Geoscience Reference
In-Depth Information
Oceans. In the north Atlantic, blocking increases extra-
tropical storm activity and enhances persistence of
patterns.
Within this general picture of anomalous circulation
are smaller features that are responsible for localized
flash floods. These features can be classified into four
categories: synoptic, frontal, mesohigh, and western
events. Synoptic events occur when an intense mid-
latitude cyclone and a semi-stationary front are linked
to an intense, low-pressure trough overhead. Rainfall is
widespread, persistent and, in local cases, heavy as
thunderstorms develop repeatedly in the same general
area. Such storms occur in the seasonal transitions
between winter and summer pressure patterns over
North America. Thus, they are common from spring
to early summer, and in autumn. Frontal events are
generated by a stationary or very slow-moving front
with zonal air circulation. Upper air stability may exist,
and rainfall is usually heavy on the cool side of any
warm front, as warm air is lifted aloft. Such events
occur most frequently in July-August in the United
States. Mesohigh floods are caused by instability and
convection following the outbreak of cold air that may
or may not be associated with a front. For instance, a
stationary front may develop supercell thunderstorms
that force out, in a bubble fashion, a high-pressure cell.
These storms occur in the late afternoon or evening
and are a localized summer feature. Western-type
events refer to a range of regional circulation patterns
over the Rocky Mountains. These events tend to peak
in late summer and are associated with either
extremely meridional or zonal circulation.
Low
Heavy rain
High
Zonal
Heavy
rain
Low
Ridge
Trough
High
Meridional
Low
Heavy
rain
L
H
High
Blocking
Max
imum probable rainfall
(Australian Bureau of Meteorology, 1984)
Abnormal patterns of airflow, within general air circulation,
leading to flash flooding (adapted from Hirschboeck, 1987).
Fig. 6.5
There are limitations to the concepts of recurrence
interval and probability of exceedence presented in
Chapter 3. In many cases, calculations of recurrence
intervals for flash flooding events within the historical
record are erroneous because they give no idea of the
possible rainfalls within a given time interval. The use
of probability of exceedence graphs as a technique to
hypothesize rainfall amounts is not viable for four
reasons. First, the occurrence of a large and rare event
does not exclude one of the same magnitude or larger
happening soon after. Second, large and rare events are
clustered in time. Third, rare events may or may not
appear in rainfall records because the rain gauge
network may sample rainfalls at too gross a scale to
path of the polar jet stream, especially over continents
in the northern hemisphere. As stated in Chapter 2,
the jet stream path can be perturbed by El
Niño-Southern Oscillation events, and can be associ-
ated with persistent weather patterns giving rise to
droughts. Exaggerated meridional flow also appears to
be one of the most frequent atmospheric patterns gen-
erating flash flooding in the United States. Frequently,
it is associated with blocking of a low- or high-pressure
cell. The former occurs over warmer water while the
latter is associated with the stalling or agglutination
of mobile polar highs. Blocking in North America
commonly occurs over the north Atlantic and Pacific
