Christmas Weather and the Arctic Oscillation

UPDATE: Thu 16 Dec 2010 / 5:45p.m.

EXPLANATION: Computer “models” are nothing more than complex pieces of software which crunch a program and provide a numerical guess of what the weather might do. As one ventures further out in time, errors which are generally small for a two or three day forecast might become huge by 8 or 9 days. That’s why confidence in them generally improves the closer you get to the day in question.

Sometimes a pattern will be very stable and we can nail a frontal passage or mid-level trough 5 or 6 days out, but this is less than half of the time. A changing pattern like we’re in makes such a forecast much more difficult.

MAP DISCUSSION: Now to Christmas 2010: Trends continue to suggest a blocking pattern re-developing, which would steer cold air into our area between the 25th and 27th. This does not mean they are right, but they are somewhat consistent with the past three days’ worth of output.

The Arctic Oscillation (scroll down in this blog for more on the AO) is a key factor in whether we have a cold or mild Christmas; the newest output (and latest weather analysis) shows the block breaking down over the next four days, and rebuilding by Christmas Day. A cold front would then follow with northerly flow on the western side of the trough in the Dec 25th-27th time frame.

As for timing, a shot of colder air could arrive by Christmas Day, but more likely the 26th or 27th. Given this, I’m expecting Christmas Day temperatures chilly in the morning (30s) and in the 50s or possibly 60s by afternoon.

The trough in the east/ridge in the west steering flow does not favor any precipitation due to sinking air aloft. So, a white Christmas still looks “very unfavorable” here.

The model resolution increases once we get below 192 hours out, so I should be able to become a little more confident by my Sunday post (no post Saturday).

Stay tuned -- Steve

 

Map above continues blocking trend with low pressure to the east Christmas Day. The block is a little weaker than in Wednesday's run, but it's still too far out in time to claim this as a new trend.

================WEDNESDAY DISCUSSION===================

 

 

 

UPDATE: Wed 15 Dec 2010 / 5:35p.m.

Note: I will not make a habit of looking this far ahead as the errors are too large; this is rare occasion for the sake of your interest and Christmas planning. I will leave older posts below so that you may track how a forecast this far out changes with time.

The overall trends have not changed; see original post below.

The models continue to advertise a blocking pattern to our east with chilly, but dry, northerly flow through Texoma. Here's the latest map (above).

  ====================Post from Tue 14 Dec==================

This blog begins as a VERY optimistic attempt to provide an idea of what our Texoma Christmas weather might be like. Please understand it’s quite general and really won’t be worth much until Monday or so. No, I'm not crazy; I figured, why not have some fun? There's some science in here too...

So...let's talk weather.

It’s useful to look at the larger-scale pattern when forecasting beyond a week ahead.

There’s been a persistent blocking pattern over the North Atlantic, which means a low pressure south of a high pressure, with neither moving much. This is why the eastern 1/3 rd of the nation has been in the deep freeze for some time: the high is keeping upstream low-pressure systems nearly stationary. The counterclockwise flow around the low has been pulling very cold air into the eastern half of the nation. We have been on the western fringe of this and have been spared the brunt of the cold.

Orlando, Florida reached 28 degrees Tuesday morning and Miami hit 36. This kind of cold in the Sunshine State” is usually the result of such a pattern, known as a negative phase of the Arctic Oscillation, or “AO”.


Texoma weather tends to be cold and windy but rather dry during a negative phase of the AO, like we saw at the end of November and again last weekend. The AO has temporarily weakened but is predicted by long-range models to re-develop Christmas weekend. This creates a "traffic jam" in the flow upstream, which makes the jet stream buckle, and it may push more arctic air southward.

Surface and upper level pressure patterns guide cold air further south during the negative AO episode (right side of above graphic).

Long-range indications bring the AO back into a blocking (negative) phase around Christmas time…which would favor another chilly Canadian visit between Dec  25-27. So my first ultra-risky stab at the Christmas 2010 forecast for Texoma is for a fairly strong front to pass Christmas weekend, most likely without precipitation.

We had a White Christmas last year.  Given this year’s pattern, I’d put the odds of one for 2010 in the “unfavorable” (but not impossible) range.

Of course, I’ve been wrong before…such is the nature of trying to forecast this far out. Like I said, I figure I’d have a little fun; or perhaps I am crazy!

This blog will be updated each evening through the 23rd.

The daily discussion will take care of Christmas Eve.


Here’s Hoping You Have A Wonderful Christmas!

Steve LaNore

Chief Meteorologist / KXII-TV

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