Severe Weather Watcher Handbook

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42  N/SE,  43  NW/SE   In 42, the flanking line of an HP supercell (in which the inflow is turned forward and precedes the heaviest rain) has been converted into a surging gust front. Inflow air still flows toward the distant core and lowering along this axis, but it is being pushed quickly forward during this outflow phase. If the outflow weakens and the storm regains control, the axis will darken, widen and slow down again. A higher-based (drier air), slow-moving gust front in 43 has separated from the storm in the background. It is becoming a roll cloud, a totally distinct cloud roll or tube below and ahead of the higher anvil layers. It stands out as a lighter cloud (reflected light) against the darker storm.

The diagram shows a cross-section through a typical gust front/shelf cloud (similar to photos 42, 48 or 50) which has moved just ahead of the rain curtain. Cool air emanating from the rain core is shaped like an elongated bubble. Its leading edge forms the lowest, darkest part of the shelf. The dashed line is the cool-warm boundary.

44-45  NW/E and NE/E  Photo 44 (most often seen in Alberta) is like a dry weather version of 42 in which the inflow area has been turned forward so that it is located on the storm's E or NE side with winds entering from the E or SE. This only happens after a strong initial phase drops enough precipitation to create a pool or cool outflow that then redirects inflow to the forward side (seen here as the distant darker base). The original flanking line is now becoming a gust front and is beginning to separate (note the light streaks along the axis, lower left). Despite this, the storm is still well-organized, as seen by the control over the regional flow (radiating bands) and the dark, regenerating core region. A short while later, in 45, the gust front has moved far ahead of the storm, but is still connected to it far to the left. It consists of a sloped band of mid-cloud and a lower roll cloud drifting away from the anvil (top left) into a clear sky. An hour later, this outflow boundary exploded into a line of new storms after it slowed down and began sprouting new towers.

46  NW/NW  Sunset silhouettes this roll cloud, which formed along a gust front surge out of the backside of a collapsing storm. As cool outflow slides under warmer air, the lift creates a cloud that rolls forward in reverse (the bottom moves forward but the upper surface arcs back). Despite hints of new convection within it, this one soon evaporated.

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