Cooling Trend ends with a COLD
SNAP this weekend
Then milder weather returns!
The Arctic Oscillation (AO) is something that is often linked
to changes in the weather in North America. When the numerical
value associated with the oscillation trends toward more
positive values, a gradual trend toward warmer conditions is
often observed in Middle Tennessee temperatures. Conversely,
when the values trend into "negative territory" temperatures in
the Mid-State often show a cooling trend.
There is usually a lag of at least several weeks between an
established trend in the AO and an established trend in
temperatures in Middle Tennessee. For instance, when the
AO headed toward its maximum value back in December, we
started experiencing very warm average temperatures in our neck
of the woods. However, in recent weeks, the AO trend has
been toward "negative territory," indicating a potential turn
toward cooler weather. Indeed, if you look at the latest plot
of the AO (shown below) you can see this trend quite clearly.
The observed conditions (in black) trended below the
"zero line" after the middle of January, and suggested
that a cooling trend might be in store sometime over the
following few weeks. The cooling trend this week and cold
snap would seem to support this general correlation.
However, latest forecast trends in the AO (shown in red )
reveal a general tendency for the AO to rise back toward
positive territory over the next couple weeks, implying a
possible trend back to milder weather in the Mid-State
(especially if it manages to move back into "positive
Considering how warm it has been so far this winter, a cooling trend
merely implies temperatures returning to more seasonal levels...
and that's exactly what's happening this week (with highs in the mid
40s to lower 50s, rather than in the 60s).
The cooling trend experienced this week will be capped off by a
genuine Canadian cold snap this weekend, as a strong cold front sweeps
in Friday night and produces highs on Saturday only in the upper 30s
to around 40 degrees across northern parts of the Mid State, and mid
40s south. Lows Sunday morning are forecast to drop to the upper teens
to lower 20s.
Much of the mild weather we've enjoyed so far this winter has been the
result of a west-to-southwesterly flow of air in the upper levels of the
atmosphere across the continental United States (similar to what is
depicted in the picture below)
Now, Let's take a look at the upper level wind pattern expected
to be in place over the United States this coming weekend.
Notice, how the lines are bulging northward into western and
Central Canada, indicating that the "refrigerator door" to
Canada is open, and allowing cold air from Central Canada
to push southward into the Tennessee Valley.
The low temperature forecast at the Nashville Airport Sunday
morning is 20 degrees. The last time temperatures were that
cold in Nashville was back on January 14th. Prior to that,
the last time the temperature hit the teens in Nashville was
way back on December 11th.
So, keep the heavy coats handy. You'll probaby need
them by this weekend.
However, the cold snap this weekend is expected to be
short-lived and temperatures will bounce back above
normal again by next Monday and Tuesday.