JPATS Weather - NETC

JPATS AVIATION WEATHER BOOKLET
visibilities are generally visual meteorological conditions (VMC), but isolated instrument
meteorological conditions (IMC) exist in heavy precipitation and near thunderstorms. Wider
areas of IMC conditions can exist in winter due to snow showers.
Turbulence — Many active cold fronts have turbulent cloud systems associated with them, but
thunderstorms may not always be present. Even when there are no clouds, turbulence may still
be a problem. As a rule, expect a rough flight in the vicinity of an active cold front, even when
flying at a considerable altitude.
Precipitation and icing conditions — Active cold fronts usually have a relatively narrow belt of
precipitation, especially if the precipitation is showery. Icing may be severe in cumuliform
clouds. Slow-moving cold fronts may have a broader area of precipitation and a greater threat of
remaining in icing conditions for a longer period.
Thunderstorms and squall lines — Severe weather is implied to exist in areas of reported
thunderstorms. Chapter 4 will detail the hazards associated with thunderstorms.
Squall Lines
Figure 3-10 — Squall Line Formation
A squall line is a line of violent thunderstorms. They are indicated on surface charts by a dashed,
double-dotted purple line. They develop 50 to 300 miles ahead of the cold front and roughly
parallel to it. They form when cold air downdrafts flowing ahead of a cold front lift additional
warm, unstable air. The uplifted air develops its own updrafts and downdrafts and starts the
thunderstorm development cycle (Figure 3-10). Sometimes, however, squall lines can be located
nowhere near a cold front, possibly from the convergence of air flows at one location. Squall
lines are usually the most intense during the late afternoon and early evening hours, just after
maximum daytime heating.
It is often impossible to fly through squall lines, even with radar, since the storms are extremely
close to one another. Similar to cold fronts, Squall lines will also have a 90° wind shift from the
SW to the NW.
Version 3.2/Dec 08 3-11