Much Colder Air by Late Week

Mild Temperatures For a Few More Days

Then...Much Colder...and a general
Cooling Trend

Middle Tennesseans can enjoy at least one more 
day of unusually mild temperatures. However, after 
that you can expect another blast of cold Canadian
air to arrive for the latter part of the week. High 
temperatures will tumble from the 50s and 60s 
into the 30s and 40s, and low temperatures will 
likely reach the teens and lower 20s by the 

If we look at a recent temperature forecast from
one of the most popular meteorological computer
models (the "ECMWF") we can watch how
the cold Canadian air is forecast to plummet 
southward out of Canada. In the picture below,
you can see what the temperatures in the lower 
atmosphere (about 5,000 ft up) are forecast to
be on Wednesday, January 11th. The blue and 
purple colors indicate very cold air, while the yellow
and orange colors indicate warm air. You will
notice the very cold air mass hovering over western
Canada , while Middle Tennessee continues to
enjoy relatively warm temperatures:

In the next picture you can see that by Saturday, 
January 14th, the coldest air has plunged southward into 
Middle Tennessee:

By next Tuesday, January 17, we notice another cold 
air mass, even more frigid than the one before,  poised 
up across Western Canada, and starting to plunge 
southward into the northern Plains. The red and green 
colors represent the coldest air:

The way things look now, this cold air mass could reach
Middle Tennessee during the last half of next week.

So, i
nstead of a pattern of largely uninterrupted 
warmer-than-normal temperatures, that we've grown 
accustomed to during the first part of this winter, we 
are now looking at a trend toward more frequent periods
of colder-than-normal temperatures for the latter half of

Another factor that points toward the beginning of a general
cooling trend is the forecast change in the Arctic Oscillation
(AO). A positive value for the AO generally is associated with mild
conditions in Middle Tennessee. A negative value indicates an
increased likelihood for cold weather. Since the start of this winter
we've seen nothing but positive (i.e., "warm") values. In fact, the 
last time the AO was negative (i.e., "cool") for more than two
consecutive days was back in October. The first plot on the 
graphic below shows various computer forecasts for the AO 
for the next several weeks. The previous observed values are 
in black, and the various forecasts are in red

As you can see in the plot below, some of the most 
extreme forecast values for the AO are as low as
 -3 to -4. Values that low haven't been observed since 
January of last winter. 

One of the things we typically look for in a plot of 
AO trends that can sometimes suggest the onset
of a long-term change in the weather patterns are 
successive changes in the amplitude of the highest 
"peaks" and the lowest "valleys". For instance, if you 
look at the plot in the graphic below you will notice that 
the "peak" observed in the plot for today is lower than 
the "peak" that occurred back during the latter part of 
December. If the next "peak" that occurs turns out to be 
even lower than the one today, we may be witnessing
the onset of a long-term cooling trend for the last half
of January. However, since the next "peak" in the AO 
plot probably won't occur for at least another week, it's 
hard to know where things are headed for the 
long term. All we can say now is that we might be 
witnessing the start of a long-term cooling trend (lasting
for 2 to 3 weeks or more).


 Although it's impossible to know if the AO will 
(a) eventually trend below zero, or
(b) reach values as low as -2 to -4,
there are certainly indications that the trend of the AO is down--
which implies a general trend over the next couple weeks 
toward temperatures more seasonably cold for this time of year. 

Will this general cooling trend eventually lead us to a significant 
winter weather event in the next few weeks? Only time will tell. 
However, there are certainly reasons to believe that Middle
Tennessee weather  could become a bit more interesting as
we head into the last half of January.

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