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Friday, January 6, 2012

Pattern change

1-12 update.

Euro and GFS ensembles losing the cold in week two. GFS op is all over the place, hence the focus on the ensembles. Next ten days are still mostly cold. After that, a more zonal look, indicative of a neutral PNA and NAO. I knew the PNA would be a problem, as noted well below. I still note there is a pattern change, and while it looks warm again in the end of week two, its not a huge ridge that's causing it, just a zonal flow, with a slight dip/trough centered in the Midwest. Even the anomalies are not that far off. But the ensembles do trend towards climatology in the long run. And indeed, the NAO is forecast somewhat neutral, AO is negative and PNA is neg to neutral.

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1-10 update. Euro and GFS ensembles still showing a change to normal to below normal 850 temps. Operational Euro still mainly cold, but with quick warmups. Operational GFS is signficantly warmer than the ensembles and looks very much like the same pattern after a few cold outbreaks.

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1-6

With stratospheric warming over the poles, the polar vortex should be disturbed and the AO should go negative. This along with the southern oscilation index going neutral ( ie La Nina going neutral) rapidly, should shake the pattern up a little.

We've seen the warming event modeled for a few weeks now, and the observations are matching the prediction. So I've been watching the long range models for signs of a change.

The first thing I noticed a while ago, in another post somewhere, is that the storms and highs are now sliding out of Canada. While the troughs are still lifting out over us, resulting in a cold then warm pattern. This is actually the pattern I thought most of the winter would have. The models, for weeks now, have shown a return to normal to colder than normal temps and a broad trough in the last 5 days of the run. But those temps always moderated or were replaced as the event neared. So once past 192 hrs, instead of 5 days of cold, it becomes 2 days of cold, 3 warm.
Example - todays Euro ensemble control (mean) has the following temps for CT at 850mb. Normal is around -3 or so.
Today - -10
Sat +7
Sun 0
Mon-wed - 5 to 0
Wed to Fri +5 to +10
Fri to Tues - 10 to 0
Tues and wed +5
Wed-Sat dropping to -25 (real cold)
But you can see the longest stretch of cold is at the end.

The GFS operationals have now been consistently showing colder air (albeit normal)
18z yesterday is below 0 at 850 from 192 hrs to 384, with only a 6hr period above.
oz has below 850 from 168hrs to 384 with one day above
6z also starts at 168 hrs and is below 0 all the way to 384, with som -20 to -25 showing up
12z run goes from 156hr all the way through, though less extreme
18 z went to more of a zonal flow after a brief cold break.

Only problem is that this look is more typical for a positive PNA/neg AO, and a strong negative PNA is being forecast by the ERSL, with CPC showing more of a neutral, wavy look.


1 comment:

SW CT WX said...

Euro shows 10 day cold outbreak in the ensembles then back to normal to slightly above normal after hrs 240. GFS does not let go of the cold. Both have 12 hr periods when 850 temps are above 0, which is when it will rain.

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