National Weather Service Weather Forecast Office

What the 90-day outlook says, and doesn't say


The Climate Prediction Center (CPC) has issued its national three-month
outlook for October, November, and December 2013.

So, what does this outlook mean to someone here in Middle
Tennessee? you ask. Well, it depends on how you interpret it.

For instance, take a look at the temperature outlook:

There are three categories used by CPC to define the type
of weather expected:  "A,"  "B," and "EC."

These categories are defined thusly:

A= There is at least a 33.33% probability that the
      temperatures will trend above normal,
      and this is deemed the most likely of the three
      categories to be experienced.

B= There is at least a 33.33% probability that the
      temperatures will trend below normal,
      and this is deemed the most likely of the three
      categories to be experienced.

EC= There are equal chances that temperatures
         will trend below normal, trend above normal,
         or trend near normal.

So, based on the above map, there are "equal chances
that temperatures will trend below normal, trend above
normal, or trend near normal" in Middle Tennessee
In other words, statistics would suggest that there's
not much you can bank on there--so it may be too
early to make plans for on stocking up on extra firewood.
Of course, that's not the case back over the western
United States--where there is a 33.33-40.00+%
probability that the temperatures will trend "above
normal" and it is the most likely of the three
categories to be experienced. 

Now, if we introduce some additional meteorological
reasoning, one might begin wonder if temperatures
will average a bit cooler than normal in Middle
Tennessee for the upcoming late fall and early winter
months. For instance, generally speaking,
whenever a large area of warmth is observed
over the western United Stated, cooler-than-normal
weather is simultaneously observed over the east.
This is because warm temperatures over the western
U.S. are usually associated with an upper level ridge of
high pressure, located between the West Coast and
the Rockies. A ridge over the West would tend to
support a cool northwest flow of air over the
East (including the Tennessee Valley). Thus, there
is at least a hint in the current 3-month outlook
that we might be looking at some frequent cold front
passages during the upcoming late fall and early
winter. However, exactly how "cold" those cold fronts
might be remains to be seen.

The precipitation forecast offers pretty much the same
"flip a coin" type option for the Mid State. However,
if you happen to live over parts of the Pacific Northwest
and Northern Rockies, you'll stand a 33.33-40.00+%
chance of experiencing precipitation that trends
above normal (which, incidentally, would also
be the type of weather one might expect when
an upper level ridge exists near the spine of the
Rockies.)

That precipitation outlook is provided below:

So, for Middle Tennessee, this three-month outlook is,
statistically-speaking, a bonified "toss up": In the
temperature department, things could trend warm,
cold, or near normal. However, there is at least some hint
that cold air masses will be a frequent visitor to the
Tennessee Valley. In the precipitation department, things
could trend wet, dry, or near normal.