Cold Spell General Discussion

[snowmadchrisuk]

Interesting the Meto are going for northerly winds.

[*Stormforce~beka*]

Friday is definately the day when the weather will change.

http://www.wetterzen...cs/Rtavn961.png

Expect some max temps to be supressed with that chart as fog will prove rather stubborn to clear.

That is sooo close! Just look at it ... only 2 inches away from the other high ... only 2 little inches ...

Puff and blow everyone to push it up there ... if we all blow together then the 2 inches can be done.

[doublebarrel]

looking at this weeks charts ( fri/ sat) will we be cold or will there be fog and maybe frost ?

[Osbourne One-Nil]

Paul and mods, apologies, but this cannot go without comment

if we all blow together then the 2 inches can be done.

[chionomaniac]

Before I say something about the models, can someone point me to some info on 'Short Waves'?

Looking at the panel maps put up earlier, there are a few cold ensemble members in there, but there are also quite a lot that are pretty average. From that run of ensembles, I'd say that it's only 30-40% chance of really cold weather getting further than the south east within a reliable timeframe

Try this for some good background info:

http://www.geologywales.co.uk/storms/upthere.htm

[cooling climate]

I have just been reading up on shortwaves

(3) decreasing the mass of the air (i.e. upper level divergence).

Causing the air to rise counteracts some the downward force created by gravity. This lowers pressure just as if someone started pushing up on you when standing on a scale; your weight would decrease. Fronts force the air to rise. This causes the surface pressure to decrease in the vicinity of the front. Cold fronts have a more defined pressure trough than warm fronts because the average cold front has a steeper slope and stronger temperature gradient than the average warm front. A warm front raises the air gradually while a cold front lifts the air more quickly in the vertical. The faster the air rises, the more pressure will lower. A mid-latitude cyclone and a front will both cause the air to rise and pressure to lower. The stronger the front, the more well defined the pressure trough will be.

Now to shortwaves. A shortwave is an upper level front or a cool pocket aloft. Just as a surface front causes the air to rise, upper level fronts can do the same. First, let's start with a general description of a shortwave:

(1) It is smaller than a longwave trough (shortwave ranges from 1 degree to about 30 degrees in longitude (the average one is about the size of two U.S. states put together (Iowa and Missouri put together is a good example)

(2) Isotherms cross the height contours (if it is a baroclinic shortwave). This creates an upper level temperature gradient and therefore an upper level front

(3) They are best examined on the 700 and 500 millibar charts

(4) They generate positive vorticity (mainly due to the counterclockwise curvature within the shortwave). This creates positive curvature and positive shear vorticity. If PVA occurs with the shortwave then the shortwave will deepen and strengthen due to lift created by upper level divergence.

(5) They can create an environment conducive to surface based convection or elevated convection due to the cooling aloft.

It is important to see how much moisture is associated with the shortwave. A shortwave moving over a warm and moist lower troposphere has a better chance of producing precipitation and strengthening than one moving over a dry lower troposphere. If the low level dewpoint depressions are low, the instability and lift associated with the shortwave can enhance cloudiness and precipitation. In summary, a pressure trough is associated with a low-level front while a shortwave is associated with an upper level front or a cool pocket aloft. Both are associated with rising air and can add instability to the atmosphere.

The diagrams below show an upper level shortwave and a surface pressure trough. The pressure trough is the kinking along a front.

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If I have understood it correctly then I would say that in the model time frame of t120-144 it would be

more probable to get a shortwave develop on the east side of the ridging high where you also have the

largest temperature gradient.

I am confident now there will be no spoiler shortwave development but rather one on the Scandinavian side

that will aid in the CAA westward toward us.

Edited December 7, 2009 by cooling climate

[*Stormforce~beka*]

Very interesting read. Thank you.

[Dingwantssnow]

So the short wave could in fact aid the westwards push of the cold air?

From what i can gather its a cert for some cooler times ahead but were non the wiser on just how cold its going to get? TBH anything is better than what we have had upto now.

Im just dreading that within the next day or 2 its all going to go pete tong and its all going to slide right by us like every other year before. Having said that am i right to to believe that we havent had as good a setup as we have now for a long long time?

[Coast]

Ah yes, the place to discuss the will it's, won't its, I've heard that, can you tell me if, my granny says and such like.

[Cecil Hogmanay Melchett]

Not stirctly model related but apparently the bbc are talking about a global pattern change which is

going to bring about some increasingly colder weather for us.

[Thundery wintry showers]

Reading the Met Office update it seems clear to me that they expect one of those shortwaves to plunge south to the east of Britain introducing northerly winds, gradually pulling in colder and colder air from the northeast. This implies to me that they are siding more with their own model and the GFS and yesterday's ECM, rather than today's ECM.

[NorthNorfolkWeather]

Also- the GFS operational out to T+144 supports an odds-on evolution to a cold easterly or north-easterly. The most important bit, I feel, is not how things evolve from there, but whether we get there in the first place. T+144 is still a bit too far out to be certain, but I suggest that the building blocks for this easterly actually arise within the higher resolution part of the GFS.

As is often the case the 12Z runs will be quite important. We really need to see the ECMWF show a similar scenario out at T+144 to have much confidence in this cold easterly coming off, as opposed to a moderately cool cloudy one or a stagnant high pressure scenario.

The strength of any Easterly or North Easterly that develops will determine what happens this close to the coast, January 1987 is a great example of a good blow from the East, bitter coming off the continent, picking up just enough moisture crossing the North Sea to drop a level 20 cm's of snow, but the wind was enough to push drifts up to 8 ft in places. Also I think it was 1982 where we ostensibly had a cold HP with gentle North Easterlies. The Coastal strip had up to 25cm lying snow, but ten miles inland virtually nothing.

So where do we go from here? For the period before Christmas, if we keep shortwaves out of the picture in the short term, it's obviously going to cool. Don't think there'll be much snow to start with at least. Then we hit the shortwave possibility, if one tracks towards Iceland, is blocked and comes down the North Sea, or even better passes over Copenhagen on the 18th, it's likely to drag much colder, -15 uppers behind it. What happens then is conjecture, but it the block maintains we could be in for one of the scenarios that TEITS has spoken about, a Greenland/ Scandi high that swops us between Easterlies and Northerlies. But, we shall see

[Just Before Dawn]

The strength of any Easterly or North Easterly that develops will determine what happens this close to the coast, January 1987 is a great example of a good blow from the East, bitter coming off the continent, picking up just enough moisture crossing the North Sea to drop a level 20 cm's of snow, but the wind was enough to push drifts up to 8 ft in places. Also I think it was 1982 where we ostensibly had a cold HP with gentle North Easterlies. The Coastal strip had up to 25cm lying snow, but ten miles inland virtually nothing.

True - strength of the wind can make a difference, as can wind direction - less of an issue along the North Norfolk Coast, where even stright northerlies can deliver - same for parts of East and North Yorkshire and Kent, however for us, there needs to be an easterly component. Straight northerlies can deliver, but often keep precip out in the North Sea. The 1982 event for us, with long sea-tracks generated a fair bit more snow than the easterly of 1987 did for us, and penetrated well inland on a fairly benign NE breeze.

SST at the moment are relatively warm, which also can create issues in coastal areas during marginal events - feb 2005 I think we had sleet when 10 miles inland they had snow. They can also, of course beef up convective activity, so it's swings and roundabouts really.

Edited December 7, 2009 by Just Before Dawn

[snowmadchrisuk]

If the UKMO and GFS 06Z are correct at t144 then its definitely game on.

The BBC weather man at 13.30 said high pressure could be around for some

considerable time to come from this weekend onwards.

Yes if you live on the Eastern side of the country then enjoy the rain that is currently falling , because the next time precipitation falls out of the sky with a bit of luck it should be Snow.

[doublebarrel]

can anyone say whats happening with the charts yet? or is this message invisible like the other one again ( deleted ) when a question was asked its ok for some and not others eh.

[guitarnutter]

can anyone say whats happening with the charts yet? or is this message invisible like the other one again ( deleted ) when a question was asked its ok for some and not others eh.

[Coast]

can anyone say whats happening with the charts yet?

Everyone has the opportunity to see what the charts are indicating and you can join in too, here:

http://www.netweather.tv/charts

[Dingwantssnow]

So what do you thing the chances are for the manchester are getting anything out of the current trend?

[doublebarrel]

Everyone has the opportunity to see what the charts are indicating and you can join in too, here:

http://www.netweather.tv/charts

sorry coast after all the years reading i`m still not clever enough !!

[Coast]

sorry coast after all the years reading i`m still not clever enough !!

Neither am I!!! But if you keep an eye on what the clever people show and say in the Model Output Discussion thread, you will eventually get there.

Patience, my young Padawan

[glasgow-guy]

So basically if you live on the western side of the uk your not gonna get anything?? All this talk of easterlies being the best yadda yadda is mainly for the peeps who live on the east side of the uk and for us up in the north west are going to get pretty much zilch.

I think all this hype is a bit biased to the people who live in the east....i mean theres other parts of this country apart from london east anglia etc and when people say this is goingot deliver what is really mean its going to deliver for the east.

depends on how far west you are inland i live south west Glasgow but have caught many a easterly so dont be so down hearted just yet

[Alex95]

So basically if you live on the western side of the uk your not gonna get anything?? All this talk of easterlies being the best yadda yadda is mainly for the peeps who live on the east side of the uk and for us up in the north west are going to get pretty much zilch.

I think all this hype is a bit biased to the people who live in the east....i mean theres other parts of this country apart from london east anglia etc and when people say this is goingot deliver what is really mean its going to deliver for the east.

Not necessarily, convective snow showers will be concentrated in the east, but if any persistent snow comes along it may reach the west. Also as low pressure systems come up against the easterly flow the west would probably face the brunt of the snow. Even if a cold spell gets nailed on by, say, the weekend, it might still be unclear where the most snow is likely to fall until it has already started. Back in February the east was forecast the most snow, but in the end large parts of the South west got deeper snow than large parts of the East (55cm or thereabouts in Okehampton)

Edited December 7, 2009 by alza

[Nick L]

For those who are completely new to model watching, the best thing to do is watch the run as it comes out and keep an eye on the model thread as people make comments. When they make an observation about it, look at the time period they are talking about. After a while it will probably stick!

I am still a complete amateur compared to most on here, but everybody starts somewhere!

Edited December 7, 2009 by nick2702

[JamieUK]

I am liking this very much, just look at what is sinking south

http://212.100.247.1...3e582a017b916f9;

http://212.100.247.1...3e582a017b916f9;

Hey Guys,

Im really sorry, but when i look at these charts together im a little confused as to what they are.

Erm, what i mean is they are both 12Z charts and at the same time (Mon 14 Dec 06:00). Is 1 a pub run? I dont even know what is either lol

[guitarnutter]

Roll up Roll up, Place your bets on when the models will stop showing the potential cold spell and the forum is flooded with 'winter is over' whining posts!

I'll say the 9th when the -5 850 air no longer makes it to the UK!