Thursday, February 03, 2011

Severe Weather Awareness Week: Hurricanes and Flooding

The Atlantic hurricane season officially begins on June 1st and continues through November 30th.  Although the number of tropical storms and hurricanes typically peaks during August and September, it is important to remember that Florida can be impacted by tropical weather systems any time during the six-month-long season. Hurricanes and tropical storms can bring very dangerous weather to areas near the coast, including strong winds, storm surge and coastal flooding, flooding from heavy rain, and tornadoes. For those away from the immediate coastline, inland flooding and tornadoes are often the most hazardous impacts from these systems.

What's it like to go through a hurricane on the ground? What are the early warning signs of an approaching tropical cyclone?

Just as every person is an individual, every hurricane is different so every experience with a storm will be unique. The summary below is of a general sequence of events one might expect from a Category 2 hurricane approaching a coastal area. What you might experience could be vastly different.
  • 96 hours before landfall
    At first there aren't any apparent signs of a storm. The pressure is steady, winds are light and fair weather clouds dot the sky. The perceptive observer will note a swell on the ocean surface of about three feet in height with a wave coming ashore every ten seconds. These waves race out far ahead of a storm at sea, but could easily be masked by locally wind-driven waves.

  • 72 hours before landfall
    Little has changed, except that the swell has increased to about six feet in height and the waves now come in every nine seconds. This means that the storm, still far over the horizon, is approaching.

  • 48 hours before landfall
    If anything, conditions have improved. The sky is now clear of clouds, the pressure is steady, and the wind is almost calm. The swell is now about nine feet and coming in every eight seconds. A hurricane watch is issued, and areas with long evacuation times are given the order to begin evacuating.

  • 36 hours before landfall
    The first signs of the storm appear. The pressure is falling steadily, the winds pick up to about 10-20mph, and the ocean swell is about 10-12 feet in height and coming in every five to seven seconds. 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 warning is issued and low lying areas and people living in mobile homes are ordered to evacuate.

  • 24 hours before landfall
    In addition to the overcast, low clouds streak by overhead. The pressure continues to fall and the wind picks up to 35 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.

  • 18 hours before landfall
    The low clouds are thicker and bring driving rain squalls with gusty winds. Winds are whistling by at 40 mph. It is hard to stand against the wind.

  • 12 hours before landfall
    The rain squalls are more frequent and the winds don't diminish after they depart. The pressure is falling rather rapidly. The wind is howling at hurricane force at 74 mph, and small, loose objects are flying through the air and branches are stripped from some trees. The sea advances with every storm wave that crashes ashore and the surface is covered with white streaks and foam patches.

  • Six hours before landfall
    The rain is constant now and the wind, now around 90 mph, drives it horizontally. The storm surge has advanced above the high tide mark. It is impossible to stand upright outside without bracing yourself, and heavy objects like coconuts and plywood sheets become airborne missiles. The wave tops are cut off and make the sea surface a whitish mass of spray.

  • One hour before landfall
    It didn't seem possible, but the rain has become heavier, a torrential downpour. Low areas inland become flooded from the rain. The winds are roaring at 105 mph, and the pressure is falling rapidly. 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. However, the pressure reaches its lowest point and the storm surge reaches the furthest inland. Wild waves crash into anything in the grasp of the surge. Soon the winds fall to near calm, but the air is uncomfortably warm and humid. Looking up you can see huge walls of cloud on every side, brilliant white in the sunlight. The winds begin to pick up slightly and the clouds on the far side of the eyewall loom overhead.

  • One 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 the monstrous waves continue to crash ashore. The pressure is now rising, the winds top out at 105 mph, and heavy items torn loose by the front side of the storm are thrown about and into sides of buildings that had been facing away from the storm’s winds before the eye passed.

  • Six hours after landfall
    The flooding rains continue, but the winds have diminished to 90 mph. The storm surge is retreating and pulling debris out to sea or stranding seaborne objects well inland. It is still impossible to go outside.

  • 12 hours after landfall
    The rain now comes in squalls and the winds begin to diminish after each squall passes. The wind is still howling at near hurricane force at 70 mph, and the ocean is covered with streaks and foam patches. The sea level returns to the high tide mark.

  • 36 hours after landfall
    The overcast has broken and the large mass of white cirrus clouds disappears over the horizon. The sky is clear and the sun seems brilliant. The winds are a steady 10 mph. All around are torn trees and battered buildings. The air may smell of vegetation and muck that was pulled up by the storm from the bottom of the sea to cover the shore. Local officials begin response efforts and some municipalities may give notice for residents to return.

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