Irene Watch: Hurricane Entraining Dry Air, Weakening Slowly

Well…this isn’t rthe first time I’ve gotten fooled by a hurricane undergoing the start of a rapid deepening in the presence of dry air nearby. Here’s how it works:

Step 1: The storm fires convection as its’ structure improves…the new storms are initially very impressive, giving the hurricane a potent look and lowering the central pressure of the cyclone.

Step 2: The winds increase temporarily in the outer parts of the storm (the size of the tropical storm force wind region expands)

Step 3: These stronger winds suck in dry air from neighoring regions, creating gaps and instabilities in the central dense overcast of the storm.

Step 4: The dry air intrusions lead to a cooling of the atmosphere in the center of the storm, reducing the storm’s power and weakening it. The pressure rises.

Step 5: Eventually, the storm finds an equilibrium between active convection (feeding on the healthy structure of the storm) and dry air intrusions. Less convection = less dry air intrusion = more favorable environment for convection. It’s a negative feedback cycle that will cause Irene to hold at a lower intensity until she makes landfall in North Carolina and again on Long Island.

This somewhat reduces the likely impacts of the storm, but it is still jsut as powerful overall as it was yesterday at this time…the wind field is simply wider and slightly weaker in the middle and the rain will simply be more episodic to the east and south of the storm. This means less total rainfall in Eastern New England and far Eastern Long Island, no change in rainfall forecasts further west, weaker winds near the point of landfall (less hurricane force wind affects and less catastrophic damage), stronger winds even in dry quadrants of the storm out to 300 miles from the center…more tropical storm force winds early in the event…perhaps more tree damage to the west of the center than previously forecast.

The track has not changed since yesterday’s forecasts were issued…a landfall in western Long Island seems most likely (along with the Outer Banks fo NC). You shouldn’t focus on the track, however. The effects will be much more widespread and uniform than a typical hurricane.

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