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72-77 W to N/NE Sequence showing formation and evolution
of a wall cloud. 72 shows an ordinary thunderstorm with flaking line towers
leading up to the rain core at right. In 73, after 15 minutes, the updraft has
become more compact. Although there is no typical lowering, the whole base is
lower (like photo 69) and draws in some of the outflow (seen by tilt toward
rain). The grey area lower left is another, unrelated shower. By 74, (7 minutes
later), the base is even lower and a circular structure is emerging. The storm
has become an LP supercell with a very short flank and a narrow updraft column
at the back atop the lowered wall cloud. Only 7 minutes later (75), a fully-formed
mesocyclone spins the air upward. Tufts of scud and a brief funnel were seen
below the base. A line of scud from the rain flows into the wall cloud. This
intense phase lasted about ten minutes, until a surge of outflow swept around
the storm's south side and undercut the mesocyclone. The storm, meanwhile, moved
past and, after another 13 minutes, in 76 it is much weaker. The lowering, squeezed
forward (to the right) by outflow is elongated and no longer rotating. You can
see the gust front axis as an arc from top left toward the lowering. But with
a very unstable atmosphere, it recovered as soon as outflow weakened (77, 8
minutes later) and became an even larger, fully rounded, low wall cloud. The
storm was becoming a more dangerous classic supercell. A dust cloud below it
may have been a brief tornado touchdown.
78-83 NW/SE Formation and dissipation of a classic supercell
wall cloud. The rain-free base of this storm (78) has no lowering and is turned
slightly forward from the rain core to its north. Two minutes later (79), a
rain foot has kicked up a chunk of scud below the base. After another two minutes
(80), a lowering is taking shape. It is already compact (likely circular) and
has pointy tufts where the inflow air rises straight up into the base. A closer
look in 81 (3 minutes later) still shows the rain foot connected to a gangly
wall cloud that spins and gyrates against the bright sky beyond. But the wider
view (82) tells another story. Outflow is also pushing south from the rain core
(dark line, right centre) and the lowering is elongating as outflow there finally
pushes beyond the strong updraft. By 83 (2 minutes later), the wall cloud is
flattening and a gust front is beginning to separate from the rain area (note
light behind forward edge, top right) as outflow takes over the scene. At peak
intensity, this storm was a classic supercell. Later, it evolved into a raging
HP supercell with a long swath of damaging winds and baseball hail.
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Created :
2002-08-26
Modified :
2002-12-19
Reviewed :
2002-12-19
Url of this page : http://www.msc.ec.gc.ca /education/severe_weather/page29_e.cfm
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