AOPA Pilot Magazine - January 2015 - 89

Jet streaks

Pacific storm system

L

L

THESE LARGE LOW complexes typically form in the far

North Pacific, deepen over the Aleutian Islands, then track
into the northwest United states, as represented by the
arrow.

JET STREAMS AND TROUGHS ALOFT. If
you're someone who leans on shorter-term
36- to 48-hour surface analysis and prog
charts for future synoptic situations, you
may miss a big warning flag-deep troughs
aloft. These U-shaped patterns can easily be identified on the 500-millibar (about
18,000 feet) and 300-millibar (about 30,000
feet) constant pressure charts. The fastest
jet stream winds are most often found in
the areas east of a trough's apex, where the
height contours (lines depicting heights
of equal pressure surfaces) are most compressed. Like isobars on surface analysis
charts, tightly-spaced height contours indicate the strongest winds aloft. Bands of the
strongest (50 to 150 knots or more) winds
impart lifting forces to the air below-and
if that air is cold and moist enough, you can
expect major-league winter weather. This
includes deep low pressure systems with
both cold and warm fronts, each containing plenty of snow, icing conditions, low
ceilings, and low visibilities.
WINTER WARM FRONTS. Does the mention of warm fronts make you think of balmy
temperatures? Forget that. Winter warm
fronts are associated with some of the
worst icing conditions. Warmer air riding
up over colder air below can create layers where temperatures can run between
plus 2 and minus 10 degrees Celsius-right
where clear and mixed icing conditions
prevail. And any above-freezing precipitation falling from a warm frontal surface
into subfreezing air beneath it can produce
freezing rain-even if that colder air consists
of good visual meteorological conditions!
Worst of all, warm frontal weather can
be spread over several states, creating not

L

SHADED AREA SHOWS the zones of highest winds aloft,

which help create and intensify surface lows and fronts.
Lows frequently originate east of the Colorado Rockies.

just icing conditions but plenty of instrument weather. If you miss an instrument
approach, a suitable alternate may be a long
way away, and the weather en route could
be a challenge, to say the least.
TROUBLE SPOTS. Large-droplet icing
conditions have caused a number of highprofile fatal accidents. Rime ice is created
by the smallest supercooled droplets and
colder (say, minus 10 to minus 20 degrees
Celsius) temperatures. Clear ice is a product of larger droplets and warmer (minus
10 to plus 2 degrees Celsius) temperatures.
But other types of clear icing-large-droplet icing, as well as freezing drizzle-consist
of the largest droplets and highest liquid
water contents of all (each one about the
size of a raindrop). When these droplets
strike a leading edge, they run far back along
the wings and other airframe parts before
freezing. This runback ruins lift even more
severely-and quickly-than "ordinary"
clear ice. Not surprisingly, these conditions
prevail most often near the Great Lakes,
the Pacific Northwest, and the coastal New
England states-areas already heavily saturated by the moisture from their associated
large bodies of water.
NOR'EASTERS. Every winter several
coastal storm complexes make their way up
the eastern seaboard, some of them reaching farther inland than others. Strong winds
out of the northeast announce their arrival,
hence the name. Depending on the locations
of their lows and fronts (inland storm tracks
are worst), nor'easters can draw in enough
cold air to produce widespread icing conditions and instrument weather. And with
their origins and tracks over the Atlantic
Ocean, all that moisture can mean rain,

snow, ice pellets, freezing rain-or all four.
Sometimes, lots of it.
CUTOFF LOWS ALOFT. By far, the longestlasting adverse winter weather is caused by
closed lows aloft. These can be identified
on 500-millibar constant pressure charts
as islands of low pressure lurking south of
the main upper-air wind flows. However,
their closed circulations can extend from
the surface all the way to the tropopause,
which in meteorology-speak is called vertical stacking. Because they are detached
from the upper winds, closed lows don't
move with them. So they remain stationary
for days, covering thousands of square miles
with instrument meteorological conditions
in rain, snow, or ice-depending on the vertical temperature profile. And things won't
change until the next trough aloft comes
along and shoves it downwind. Some use
the term "closed low aloft" interchangeably
with "cutoff low," but there is a difference.
Cutoff lows are truly separate from the
upper-air flows to their north. Closed lows
exist within an upper trough. Either way,
these closed circulations mean you'll be
filing IFR flight plans for the duration. To
make things worse, cutoff lows are unpredictable in their movement. Sometimes they
even move westward!
Do you need a flight service weather
briefer to warn you of these winter systems? Yes and no. There's no substitute
for a good, official weather briefing, but
there's nothing to prevent you from doing
some of your own probing on the many
weather websites that depict both general and aviation weather.
AOPA
EMAIL tom.horne@aopa.org

www.aopa.org/pilot AOPA PILOT | 89
http://www.aopa.org/pilot
AOPA Pilot Magazine - January 2015

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2015
