Spring Model Discussion 21st March 2013 006z onwards...

[bluearmy]

the nae clearly shows the dp remaining sub zero further south than earlier runs. looking at the uppers and 850/1000 thicknesses, there is the capacity for precip to be white all the way down to n london on a line running nw to mid wales. however, without intensity i suspect areas wont see troublesome accumulating snow. that makes it terribly difficult to forercast as we all know that rainfall intensity is the most tricky aspect of forecasting.further north into the midlands and the colder entrenched temps will probably encourage easier settling.

[A.J]

Now we have the fine scale models available be it NMM from Net Wx or NAE UK Met it might be a good idea for someone, not me, to look at the parameters for snow, aka the Net Wx Guide for your locality and see just what your chances are assuming of course the model shows precipitation over you?

It does work quite well for lowland areas, remember to factor in your height, if you have height it will give you a better result if it is snow you want and also how close to any windward coast you are, in that case look VERY closely at parameters like Td and 850mb temperatuures and scan the skew-t for your area for temperatures from 850mb down to the surface.

Good luck everyone if you are wanting snow.

spare a thought for anyone in the SW on low ground though, and do remember IF the models are correct for the large snowfall totals being predicted for higher ground that it will have implications for anyone near the major rivers that flow away from those areas when the thaw arrives. It is not just the immediate effect for low areas in the SW but a much wider threat once the cold eases.

John, is there a formula for equating thicknesses with T850 temps to calculate in altitude terms where the T850 boundary is? ....and thus lapse rates from sea level to the T850 boundary?.....simply from an IMBY point of view I have some elevation (approx 500ft) so I'm curious as to where the T850 boundary will be at any given time.....My understanding (if correct) is that in the coming scenario with relatively high thickness values that the T850 boundary is quite high up so the air below that would have to be dry (low dew points) to promote snowfall

Edited March 21, 2013 by ajpoolshark

[snowking]

GFS Ensemble mean snowfall projection:

SK

[Mucka]

The same void is also progged in EURO4 & UKPP in same area.... exact replica. Clear topographic effect there.

I was going to mention this since I am right in that area (SW Manchester)

Many times we have amber alerts and heavy snow/blizzard warnings and barely get a few spots of sleet - it has been that way all this Winter with no lying snow despite the repeated forecasts of snow and accumulations.

Very frustrating for a snow lover like me and I expect a repeat performance while areas to the N, E S and W of me get good dump of snow.

Obviously it is the shadow effect but I remember many good dumps of snow for this location from attempted breakdown scenarios in the 70's and 80's but perhaps they were more W/E aligned attacks rather than SW/NE?

Anyway, many living in South Manchester may well be very disappointed after all the ramping and hearing they are in the ideal position/firing line although some areas only a few miles away will do very well.

Chester looks like getting hammered to me and places like Oldham and other North Manchester areas.

[johnholmes]

John, is there a formula for equating thicknesses with T850 temps to calculate in altitude terms where the T850 boundary is? ....and thus lapse rates from sea level to the T850 boundary?.....simply from an IMBY point of view I have some elevation (approx 500ft) so I'm curious as to where the T850 boundary will be at any given time.....My understanding (if correct) is that in the coming scenario with relatively high thickness values that the T850 boundary is quite high up so the air below that would have to be dry (low dew points) to promote snowfall

hi aj

I suspect there is if not a formula some kind of rule but I have to say I am not aware of it.

looking in my VERY ancient Forecasters handbook, issued in the late 60's and I find the following

p0-1000/4-h/30 to give the 1000-850mb thickness

and subsequently

values of 1281=90% snow

1290=70%

1293=50%

It may be that when MOS (Model Output Statistics) were fed into the computer model that this has been modified and probably since then but it is a good enough way of looking at your specific location

The figures relate to

Boyden recommends that adjust the MSL pressure (p0 mb) and station height above MSL by adding to the 1000-850 mb thickness the correction factor shown in the above formula.

Hope that makes sense to you or anyone else-it does work and is almost certainly something like that which is in the UK model outputs for both their GM and finer mesh models

[johnholmes]

Obviously it is the shadow effect but I remember many good dumps of snow for this location from attempted breakdown scenarios in the 70's and 80's but perhaps they were more W/E aligned attacks rather than SW/NE?

quite correct and the latest fine mesh models are becoming quite good at showing these rain/snow shadows, Manchester is a classic downwind of the Welsh hills and this area for the Peak District effect.

[Summer Sun]

GFS Ensemble mean snowfall projection:

SK

Confirms my thoughts that the NE would avoid the worst of the snow the main action looks to be around the midlands and parts of wales

[A.J]

hi aj

I suspect there is if not a formula some kind of rule but I have to say I am not aware of it.

looking in my VERY ancient Forecasters handbook, issued in the late 60's and I find the following

p0-1000/4-h/30 to give the 1000-850mb thickness

and subsequently

values of 1281=90% snow

1290=70%

1293=50%

It may be that when MOS (Model Output Statistics) were fed into the computer model that this has been modified and probably since then but it is a good enough way of looking at your specific location

The figures relate to

Boyden recommends that adjust the MSL pressure (p0 mb) and station height above MSL by adding to the 1000-850 mb thickness the correction factor shown in the above formula.

Hope that makes sense to you or anyone else-it does work and is almost certainly something like that which is in the UK model outputs for both their GM and finer mesh models

thanks John....so in my case, going by GFS and hi-res output, at 0600z Sat, the 850-1000 hpa value is 1302m. Would I be correct is assuming that the '1302m' value equates to 1302 meters between the 2 boundaries, so factoring in the SLP reading and adjusting for my altitude using your formula would give me my answer?.....Apologies to the thread mods for taking this point somewhat off topic, but just trying to clarify that I've understood correctly

just found this brilliant resource for calculating thickness values and indeed more formulae than you can shake a stick at... http://weatherfaqs.org.uk/node/152

It's a bit of a read, but for anyone interested in calculating snowfall probability for their location, it's a must read!

Edited March 21, 2013 by ajpoolshark

[johnholmes]

thanks John....so in my case, going by GFS and hi-res output, at 0600z Sat, the 850-1000 hpa value is 1302m. Would I be correct is assuming that the '1302m' value equates to 1302 meters between the 2 boundaries, so factoring in the SLP reading and adjusting for my altitude using your formula would give me my answer?.....Apologies to the thread mods for taking this point somewhat off topic, but just trying to clarify that I've understood correctly

will pm you

[Summer Sun]

I know I sound like a broken record but GFS continues to show milder air pushing in just in time for the Easter weekend

To go with the milder air it looks like rain will arrive with it but where and how much is impossible to pin point at this stage

Something to cling onto if your fed up of this what seems never ending cold

Edited March 21, 2013 by Gavin.

[SomeLikeItHot]

I know I sound like a broken record but GFS continues to show milder air pushing in just in time for the Easter weekend

Something to cling onto if your fed up of this what seems never ending cold

ECM also now, although about a day later. Its all in FI so I wouldn't count on it and it looks very like what the models were all saying at the similar time frame forward 6 days ago. Personally I am expecting heavy snow at Heathrow when I attempt to fly out on the 1st of April!

Edited March 21, 2013 by SomeLikeItHot

[johnholmes]

I know I sound like a broken record but GFS continues to show milder air pushing in just in time for the Easter weekend

To go with the milder air it looks like rain will arrive with it but where and how much is impossible to pin point at this stage

Something to cling onto if your fed up of this what seems never ending cold

not that different from what the anomaly charts are beginning to suggest?

[Summer Sun]

Been busy this morning so still catching up but ECM is also going down the route of shifting the coldest air away just not as quick as GFS at this stage

What both agree on as well is the pesky northern blocking erodes away

[Radiating Dendrite]

No it was a few days earlier.

Rain turned to snow and then it continued all night.

Think is was around the 10th April 1998.

[johnholmes]

just found this brilliant resource for calculating thickness values and indeed more formulae than you can shake a stick at... http://weatherfaqs.org.uk/node/152

It's a bit of a read, but for anyone interested in calculating snowfall probability for their location, it's a must read!

the link is probably one of if not the best on the web for information that is clear and objective, most of it was provided by a senior forecaster with UK Met.

[Troubleatmill]

I know I sound like a broken record but GFS continues to show milder air pushing in just in time for the Easter weekend

To go with the milder air it looks like rain will arrive with it but where and how much is impossible to pin point at this stage

Something to cling onto if your fed up of this what seems never ending cold

Would love to see some yellows over me now, next few days are quite exciting for snow and very rare conditions, 30-40cm of snow at higher levels in some areas being touted, but will be glad when its out of the way and i can walk the street with just a shirt and pair of jeans on, its that time of year. Winter has gone on way too long now lol

[Summer Sun]

06z ensembles (London and Manchester) also back up milder air pushing in later next week with a steady rise from around the 29th notice the lower precipitation on the London ensemble from the 3rd

More scatter on the Aberdeen ensemble compared to London and Manchester but Aberdeen remains the one with the lowest precipitation through-out

[Captain Shortwave]

Well I have come to the conclusion that the NASA model is not very good to be honest.

The model last night put the rainband about 200 miles or even more further north and never had the -8 isotherm getting any further south than Scottish borders.

Today

That's a correction of about 500 miles south and more in line with other models.

Pick of the bunch of the models for coldies longer term is the GEM which brings another low pressure through France which then pivots up through Belgium/Holland

Last frame just as a casing point

Edited March 21, 2013 by Captain shortwave

[rory o gorman]

yes gavin we may get a nice WWA from the southeast later next week or weekend. The ensembles giving some good backing to this as you alluded too.

[Chris W]

Looks like the GFS returns to blocking and an easterly in FI, but a milder one. Re: the snow projections for Cheshire earlier, despite the Clwydian snow shadow we do suffer I can see a decent snowfall being possible. In Jan it made it to us under what I would call a slightly less promising situation - one thing is for sure though, I will be telling my colleague in Cerrigydrudion that she might want to work from home tomorrow!

[joggs]

Surely there would be alot of heavy snow showers with what the models are showing early next wk? .The cold uppers are there and instability etc and the sun been higher? Anybody?!?!? .

[SomeLikeItHot]

Amber warning for an area roughly the triangle encompassed by Liverpool, slightly north of Birmingham and slightly south of Sheffield. Bit surprised it didn't stretch more into wales and slightly further east to near derby where the peak precipitation was progged on some other model output (eg GFS which seem to peak at around Derby.

[Mucka]

Looks like the GFS returns to blocking and an easterly in FI, but a milder one. Re: the snow projections for Cheshire earlier, despite the Clwydian snow shadow we do suffer I can see a decent snowfall being possible. In Jan it made it to us under what I would call a slightly less promising situation - one thing is for sure though, I will be telling my colleague in Cerrigydrudion that she might want to work from home tomorrow!

Made it to you perhaps but not to me.

As I say places like Chester, Crewe, perhaps Warrington further South and West may well still do well (I'm expecting Chester to get a heavy fall) as will places a little further East such as Stockport probably but those urban areas just SW of Manchester, Sale, Altrincham, Stretford, Irlam for example are well sheltered in such situations.

I expect these sheltered locations will see some snow and perhaps a temporary covering but not the heavy falls some surrounding areas may well receive.

Hard to be sure but that's the history of such events and why much less ppn is modelled for these areas.

Sorry a little OT, this really belongs in regional thread now but given an Amber warning has just been issued for the area as previously discussed here perhaps worth mentioning once again.

Edited March 21, 2013 by Mucka

[Blake]

No it was a few days earlier.

Rain turned to snow and then it continued all night.

Think is was around the 10th April 1998.

15th I believe. Sister's 8th birthday, woke up to an inch of snow when I lived back in Bournemouth. Was just after Easter weekend and during Easter hols?

As for this weekend, didn't expect a few days ago that there would be borderline chance of snow here in Guildford, if these southward corrections continue we could end up a coupe of inches, just hope ground is dry enough for snow lovers here.

Charts posted in last few hours show risk from as early as Sat morning here now.

[Radiating Dendrite]

15th I believe. Sister's 8th birthday, woke up to an inch of snow when I lived back in Bournemouth. Was just after Easter weekend and during Easter hols?

As for this weekend, didn't expect a few days ago that there would be borderline chance of snow here in Guildford, if these southward corrections continue we could end up a coupe of inches, just hope ground is dry enough for snow lovers here.

Charts posted in last few hours show risk from as early as Sat morning here now.

Just to put this to bed:

"North Wales saw particularly heavy snowfalls on the 10-11, and northern England on the 14-15th. Mold had 37 cm on the night of the 14-15th". - Taken from the excellent Trevor Hartley Website.

Being close to the border in NE Wales it looks like we had snow from both events. Certainly remember there being around a foot lying.

Anyway - back on topic

Edited March 21, 2013 by Radiating Dendrite