Model Output Discussion 29th December - Into mid-Winter.

[phil nw.]

The pattern at T120hrs looks better for snow chances further south on the UK model.The Azores ridge held a tad further west probably as a result of a stronger ridge upstream off Newfoundland earlier on.

so the trough ahead a bit sharper so a more direct flow of the cold south as we get to day 5.

Following on we see the AH trying to move in but at this stage progress is uncertain.We may squeeze more than 2/3 days of cold out of this if we see further trough disruption over Europe.

Edited January 7, 2017 by phil nw.

[Johnp]

Just regarding polar lows....

These charts we have seen are not showing  polar lows, but are regular depressions heading southwards.

Proper polar lows are generally only picked up in now cast situations using satellite imagery.

[ArHu3]

11 minutes ago, Rocheydub said:

With the greatest respect, those charts are chalk and cheese!

That's what I said already, maybe you have a better comparison?

 

[knocker]

Great example of a N/S split Keep still my beating heart

Edited January 7, 2017 by knocker

[radiohead]

12Z ARPEGE is out to 114 hours. Uppers not as cold as the GFS or UKMO. -8 uppers on the GFS at T114 are -4 uppers on the ARPEGE.

[Summer Sun]

What a difference 24 hours makes

D9

D10

[NorthernRab]

1 minute ago, Johnp said:

Just regarding polar lows....

These charts we have seen are not showing  polar lows, but are regular depressions heading southwards.

Proper polar lows are generally only picked up in now cast situations using satellite imagery.

I would agree with that - polar lows are generally very small 'comma' features which form within cold northerly flows at short notice. In saying that, a polar low could form in such a set up, but we'd be talking about it 12-24 hours before it hits, not 150+.

[Gael_Force]

3 minutes ago, Johnp said:

Just regarding polar lows....

These charts we have seen are not showing  polar lows, but are regular depressions heading southwards.

Proper polar lows are generally only picked up in now cast situations using satellite imagery.

Yes, but they are snow bearing, regular lows of true polar origin and that makes them as rare a visitor in recent winters as the true little arctic 'hurricane'.

[knocker]

2 minutes ago, NorthernRab said:

I would agree with that - polar lows are generally very small 'comma' features which form within cold northerly flows at short notice. In saying that, a polar low could form in such a set up, but we'd be talking about it 12-24 hours before it hits, not 150+.

That is also the problem with the great looking set up around t160 with that shortwave arriving from the NW hitting a UK which already has some very cold air in the lower layers so the potential for significant snowfall on the leading edge is very high but cannot be taken seriously at that range.

[phil nw.]

re Polar lows.I read sometime ago we need the 500hPa temperatures to at least -40C or lower to be in with a shout so current modeling is of some interest

But as NorthernRab has just said above these are very short term forecasted features,small scale but can be quite prolific for snowfall where they occur. I think the UK model this evening may be an even better bet with that more direct flow but of course at this stage it's all speculation.

 

[Rocheydub]

9 minutes ago, ArHu3 said:

That's what I said already, maybe you have a better comparison?

 

No, you said: "Feb 1979 looks a bit similar but now very". Ver batim!

i don't think they're similar at all. 

 

[ArHu3]

1 minute ago, Rocheydub said:

No, you said: "Feb 1979 looks a bit similar but now very". Ver batim!

i don't think they're similar at all. 

 

typo I meant "not very "

[Singularity]

2 hours ago, bluearmy said:

Careful re the MJO nick

it's a few weeks since it was in any amplitude and I am suspicious that without any clear drivers (strat zonal wind and qbo aside, ) people tend to adopt it as an indicator when it isn't actually likely to have any influence 

unless we see some x model agreement and amplitude then I suspect it's irrelevant.  

http://envam1.env.uea.ac.uk/mjo_forecast.html

Actually, the Hendon-Wheeler phase diagrams are doing a poor job with the MJO signal at the moment. Looking at the infra-red imagery for the tropics (not shown), there's residual convection in the eastern Indian Ocean even as the actual MJO-associated convection continues in the W-Pacific. This activity is not very strong, but it's enough to perturb the Pacific wavetrain with downstream impacts. 

I believe that via the N Pacific-Aleutians ridging, this activity is the main reason why we have such a notable digging trough situation on the cards for later this coming week. As the MJO propagates east, that ridge should tend to be replaced by a trough, at which time the level of MJO activity will become important for whether this sustains a more amplified downstream pattern or acts to flatten it.

Model projections for the MJO amplitude through mid-Jan are currently extremely inconsistent; yesterday they were exploring modest activity as it crosses the Pacific, but today they show much less although there's some suggestion it could wake up again as it reaches the C-Pac. For the time being, I've lost a lot of faith in the guidance tools with respect to the strength of tropical forcing.We are sailing through a sea of fog.

[chris55]

That slider attempt on the GFS 12z caught my eye so i thought id compare the GFS and UKMO at 144 to see how they were both dealing with that time frame.

More often than not these sliders trend further southwest as the verification date comes forward so if something akin to the GFS 12z did come off then the possibility of a boundary snow event is possible.

We can see the differences between the GFS and UKMO at 144 clearly, the UKMO looks cleaner and potentially with colder 850s, but the GFS develops the energy "between" the highs (out in the Atlantic ) whereas UKMO keeps it to the West. Subsequently GFS develops that "potential slider" where UKMO offers the Iceland low alone which could well drop south east, but hard to be confident without a 168 chart.

I have got my Crayons out to show what I mean

GFS 144, developing the energy "between the highs"

Which leads to the Low pushing east and offering that potential "slider"

However UKMO ridges the Atlantic/Azores high keeping the two Lows separate, no slider but a colder feed, with the prospect of the Iceland Low pushing south east.

Not sure which option is better longer term TBH but does highlight that with this complex troughing in a cold flow nothing is nailed down and plenty of wintry prospects, even for those down south are available!!

 

Edited January 7, 2017 by chris55

[nick sussex]

The GFS has made some improvements during the day but is still not as good as the UKMO at T144hrs. I'm not sure whether to have one of my shortwave tirades though as the UKMO does have one just near Newfoundland. I'm still trying to work out what that will do at T168hrs.

Even with that shortwave the UKMO has potential to extend any cold. After the 00hrs outputs which apart from the UKMO were a bit disappointing I think we've made a little progress in terms of cold.

Theres enough there to work on and although an easterly might be hard work to get to you can't really rule it out just yet because if we keep a ridge in the Atlantic as the PV starts heading south again it could force this ne towards the UK.

[Summer Sun]

The 850's don't get too low on this run from GEM

Some snow in places then gradually turning less cold as we move into the 2nd half of winter

[IDO]

Looking at post D8 still two main clusters as per this morning's 0z. The op and control in one group and the other closer to the ECM 0z solution. I would say 60:40 favouring the op solution.

So after the colder few days no clear path just yet.

Clearly the op and control take us into a zonal path, so we would like ECM to stay with its take on the 12z.

GEM is in the other cluster (a la ECM):              

More runs needed for FI, but a cold spell now nailed. For the south,still  odds against for snow, but probably won't resolve till closer.^^^

[booferking]

1 minute ago, Summer Sun said:

The 850's don't get too low on this run from GEM

Some snow in places then gradually turning less cold as we move into the 2nd half of winter

Gem 850 temps are never low there is issues with the modelling of 850 temps on Gem i believe this was mentioned start of the week also.

[SMU]

44 minutes ago, Johnp said:

Just regarding polar lows....

These charts we have seen are not showing  polar lows, but are regular depressions heading southwards.

Proper polar lows are generally only picked up in now cast situations using satellite imagery.

Gone are the years where a polar low was a nowcast-

The GFS develops it @ 102 south of svalbard ( the kink north of 985 ) - you will note thats not a depression...

the 500MB temps are perfect @sub -40c

 

At the timeline I highlighted @ 156 the PPN field is a perfect comma shape

Note the same shape

best

s

Edited January 7, 2017 by Steve Murr

[Scandinavian High.]

5 minutes ago, Summer Sun said:

The 850's don't get too low on this run from GEM

Some snow in places then gradually turning less cold as we move into the 2nd half of winter

Cold enough for me.

[Singularity]

@Summer Sun GEM overdoes the modification of 850 hPa temps (and those at lower levels too of course) by the underlying ocean. So in reality it likely would bring temps akin to GFS.

 

As for snow in the south, I reckon there's a good chance of at least brief falling snow, but not lying snow unless a more organised disturbance moves through and without drawing in much of a flow from the waters around the UK (though a snow-rain-snow event could in theory take place...).

A common outcome for the (far in my case) south in a NW-N flow during winters past is a bright, mostly sunny day with a raw wind and, once or twice in the day, some interesting looking clouds approaching fast and then delivering a short spell of snow that can look quite impressive (due to decent atmospheric moisture in an Arctic/polar maritime airmass) yet rarely amounts to any accumulations of much note. This being because disturbances in the flow have a habit of giving me a wide berth! 

[NorthernRab]

2 minutes ago, Steve Murr said:

Gone are the years where a polar low was a nowcast-

The GFS develops it @ 102 south of svalbard ( the kink north of 985 ) - you will note thats not a depression...

the 500MB temps are perfect @sub -40c

 

At the timeline I highlighted @ 156 the PPN field is a perfect comma shape

Aye those 500MB temps look bang on. Hadn't even realised we'd moved to the point that we can forecast polar lows out to 100-150+ hours. Probably a lot to do with the lack of polar lows hitting our shores in the interim! 

[SMU]

^^ The GEM has 850 depth issues - there always between 2 & 4c to warm...

[The Enforcer]

8 minutes ago, knocker said:

That is also the problem with the great looking set up around t160 with that shortwave arriving from the NW hitting a UK which already has some very cold air in the lower layers so the potential for significant snowfall on the leading edge is very high but cannot be taken seriously at that range.

A Low coming in from the NW will contain too much of a tropical maritime air that would undermine the potency of the cold spell, like it did in the token cold snap of last winter and the umpteen times before that when a Low has approached from that trajectory. Disturbances like troughs embedded in the polar flow with no mild sector are what is required, February 2015 being the last time snow settled here courtesy of such an occurrence - only forecasted by NMM.

[knocker]

7 minutes ago, The Enforcer said:

A Low coming in from the NW will contain too much of a tropical maritime air that would undermine the potency of the cold spell, like it did in the token cold snap of last winter and the umpteen times before that when a Low has approached from that trajectory. Disturbances like troughs embedded in the polar flow with no mild sector are what is required, February 2015 being the last time snow settled here courtesy of such an occurrence - only forecasted by NMM.

Two points. Where exactly has the Tm come from because from  where I'm sitting it looks very much like Pm. Of course eventually it undermine the cold spell (or at least could do, rather depends on the track) but in the mean time you can get some significant snowfalls as the warmer air overrides the colder air in the lower and boundary layers.It's all academic anyway.

Edited January 7, 2017 by knocker