C. Boundaries Not Involving Air Masses
and satellite information, the analysis and prognosis of the boundaries are relatively straightforward, provided the analyst is aware of the basic types of structures seen.
1. Wind Shift Lines
As a final word on outflow boundaries, the reader needs to be aware of the flash flood potential in situations involving this phenomenon (also see III.F and II.III.C). Maddox et al. (1979) have noted that many of the flash floods which they studied (34%) were associated with storms developing along an outflow boundary. Therefore, the analyst needs to be concerned with factors relating to flash flood potential. A significant factor in the frequent occurrence of flash floods during the nighttime hours is the low-level jet phenomenon. Although the low-level jet will be discussed more fully later, the analyst should note the axis of the low level flow and where the speed maxima are along that axis. Any situation wherein strong lowlevel flow impinges on an outflow boundary has flash flood potential, when the undisturbed lowlevel flow is moist and unstable.
The careful analysis of surface data frequently reveals organized windshift lines which are apparently non-frontal in character. Since, by definition, no clear change of air mass is involved, it is often difficult to explain or understand their origins. Nevertheless, they do occur and, since they may give rise to surface convergence, they can be involved in severe weather events. At times, the wind shift may be traceable to an old outflow boundary from thunderstorms occurring, say, the previous day. Modification of the outflow air may have erased any apparent temperature
Fig. 3.25. Time averaged precipitation produced in several periods during the life of the mesosystem in Fig. 3.24 (after Fujita and Brown, 1958). Reported wind gusts are plotted in the upper left.
Fig. 3.24. Relationship between radar echoes (black areas) to overall mesosystem produced by the storms associated with the echoes (after Fujita and Brown, 1958).
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