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December outlook

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Mark Bayley

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[b]December outlook [/b]
The following forecast is based on the latest stratospheric output, MJO, CFS, AO and NAO ensembles means, as well as 500hpa and 850hpa means.

[b]Stratospheric outlook[/b]

Stratospheric temperatures at both the 10hpa and 30hpa level are set to increase, as shown on the following diagrams. This should strain the vortex and encourage blocking.
[attachment=146385:ECM current temp. conditions.gif][attachment=146388:ECM T240 temp. conditions.gif]
The GFS also indicates a warming event. I note how the warming on the pacific side is extending into North America, pushing the coldest temperatures to the East of Greenland. This would support a large segment of the PV shifting into this region later in December, and lend support to a -NAO. It is important to note however that the troposphere and Stratosphere interactions seem to have been different this year, which would cast some doubt on this assumption. The extended cool period and higher zonal winds did not lead to strong and organised PV that some may have expected as we progressed through November. Why this occurred is beyond my knowledge.
[attachment=146408:Warming forecast current GFS.gif][attachment=146409:Warming forecast T240 30hpa.gif]
Mean zonal temperatures are forecast to decrease which again would strain the vortex and encourage blocking
[attachment=146386:ECM current zonal conditions.gif][attachment=146389:ECM T240 zonal conditions.gif]
I have limited understanding of waving breaking. I believe it stands for reflection of large-scale planetary waves off mountain chains that break into stratosphere, which in turn places pressure onto the vortex. If we use CH’s analogy of the balloon then it is perhaps simpler to understand how two wave breaks in the NH (placing pressure on two points of the vortex) can lead a split vortex, while one wave break (placing pressure at one point of the vortex) can lead to disruption,
We can see from the ECM forecast that wave two activity is set to peak over the next few days, before petering out.
[attachment=146390:ECM wave break T144 (2).gif][attachment=146392:ECM wave break T240 (2).gif]
However, wave one is forecast to increase substantially at the day ten. So using the CH’s analogy, this should lead to further disruption (of an already disorganised) PV.
[attachment=146391:ECM wave break T240 (1).gif]
I should probably pass over the wave breaking section, I’ve only included it with the hope that one of the experts may correct me if any of my interpretation is wrong.

The latest ECM geopotential height charts support higher pressure in the mid-atlantic with a NW west flow, as does the GFS. Based on these charts I would suggest height rises over Greenland are some way off (i.e. past day ten).
[attachment=146393:Geo height 100hpa t240.gif][attachment=146398:GFS t240 geo height.gif]

[b]MJO outlook [/b]

The MJO signal is weak, with most models indicating that will remain in low amplitude. The UKMO seem keen on a weak phase one or two, the ECM on a weak phase one or eight while the GFS predicts a higher amplitude phase one (although briefly skirting through phase eight). Based on the forecasts I would have probably discounted the MJO. The GFS however has been keen on a higher amplitude phase one, and has for some days. The ECM would support phase one, albeit in much lower amplitude.
[attachment=146407:UKMO MJO phase forecast.gif][attachment=146387:ECM MJO phase forecast.gif][attachment=146396:GFS MJO.gif]
Phase one MJO would give us the following composite, heights over Greenland, and a trough over the UK. Note that these composites should only be used as a guide.
[attachment=146383:DecemberPhase1500mb.gif]

[b]CFS charts[/b]

Not a model I normally use however the trend has been for higher heights in the mid-Atlantic and over Siberia for weeks one-two, with some support for higher pressure in these regions for weeks three-four. I should note there has been variability in these runs over the past week, although there is a theme for higher heights in the said areas.
[attachment=146381:CFS outlook week 1-2.gif][attachment=146382:CFS outlook week 2-4.gif]
The monthly CFS, for some time now, has indicated a below average December, with considerable blocking to our North.
[attachment=146380:CFS heights forecast december.gif][attachment=146379:CFS global temp forecast december.gif]

[b]AO and NAO Ensembles[/b]

Both the GFS and ECM ensembles support a –NAO, after brief close to/positive spell, as we progress into and past the middle of December.
[attachment=146384:ECM AO outlook.png][attachment=146397:GFS NAO forecast.png]
The GFS also supports the general continuation of a -AO
[attachment=146395:GFS AO Forecasrs.png]

[b]Mean height ensembles[/b]

The ECM ensembles are supportive of higher pressure in the mid-Atlantic, and over the Scandinavian region, lending support to some sort of easterly by day ten.
[attachment=146399:Mean ECM T240.gif]
The ECM 850hpa mean temperatures are shown to be below average.
[attachment=146402:mean t850 ecm T240.gif]
The GFS ensemble mean is also supportive showing higher pressure in the mid-atlantic retrogressing northeast.
[attachment=146400:Mean height anoms t180.gif][attachment=146401:Mean height anoms t364.gif] [attachment=146404:Mean t850 t384.gif]
Which is also supported by the NAEFS and NOAA
[attachment=146405:NAEFS T198.png][attachment=146406:NAFES T364.png][attachment=146394:GEO height outlook next two weeks.gif]

[b]Overall[/b]

Overall im finding it rather difficult to produce a forecast, but then isn’t it always! What’s apparent is the PV shall remain disrupted and disorganised as we progress through December. I am generally supportive of height rises to the NE given the latest runs, although feel that these will transfer nearer Greenland as we progress through the month when we see further wave breaking disrupting the vortex and leading to more movement of the PV segments. This would also be supported by the GFS MJO outlook, and ensemble forecasts for the NAO from both the ECM and GFS. The CFS could also be included. Weatherwise a cold and unsettled start, with the potential for height rises to out NE, and some sort of easterly flow during the first third. As we progress into midmonth and beyond I feel higher pressure would transfer closer to Greenland, bringing a more N/NE’rly flow.
I intend to update midmonth.
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