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AIRMET Zulu does not catch all
widespread icing events 
AIRMET Zulu is issued by the Aviation Weather
Center for a
widespread area of icing. Widespread is defined as icing that
is
affecting or is forecast affect an area greater than 3,000 square
miles. For reference, 3,000 square miles is a little over
half
the size of the state of Connecticut.
AIRMET Zulu isn't designed to put a
fence around
you all possible areas of structural icing, even widespead
icing.
One example, is icing associated with deep moist convection,
better known as thunderstorms. Even before towering cumulus
reach
the point of letting go of that first lightning strike, you won't
likely see an AIRMET issued for this field of cumuliform clouds.
In many cases, these areas of potential icing don't typically
meet the AIRMET criteria. This is a frequent occurrence
throughout the summer.
Notice that in the icing AIRMETs image above (click here
for a larger image) that AIRMET Zulu isn't present anywhere in
Illinois, Missouri or Arkansas despite the high probability of icing as
shown on the Current Icing Product (CIP) shown on the left (click here for a larger
image).
Turns out that most of the icing in the
Mid-Mississippi Valley area is due to moist deep convection as shown on
the image below (click here
for a larger image). The freezing level is likely in the the
low
teens, so flight at or above the freezing level will result in heavy
icing and possible supercooled large drop (SLD) icing.
Once thunderstorms reach convective
SIGMET criteria, the Aviation
Weather Center will put a fence around this area and call it a
convective SIGMET. Convective SIGMETs always imply the possibility for
severe icing so the Aviation Weather Center doesn't issue AIRMET Zulu
or a SIGMET for severe icing since it would be redundant.
Keep in
mind that icing still may exist around any convective cells at an
altitude above the freezing level. Often thunderstorms
produce
convective debris that isn't likely turbulent, but may contain
supercooled liquid water. All pilots should anticipate a high
probability of intense icing flying into visible moisture around
convectivity activity and there will not likely be an AIRMET or SIGMET
telling you about it.
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