A simple way to calculate this is to average the 700 and 500 mb winds. Otherwise, derecho development is not likely. If yes, be alert for derecho development for areas downstream for at least 250 nm. Otherwise, the potential for wind damage will probably be too localized to meet the areal criterion for a derecho. Squall lines may form on some surface or low level discontinuity line and may or may not be parallel to a front.
Squall lines produce a variety of severe weather with strong damaging winds the main threat.
Initial Outbreak Area Along and just ahead of a cold front or along an advancing dry line Along a low level trough, within or just above the moist layer Bounded by 700 mb cold front and east from there to a point where air is too stable to produce severe weather Where the dry air at 700 mb meets the overrunning warm moist air 4. Broken Line Forms along cold front Relatively weak wind shear Large CAPE and large BRN Low relative helicity 2. Back Building Can occur with or along a number of different types of surface boundaries Environments with large CAPE, small BRN Usually unidirectional flow through a deep layer with minimal shear 3. Broken Areal Formation appears to result from the interaction of outflow boundaries Low CAPE 4. If severe thunderstorms are forecast, use Figure 3 to calculate maximum wind gusts.