[Model output discussion - 7th January onwards]

T180:

Three jet streaks in play across the North Atlantic. Ridge number 1 there beneath the poleward streak.

T222:

Trough carving through Texas, next amplification coming through

[Model output discussion - 7th January onwards]

Your challenge Nick answered by another question. How many in that period have had Modki / weak / moderate end Ninos / low solar / coming after strong sustained weakening of the SPV ?

[Model output discussion - 7th January onwards]

Agree with this Nick, although it's noticeable how much sub-tropical flow we get into the SW Atlantic through days 5-12 which is in line with tropical forcing signals. That, allied to the Euro trough gives a strong below average signal for us care of repeated ridge building in the Atlantic. If we get the cold entrenched, these become self reinforcing for cold.

The impacts of the SSW this time are a lot more nuanced and subtle, but they're clearly having a big impact in the pattern.

[Model output discussion - 7th January onwards]

Blocking features are not necessarily Greenland Highs. Within the context of a deep cold trough across NW Eurpe, you don't need strong blocking features. These are however still potentially on the table for the week 3 period, and would represent the icing on the cake.

We go below average on the 16th, and remain there after.

[Model output discussion - 7th January onwards]

Extended EPS T2s and T850s dipping at the very end of the run, which would be against model bias to drift back to climatology.

I'm struck at the moment by the Scandinavian trough and daughter vortex over Canada. Perfect mirroring of the stratospheric pattern. This is a clear influence which does not fit troposheric drivers. Between the two features, uncertainty abounds, but you can't escape the model trend for deep cold uppers forecast within the Canadian vortex. That should, against other signals, continue to remove the Greenland High option. I think more +ve height anomalies and a more likely mid Atlantic ridge rebuilding but not necessarily Greenland Highs are of the order, and tonight's EC run will be viewed with interest - there could well be a very slow downwell of the stronger -U wind anomalies from the warming to come which might be the trigger, although I'd like to see some more NAM plots to get a proper handle on this.

The tropical wave signal is moving extremely fast. By week 3, we could well be back to Dateline convection and Eastern Pacific. Again, good signal for sustained cold.

Other features catching my eye: the deeply negative state of the Arctic Oscillation (not unexpected for the second half of the winter); and, related to this, the ridge extending into the Arctic from Alaska modelled to break off and move towards Siberian sector. With the clockwise rotation developing in the Arctic lower tropospheric flow, this shows signs of moving towards Svalbard by month's end. With the European trough embedded, that makes for some interesting permutations for February.

[Model output discussion - 7th January onwards]

...and there's a difference between low tropospheric heights over Greenland with a deeply entrenched SPV and without it. The latter instance results in rapid changes in the lower circulation pattern.

[Stratosphere and Polar Vortex Watch]

On 09/01/2018 at 16:56, chionomaniac said:

Absolutely want to see the upper strat u wind resume westerly to aid in flushing through the -u wind signal

[Model output discussion - 7th January onwards]

CFS weeks 3 and 4 identical to EC:

 

[Model output discussion - 7th January onwards]

4 minutes ago, Nick F said:

Interesting article in the Washington Post suggesting that GFS performance is being affected by US government shutdown because it’s not being maintained because of employees on furlough:

https://www.washingtonpost.com/weather/2019/01/07/national-weather-service-is-open-your-forecast-is-worse-because-shutdown/?noredirect=on&utm_term=.189acfbd0823

not sure if I believe the claims or whether the GFS model output would be really that affected given it’s probably mostly automated and fed data from around the world. Fake news?

Back to the 18z, as with recent runs, signal increasing for cold zonality at times through next week, thanks to low heights and colder T850s moving from the west, would favour northern hill snow and perhaps lower levels of Scotland at times. Cold rain for the rest of us. No signs of pattern flip this run.

 

Look again Nick. STJ becoming evident across N Atlantic with split flow and ridge signal off the Eastern US

[Stratosphere and Polar Vortex Watch]

Effectively the -U wind signal waxes and wanes as it downwells appearing as a series of nodes at descending heights. It is a good signal if you're looking for tropospheric blocking, and would be difficult to model correctly *If* the GFS is correct. 

[Stratosphere and Polar Vortex Watch]

and a hint on nodal propagation going on there ....

[The Hunt for Cold continues 02/01/19]

Interesting to note the key phase around days 8-9 when a tropical wave forces a ridge over the Southern US and enhanced sub-tropical flow develops in the SW Atlantic. That seems to be the trigger to the sudden slowing in the mid latitude flow, but likely to be related to downwelling zonal wind signal.

[The Hunt for Cold continues 02/01/19]

Significant adjustments to the eps tonight, much more amplified in the North Atlantic and across North America. Still got some catching up to do with the GEFS, and confirms it takes the EPS 2 days to react to GEFS when it switches.

All model suites now carving out a deep trough across Scandinavia and West Russia in the extended. Significant as it cannot be attributed to upstream ridging or nascent polar blocking. First impacts of downwelling being felt?

[Hunt For Cold: Model Discussion - Heading Into Christmas]

4 minutes ago, shaky said:

Gfs p flatter again between 120 and 144 hours!!look forward to more boring uk high!!its never ending!!

.. back to views out of hotel windows in Torquay....

Unless and until upstream Pacific flow is resolved, and until strat downwelling takes place, out to day 10 will have a strong mid latitude high component for NW Europe.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

GFS, GEFS and GGEM starting to find some consistency on the building Alaskan ridge around day 10. That allows ridge development in the North Atlantic.

Notably end of GFS (p) and op also starting to develop sub-polar lows days 12+ across the NH, suggestion of slowing zonal winds aloft.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

Noted difference between EPS and GEM/GEFS over handling of Pacific trough and downstream ridge over Alaska. GEM/GEFS more amplified on the trough and less flat across the North Atlantic.

[Stratosphere and Polar Vortex Watch]

21 hours ago, Catacol said:

Thanks Stewart. You looked in on 33andrain yet? Your views would be interesting over there.

Yet more delay - but I think we are all beginning to understand that any propagation this time around is going to slow, unlike the lightning quick response earlier this year. Is this wQBO related in your mind - given the QBO is westerly now probably down to around 50hpa? How closely does QBO shear at various levels relate to activity over the pole?

And as a teleconnection junkie can I ask you to elaborate on what you mean by "pattern wants to go cold, but is prevented from doing so by the atmospheric field over the polar region....a clear cut signal for mid latitude highs". I've been expecting a GSDM driven move towards high lat blocking for around a week now, and been scratching my head as to why it hasn't occurred and why the models continue to sustain the limpet UK high. What do you mean by "atmospheric field" - never encountered this phrase before. Is this something to do with the magnetic field, and if so how can we read it, and how does it help conduct the teleconnective orchestra?

wQBO extends down to just above 50hPa I think, but will be ‘footprinted easterly below that, so reduced shearing.

The GSDM has to my eye been typically low amplitude (reduced tendency in AAM) in response to potential drivers through reduced equatoward eddy flux, but that should change in the coming 2 week period to allow for more extreme tendencies to develop.

Polar field = all layers  1000hPa up to 1hPa, typically viewed as cross section

[Hunt For Cold: Model Discussion - Heading Into Christmas]

... all of a sudden, some notable model output beginning to appear...

Progression of mjo towards favourable tropical forcing phases not to be overlooked here.

[Stratosphere and Polar Vortex Watch]

With the central warming date set at 1st Jan, earliest trop response seems to be 13th onwards. To restate - Stratospheric. Timescales. Are. Extremely. Slow.

I suspect we may be at the longer end of the envelope for trop response (so more like 15-20 day) given that the duration of this warming event is extremely long. For downwelling signature I would normally look for a reorganisation of the upper SPV. That's clearly not evident just yet, and it may well be we are reliant on a massive -U wind signature muscling its way down.

In the meantime, we are in what is a clear teleconnective phase, one where the (enso lead) pattern wants to go cold, but is prevented from doing so by the atmospheric field over the polar region not being conducive.  A clear cut signal for mid latitude highs. The unfavourable AO state is being driven by constructive interference from the stratospheric warming and tropical forcing in the Pacific.

looking at the tropical forcing signal for weeks 2 and 3 with a return of forcing to the far Indian Ocean, that would suggest a slowing and retraction of the Pacific Jet. 

We therefore have a plausible favourable time line for downwelling stratospheric signal and tropical forcing opening up end week 2 into week 3.

For those on the 2001/02 bandwagon, no propagation, I offer this. This was one of my analogues that performed well during the summer and early autumn. Atmospherically it punched above its weight during the autumn with AAM running well ahead of the enso indices. However, this analogue was rejected for the winter as it fell at the complete opposite end of the solar cycle, and could not mirror the enso values experienced this winter. Additionally, and crucially, I cannot see this analogue holding a candle the scale and extent of the major mid winter warming we are about to embark upon. That gives me a 70% chance of downwelling taking place, but more likely on a much longer time scale that many are anticipating.

the broad scale evolution looks to remain as mid latitude high retrogressing north and northwards over time.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

Agree with timings there Nick. Warming date looks to be 31st December and split on 3/4th January. That gives 11th January as first real opportunity to gauge any impacts on the tropospheric pattern. Suspect that the continued warming forecasts are a hindrance to fast downwelling.

It's all related, but the destruction of the SPV is possibly constructively interfering with the tropics. That MJO signal for week 1-2 is well beyond what we would expect for an +ENSO pattern. 

As the atmosphere choreographs itself, an MJO in Phase 8/1 and downwelling -U wind continues to be the end destination. Expectation would however be for the MJO to weaken significantly through phase 7 but regain intensity in phases 8 / 1.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

4 minutes ago, karyo said:

Out to 300 hours is a small improvement to the 6z but still not the perfect split.

Take a look further down. 12z op likely to be a significant outlier synoptically in our quadrant based on this. Placement of the vortex west of Greenland the vital thing here. GFS likely to have it too far east and north here.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

..think you'e going to like the 06z GEFS !

Whilst the fixation with 10 hPa layer is understandable, analysis of 50 hPa layer proving to be most useful at this stage, particularly regarding ridge development.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

1 hour ago, Weather-history said:

Any ideas why Michael Ventrice has mentioned 2009-10 in this tweet ?

 

[Hunt For Cold: Model Discussion - Heading Into Christmas]

06z Parallel much more aligned to what EPS and GEFS means are showing (and also splits the SPV). Not wholly surprising the op GFS turbo charges the +NAO and +AO in low resolution. 

Week 2 tropical modelling looks not uninteresting as a starting point to migrate the mean ridge NW.

[Hunt For Cold: Model Discussion - Heading Into Christmas]

12z eps doubles down on that split idea. Now clearly apparent on the means. We sit here waiting for downwelling.