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December 16, 2009 / Dave Gorham

Long Range Outlook for Christmas Week

Discussion: Long-range data continues to advertise a major pattern change toward much colder and potentially stormy weather near or shortly before Christmas over a large part of the Lower 48.  Operational computer models have been hinting at this for that past few days while other longer range signals we use here at ImpactWeather identified this potential weather change about 1 week ago.  Since both sets of data are trending in this direction we are gradually gaining more and more confidence that much colder air will be pouring southward out of Canada into the Lower 48 and all the way south to the Gulf Coast centered on next week. 

Before we take a look at the general flow pattern we are expecting Christmas week let’s take a quick look at the latest snow cover and temperature anomaly maps over the Continental U.S.  Remember that snow cover allows less modification of colder air surging southward from Canada increasing the likelihood of freezing temperatures reaching the Gulf Coast.  

  • Latest Snow Cover Maps:

As you can see there remains a substantial snow cover all the way south to southern Kansas, northern Missouri and northern Illinois.   Now let’s take a quick look at the current temperature anomalies over the source region of western North America and see if anything is pooling over this region and poised to move southward into the Lower 48. 

  • Latest Temperature Anomaly Maps:

Once again there remains a vast area of below to much below normal air over much of southern and western Canada thanks in large part to a vast and deep snow cover over this region of North America. 

Based on the snow cover and temperature anomaly data there remains quite a bit of cold air over the northern U.S. and most of western Canada poised to move southward.  As we mentioned last week the only thing that is lacking is some type of forcing mechanism to drive this colder air southward toward the Gulf Coast.   Let’s take a look at the flow pattern projected for next week and try and derive some conclusions from it.  

  • Projected Mean Flow Pattern for Christmas Week:

As described last week all of our long range signals remain about the same for Christmas week.  Strong high pressure is forecast to form across western North America causing the storm track to buckle around the upper high north of Alaska, pulling down cold air from the Arctic and shoving it southward into the Lower 48.  The intensity and timing of the cold air is still on the low side; however, confidence is on the increase in favor of a significant thrust of very cold air plunging southward toward the Gulf Coast and Atlantic Seaboard near or shortly before Christmas Day possibly setting the stage for a hard freeze affecting much of the Deep South, and may be even the Gulf Coast, centered on the last week of December.   As more data becomes available we will be able to pinpoint the timing of this potential threat in the upcoming days.  

  • What about wintry precipitation?

In addition to the colder air, the southern storm track remains very active thanks in large part to a powerful El Niño currently dominating the weather pattern across the Pacific Ocean.  El Nino’s are notoriously known for bringing multiple periods of cloud and cool weather over Texas and the rest of the Deep South during the winter months.  This year appears to be no different, but in our case we have a potential worst case scenario with a powerful El Niño interacting with the possibility of multiple surges of very cold air over the Deep South centered on the last week of December.   

Even though our confidence is still on the low side, some of our longer range signals are indicating  one or two disturbances may interact with the colder air bringing a risk of wintry precipitation over parts of the Deep South, including parts of Texas starting around the 24th or 25th.   Of course the timing of the disturbances and cold air has to be just right for the wintry precipitation to materialize, so there still is considerably uncertainty if and exactly when this will occur.  If wintry precipitation does occur across the Deep South we think the best chance for that will occur in between the 23rd and 27th of December.

We will continue to monitor the weather trends very closely!


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