Interpretation of Hurricane Watches and Warnings

Hurricane Warning (red) and Tropical Storm Warning (blue)

Cone of uncertainty with watch/warning areas delineated

The hurricane watch and warning system combines the storm- and location-oriented perspectives of forecasts involving tropical cyclones that approach land. It utilizes the cone of uncertainty, but the warned area will sometimes extend to areas outside the cone, as in this graphic that was issued for Hurricane Rita. There are two reasons for this.

Hurricane Watch (pink) and Tropical Storm Warning (blue)

First, if the cone includes two-thirds of the statistical uncertainty about the track of the storm center, the warning system is aiming to include closer to 95 percent of that uncertainty.

Second, if the cone is only making statements about the probable track of the center, the warning system has to take into account the storm's entire wind field. It has to be capable of warning people if any part of the storm's hurricane-or tropical storm-force wind fields might be likely to intersect their specific location and produce hurricane or tropical storm conditions there. As the cone of uncertainty predicts the position of the storm center only, the purpose of hurricane and tropical storm watches and warnings is to alert populations and emergency management agencies of the possibility of hurricane or tropical storm conditions arising from any part of the storm circulation.

The storm need not itself even make landfall (defined as the center making landfall) for it to produce hurricane- or tropical storm-force winds at a location, because the winds around the center of low pressure could still intersect land. How far offshore the storm can be centered can be and still do this depends on its wind structure (radii of the hurricane- and tropical storm-force winds) of the particular storm. A place is officially considered to have endured a "strike" if the center passes to within 50 nautical miles (58 miles) to the right or 75 nautical miles (86 miles) to the left of it. The difference is to account for the normally larger wind fields on the right side of the storm (northern hemisphere).

Graphical depiction of size forecast with labeling of storm's four quadrants

Watches and warnings utilize projections of track (direction and forward speed), intensity (maximum sustained wind speed), and size (wind radii), together with observed data to predict the likelihood that certain wind speeds will be found at specific coastal locations as a result of the close approach or landfall of a tropical cyclone.

Although the hurricane-force winds in a medium-sized storm might intersect a stretch of coastline around 100 miles long, stretches of more than 300 miles are typically placed under hurricane watches and warnings. Such "overwarning" is necessary to take into account the uncertainty in the track forecast as well as in the size and structure of the storm's hurricane- and tropical storm-force wind fields.

Coastal areas under hurricane warnings are marked in red on the cone of uncertainty product; hurricane watches, which are farther out in time, are in pink (as in the cones for Hurricane Rita, top of page). Tropical storm warning areas are marked in blue, watch areas in yellow. Very often an area under a watch one day will be under warnings the next, that is, if the storm's actual direction, speed, and size were close enough to what had been predicted the day before.

Hurricane Watches and Warnings: Technical Definitions
	Hurricane Warning		Hurricane conditions expected somewhere inside the warned area. Because the arrival of tropical storm-force winds may be enough to block some coastal evacuation routes, the hurricane warning is posted 36 hours in advance of the anticipated onset of the tropical storm-force winds.
	Hurricane Watch		Hurricane conditions possible somewhere inside the watch area. Because the arrival of tropical storm-force winds may be enough to block some coastal evacuation routes, the hurricane watch is posted 48 hours in advance of the anticipated onset of the tropical storm-force winds.
	Tropical Storm Warning		Tropical storm conditions expected somewhere inside the warned area within the next 36 hours.
	Tropical Storm Watch		Tropical storm conditions possible somewhere inside the watch area within the next 48 hours.

Actual watch and warning lead times may be greater than the officially-defined times, and the lengths of warned coastline correspondingly longer, to accommodate the reality of steadily increasing coastal populations and consequentially longer evacuation clearance times.

The lengths and placements of warned areas are based on many factors. These include the current position and projected track of the storm - with a view toward the landfall of the center - as well as the actual and projected intensity, the estimated and projected wind radii, the uncertainties in those estimates and forecasts, and the amount of lead time required in the specific area under consideration. The warning system attempts to achieve the right balance between the cost in money, effort, and credibility, versus potential lives saved or lost in case of a smaller warning area. This is the overwarning dilemma faced by meteorologists and emergency managers alike, and those are the trade-offs.

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Interpretation of Hurricane Watches and Warnings

Hurricane Watches and Warnings: Technical Definitions