AOPA Pilot Magazine - January 2020 - 88

P&E

WX WATCH

Winter jet
streams
Pulsing winds aloft mean big trouble below
BY THOMAS A. HORNE

CONVERGENT

DIVERGENT

CONVERGENT

AT HIGH ALTITUDE, air accelerates as it enters the core of strongest winds aloft. Pressure

gradient forces move air first to the north, then Coriolis force takes over as the air slows
after exiting the core. Surface lows form under areas of divergent air aloft, at the right
entrance and left exit quadrants.

88 | AOPA PILOT January 2020

streaks and cores generate much of the
adverse weather at lower altitudes. How
so? Re-read the first sentence, please.
Areas of diverging air aloft typically
occur just east of a jet stream's troughs,
which are U-shaped southerly extensions
of very cold, low pressure. The air flowing
within the jet stream may be fast, but as it
rounds the U in the trough, it leaves the
trough's axis and speeds up dramatically in
jet streaks. Within the streaks core speeds
can reach 200 knots or more in the winter
months. These cores are flanked by accelerating and decelerating air, so it's best to
think of the jet streaks as pulsating bands
of wind. And it's that pulsing that sets up
the weather-makers.
I know that any mention of Coriolis
force is a stunning, sleep-inducing blow

EARLY NOVEMBER JET STREAM WX

As I write this it's November 5, 2019, and
I'm thinking about the short-term forecast. It features jet stream weather over
the Midwest and Northeast. By November
7, winds at 300 millibars/30,000 feet will
reach 100 to 150 knots in a jet streak running from the Great Lakes to Maine. The
surface forecast is for instrument conditions in snow over the northern Midwest,
the Appalachians, and northern New
England-all from a surface low traveling
from Iowa to Maine. That low may well exit
the United States and become a nor'easter
as it enters the Atlantic Ocean. It will be
interesting to see if the forecast verifies,
because this will be the first snow of the
season for many. Depending on location,

KEVIN HAND

REPEAT AFTER ME: Divergence aloft means
convergence at the surface. This fact is central to the dynamics within the areas of
fastest winds aloft. These are found in the
streaks and cores of 100-plus-knot winds
embedded within the jet stream, which
encircles the poles and undulates as it
flows from west to east (in the northern
hemisphere). Owing to the tilt of Earth's
axis, in the colder months of the year its
average track runs anywhere from 35 to
50 degrees north latitude; in the warmer
months it migrates farther north. Look on a
300-millibar (about 30,000 feet) chart and
you can see its flow.
Right about now I hear you saying, "But
I fly well below 10,000 feet, why should I
care about what's happening deep in the
flight levels?" Because those jet stream

to most of us. And yet, it comes up frequently in weather theory-because it's so
important to understand. Here's another
situation where it directly plays into the
creation of surface lows.
Aloft, as at the surface, Coriolis force
acts opposite to the pressure gradient
force, which is the force that wants to
draw air directly toward lower pressure.
Pressure gradient and Coriolis forces are
in balance when air moves in parallel isobars (on surface charts) or height contours
(constant pressure charts). But when the
air around jet cores slows and speeds up in
pulses, there's a fight between the forces.
When the air slows, pressure gradient
force asserts itself, drawing air toward low
pressure to the north. But the moment it
speeds up again, Coriolis force comes into
play, drawing air toward higher pressure to
the south. It's this back and forth motion
that creates the setup for the lousy weather
shown here. Because as it flows into the jet
core there's diverging air in two zones-
one where the air enters the core going
north, and one where it exits going south.
The huge columns of air below these
divergent zones aloft respond by drawing in
air and converging air at the surface. The only
way it can go is up, so surface pressures fall.
This creates surface lows-and the warm and
cold fronts that extend from their centers.
Congratulations! You just completed a
second-year meteorology crash course in
jet stream dynamics.

AOPA Pilot Magazine - January 2020

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