Hurricane timeline: Counting down the hours

Excellent summary of a hurricane's life cycle from the Herald Tribune:

Tropical cyclone is the meteorological term for a storm system characterized by a low-pressure center and thunderstorms, producing strong wind and flooding rain. Tropical cyclones form over warm waters from pre-existing disturbances. These disturbances typically emerge every three or four days from the coast of Africa as tropical waves that consist of areas of unsettled weather. Tropical cyclones can also form from the trailing ends of cold fronts and from upper-level lows.

In the Atlantic Basin, tropical cyclones are called hurricanes and must have at least three conditions to form: A pre-existing disturbance with thunderstorms; warm (at least 80 degree) ocean temperatures to a depth of about 150 feet; and light upper level winds that do not change much in direction and speed throughout the depth of the atmosphere.

If weather and ocean conditions continue to be favorable, the system can strengthen first to a tropical depression, tropical storm and then a hurricane.

Every hurricane is different and there are many factors that contribute to its development. However, the following is a general sequence of events that could occur during the development of a Category 2 hurricane (wind speed 96-110 mph) approaching a coastal area.

96 hours before landfall

At first there aren't any apparent signs of a storm. The barometer is steady, winds are light and variable, and fair-weather cumulus clouds appear.

72 hours

Little has changed, except that the swell on the ocean surface has increased to about six feet and the waves come in every nine seconds. This means that the storm, far over the horizon, is approaching.

48 hours

The sky is now clear of clouds, the barometer is steady, and the wind is almost calm.

The swell is now about nine feet and coming in every eight seconds.

36 hours

The first signs of the storm appear. The barometer is falling slightly, the wind is around 11 mph, and the ocean swell is about 13 feet and coming in seven seconds apart. On the horizon, a large mass of white cirrus clouds appear.

As the veil of clouds approaches, it covers more of the horizon.

A hurricane watch is issued, and areas with long evacuation times are given the order to begin.

30 hours

The sky is covered by a high overcast. The barometer is falling at .1 millibar per hour; winds pick up to about 23 mph.

The ocean swell, coming in five seconds apart, is beginning to be obscured by wind-driven waves, and small whitecaps begin to appear on the ocean surface.

24 hours

Small low clouds appear overhead. The barometer is falling by .2 millibars per hour, the wind picks up to 34 mph. The wind driven waves are covered in whitecaps, and streaks of foam begin to ride over the surface. Evacuations should be completed and final preparations made by this time. A hurricane warning is issued, and people living in low-lying areas and in mobile homes are ordered to evacuate.

18 hours

The low clouds are thicker and bring driving rain squalls with gusty winds. The barometer is steadily falling at half a millibar per hour and the winds are whistling by at 46 mph. It is hard to stand against the wind.

12 hours

The rain squalls are more frequent and the winds don't diminish after they depart. The cloud ceiling is getting lower, and the barometer is falling at 1 millibar per hour. The wind is howling at hurricane force at 74 mph. The sea advances with every storm wave that crashes ashore, and foam patches.

6 hours

The rain is constant and the 92 mph wind drives it horizontally. The barometer is falling 1.5 millibars per hour, and the storm surge has advanced above the high tide mark. The sea surface is a whitish mass of spray. It is impossible to stand upright outside without bracing yourself.

1 hour

The rain becomes heavier. Low areas inland become flooded. The winds are at 104 mph, and the barometer is falling at 2 millibars per hour. The sea is white with foam and streaks. The storm surge has covered coastal roads and 16 foot waves crash into buildings near the shore.

The eye

Just as the storm reaches its peak, the winds begin to slacken, and the sky starts to brighten. The rain ends abruptly, and the clouds break and blue sky is seen. The barometer continues falling at 3 millibars per hour and the storm surge reaches the furthest inland.

The winds fall to near calm, but the air is uncomfortably warm and humid. Huge walls of cloud appear on every side, brilliant white in the sunlight.

At this point, the barometer stops falling and in a moment begins to rise, soon as fast as it fell. The winds begin to pick up slightly and the clouds on the far side of the eye wall loom overhead.

AFTER THE STORM

1 hour after landfall

The sky darkens and the winds and rain return just as heavy as they were before the eye. The storm surge begins a slow retreat, but waves continue to crash ashore. The barometer is rising at 2 millibars per hour, and the winds top out at 104 mph.

6 hours

The flooding rains continue, but the winds have diminished to 92 mph. The storm surge is retreating and pulling inland debris out to sea.

12 hours

The rain now comes in squalls, and the winds begin to diminish after each squall passes. The cloud ceiling is rising, as is the barometer at 1 millibar per hour. The wind is still howling at near hurricane force at 69 mph, and the ocean is covered with streaks and foam patches. The sea level returns to the high tide mark.

24 hours

The clouds break into smaller fragments and the high overcast is seen again. The barometer is rising by .2 millibars per hour, the wind falls to 34 mph. The surge has fully retreated from land, but the ocean surface is still covered by small whitecaps and large waves.

36 hours

The overcast has broken and the large mass of white cirrus clouds disappears over the horizon. The barometer is rising slightly, the winds are a steady 11 mph.