_____ Cold air is flowing over a water surface _____ Large air/water temperature difference - greater than 20°C favourable. _____ Cyclonic curvature or contours at the 700- and 500-hPa levels _____ The formation of low-level cloud streets _____ Surface winds of 15 knots or less _____ Development and or movement of upper level negative height anomaly. For the Bering Sea and Gulf of Alaska the Navy Tactical Applications Guide suggests a 200-metre anomaly. _____ Formation of low-level vortices. These can be individual vortices or may occur in families along a shear line or low-level baroclinic zone. Within the cold air mass look for evidence of low-level baroclinic zones.
These can be induced by:
_____ Converging of flows with different over-water trajectories _____ Wrap around warm air from a synoptic low _____ Cold air flowing parallel to the water land/ice edge
_____ Approach of a 500-hPa trough or vortex with temperatures below -30°C. This is a minimum value, and most of the literature cites values 5 to 10 degrees colder. _____ Approach of a comma cloud, jet streak, or area of positive vorticity advection
_____ Rapid or unexpected pressure falls in an otherwise benign environment _____ Rapid or unexpected increase in surface winds _____ Increased snow shower activity and possibly thundershowers at shore, island, or ship locations _____ Satellite imagery showing enhanced and organized convective cloud development _____ Development of a symmetrical, spiral-shaped cloud formation _____ A clear eye may be visible during the mature stage _____ To reach "Arctic hurricane" status requires increased low level convergence or upper-level divergence resulting in the cloud pattern becoming asymmetrical and an anticyclonically-curved cirrus shield reflecting the outflow region
_____ Extrapolation - only good for short range forecast _____ Using a steering flow - use 850 to 700-hPa wind field at 1/3 to 1/2 speed _____ As applicable, consider the possibility of binary rotation _____ Numerical models _____ Climatology
_____ Will quickly dissipate once the heat source is removed _____ The majority will begin to weaken within 24 hours _____ If it remains over water the system will gradually become an expanding spiral cloud formation as it weakens _____ May merge with frontal wave