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Stratosphere and Polar Vortex Watch

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Just now, Yarmy said:

 

Presumably this is data from the current GloSea runs (or run singular), and not those that have generated the DJF forecast (which would have been from the October runs)? It looks like an SSW for early Dec, but hard to tell if it's a major one without seeing the actual numbers.

As for the westerlies at the surface...well yes, but if it ran forward a few more days/weeks...?

Yep, be interesting to see if they continue to show that, higher chance of unsetlled early Dec, but then  possible downwelling in time for Christmas. Interesting stuff, so many things going on this winter!

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From Cohen

I do believe that a significant PV disruption is becoming more and more likely but is not guaranteed. As I have discussed over the past year there is vertical energy transfer that can result in a “reflective” PV disruption and alternatively vertical energy transfer that can result in an absorptive PV disruption. A reflective event is characterized by positive vertical energy transfer anomalies or vertical wave activity Flux (WAFz) quickly followed by negative WAFz anomalies. The PV itself is stretched but often not displaced. Also, the cold temperature response to a reflective event are focused in North America east of the Rockies. If you look at Figure 12 below you can see a reflective event at the end of October and early November identified by a red blue striation or positive anomalies quickly followed by negative anomalies. I do believe this reflective event is at least partially responsible for the cold air outbreaks of last week and this week.

In contrast during absorptive events the WAFZ is consistently positive until the climax of the PV disruption. Also, the cold temperature response to an absorptive event are focused in Eurasia. Also, I have discussed many times that during positive WAFz leading to an absorptive PV disruption, temperatures tend to be mild in eastern North America but cold in East Asia. Again, in Figure 12 the predicted WAFz anomalies are predicted to be positive only over the next two weeks. Also, the PV is predicted to be displaced towards Eurasia (see below Figure 13). This is looking to me like we are transitioning from a reflective PV disruption in early November to potentially an absorptive PV disruption in late November. The biggest immediate impact on Northern Hemisphere (NH) temperatures would be a shift of cold temperatures from North America to Eurasia. The positive WAFz leading up to an absorptive PV disruption can last up to six weeks. So potentially we are at the beginning of a multi week period of overall mild temperatures in eastern North America.

When I tweeted on Friday about the potential for more cold air for eastern North America based on a minor PV disruption, I had assumed another reflective event. Much of this was based on the time of year. It seems to me that positive WAFz in November most often is associated with reflective events and only later in December and January are the positive WAFz anomalies associated with absorptive events. This was the case the last two years. These events are much easier to diagnose in hindsight, so I am not committing to one type of event or another. As of today, it is starting to look like more of an absorptive event to me but I don’t rule out a reflective event just yet. If it is a reflective event, then I do believe it can turn colder in eastern North America. And even if it is an absorptive event, short-lived cold outbreaks are still possible.

Longer term if it is the beginning of absorptive positive WAFz, this could be the initiation of a significant PV disruption. But this takes time and I would guess the earliest would be the second half of December. So far, our speculative polar vortex model does not indicate a significant PV disruption through December 11, but I think over time it will show greater confidence in one based on the predicted NH atmospheric circulation

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Interesting, sebastiaan1973. I'm thinking an absorptive disruption would be better for cold in the UK if it leads to warmer temperatures in the Eastern US.

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1 hour ago, Yarmy said:

 

Presumably this is data from the current GloSea runs (or run singular), and not those that have generated the DJF forecast (which would have been from the October runs)? It looks like an SSW for early Dec, but hard to tell if it's a major one without seeing the actual numbers.

As for the westerlies at 500hPa...well yes, but if it ran forward a few more days/weeks...?

Not sure what that glosea chart shows ?

according to that we should currently have a high anomoly of zonal flow 65N in the strat/trop coupled area?  We don’t. Or if it predicts a big weakness low down (which we do have) then it shows a strong zonal flow for Dec.  either way, the model shows a reversal of what we have now for first part Dec which won’t be any good for the trop re zonal flow - unless that forecast is a bust from last month but would be odd given it’s within a month !!

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14 minutes ago, bluearmy said:

Not sure what that glosea chart shows ?

according to that we should currently have a high anomoly of zonal flow 65N in the strat/trop coupled area?  We don’t. Or if it predicts a big weakness low down (which we do have) then it shows a strong zonal flow for Dec.  either way, the model shows a reversal of what we have now for first part Dec which won’t be any good for the trop re zonal flow - unless that forecast is a bust from last month but would be odd given it’s within a month !!

I think the title 'zonal mean' on the chart is misleading as it appears to be the overall geopot height anomaly for the entire H Hem from 65N to 90N at each pressure level, i.e. it's not the U component of the zonal mean at 65N. So it's saying we currently have below average heights from 50hPa upwards and above average heights below that, which is in agreement with what we see elsewhere?

As you say, it's then a complete reversal for late Nov/early Dec indicating an SSW or at least significant strat pv disruption. It doesn't run out far enough for us to see if that anomaly would then propagate down to the surface later in Dec.

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Certainly current data forecasts continue with the slowing down of mean zonal winds as strong wave 1 activity is projected to continue.

2143277024_wave_serie10(2).thumb.png.63bf0fc9b527746d7266d59f9d1eb5a4.png303403951_epsmean10hPa60N(1).thumb.png.edcc5d2da3d349c3f1e79c3b15443429.png

The latest 00z gefs continues with this pronounced downward trend for the next 14 days in it's forecast.A big reduction too currently at 10h Pa and 60n we do actually have a record 41m/sec expected to come right down below 20m/sec near months end.

The last ECM update which only goes out to 10 days also going with a decline in wind speeds

1916256082_ecmwfzm_u_f240(1).thumb.gif.1942c6c0b1cc9455968ad37088155eb3.gif

Whether we see a split or displacement or even a reversal with the consequent effect on our 500 hPa pattern remains to be seen.Certainly a weakening of SPV should start from now on as the current record speeds start to come down.

 

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Well if looking at a split then one tropospheric lobe could could set up shop over N. Canada. Now haven't I seen this someplace today ? 8)

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5 minutes ago, knocker said:

Well if looking at a split then one tropospheric lobe could could set up shop over N. Canada. Now haven't I seen this someplace today ? 8)

Quite true Knocker.

A split does not mean it will bring cold here if we end up on the wrong side of it.A warming and slowing down of the vortex takes time anyway and the forecast period where this kicks in is only just commencing.A while yet before any dramatic effect on the 500 hPa pattern manifests itself in the daily NWP.

If wave action continues into December then interest will certainly rise i think. 

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genuine question for the big guns in here. at the moment the top of the strat is spinning at record pace and is not coupled with the bottom of the strat. its forcast to slow significantly over the next couple of weeks. everyone is predicting this may lead to a SSW but couldnt it just make it easier for the trop/strat coupling to take place and that would be that??

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On 10/11/2019 at 16:58, Catacol said:

What’s he mean by weak or strong? Swift turn around within 48 hours 13th to 15th. 

Strong is +41.2M/S 

 

Cant remember the weak value - Probably sub 15 M/S

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Predictability of Sudden Stratospheric Warmings in the ECMWF Extended-Range Forecast System

WWW.RESEARCHGATE.NET

ResearchGate is a network dedicated to science and research. Connect, collaborate and discover scientific publications, jobs and conferences. All for free.

From the abstract:

Quote

Using the spread of ensemble members to estimate forecasted SSW probability, it is shown that some SSWs can be predicted with high (>0.9) probability at lead times of 12-13 days if a difference of 3 days between actual and forecasted SSW is allowed. Focusing on SSWs with significant impacts on the tropospheric circulation, on average, the forecasted SSW probability is found to increase from nearly 0 at 1-month lead time to 0.3 at day 13 before SSW, and then rapidly increases to nearly 1 at day 7. 

For me, the 2 points to take away are:

i. If the ECM extended range model is forecasting an SSW 2 weeks away, you can be fairly certain it will happen. However, those that affect the trop (which is what we are mostly interested in) are harder to forecast, and you can only be confident a week away.

ii. At the 4 week range, trop-affecting SSWs are essentially impossible to forecast. (Something to consider when looking at all those LRFs).

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I presume  the Glossea forecast for the following three months  takes a warming Strat into consideration?.  If so  ( and i know it may be difficult to gleen from a mean)   but why does it scream a raging + nao   and mild temps?.   is it just that it does not see a SSW coming to fruition  

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5 minutes ago, weirpig said:

I presume  the Glossea forecast for the following three months  takes a warming Strat into consideration?.  If so  ( and i know it may be difficult to gleen from a mean)   but why does it scream a raging + nao   and mild temps?.   is it just that it does not see a SSW coming to fruition  

It can't. At least if we assume it has roughly the same limits on predictability as the ECMWF extended range model in the paper above.

The Feb 18 SSW wasn't picked up by any of the extended models at a range of just 12 days, according to the paper by Lee et al. posted further back in the thread.

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3 minutes ago, Yarmy said:

It can't. At least if we assume it has roughly the same limits on predictability as the ECMWF extended range model in the paper above.

The Feb 18 SSW wasn't picked up by any of the extended models at a range of just 12 days, according to the paper by Lee et al. posted further back in the thread.

Aplogises  Yarmy i didnt read the paper   i saw a few words that consisted of more than two syllables  and left it   i shall take a better look later.   On a further note  if this is the case   why do people hold so much weight in this type of seasonal forecast   

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5 minutes ago, Yarmy said:

It can't. At least if we assume it has roughly the same limits on predictability as the ECMWF extended range model in the paper above.

The Feb 18 SSW wasn't picked up by any of the extended models at a range of just 12 days, according to the paper by Lee et al. posted further back in the thread.

Glosea is a high top model & will be able to factor in the SSWs into the seasonal forecast however its only in the last week or so this has become more visible in the models - based on that the next seasonal run of the models in particular Met office & ECMWF will be very interesting...

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Quote

The atmospheric model (UM) uses a resolution of approximately 0.83 x 0.56 degrees and extends from the surface to the mesosphere in order to include stratospheric processes in our forecasts.

That's regarding GloSea5.

I guess it can't take an SSW in to account because the data has already been assimilated?

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The norm for early SSW events is actually for the stratospheric vortex to be strong in late winter (though small sample) so it may be that it simply expects Jan-Feb to revert.

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After a bit of a lull it appears that the GEFS are picking up the signal for some warming again gensnh-1-7-384.thumb.png.ed33851fa9b051de34f7ff82737bf7e8.png447872263_gensnh-16-7-384(1).thumb.png.8ec02b9ed61326ecfbcd9a3520bf6be7.pnggensnh-14-7-384.thumb.png.4af52e4ebb657ad5bfd2af6544ba1d74.pnggensnh-8-7-384.thumb.png.88ff78121488eec912ea21edf0b1b970.png still lots to be resolved but the interest continues as we move closer to winter..

 

Edited by Kirkcaldy Weather

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20 minutes ago, Recretos said:

The problem is that it does raise heights over the pole, and breaks up the tropospheric vortex, but the residual lobes end up in Pacific, Atlantic and India/Himalayas. So essentially we end up with a +NAOish type pattern.

Not great news, then, although I suppose it’s subject to change?!

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Of course. Trop-Strat coupling is not such an easy matter for the medium range, harder for the extended range, and very hard for the seasonal range.

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