Model Output Discussion - cold spell to end Autumn

[comet]

Loving this run. What a difference a slight easterly push and orientation of the low in the north sea to the precipitation across the country with many areas back in the firing line for some significant snow perhaps.

[Man With Beard]

Look at the slider in the Atlantic on the GFS T138 - still moving towards the UK at this point...

[Methuselah]

Not bad for Friday night...colder uppers than last Friday...

[Derecho]

Just now, Man With Beard said:

Look at the slider in the Atlantic on the GFS T138 - still moving towards the UK at this point...

Yup no major hiccups on the GFS 12z so far, key thing is where that low slides to....

Ideal scenario would be to pass around the channel and move to Italy 

[Winter Hill]

Gfs T+ 120 chart.. very nice.. WAA being pumped up to Greenland. 

Heavy shower into northern & western areas.

 

[weirpig]

Snow for quite a bit of the uk on the gfs12z  

[Guest]

WINTER 2017/18 REPORT No. 3 WITH DECEMBER 3rd INPUT

PART 1: As this will be a long report (even by my standards!) I am splitting it into to two parts. Part 2 will probably be posted much later on this evening. It will include an updated analysis of the Arctic, European and Asian snow, ice and temperature profiles.

A REVIEW OF THE MODEL OUTPUT OVER THE LAST FEW DAYS

Another extraordinary week in the volatility of the model output and continuing uncertainties even for later this week. In response to all this the general mood on this forum has gyrated from agony to ecstasy and back again – at least amongst many of those seeking cold and particularly snowy weather.  I broke one of my golden rules during the week and produced a one liner with a single chart while an ECM run was in the middle of being churned out. The comment proved pretty inaccurate on the very next chart! I’ll endeavour to leave the instant reactions to others but for any cold and snow starved “coldie” it’s so easy to get sucked into the frenzy. I shall stick to these occasional (perhaps weekly) long reports with some shorter interim posts when I feel that I can add something to the overall discussion which hasn’t already been covered.

There is much still to be decided in the short term. The period around Thursday to Saturday, December 7^th to 9^th had been identified a few days ago as being critical in terms of determining the direction of a broader pattern change. Most of the models (including the big 3) swung at some stage between a longer lasting northerly, a much briefer northerly and no northerly at all for the end of this week. Just as one or more of the models moved in one direction, at least one other moved in the opposite direction. Then about 2 days ago it at last looked like we had some general agreement on a decent Arctic outbreak towards the end of this week. There continued (and still does now) to be considerable variation on its longevity and its potency.

For several days now, most of the models have shown a LP crossing the country from south-west to north-east on different paths and at different strengths with the northerly sweeping in behind them. Just small variations in the path, speed and intensity will make huge differences in terms of how much precipitation there might be, how much (if any) cold lies ahead of the system, how quickly the cold moves in from the north, how much it might undercut , how much snow there might be and how windy it might get.

Several GFS runs showed the LP deepening rapidly and crossing the centre or south of the UK. At one stage it showed an intense LP in the south North Sea which, if it verified, would probably have led to a substantial tidal surge with high Spring tides due mid-week and still high tides during the critical period.  I did a short post on this potential threat on page 81 of this thread. Fortunately for those in the firing line, the intensity of the LP has been considerably downgraded.

Just ahead of the Arctic air stream the south in particular, is likely to see some pretty mild conditions.  Whatever path the LP takes (unless it goes much further south)  it has really looked like rather too much of this mild air would initially mix into the system which would almost eliminate any “early” snow prospects over lowland areas (at least south of the Scottish border) ahead of the Arctic air arriving. Again, several GFS runs showed much lower temperatures at an earlier stage with considerable widespread snowfall. Many on here (including me) did get very excited about this prospect but a number of the wisest posters (and perhaps also several who are more pessimistic or even “warmies”) did warn us to manage our expectations. Even the GFS (at least at the time of writing this part of my report – it’s currently 1030 with the 6z output still rolling out) has moved away from the early snow scenario. Much will then depend on how quickly any mild air is forced away by the surge of Arctic air and the depth and intensity of this incoming cold air and also how long the LP hangs around close enough to our east to produce more widespread wintry precipitation and not just wintry showers close to exposed western, northern and eastern coasts. Another factor will be whether there are any troughs or minor disturbances (even the outside chance of a Polar Low) embeded in the northerly flow. At this stage most of the potential for “widespread” snowfall for next weekend has been downgraded by GFS and the other models, several of which never really showed it in the first place. Obviously there is still huge uncertainty and things can easily change again even within the 24 to 48 hour period particularly in terms of snow events. Nobody can seriously rule it in or out at this D4 to D7 range given the current set up. It still looks like there will be some snowfall but how much of the country will see it? Stay tuned to this forum all week for comments and analysis from our regular (and occasional) posters for the latest developments!

LOOKING FURTHER AHEAD

Things still look very interesting going beyond next weekend and current uncertainties continue which is hardly surprising given that the next few days are still to be decided. There seem to be far greater than usual complexities. The main overnight runs have shortened the direct northerly to just several days. The GFS and ECM 0z runs both continue to show a pretty cold pattern even though the jet stream appears to strengthen considerably with a lot of energy moving over the top of the Atlantic HP and flattening it to a lesser or greater extent. With a likely north-west to south-east jet and the UK remaining on the eastern side of it both models in their own way show a succession of LPs moving down over the vicinity of the UK and engaging some of the colder air remaining over us. The  “potential” for snowfall almost anywhere in the UK will continue as long as this broader pattern is maintained. We cannot see what the UKMO shows beyond T+144 and their longer range outlooks have varied somewhat in recent days.  Several models like GEM as well as a few of the GFS and ECM ensembles have gone for a much flatter flow with a much more direct jet stream pushing almost all before it.

Let’s have a look at some of the different possibilities predicted for the jet stream using the control run and several perturbations from the GEFS ensembles from today’s 6z run which has just rolled out (currently 1145).

GEFS 6z on December 3^rd Jet Stream Predictions:

T+0:        Control – current:

       

 

T+144:         Control                                      Perturbation 5                              Perturbation 7                             Perturbation 13

                      

 

T+180:         Control                                    Perturbation 5                               Perturbation 7                             Perturbation 13

                                    

 

T+264:         Control                                    Perturbation 5                               Perturbation 7                              Perturbation 13

                           

 

T+384:         Control                                    Perturbation 5                            Perturbation 7                               Perturbation 13

                            

I selected these particular perturbations as they show quite (or very) different outcomes and routes of getting there. The time periods chosen are to show the position around the start of the so-called critical period for a possible pattern change at T+144 and the interesting changes just 36 hours later at T+180.  I found more variations around T+264 and towards the end of the run at T+384.

I will not comment on all the detailed charts (until part 2 of this report), they are simply there to show the significant range of variations going forward after around T+144.  One of the causes frequently cited of a strengthening jet stream is the North American temperature profile with the conditions down the eastern side contrasting to the adjacent much warmer Atlantic Ocean. The greater this contrast, the greater the likelihood of establishing or maintaining a stronger jet stream.

It is much more complicated than just this and this is in danger of getting rather above my pay grade and technical ability, so I shall keep it simple for my sake as much anyone else’s! In recent winters we have got very used to seeing the jet stream frequently roaring across the Atlantic and blowing away everything in its path, although last winter saw long periods of MLB but very little in the way of any deep cold (just surface cold at times). This winter does seem to offer something rather different. The strong and quite persistent Pacific and Atlantic ridges have not only caused significant MLB but, unlike last winter, have so far caused the jet stream to buckle leading to some greater amplification and a much more meridional pattern.

Now let’s have a look at the North American temperature profiles for the same time periods as I used for the jet stream predictions. I could have taken the temperature predictions several days ahead of each jet stream chart as there is usually a delayed response from temperature changes but the five time periods from T+0 to T+384 should give a rough idea of the predicted changes for each model run. Again I shall comment on these in part 2 of this report.

GEFS 6z on December 3rd 2m Surface Temperature Predictions:

T+0:              Control – current:

           

 

T+144:                Control                                      Perturbation 5                                 Perturbation 7                                 Perturbation 13

                                                   

 

T+180:               Control                                       Perturbation 5                                Perturbation 7                                  Perturbation 13

                                                  

 

T+264:              Control                                        Perturbation 5                                Perturbation 7                                 Perturbation 13

                                       

 

T+384:              Control                                      Perturbation 5                                Perturbation 7                                  Perturbation 13

                                    

 

GEFS 6z on December 3rd 850 Temperature Predictions:

T+0:               Control – current:

           

 

T+144:             Control                                    Perturbation 5                                Perturbation 7                               Perturbation 13

                                       

 

T+180:             Control                                    Perturbation 5                                Perturbation 7                                Perturbation 13

                             

 

T+264:             Control                                    Perturbation 5                                Perturbation 7                                  Perturbation 13

                                         

 

T+384:             Control                                    Perturbation 5                                Perturbation 7                                  Perturbation 13

                                         

I think that this will be the first and last time that I do such a detailed overview with so many different charts! Apart from the time to prepare and post them, it is difficult to look at all the changes and comment on each run. Hopefully the end product will have made it all worthwhile!

Part 2 to follow this evening – quite late.

Edited December 3, 2017 by Guest
Correct typos and check charts

[Singularity]

UKMO is surprisingly far east with the trough on day 6... shorter-lived run of snow showers from that.

Upstream is much improved though with signs of a trough splitting against SW Greenland which tends to then see a breakaway low move SE to the UK I.e. a 'slider low'. Sometimes this helps build heights NE of the spit point too so across and to the east of Greenland in this case.

GFS as you were really - halfway between 00z and 06z as of day 7. Jet track a little S of the 06z with more chance of retaining a wedge of higher heights very near to the N or NE with LP stretching out south of that.

Edited December 3, 2017 by Singularity

[Tracy Flick]

2 minutes ago, ShortWaveHell said:

What’s becoming apparent in the 12z is a confirmation of how well the GFS has done with this , it really hasn’t let it go,  day after day , run after run , for nearly a week , the latest 6-30 day MetO update falls in line with the GFS , moral victory for a model verifying 5th this year ! 

It had Thursday's low going south of the M4.  Now it's going over Shetland.

[Derecho]

Would be great if those heights in the arctic helped prop up the high pressure to our west , but a good GFS

UKMO is closer to the GFS and ECM with the exception of less energy in the Atlantic being developed so undercutting is not really a feature here. The GFS however still being consistent.

[GeordieX9v]

Consistency from the GFS.

[Daniel*]

No one is really discussing the threat of severe gales and blizzard conditions for the far north will need to be monitored as there could be some rather severe wintry weather. 

[cheshire snow]

@Bring Back1962-63

What a very informative and well presented post keep up the excellent work.

C.S. 

[nick sussex]

More amplified UKMO output and it could be that longer range the set up going forward is better with a shortwave running closer to the UK than heading for Iberia.

Whilst you've got that embedded cold over the UK you want some fronts to engage that.

I think the shortwave exit point for the UK is more condusive for that, so it might come as a surprise but for forward evolution I prefer the UKMO.

What you want is the energy heading towards the UK and diverting into France not Iberia.

[knocker]

Now who was it mentioned the possibility of a cut off high cell with low pressure to the south a couple of days ago?  Definite signs of an upgrade if transient.

Edited December 3, 2017 by knocker

[A.J]

3 hours ago, Frosty. said:

Haha, well at least I'm looking for the good news and not moaning about breakdowns, I like to stay positive ??

Plenty to be positive about too

Aye, as that well known NetWeather saying goes......."Get the cold in place..................................and the inevitable breakdown moans will come" 

[sundog]

10 minutes ago, Daniel* said:

No one is really discussing the threat of severe gales and blizzard conditions for the far north will need to be monitored as there could be some rather severe wintry weather. 

 

Yes ,surely whiteout conditions for northern Scotland. Ah well they are hardy bucks up there.

 

9 minutes ago, Daniel* said:

 

[nick sussex]

After an iffy middle part the GFS once again re-loads the positive PNA. Just as this morning this seems a solid signal to do that which increases confidence.

With any lucky you can avoid the iffy bit keep the UK cold and wait for the next re-load to develop a stronger high to the ne.

 

[feb1991blizzard]

6 minutes ago, nick sussex said:

More amplified UKMO output and it could be that longer range the set up going forward is better with a shortwave running closer to the UK than heading for Iberia.

Whilst you've got that embedded cold over the UK you want some fronts to engage that.

I think the shortwave exit point for the UK is more condusive for that, so it might come as a surprise but for forward evolution I prefer the UKMO.

What you want is the energy heading towards the UK and diverting into France not Iberia.

Agree with you re- forward evolution and needing fronts for precipitational reasons, BUT with the caveats that we are on the right side of them, its not a 3 inch slushfest and the track being further East isn't either a prelude to the Atlantic breaking back through with a return to mild muck / being stuck with a really frigid cold pool for weeks tantalisingly just out of reach.

[Methuselah]

Oh dear. I think that this one might be headed for the greengrocer's:

Edited December 3, 2017 by Ed Stone

[Singularity]

4 minutes ago, nick sussex said:

More amplified UKMO output and it could be that longer range the set up going forward is better with a shortwave running closer to the UK than heading for Iberia.

Whilst you've got that embedded cold over the UK you want some fronts to engage that.

I think the shortwave exit point for the UK is more condusive for that, so it might come as a surprise but for forward evolution I prefer the UKMO.

What you want is the energy heading towards the UK and diverting into France not Iberia.

With you on that one Nick; GFS' idea of cutting-off the low west of Europe rather than merging it with the Euro trough serves to encourage a flat jet to the north of the cut-off feature with a ridge pushing across to Europe. I had hoped that the wedge of higher heights to the N. would be enough to disrupt the next trough and send it sliding, but it didn't quite make it on this run. The model is up to its old tricks into lower-res with a low hanging back west of Iceland to delay the recovery even as the major build of HLB gets nicely underway... it may yet turn out slightly better than the 06z by the late stages, but the earlier troubles may in any case be invalidated if the track of the first slider can be improved upon.

If one looks back to the 00z GFS and ECM runs, they had the low sliding into Europe and the connection with the polar jet being retained into next week, keeping the Euro trough fed by further disturbances sliding along the boundary.

The angle is a bit precarious from UKMO here though. Looks like a bit much of a due-east push on the S. flank of the trough, leaving us dependent on the shallow feature NE of the Azores for cutting off a wedge of higher heights near Iceland.

 

Oh okay then GFS, I'll bite...

The potential for 2nd half of Dec that Steve M has referred to is hard to miss. This is only just outside higher-res too. Imagine if we could get there without the cut-off low causing trouble beforehand... well now I'm just speculating for the sake of it 

[Mucka]

I'm still not buying the way GFS handles that low and send it South instead of SE but nice to see a strong signal for continued amplification in FI.

[Frosty.]

Well..I'm seeing upgrades since the 00z, ukmo  looks better and the gfs is even better too!..great time to be a coldie ???..?the current output!..bring on the arctic..again..hopefully more potent and longer lasting!

Edited December 3, 2017 by Frosty.

[nick sussex]

2 minutes ago, feb1991blizzard said:

Agree with you re- forward evolution and needing fronts for precipitational reasons, BUT with the caveats that we are on the right side of them, its not a 3 inch slushfest and the track being further East isn't either a prelude to the Atlantic breaking back through with a return to mild muck / being stuck with a really frigid cold pool for weeks tantalisingly just out of reach.

Because the re-load positive PNA looks a strong signal then mild muck is very unlikely. The PV will get pulled back nw and low heights seep away from Greenland.

Overall I'm happy with tonights outputs so far.

 

[karlos1983]

Nice improvement from UKMO, GFS still good as we edge closer and closer. 

Got to be happy with that.

GEM seems a bit quicker though?

Edited December 3, 2017 by karlos1983