Tornadoes - Severe Convective Storms and Tornadoes: Observations and Dynamics - page 328

California tends to get tornadoes when cold upper-level troughs/cyclones from
the Pacific enhance vertical shear and onshore flow from the Pacific provides the
necessary moisture, which in tandem with the relatively cool air aloft associated
with the upper-level troughs/cyclones, provides the necessary CAPE for supercells
(cf. Figure 4.49 ). These supercells are usually shallower in depth than most
tornadoes in Tornado Alley because the tropopause is relatively low. Such con-
ditions tend to occur more frequently during the late winter months. It is beyond
the scope of this text to enumerate the synoptic conditions necessary for tornadic
supercells everywhere on Earth. The preceding examples of highly different
synoptic patterns were highlighted in order to make the point that there are wide
variations in synoptic patterns that can support tornadic supercells.
In the U. S., tornado activity begins along the Gulf states in late winter and
activity peaks first at low latitudes east of the Rockies and in much of the south-
eastern U. S., except for Florida, and peaks progressively later in the season with
latitude along the Canadian border in the Great Plains and Midwest in July
( Figure 6.20 ), when the overall probability of there being a tornado somewhere in
the U. S. is greatest.
Tornadoes have been well documented in China, Japan (where a tornado is
called a tatsu maki), all over Europe (including especially the U. K., Spain,
France, the Netherlands, Romania, Italy, and Germany), Russia, Australia, New
Zealand, Argentina, Brazil, Bangladesh, South Africa, and Canada, just to name
some, even though they are not as common as they are in the U. S. In most of the
mid-latitudes over land areas conditions are sucient for the formation of super-
cells on some days.
Tornadoes can strike at any time of day or night, but in ''Tornado Alley'' in
the U. S., part of the Great Plains, they are most common late in the afternoon
and early evening, which indicates that there is a connection between most
tornadoes and the diurnal heating cycle: convective storms are most likely to form
late in the day after maximum surface temperatures have been reached and shortly
thereafter; any supercells that form may go on to persist for hours into the early
evening. Other factors must also be taken into account: during the early evening,
as surface heating decreases and ceases altogether by sunset, the boundary layer
becomes decoupled from the free atmosphere above as vertical mixing decreases.
As a result, surface winds back (mixing down of westerly momentum from aloft,
when the winds have a substantial westerly component aloft, which is typical in
mid-latitudes, ceases) and low-level vertical shear increases. When this happens,
inertia gravity waves may be excited and may induce additional upward motion.
Other storm-chasers and I have colloquially referred to the apparent increase in
the likelihood of storm initiation and the formation of tornadoes beginning at
6 pm local time as ''6 o'clock magic''. (I recently learned from Jim Wilson at
NCAR that the same term has also been used to describe the behavior of
convective storms in eastern Colorado.)
In addition to tornadoes that are spawned by supercells, tornadoes also
occur in other types of storms. Some of these convective storms are tied to the
diurnal cycle, which varies from location to location. In mountainous terrain, the