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December outlook update

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Mark Bayley

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[b]Initial forecast[/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.

[b]Review so far[/b]

The month started on a cold and unsettled note, while expected height rises to the north east materialised. Unfortunately we did not see the easterly flow that i expected, with low pressure from the west taking control, introducing unsettled weather with temperatures close to the average. Despite this the PV has remained disrupted, with a -AO and -NAO which has prevented us from experiencing something from winters past (i.e. Bartlett type weather). It’s apparent that my forecast of midmonth height rises over Greenland via retrogression from scandi will not materialise, and thus a north/north east flow.

[b]Update and initial January thoughts[/b]

The following update and forecast is based on the latest stratospheric output, MJO, CFS, AO and NAO ensemble means, as well as 500hpa and 850hpa means.

[b]Stratospheric outlook[/b]

[font="helvetica, arial, sans-serif"][color="#282828"]Stratospheric temperatures at the 10hpa and 30hpa level are running just below the average, and have done since a small warming event at the beginning of the month. They are forecast to remain just below average for the next 10 days, with temperatures rising at 30hpa by day 10 (as shown on the ECM temperature charts). Note i have used the graphs for simplicity.[/color][/font]
[attachment=149126:temp graphs.gif]
[color=#282828][font=helvetica, arial, sans-serif]Although not as apparent on the temperature graph, a major warming at the 10hpa level is forecast from day ten onwards (Siberian sector), this is shown the ECM 10hpa geopotential height charts at day and ten, and the GFS strat charts.[/font][/color]
[attachment=149116:ECM t240 10hpa.gif][attachment=149130:gfs_t10_nh_f240.gif]
[color=#282828][font=helvetica, arial, sans-serif]The most significant warming is still out of range for above charts, however the GFS 12z strat charts show this nicely, and have done for some time (and are not pushing back)[/font][/color]
[attachment=149123:gfsnh-10-348.png]
I suspect this will lead to major disruption of the PV, or the killer blow seen as its not exactly rampant at present. This would be expected as we progress into January. For the rest of December, I expect the continuation of the relatively weak PV, centred towards the Siberia, which opens up potential for height rises over Greenland.
[attachment=149113:ECM geo t240.gif]
Zonal winds have risen slightly since a drop at the beginning of December, and are forecast to decrease again. Given their current and predicated strength I would expect the continuation of a weak(ish) PV.
[attachment=149124:graphs zonal.gif]
Further wave breaking is forecast by day ten, most notable wave one. This should displace the PV, and further stress it. It is also noted that wave two activity shows signs of increasing, which would place even greater stress on already strained PV.
[attachment=149128:wave one.gif][attachment=149129:wave two.gif]

[b]MJO outlook[/b]

There seems consensus to progress the MJO into phase two, although not of high amplitude.
[attachment=149119:GFS mjo.gif][attachment=149114:ECM mjo.gif][attachment=149127:UKMO mjo.gif]
The UKMO is interesting in that wants to progress the MJO through higher amplitude phase one beforehand, which would give the following composite which has height rises over Greenland and a trough over the UK (although bear in mind these are to be used as guides).
[attachment=149111:DecemberPhase1500mb.gif]
The composite for phase two MJO is the following
A trough with the core of lower heights situated over the UK (i.e. unsettled)
[attachment=149112:DecemberPhase2500mb.gif]

[b]CFS charts[/b]
[color=#282828][font=helvetica, arial, sans-serif]Weeks one and two show a trough situated in the Atlantic, unfavourably for us, leading to unsettled and probably mild (at times), weather. Looking at all forecasts from Sunday 9[/font][/color][sup]th[/sup][color=#282828][font=helvetica, arial, sans-serif] there seems a trend for height anomalies to continue above the norm around Greenland as progress past xmas day and into new year, with the core of lower heights transferring from the Atlantic into Europe. This would open the door to something a little more seasonal (e.g. northerly airstream).[/font][/color]
[attachment=149110:CFS week one-two.gif][attachment=149131:weeks three-four.gif]

[b]AO and NAO Ensembles[/b]

The ensembles support the continuation of a –AO, although trending towards positive as we move towards months end and then negative again.
[attachment=149118:GFS AO.png]
Further to this there is support for the continuation of a mostly –NAO, although trending towards positive for months end.
[attachment=149120:GFS NAO.png][attachment=149115:ECM NAO.png]

[b]Mean height ensembles[/b]

The ECM ensemble mean supports the continuation of higher pressure to the north (Greenland area) and the core of lower heights in the Atlantic, with the continuation of unsettled weather.
[attachment=149117:ECMWFENS6-10day500mbHeightAnomalyNH.gif]
Analysis of the 850hpa anomalies would indicate that temperatures should remain close to average, despite being unsettled.
[attachment=149108:00zecmwfenshourly850mbTempAnomalyNH216.gif][attachment=149109:00zecmwfenshourly850mbTempAnomalyNH240.gif]
The GFS is similar, although perhaps more interesting by day ten to fifteen, with the core of lower heights into Europe and indication that UK might experience something a little more seasonal (i.e. some sort of northerly) past xmas day.
[attachment=149121:GFS6-10day500mbHeightAnomaly.gif][attachment=149122:GFS11-15day500mbHeightAnomalyNH.gif]
This is also supported by the NAEFS
[attachment=149125:naefs-0-0-360.png]

[b]Overall[/b]

I expect the continuation of unsettled conditions, with on average temperatures remaining close to the norm. Given the expected continuation of a –NAO, and –AO I would not bet against colder interludes, in the form of brief northerlys, as we progress through the xmas period (something which has shown around xmas day on the latest model runs). I do not see great support for something prolonged this side of New Year. Given developments in the stratosphere I think the first half January, probably past week one, is when it could potentially come very interesting..!
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