Model Output Discussion - 28th November - 4th December

[nick sussex]

Probably the hills in NW Scotland. Nothing further south and lower down in the most populated areas.

I'd be surprised if there wasn't at least a temporary covering of snow on lower ground especially in the overnight period, because of the strength of the flow there would be less modification and the upper air profile looks okay.

The direction of the flow of course means its areas exposed to the nw to westerly flow that would be most at risk.

There is however a big question mark about the shortwave at 120hrs and how much cold air will get pulled south as it clears away. If you look at the ECM ensemble maps upto 168hrs theres quite a range of solutions in terms of how far south the cold air is likely to get.

http://www.ecmwf.int/samples/d/inspect/catalog/samplers/banner/mean_sea_level_pressure_and_temperature_at_850hpa!168!pop!od!enfo!enplot!2009112700!!/

[johnholmes]

for anyone trying to predict where, when/how much snow will fall - beware.

predicting precipitation is the most tricky thing to do, makes forecasting wind strengths seem easy. Believe me I did it for 20 years. Add to the mix will it snow and it really becomes hugely difficult even as near as T+12 let alone T+120.

Once it gets to within T+48, use the data from Net Wx NMM model or the Met Office output, and try the 'will it snow' Guide I prepared a year or more ago. Remember to factor in height and how much effect the sea track to you will make if any.

It does work IF a big IF the model has the basic precipitation correct for place/time/intensity etc.

Edited November 29, 2011 by johnholmes

[J10]

For Scotland I think theres a decent chance that the cold air will get far enough south so snow prospects there are much better than for the rest of the UK.

It's more marginal further south and so its here where any downgrades would make more difference.

We'll just have to wait and see how much amplification there is upstream and what the models do with that shortwave between 120 and 144hrs, for Scotland I'm reasonably optimistic about snow chances.

It's always risky saying too much in here regarding snow chances as this can cause bedlam to break out!

If we look at the profile, for southern areas we have 850HPa at -5 to -6c, thickness around 522-525 and 1300m. For overnight snow that is really on the borderline, and any slight GFS downgrade,and we very quickly go on the wrong side of marginal, and that is excluding any shortwave interference.

As a result, the most likely outcome for southern areas, is for some wintry showers.

[nick sussex]

If we look at the profile, for southern areas we have 850HPa at -5 to -6c, thickness around 522-525 and 1300m. For overnight snow that is really on the borderline, and any slight GFS downgrade,and we very quickly go on the wrong side of marginal, and that is excluding any shortwave interference.

As a result, the most likely outcome for southern areas, is for some wintry showers.

I was talking just about Scotland, the rest of the UK is very marginal.

We'll know more after this evenings model runs.

[J10]

I was talking just about Scotland, the rest of the UK is very marginal.

We'll know more after this evenings model runs.

II was largely agreeing with you, and it does remain quite a long time away.

[Cheese Rice]

UKMO still not -really- buying into the shortwave. There is a shortwave to our south west on the current run but nothing like the other models are projecting.

UKMO-

GFS/ECM/JMA/GEM all going for a deep low. Interestingly the track is very similar on all the models, passing over Northern England.

GFS-

ECM-

JMA-

GEM-

A few ensemble members (GFS) don't go for a low, which is expected really a variation always occurs, perhaps due to the resolution.

The met office now are pro low too so expect the UKMO to back down over coming runs. Even if it just a stronger shortwave.

deep Atlantic depressions run close to, or over the UK

Those lucky enough to be on the northern flank of the low could see a significant snow event. As it stands Southern Scotland and the far north of England are in line for some snowfall, perhaps down to lower levels especially as the colder 850s undercut the precipitation.

Edited November 29, 2011 by Cheese Rice

[SMU]

http://176.31.229.228/mo...s/run/gfsnh-0-114.png?12

polar low alert-

S

[Eskimo]

http://176.31.229.228/mo...s/run/gfsnh-0-114.png?12

polar low alert-

S

Being an amateur, what does this signify?

[Paranoid]

This might not be quite on topic for the model discussion thread, so apologies in advance if it isn't, but that low on the JMA looks like it could bring some quite intense winds for the Northeast. Are there any estimates on the sort of winds that could be expected from that?

[SMU]

Being an amateur, what does this signif

y?

polar lows usually develop off the greenland shelf & slide SSE in a strong Northerly or Northwesterly airflow, They require deep cold flowing off the shelf to develop-

When they slide SE across the UK they enhance the PPN activity & have been known to deliver some good snow accumulations

[A Winter's Tale]

Was the December 20th 2009 event a Polar Low?

Got to say Steve M that's a great chart and I really hope this cold spell does happen. I'll probably feel like putting up the Christmas Tree come Sunday Night or Monday evening!

[Marie]

polar lows usually develop off the greenland shelf & slide SSE in a strong Northerly or Northwesterly airflow, They require deep cold flowing off the shelf to develop-

When they slide SE across the UK they enhance the PPN activity & have been known to deliver some good snow accumulations

Sorry if this is a silly question im still learning,if this ploar low does come off could it give good snow accumalations almost anywhere in the Uk or just up North?

[Radiating Dendrite]

Sorry if this is a silly question im still learning,if this ploar low does come off could it give good snow accumalations almost anywhere in the Uk or just up North?

The North mainly.

It will be very marginal in the south and a rain event near the South Coast.

[Thundery wintry showers]

I doubt that the chart shown by GFS is a polar low- there may be potential for polar lows in the north-westerly airstream, but forecast models rarely pick out polar lows until within a couple of days of the event. Chances are we're looking at troughs, though these can bring plenty of shower activity well inland.

[johnholmes]

http://176.31.229.228/mo...s/run/gfsnh-0-114.png?12

polar low alert-

S

come off it Steve...

Incidentally polar lows can form and most that affect the Uk do form to the lee of a strong northerly south of Iceland, also in the Norwegian Sea, indeed there are cases where they have formed in the approaches to the west coast of Scotland and the entrance into the North Channel(NOT to be confused with the English Channel!)

Edited November 29, 2011 by IanM
tidying up to save just deleting two pages

[Marie]

The North mainly.

It will be very marginal in the south and a rain event near the South Coast.

Many thanks

[johnholmes]

I doubt that the chart shown by GFS is a polar low- there may be potential for polar lows in the north-westerly airstream, but forecast models rarely pick out polar lows until within a couple of days of the event. Chances are we're looking at troughs, though these can bring plenty of shower activity well inland.

agreed I too doubt it is a polar low and the comment about models having difficulty picking them up, at least in course mesh, even in finer mesh they are hard to predict. Sat piccs are the best indicators quite often that they are starting to form then they can be tracked on sat piccs.

[Cecil Hogmanay Melchett]

i was under the impression the fax charts would be best to pick up PL formation?

[Isolated Frost]

Yes my first suspicions were a shortwave/trough developing again, as the models have been scheduling frequently, not a polar low, that is better suited to mesoscale forecasting and n'ly airstreams... I'd think Steve has got many hopes up, but it's a run of the mill run...

[Fozfoster]

http://176.31.229.228/mo...s/run/gfsnh-0-114.png?12

polar low alert-

S

Steve, I've circled what I think is the start of it... or am I talking total tosh?

Edited November 29, 2011 by Fozfoster

[johnholmes]

i was under the impression the fax charts would be best to pick up PL formation?

remember that the Fax chart is the best mix for forecasting weather=man and machine. Its not usual to see a GENUINE polar low shown much more than 24-48 hours in advance.The forecaster will obviously be closely watching for any signs of such when the charts are conducive to their formation. They certainly are not at the moment.

Reasons as given before.

[Strider]

The two times I can remember a polar low occuring (one being very early last winter/late autumn - NOT 30th Nov) neither were picked up by the models before hand.

You have to watch sat24.com and keep an eye on it at the time

[A Winter's Tale]

Steve, I've circled what I think is the start of it... or am I talking total tosh?

That's a bartlett! It's amazing for bringing snowfall!

[A Winter's Tale]

Steve, I've circled what I think is the start of it... or am I talking total tosh?

Actually that chart looks very similar to the one I posted earlier about December 4th 1999.

[Isolated Frost]

That's a bartlett! It's amazing for bringing snowfall!

I assume you are pulling my leg a tad old chap?