AOPA Pilot Magazine - February 2019 - 92

P&E

WX WATCH

MD #1648 (above left) shows a rain-snow line. Snow falls north of the line, where subfreez-

ing air extends to the surface. South of the line, precipitation falls through a warm layer,
then re-freezes as a mix of snow, ice pellets and freezing rain. MD #1651 (above right) has
the rain-snow line moving north, but a warm layer aloft and a subfreezing layer beneath
it remain over southern New England.

snow, and freezing rain covered much of
the mid-Atlantic and Ohio Valley. This
whole mess-and it covered entire states-
was tracking toward New England.
Temperatures aloft were critical.
Remember, with a warm front, warmer
air (a relative term) lies over cold. Any
precipitation from the warm frontal surface aloft will fall into that colder air. If
that colder air is below freezing-and in
winter systems, it usually is-then the
result is snow, ice pellets, or freezing
rain. It all depends on how long the falling precipitation spends in subfreezing
temperatures on the way down. If the
precipitation is in subfreezing air all the
way to the surface, then snowfall occurs.
But what if it falls through layers of
warmer, then suddenly subfreezing air?
Then snow can melt into ice pellets (aka

sleet), graupel (where supercooled water
droplets freeze on snowflakes, making snow pellets), or freezing rain. That
is exactly what happened with Avery
in the mid-Atlantic. In many locations,
there was a layer of above-freezing temperatures at 2,000 to 5,000 feet msl, and
beneath it a subfreezing layer hugging
the surface. This is the classic recipe for
freezing rain.
As Avery moved north, the National
Weather Service updated significant
events through its mesoscale discussions
(MDs). There were plenty during this
storm's trip through the eastern United
States, and no pilot would want to fly
through areas affected by any of them.
You can check for any mesoscale discussions at the Storm Prediction Center
website (www.spc.noaa.gov). Click on

KHGR 160422Z AUTO 33009KT 2SM -SN BR BKN004 OVC015 01/M01
A2974 RMK AO2 P0001 T00111011
KHGR 160413Z AUTO 33009KT 1 3/4SM -SN BR BKN004 OVC014 01/M01
A2974 RMK AO2 P0000 T00111011
KHGR 160353Z AUTO 35009KT 2SM -SN BR BKN004 OVC06 01/M01
A2974 RMK AO2 UPE05FZRAB05E16B37E48SNB48 SLP086 P0001
I1003 T00111011
KHGR 160343Z AUTO 35010KT 2 1/2SM -FZRA BR BKN004 OVC016 01/
M01 A2974 RMK AO2 UPE05FZRAB05E16B37 P0000 I1002 T00061011

METARS FROM CENTRAL MARYLAND feature truly horrible weather in low IMC. Notice how

temperatures and dew points hover around 0 degrees Celsius.

92 | AOPA PILOT February 2019

A SKEW-T CHART shows a freezing rain

setup at work. There's above-freezing air
at 850 to 700 millibars (about 5,000 to
10,000 feet) and subfreezing air below
850 millibars. Any rain falling through the
warm layer becomes supercooled and
flash-freezes as clear ice on trees, roads-
and aircraft. Red line is temperature; green
line is dew point; black line is zero celsius.

the "SPC Products" dropdown menu,
then select Mesoscale Discussions. MDs
are worth checking during any preflight
briefing-and they are posted all year,
not just the winter months. Sure, their
text portions include some meteorological jargon, but together with their
accompanying chart illustrations they
can help you understand storm dynamics. In Avery's case, after a quick glance at
MDs and the Aviation Weather Center's
Current and Forecast Icing Potential
charts, you instantly know you should
fly another day.
AOPA
EMAIL tom.horne@aopa.org

http://www.spc.noaa.gov

AOPA Pilot Magazine - February 2019

Table of Contents for the Digital Edition of AOPA Pilot Magazine - February 2019