Severe Weather Watcher Handbook

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47-48  W/E & N/E  In humid, hazy weather there's not much to see before a storm arrives. The anvil darkens the sky, and if the storm is severe, you may see lines or bands that seem to converge to where the storm is coming from (like photo 44 but lower, softer). These lines of converging inflow show you that the storm has control over the air around it. When the gust front arrives, it will appear rather suddenly out of the haze because it is very low. This one (47) has a long, soft ledge along the front side. Within a few minutes (48) it is already overhead and cool, gusty winds have replaced the muggy stillness beforehand. The gust front's underside has tiny cracks and holes in it where small downdrafts puncture the cloud base. This is the wake area - a region of turbulent skies behind the lowest, forward edge of the shelf cloud. The heavy rain core will arrive shortly (lead-grey curtan, left). In this case, the storm was one of several in a squall line and the connecting gust front had a wavy shape along the front from alternating sections of stronger and weaker outflow.

49-50  NW/E & NE/E  In 49, conditions are less humid so the view is clearer. An approaching gust front shows up as a dark cloud bank with a crinkly underside, and heavy rain behind. At the moment, it is in a weaker phase. Gust fronts regularly alternate between weaker, slower periods and stronger, faster ones as new downdrafts reinforce the outflow. By the time it passed overhead in 50, a much stronger phase was underway and the shelf's forward edge was much closer to the ground - digging forward, plowed by an intense wind squall (downburst). In its wake, a vaulted appearance shows you the shape of the cold pool: the vault edge is the boundary where the cooler air is lifting and condensing warm air along a sloped surface (see diagram, opposite). Watching even the non-severe stages (as in 48 and 49) is useful because a sudden change to severe, damaging winds may only be a minute away.

51-52  W/E & SE/E  In less humid areas, the gust front will be higher-based and visible well before arriving. Here (51), the forward edge is lined with many little spikes in rising, condensing air. As the moister cool air advances, it mixes unevenly with the warmer (drier) air it is moving into, creating places where updrafts are either stronger or wetter than other parts. These "teeth" can look briefly like funnels, but are not (see photo 136). In rare, extreme cases a single, intense vortex can become a gustnado (see pg. 40) and you may then see a column of dust below the cloud tuft there. After the gust front passes (52), these cloud spikes stand out against the lighter background and can be very deceiving. Behind the leading edge, this more typical shelf cloud is flat and not as dark, suggesting a higher cloud base and weaker outflow than in 50.

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