AOPA Turbine Pilot Magazine - September 2014 - 93

FREE
SURVIVING

ENGINE

FAILURE

But not all cold fronts are turbulent
meanies. For example, cold fronts that
have exhausted their energy after travelling across the United States are less likely
to have widespread thunderstorms. Also,
slower-moving cold fronts generally are
less hazardous than fast-movers, and so
are fronts with relatively small temperature differentials across their boundaries.
On the other hand, if you see that surface
temperatures ahead of a cold front are in
the 90s, and drop to the 50s behind it-
well, that's when you can expect fireworks.
There's another brand of cold front
that sometimes affects the northern tier
of states. It's called a "back door" cold
front. Why the name? Well, we typically
expect cold fronts to travel from west to
east, and be aligned north-south or northeast-southwest. But sometimes, if there's a
large, strong (think high barometric pressure and altimeter settings) high pressure
system anchored in Canada or the northern
states, this high pressure's clockwise flow
can send cold air southward. The leading
edge of this cold air creates a cold front,
which most often is depicted on surface
analysis, weather depiction, and low-level
surface prognosis ("prog") charts as a front
aligned roughly east-west. Because this
isn't the normal, west-to-east path that
cold fronts usually take, meteorologists
call these back door cold fronts-the back
door being the north-to-south path of air.
Back door cold fronts usually are welcome arrivals-especially in the depths

of summer, when hot, humid air flows up
from the Gulf of Mexico and takes over
huge areas of the eastern United States.
When a back door cold front pushes this
hazy, muggy, convective mess back to the
South, the result is clearer skies, lower temperatures, and less humidity. It's a welcome
break from the end-of-summer dog days-
one that brings good VFR, light winds, lower
density altitudes, better performance, and
less sweat during the preflight!
But when it comes to the weather, take
nothing for granted. Back door cold fronts
can cause trouble. That's why the events of
May 29, 2014, caught my eye. On that day
a back door cold front came out of New
England and set up over the Mid-Atlantic
and Piedmont states. The clockwise flow
around the high brought cooler temperatures, yes, but also imported a lot of
moisture from the Atlantic. This colder,
moist, ground-hugging air pushed its way
inland until it dammed up against the high
terrain and upslopes of the Virginia piedmont. The end result was anything but clear
skies and dry air. It was more like overcast to
broken skies at 700 to 1,400 feet, with visibilities of two to seven miles in mist.
Like I said, back door cold fronts are
usually benign. Still, be on guard in the
spring and autumn months, when cold
highs from northern latitudes run into
either the earliest or latest warm days of
the summer season.
AOPA
EMAIL tom.horne@aopa.org

www.aopa.org/pilot AOPA PILOT | 93

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AOPA Turbine Pilot Magazine - September 2014

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