Interitus

Will have to have a look at this, however not sure this is the whole explanation - +ve u anomalies in the subtropics would lead to an expanded anomaly region, but the hovmoller shows that the QBO anomaly signature shifts northwards with a reduction over the equator.

Maybe the message for late autumn and early winter was in the SSTs - see my post from Sept 25th - CET anomalies, October 0.0, November +1.8, December (to the 18th) +1.9 at 6.8°C Looking like quite a good call.

Those GEOS charts are a bit SD, for HD try the animations (up to 10mb, 240h on 00z 120h on 12z) here - https://fluid.nccs.nasa.gov/wxmaps/?region=nps Update on minimum zonal winds from last 18z and 00z runs - GFS -2.0, +3.7 m/s Para +6.4, +10.8 m/s Wow!

You'd think so but a over a week after the ECM, at what strength and would it recover? An interesting test as this is replacing the current GFS remember which went for -2.8 and -4 m/s on the 06 and 12z. Maybe more importantly the date came forward from the 2nd Jan to Dec 28th.

Minimum wind speed +2.0 m/s

Double post

Still looking good for this!

Since this run the minima have been -10.6, 1.8 and 3.2 m/s ...er... The 00z GFSFV3 para was 4.5 m/s ... hmm. Would look at more from the para but the source (NOMADS) seems to miss or discard the data very quickly.

Obliteration of the strat on the GFS 06z - reversal from 264 hrs (29/12 06z) down to -18.7 m/s at t372

A couple of observations - 'uplift' from mountains doesn't produce 'vertical Rossby waves'. These would be orographic gravity waves - oscillations in the vertical which are restored to equilibrium through the density of the air parcels acting under gravity. They can increase in amplitude with height and reach a point where they break depositing their energy (possibly in the strat). But they are not planetary waves (wave 1, 2 etc) or Rossby waves. Rossby waves act in the horizontal plane perpendicular to the flow with the restorative force being the gradient of potential vorticity (typically north-south). This applies to the stationary waves from mountain torques also, where the height of the topography increases the amplitude of the wave, speed of the flow determines wavelength. For winter 2009/10, the UK did experience cold weather after the SSW with a deeply negative AO - however the lowest AO and coldest UK weather occurred before the SSW on Feb 9th, as shown in the graph below (windspeed left axis, AO and CET right axis), with a healthy mid and upper strat vortex at the time -

Strongest reversal yet for the GFS op on the 0z at -8.5 m/s. Parallel still not as interested, minimum +2.3 m/s, fascinating.

06z GFS marginal again, minimum +0.1 m/s!

Conditions seem right but it could go either way, or if there is one it could be marginal so as not to have much of an impact. Then depending on how it affects the waveguides it could be well into February before there is another shot.

26 GFS runs since and including the first to feature a reversed zonal wind - 8 runs reverse, 18 don't, only 1 reversal in last 7 runs - something's got to give.

00z minimum 9.5 m/s - end of run slightly less than 18z at 15.3 m/s, but vortex looking less like splitting -

That 18z FV3 was nothing particularly special - here is the zonal wind time series - Minimum of 7.9 m/s at 29/12 06z End of run 16.7 m/s at 1/1 18z

The SD is fine for comparison as long as it's realised the extremes aren't always as great as they at first appear. The dates with stronger 10 mb heat flux represent 9 different periods from 8 winters (with minimum u1060 in following 3 weeks) - 29/12/84 -16.38 08/01/91 7.97 10/01/92 8.95 22/01/08 19.42 19/01/09 -31.08 04/01/13 -12.29 07/02/16 11.53 05/03/16 -31.43 11/02/18 -24.24 5 of 9 had SSW in following 3 weeks (technically 4 of 9 as March 2016 treated as very early FW) Interestingly since and including the first reversal on the GFS 10/12 18z of the 20 runs 7 have had SSW but 13 without. Here are the minimum zonal winds for the runs since 09/12 0z - And here are those runs in series (coloured by day) -

Interestingly, the split vortex at the end of the GFS 12z is not from strong wave 2 - (charts weatheriscool.com)

Great degree of caution required when using the S.D. as the data is far from normally distributed. Not entirely sure on the CFS data used in the chart, but using the MERRA2 year round average and standard deviation the GEOS forecast for the 10 mb heat flux of 355.69 on Dec 23rd is indeed a shade under +6 S.D. In a normal distribution this would be 1 in 506797346 days chance i.e. over 1.38 million years! But the frequency histogram looks like this - Skewed because of practically zero heat flux in summer. Using the December values brings the SD down to 3.93 and for December 23rd excl. 2018 SD 3.68, incl. 2018 SD 3.13. This latter value would still be over 1 in 370 years but there were higher values on 28-30th December 1984 and overall it is 21st highest with most falling in January. The maximum of 564.32 from 19/01/09 is over 58% greater and using the annual mean and deviation would be 9.79 SD which for a normal distribution the chance would be greater than the age of the earth.

What big reversal is this? The displacements of 23/1/87, 15/12/98 and 21/1/6 all got below -20 m/s. edit: Note Nick's charts are at 65°N, change the urls from 65N to 60N for the 60°N graphs

181210 0z P17 10mb geop. and temp. Zonal wind -30.8 m/s at 65°N, -20.3 m/s at 60°N

Yep

Yes, the GFS 0.25° really goes for it, u1060 of just 3.3 m/s - The FV3 (0.25°) not quite as progressive, some reversal at the top but not at 60°, u1060 of 15.9 m/s (blank areas due to Panoply's handling of missing values at 15 and 40mb) Also noticeable on the GFS is a roaring subtropical jet at that time.

Not always, but at the 10mb level the predominant forcing comes from wave 1 so most feature displacement and wave breaking at some point before a spike in wave 2 may split the vortex shortly before technical SSW (or in some cases shortly after, occasionally leading to some SSW being classed as either a displacement or split by different researchers). Purely wave 2 SSW are less common, but the strongest in Jan 2009 was one such case and in this animation by Patrick Martineau it can be seen that the vortex doesn't move far from the pole. Wave 2 causes the vortex to stretch and narrow. Yes, exactly. But one thing worth noting is that for a reversal you need to be seeing these impressive looking 10mb warmings reaching down to the 30mb level where data confirms modeling studies that shows that the polar temperature gradient and implied thermal wind adjustment is more important. So for a rough estimate, take the temperature chart from instantweathermaps.com Comparing the temperatures of the 50 and 80°N latitudes (take the values at the 12 lines of longitude) shows a negative gradient towards the pole of about 15°C - data shows no wind reversals without a neutral or positive temperature gradient i.e. warmer at pole than midlatitudes - even when there might be a noticeable +ve gradient at the 10mb level. This metric can change quite rapidly, but assuming the forecast is correct the earliest an SSW has occurred from that level of gradient is 3 days and only 25% have been at that level 9 or 10 days prior to SSW.

Oh dear.