approaching has always been quite shallow & part of the trough extension - its shape has been getting flatter & flatter with time, which from an advection point of view means its increasingly difficult to mix the warmer air in-

Notice on the 24 chart -

http://www.wetterzentrale.de/pics/Rmgfs242.gif

how the WAA ( Warm air) goes along the nose of the low pressure but because of the steep gradient cannot filter properly back North-

If the system has less lateral sheer & was the usual 'Circular' shape this warm air would move northwards in abundance-

....

Best regards

Steve

Not sure I agree that the system is "shallow" Steve. The gradient to the SW is actually pretty steep and more than deep enough at the boundary. Occluded fronts by their very nature are steeper than a warm front. Where I do agree is that the boundary for significant snow is fairly narrow in the south: it would widen slightly in the north, but if the front got there it would be decaying slightly whilst also, in all likelihood, moving more quickly.

I've not got time to look at all the detailed charts but this continues to be a "too close to call precisely" event. The most favoured triangle at present does seem to be something like M40 / A5 as the southern margin, A1/M1 as the eastern flank, M6 / A14 as the northern flank. If I had to back a migration a northern movement would appear to be the more likely.

[Schnee]

As always, an interesting read there Steve.

Just how far north is the ppm expected get????

[kold weather]

A good post from Steve, in regards to the models and possibly taking systems too far north too fast, I have a theory that this may lie in the lack of data that is in the pole region, at least relative to the rest of the N.hemisphere. with less adat I've noticed that the models can sometimes under-do pressure up there by a good 3-5mbs. On a large scale thats not much but when it comes tofrontal systems that can lead to a good 100 mile shift either way een at fairly close range 12-72hrs out generally.

even now this is still a very hard set-up to make a naile don forecast though I'll agree with Steve in the main and as he said the flatter shape of the LP the less WAA will occur up its eastern flank as it tilts, the faster it can tilt the better for the whole country.

I think along Stev's line we could reasonably get 5-15ms of snow even at lower levels, for higher levels I'd add another 5cms to both set of values.

Sf- intresting you say aobut a northern migration more likely, previous years I've noticed such events tend to nearly always be progged too far north at first (I'm sad, I've been wathcing MANY such clippers over in the states...) this doesn't mean this is the case this time, i actually think the UKMO raw output is right but based on the issue about+plus the tendancy to under-do heights over the poles due to slightly less data I think ther eis a higher chance of it going further south then expected rather then north but I agree about your favored triangle region, tohugh i do think plasces tyo the east of that away from the far south could well still get a couple of cms.

By the way one wildcard for the CS/SE of england is exactly whether the development f the weak LP adds more forcing to the front. IF ity does and the timing is right then it could really pep up the front on its eastern flank for a time, UKMO raw charts had this feature as steve's charts showed, its all about timings, its very close!

Edited February 7, 2007 by kold weather

[SMU]

Not sure I agree that the system is "shallow" Steve. The gradient to the SW is actually pretty steep and more than deep enough at the boundary. Occluded fronts by their very nature are steeper than a warm front. Where I do agree is that the boundary for significant snow is fairly narrow in the south: it would widen slightly in the north, but if the front got there it would be decaying slightly whilst also, in all likelihood, moving more quickly.

I've not got time to look at all the detailed charts but this continues to be a "too close to call precisely" event. The most favoured triangle at present does seem to be something like M40 / A5 as the southern margin, A1/M1 as the eastern flank, M6 / A14 as the northern flank. If I had to back a migration a northern movement would appear to be the more likely.

SF- The 'shallow' feature was reference to the shortwave that runs ahead of the Main trough-

Im trying to get the 06Z METO but the FSU site wont upload....

I will take icebergs word for it--

Stelmer go back a page I have put my definitive line on a previous post-

KOLD- I have most of the garden still frost covered at 2pm- not often we report that in this day & Age...

S