Model output discussion - winter proper underway

[Methuselah]

It's almost time for a massive, huge, ginormous GFS 12Z...Eyes down. Let's get ready to rumble! Or crumble!

Edited December 11, 2017 by Ed Stone

[Derecho]

Hello, first post for a few days as I have been away

Looks like we have a pattern change coming this weekend and I’ll describe what I think may happen based on the model output.

However first, lets have some context… why is the pattern changing? Well a big part of the change can be attributed to the Pacific North American pattern (PNA). What is this?, well here are some basic diagrams to show its effects. The top two panels show the synoptic pattern and the bottom two show the temperature anomalies.

The synoptic pattern over the USA when the PNA is strongly positive is one where we have high pressure over the western USA (like recently). That is the main thing and usually there is lower pressure over NE Canada or the NE USA.

                     Positive PNA - SLP                             Negative PNA - SLP
    

            Positive PNA - temperature                       Negative PNA - temperature
   

So its probably little surprise that this pattern has contributed to increased wildfire activity in this region. The downstream effects (North Atlantic) are that the Azores high is more likely to ridge North which is what we have been seeing.

Due to the cold air from Canada being deposited over the SE USA, low pressure systems over around the coastal areas where cold continental air meets the warmer temperatures of the North Atlantic. Low pressure systems typically move NE off the Eastern seaboard and some of these get trapped in the far NE Canada or USA. Thus encouraging ridging of the Azores high.

However we are seeing a transition to a more negative PNA which is shown on the right hand panel. Often the high Pressure over California is not as strong allowing the jet to dive further west in the USA instead. The SE USA is often under warm air from the Gulf of Mexico. Areas of low pressure zip across Canada and carry cold Canadian air into the North Atlantic.

We therefore see a region of rapid deepening of low pressure due to the cold Canadian air meeting the warmer Atlantic. Low pressure moves past Greenland and warm air from the SW Atlantic hits us instead.

Now there are some variations over the PNA but a somewhat similar kind of pattern change explains why there is a stronger chance of mild westerlies coming into play over the UK.

Though how this Atlantic energy is split will determine how mild the UK gets. So here is the basic scenario on Sunday. Now we have mild air racing across the North Atlantic but a slightly displaced Azores high close by. From here three scenarios could develop within the following couple of days.

                    Sunday


                     Scenario 1: 15%                                   Scenario 2: 35%                                         Scenario 3: 50%



The first is a Scandinavian high which has a fair chance of happening but it probably wouldn’t occur that quickly due to the amount of energy in the North Atlantic jet. Also there is the issue of that low over Scandi blocking its Eastward progress.

The second is an anticyclonic outlook. Here the Azores high ridges across but a lot of energy goes over the top. Many parts of the UK would still be cold because of the short days. Slack winds and weak solar heating means temperatures can’t recover during the day. In these situations T850 ensembles can be misleading because it can be cold at the surface despite warmer temperatures higher up (known as a temperature inversion). Warm air will likely affect NW areas though and a slight modification could see either the whole of the UK cold or mild.

The final scenario is one where the Atlantic wins out over the following few days. This could lead to either mild SW winds if there is a pressure build to our east as well or a more alternating mild/cool pattern with westerlies.

Finally a Scandi high could develop later from the other two Scenarios. The middle panel wouldn’t be that bad if some blocking could develop from the high pressure. Overall these are just a bunch of scenarios that I think will develop from the model output. Lets see what the 12z’s bring.

Edited December 11, 2017 by Quicksilver1989

[Singularity]

Quite the turnaround from the models with the dip into the 'circle of death' almost eradicated and much better amplitude into 6/7 from GEFS and 7/8 from ECMF.

Both are good for boosting HLB starting a week or two later in time from onset... so around 10-18 days from now. Question is whether we can achieve the 'almost' situation of recent GFS runs in the 7-10 day range, or end up with more work needing to be done to rescue things as per recent ECM runs.

With the near-term reduction in MJO forcing removed, there may even be additional amplification on offer in the 6-10 day range so that's something to watch out for.

Maybe.

[Frosty.]

I'm glad parts of the southeast got some snow today, especially higher parts of surrey. Tonight looks bitterly cold with severe frost, especially over the snow fields with minus double digits celsius for some!..needless to say tomorrow looks cold after such a bitter night and then we have a midweek less cold blip before colder air returns from the north with wintry showers to exposed n / e areas and overnight frosts and ice.

Edited December 11, 2017 by Frosty.

[ArHu3]

11 minutes ago, Quicksilver1989 said:

Hello, first post for a few days as I have been away

Looks like we have a pattern change coming this weekend and I’ll describe what I think may happen based on the model output.

However first, lets have some context… why is the pattern changing? Well a big part of the change can be attributed to the Pacific North American pattern (PNA). What is this?, well here are some basic diagrams to show its effects. The top two panels show the synoptic pattern and the bottom two show the temperature anomalies.

The synoptic pattern over the USA when the PNA is strongly positive is one where we have high pressure over the western USA (like recently). That is the main thing and usually there is lower pressure over NE Canada or the NE USA.

                     Positive PNA - SLP                             Negative PNA - SLP
    

            Positive PNA - temperature                       Negative PNA - temperature
   

So its probably little surprise that this pattern has contributed to increased wildfire activity in this region. The downstream effects (North Atlantic) are that the Azores high is more likely to ridge North which is what we have been seeing.

Due to the cold air from Canada being deposited over the SE USA, low pressure systems over around the coastal areas where cold continental air meets the warmer temperatures of the North Atlantic. Low pressure systems typically move NE off the Eastern seaboard and some of these get trapped in the far NE Canada or USA. Thus encouraging ridging of the Azores high.

However we are seeing a transition to a more negative PNA which is shown on the right hand panel. Often the high Pressure over California is not as strong allowing the jet to dive further west in the USA instead. The SE USA is often under warm air from the Gulf of Mexico. Areas of low pressure zip across Canada and carry cold Canadian air into the North Atlantic.

We therefore see a region of rapid deepening of low pressure due to the cold Canadian air meeting the warmer Atlantic. Low pressure moves past Greenland and warm air from the SW Atlantic hits us instead.

Now there are some variations over the PNA but a somewhat similar kind of pattern change explains why there is a stronger chance of mild westerlies coming into play over the UK.

Though how this Atlantic energy is split will determine how mild the UK gets. So here is the basic scenario on Sunday. Now we have mild air racing across the North Atlantic but a slightly displaced Azores high close by. From here three scenarios could develop within the following couple of days.

                    Sunday


                     Scenario 1: 15%                                   Scenario 2: 45%                                         Scenario 3: 55%



The first is a Scandinavian high which has a fair chance of happening but it probably wouldn’t occur that quickly due to the amount of energy in the North Atlantic jet. Also there is the issue of that low over Scandi blocking its Eastward progress.

The second is an anticyclonic outlook. Here the Azores high ridges across but a lot of energy goes over the top. Many parts of the UK would still be cold because of the short days. Slack winds and weak solar heating means temperatures can’t recover during the day. In these situations T850 ensembles can be misleading because it can be cold at the surface despite warmer temperatures higher up (known as a temperature inversion). Warm air will likely affect NW areas though and a slight modification could see either the whole of the UK cold or mild.

The final scenario is one where the Atlantic wins out over the following few days. This could lead to either mild SW winds if there is a pressure build to our east as well or a more alternating mild/cool pattern with westerlies.

Finally a Scandi high could develop later from the other two Scenarios. The middle panel wouldn’t be that bad if some blocking could develop from the high pressure. Overall these are just a bunch of scenarios that I think will develop from the model output. Lets see what the 12z’s bring.

The scenarios add up to 115%?

[Derecho]

1 minute ago, ArHu3 said:

The scenarios add up to 115%?

Been a long day 

[mountain shadow]

9 minutes ago, Singularity said:

Quite the turnaround from the models with the dip into the 'circle of death' almost eradicated and much better amplitude into 6/7 from GEFS and 7/8 from ECMF.

Both are good for boosting HLB starting a week or two later in time from onset... so around 10-18 days from now. Question is whether we can achieve the 'almost' situation of recent GFS runs in the 7-10 day range, or end up with more work needing to be done to rescue things as per recent ECM runs.

With the near-term reduction in MJO forcing removed, there may even be additional amplification on offer in the 6-10 day range so that's something to watch out for.

Maybe.

That is possible good news Singularity, if the MJO was to occur as projected, is there anything currently showing which might override this signal? ENSO remains favourable i believe as does the lower energy from the sun.

[bluearmy]

2 minutes ago, mountain shadow said:

That is possible good news Singularity, if the MJO was to occur as projected, is there anything currently showing which might override this signal? ENSO remains favourable i believe as does the lower energy from the sun.

I think the amplitude is relevant

as far away as poss from the COD to generate a downstream response 

 

[Singularity]

1 minute ago, mountain shadow said:

That is possible good news Singularity, if the MJO was to occur as projected, is there anything currently showing which might override this signal? ENSO remains favourable i believe as does the lower energy from the sun.

Indeed it is, and I'm not seeing any particularly troublesome stratospheric behaviour at the moment, well not consistently in model projections anyway. Still signs of a strong warming event, in fact, although more uncertain since the modelled tropical forcing started dancing about.

The ENSO state is tied into the MJO and GLAAM behaviour, with the eastward bias of cold anomalies allowing the MJO to move further east than would be seen during a canonical La Nina and so send GLAAM on the rise. This has led to interesting developments in similar historical years as discussed by GP (for example) at times these past few days and weeks.

The Met Office now putting back renewed HLB chances to early January does make me wonder if something's lurking in the shadows, though...

[georgiedre]

52 minutes ago, ukpaul said:

Most of those were poor at those ranges, only in the last 24 hours did they shift the heavy snow from here elsewhere, for example. So, trust none of them!

There really is no way of certainty until the last possible moment given not only the likely track but also the numerous parameters needed to predict what will be snow, sleet or rain.

T24 and no earlier.

I agree . Met office amber warning and ALL models had us in nw Wales in inches of snow. But we only had a few flakes blowing in the wind all day. 

[Methuselah]

Midweek and on looking good for the Scottish ski resorts:

[ArHu3]

46 minutes ago, Ed Stone said:

It's almost time for a massive, huge, ginormous GFS 12Z...Eyes down. Let's get ready to rumble! Or crumble!

The 12z looks a lot more amplified by 102h compared to the 6z, now let's hope it's enough 

[Derecho]

Better chance of a block building later on this time?

This will get shunted east, but the more the Atlantic gets halted the better chance a high will build over Scandi.

                      12z                                                              06z 

   

Edited December 11, 2017 by Quicksilver1989

[georgiedre]

7 minutes ago, Ed Stone said:

Midweek and on looking good for the Scottish ski resorts:

What's the chances of another slider from that low south of Greenland ed?? 

[mountain shadow]

Just now, ArHu3 said:

The 12z looks a lot more amplified by 102h compared to the 6z, now let's hope it's enough 

I know, the Atlantic really struggling to push through with heights over the pole and Greenland evident. Could be wintry again from midweek into the weekend again for some.

[Weathizard]

Interestingly there's quite a bit more amplification in the Atlantic at day 4-5 with pressure lower in Europe.

Probably nothing but hopefully the trend can continue, it's something.

[Methuselah]

Improving prospects for another wintry weekend?

[booferking]

If we can get this next low to slide energy under then its game on for heights to build.

[Derecho]

Either way some subtle but potentially significant differences within the T120 mark. For example here is the 12z GFS and UKMO

                       GFS                                                     UKMO
  

The UKMO doesn't even have a low pressure system off NE Canada / Eastern US area...

Edited December 11, 2017 by Quicksilver1989

[Snipper]

Is there any review of how well or otherwise the various models perform over a period of time? This, that and other persons say that this or that model is good at this or that but is it ever scientifically analysed?

Thinking it was only 2 or 3 weeks ago that it was suggested, by some, that we would never have a snowflake again and winter was over before it started.

Were there any front runners of the models that predicted at anything more than a day or two, with some degree of accuracy, yesterdays snow event? Appreciate with some scenarios "Nowcasting" is the only thing that is appropriate for the fine detail of what is or going to happen in any particular location.

[mountain shadow]

2 minutes ago, Quicksilver1989 said:

Either way some subtle but potentially significant differences within the T120 mark. For example here is the 12z GFS and UKMO

                       GFS                                                     UKMO
  

The UKMO doesn't even have a low pressure system off NE Canada / Eastern US area...

Those persistent wedges of heights around Iceland could continue to aid in energy going SE rather than E or NE and help promote a Scandi ridge further down the line.

Certainly no raging zonality on offer an temperatures will remain below average for another week at least I would think.

[booferking]

Energy went under lets see were we head from here.

[Methuselah]

What about a potential ice-day next Sunday?

 

[Marcus71]

Looks like Sunday may well be another cold day (according to GFS 6z). Zonality on hold...

[tight isobar]

Am' suprised theres not a lot more members/comments via 12z 

Ukmo/gfs: 

Some decent evolutions across suites.

Block holding up very well' and certainly cool-cold preety much throughout(reliable)

Zonal IT ISN'NT!!!

Edited December 11, 2017 by tight isobar